 @@ -1,1444 +1,1444 @@
-.\"	$NetBSD: sh.1,v 1.176 2018/03/13 19:18:53 uwe Exp $
+.\"	$NetBSD: sh.1,v 1.177 2018/03/13 19:35:46 uwe Exp $
 .\" Copyright (c) 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
 .\"
 .\" This code is derived from software contributed to Berkeley by
 .\" Kenneth Almquist.
 .\"
 .\" Redistribution and use in source and binary forms, with or without
 .\" modification, are permitted provided that the following conditions
 .\" are met:
 .\" 1. Redistributions of source code must retain the above copyright
 .\"    notice, this list of conditions and the following disclaimer.
 .\" 2. Redistributions in binary form must reproduce the above copyright
 .\"    notice, this list of conditions and the following disclaimer in the
 .\"    documentation and/or other materials provided with the distribution.
 .\" 3. Neither the name of the University nor the names of its contributors
 .\"    may be used to endorse or promote products derived from this software
 .\"    without specific prior written permission.
 .\"
 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 .\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
 .\"	@(#)sh.1	8.6 (Berkeley) 5/4/95
 .\"
 .Dd October 24, 2017
 .Dt SH 1
 .\" everything except c o and s (keep them ordered)
 .ds flags abCEeFfhIiLmnpquVvXx
 .Os
 .Sh NAME
 .Nm sh
 .Nd command interpreter (shell)
 .Sh SYNOPSIS
 .Nm
 .Bk -words
 .Op Fl \*[flags]
 .Op Cm +\*[flags]
 .Ek
 .Bk -words
 .Op Fl o Ar option_name
 .Op Cm +o Ar option_name
 .Ek
 .Bk -words
 .Op Ar command_file Oo Ar argument ... Oc
 .Ek
 .Nm
 .Fl c
 .Bk -words
 .Op Fl s
 .Op Fl \*[flags]
 .Op Cm +\*[flags]
 .Ek
 .Bk -words
 .Op Fl o Ar option_name
 .Op Cm +o Ar option_name
 .Ek
 .Bk -words
 .Ar command_string
 .Op Ar command_name Oo Ar argument ... Oc
 .Ek
 .Nm
 .Fl s
 .Bk -words
 .Op Fl \*[flags]
 .Op Cm +\*[flags]
 .Ek
 .Bk -words
 .Op Fl o Ar option_name
 .Op Cm +o Ar option_name
 .Ek
 .Bk -words
 .Op Ar argument ...
 .Ek
 .Sh DESCRIPTION
 .Nm
 is the standard command interpreter for the system.
 The current version of
 .Nm
 is in the process of being changed to conform more closely to the
 POSIX 1003.2 and 1003.2a specifications for the shell.
 This version has many
 features which make it appear similar in some respects to the Korn shell,
 but it is not a Korn shell clone (see
 .Xr ksh 1 ) .
 This man page is not intended
 to be a tutorial or a complete specification of the shell.
 .Ss Overview
 The shell is a command that reads lines from either a file or the
 terminal, interprets them, and generally executes other commands.
 A shell is the program that is running when a user logs into the system.
 (Users can select which shell is executed for them at login with the
 .Xr chsh 1
 command).
 The shell implements a language that has flow control
 constructs, a macro facility that provides a variety of features in
 addition to data storage, along with built in history and line editing
 capabilities.
 It incorporates many features to aid interactive use and
 has the advantage that the interpretative language is common to both
 interactive and non-interactive use (shell scripts).
 That is, commands
 can be typed directly to the running shell or can be put into a file and
 the file can be executed directly by the shell.
 .Ss Invocation
 If no arguments are present and if the standard input,
 and standard error output, of the shell
 are connected to a terminal (or terminals, or if the
 .Fl i
 flag is set),
 and the
 .Fl c
 option is not present, the shell is considered an interactive shell.
 An interactive shell generally prompts before each command and handles
 programming and command errors differently (as described below).
 When first starting,
 the shell inspects argument 0, and if it begins with a dash
 .Sq - ,
 the shell is also considered
 a login shell.
 This is normally done automatically by the system
 when the user first logs in.
 A login shell first reads commands
 from the files
 .Pa /etc/profile
 and
 .Pa .profile
 if they exist.
 If the environment variable
 .Ev ENV
 is set on entry to a shell,
 or is set in the
 .Pa .profile
 of a login shell,
 and either the shell is interactive, or the
 .Cm posix
 option is not set,
 the shell next reads
 commands from the file named in
 .Ev ENV .
 Therefore, a user should place commands that are to be executed only at
 login time in the
 .Pa .profile
 file, and commands that are executed for every shell inside the
 .Ev ENV
 file.
 To set the
 .Ev ENV
 variable to some file, place the following line in your
 .Pa .profile
 of your home directory
 .Pp
 .Dl ENV=$HOME/.shinit; export ENV
 .Pp
 substituting for
 .Dq .shinit
 any filename you wish.
 Since the
 .Ev ENV
 file can be read for every invocation of the shell, including shell scripts
 and non-interactive shells, the following paradigm is useful for
 restricting commands in the
 .Ev ENV
 file to interactive invocations.
 Place commands within the
 .Dq Ic case
 and
 .Dq Ic esac
 below (these commands are described later):
 .Pp
 .Bl -item -compact -offset indent
 .It
 .Li case $- in *i*)
 .Bl -item -compact -offset indent
 .It
 .Li # commands for interactive use only
 .It
 .Li ...
 .El
 .It
 .Li esac
 .El
 .Pp
 If command line arguments besides the options have been specified, and
 neither
 .Fl c
 nor
 .Fl s
 was given, then the shell treats the first argument
 as the name of a file from which to read commands (a shell script).
 This also becomes $0 and the remaining arguments are set as the
 positional parameters of the shell ($1, $2, etc).
 Otherwise, if
 .Fl c
 was given, then the first argument, which must exist,
 is taken to be a string of
 .Nm
 commands to execute.
 Then if any additional arguments follow the command string,
 those arguments become $0, $1, ...
 Otherwise, if additional arguments were given
 (which implies that
 .Fl s
 was set)
 those arguments become $1, $2, ...
 If $0 has not been set by the preceding processing, it
 will be set to argv[0] as passed to the shell, which will
 usually be the name of the shell itself.
 If
 .Fl s
 was given, or if neither
 .Fl c
 nor any additional (non-option) arguments were present,
 the shell reads commands from its standard input.
 .Ss Argument List Processing
 Currently, all of the single letter options that can meaningfully
 be set using the
 .Ic set
 built-in, have a corresponding name
 that can be used as an argument to the
 .Fl o
 option.
 The set
 .Fl o
 name is provided next to the single letter option in
 the description below.
 Some options have only a long name, they are described after
 the flag options, they are used with
 .Fl o
 or
 .Cm +o
 only, either on the command line, or with the
 .Ic set
 built-in command.
 Other options described are for the command line only.
 Specifying a dash
 .Dq -
 turns the option on, while using a plus
 .Dq +
 disables the option.
 The following options can be set from the command line and,
 unless otherwise stated, with the
 .Ic set
 built-in (described later).
 .\"
 .\" strlen("quietprofile") == strlen("local_lineno"): pick the latter
 .\" to give the indent as the _ in local_lineno, and the fi ligature in
 .\" quietprofile combine to make "local_lineno' slightly wider when printed
 .\" (in italics) in a variable width font.   Probably should test the actual
 .\" widths and use the wider, but I am not sure if mandoc is up to that...
 .\" (and I don't know how to get at the font that will be used easily anyway!)
 .\" The X's just provide a little extra space.
 .Bl -tag -width \-WXXlocal_linenoXX -offset indent
 .\"
 .It Fl a Em allexport
 Automatically export any variable to which a value is assigned
 while this flag is set.
 .It Fl b Em notify
 Enable asynchronous notification of background job completion.
 (Not implemented.)
 .It Fl C Em noclobber
 Don't overwrite existing files with
 .Dq > .
 .It Fl c
 Read commands from the
 .Ar command_string
 operand instead of, or in addition to, from the standard input.
 Special parameter 0 will be set from the
 .Ar command_name
 operand if given, and the positional parameters ($1, $2, etc.)
 set from the remaining argument operands, if any.
 .Fl c
 is only available at invocation, it cannot be
 .Ic set ,
 and there is no form using
 .Dq \&+ .
 .It Fl E Em emacs
 Enable the built-in emacs style
 command line editor (disables
 .Fl V
 if it has been set).
 (See the
 .Sx Command Line Editing
 section below.)
 .It Fl e Em errexit
 If not interactive, exit immediately if any untested command fails.
 If interactive, and an untested command fails,
 cease all processing of the current command and return to
 prompt for a new command.
 The exit status of a command is considered to be
 explicitly tested if the command is used to control an
 .Ic if ,
 .Ic elif ,
 .Ic while ,
 or
 .Ic until ,
 or if the command is the left hand operand of an
 .Dq &&
 or
 .Dq ||
 operator,
 or if it is a pipeline (or simple command) preceded by the
 .Dq \&!
 operator.
 With pipelines, only the status of the entire pipeline
 (indicated by the last command it contains)
 is tested when
 .Fl e
 is set to determine if the shell should exit.
 .It Fl F Em fork
 Cause the shell to always use
 .Xr fork 2
 instead of attempting
 .Xr vfork 2
 when it needs to create a new process.
 This should normally have no visible effect,
 but can slow execution.
 The
 .Nm
 can be compiled to always use
 .Xr fork 2
 in which case altering the
 .Fl F
 flag has no effect.
 .It Fl f Em noglob
 Disable pathname expansion.
 .It Fl h Em trackall
 Functions defined while this option is set will have paths bound to
 commands to be executed by the function at the time of the definition.
 When off when a function is defined,
 the file system is searched for commands each time the function is invoked.
 (Not implemented.)
 .It Fl I Em ignoreeof
 Ignore EOFs from input when interactive.
 (After a large number of consecutive EOFs the shell will exit anyway.)
 .It Fl i Em interactive
 Force the shell to behave interactively.
 .It Fl L Em local_lineno
 When set, before a function is defined,
 causes the variable
 .Ev LINENO
 when used within the function,
 to refer to the line number defined such that
 first line of the function is line 1.
 When reset,
 .Ev LINENO
 in a function refers to the line number within the file
 within which the definition of the function occurs.
 This option defaults to
 .Dq on
 in this shell.
 For more details see the section
 .Sx LINENO
 below.
 .It Fl m Em monitor
 Turn on job control (set automatically when interactive).
 .It Fl n Em noexec
 Read and parse commands, but do not execute them.
 This is useful for checking the syntax of shell scripts.
 If
 .Fl n
 becomes set in an interactive shell, it will automatically be
 cleared just before the next time the command line prompt
 .Pq Ev PS1
 is written.
 .It Fl p Em nopriv
 Do not attempt to reset effective UID if it does not match UID.
 This is not set by default to help avoid incorrect usage by setuid
 root programs via
 .Xr system 3
 or
 .Xr popen 3 .
 .It Fl q Em quietprofile
 If the
 .Fl v
 or
 .Fl x
 options have been set, do not apply them when reading
 initialization files, these being
 .Pa /etc/profile ,
 .Pa .profile ,
 and the file specified by the
 .Ev ENV
 environment variable.
 .It Fl s Em stdin
 Read commands from standard input (set automatically if
 neither
 .Fl c
 nor file arguments are present).
 If after processing a command_string with the
 .Fl c
 option, the shell has not exited, and the
 .Fl s
 option is set, it will continue reading more commands from standard input.
 This option has no effect when set or reset after the shell has
 already started reading from the command_file, or from standard input.
 Note that the
 .Fl s
 flag being set does not cause the shell to be interactive.
 .It Fl u Em nounset
 Write a message to standard error when attempting to obtain a
 value from a variable that is not set,
 and if the shell is not interactive, exit immediately.
 For interactive shells, instead return immediately to the command prompt
 and read the next command.
 Note that expansions (described later, see
 .Sx Word Expansions
 below) using the
 .Sq \&+ ,
 .Sq \&\- ,
 .Sq \&= ,
 or
 .Sq \&?
 operators test if the variable is set, before attempting to
 obtain its value, and hence are unaffected by
 .Fl u .
 .It Fl V Em vi
 Enable the built-in
 .Xr vi 1
 command line editor (disables
 .Fl E
 if it has been set).
 (See the
 .Sx Command Line Editing
 section below.)
 .It Fl v Em verbose
 The shell writes its input to standard error as it is read.
 Useful for debugging.
 .It Fl X Em Xtrace
 Cause output from the
 .Ic xtrace
 .Pq Fl x
 option to be sent to standard error as it exists when the
 .Fl X
 option is enabled (regardless of its previous state.)
 For example:
-.Bd -compact -literal
+.Bd -literal -compact
         set -X 2>/tmp/trace-file
 .Ed
 will arrange for tracing output to be sent to the file named,
 instead of wherever it was previously being sent,
 until the X option is set again, or cleared.
 .Pp
 Each change (set or clear) to
 .Fl X
 is also performed upon
 .Fl x ,
 but not the converse.
 .It Fl x Em xtrace
 Write each command to standard error (preceded by the expanded value of
 .Dq $PS4 )
 before it is executed.
 Unless
 .Fl X
 is set,
 .Dq "standard error"
 means that which existed immediately before any redirections to
 be applied to the command are performed.
 Useful for debugging.
 .It "\ \ " Em cdprint
 Make an interactive shell always print the new directory name when
 changed by the
 .Ic cd
 command.
 In a non-interactive shell this option has no effect.
 .It "\ \ " Em nolog
 Prevent the entry of function definitions into the command history (see
 .Ic fc
 in the
 .Sx Built-ins
 section.)
 (Not implemented.)
 .It "\ \ " Em pipefail
 If set, the way the exit status of a pipeline is determined
 is altered.
 See
 .Sx Pipelines
 below for the details.
 .It "\ \ " Em posix
 Enables closer adherence to the POSIX shell standard.
 This option will default set at shell startup if the
 environment variable
 .Ev POSIXLY_CORRECT
 is present.
 That can be overridden (set or reset) by the
 .Fl o
 option on the command line.
 Currently this option controls whether (!posix) or not (posix)
 the file given by the
 .Ev ENV
 variable is read at startup by a non-interactive shell.
 It also controls whether file descriptors greater than 2
 opened using the
 .Ic exec
 built-in command are passed on to utilities executed
 .Dq ( yes
 in posix mode),
 whether a colon (:) terminates the user name in tilde (~) expansions
 other than in assignment statements
 .Dq ( no
 in posix mode),
 and whether the shell treats
 an empty brace-list compound statement as a syntax error
 (expected by POSIX) or permits it.
 Such statements
 .Dq "{ }"
 can be useful when defining dummy functions.
 Lastly, in posix mode, only one
 .Dq \&!
 is permitted before a pipeline.
 .It "\ \ " Em promptcmds
 Allows command substitutions (as well as parameter
 and arithmetic expansions, which are always performed)
 upon the prompt strings
 .Ev PS1 ,
 .Ev PS2 ,
 and
 .Ev PS4
 each time, before they are output.
 This option should not be set until after the prompts
 have been set (or verified) to avoid accidentally importing
 unwanted command substitutions from the environment.
 .It "\ \ " Em tabcomplete
 Enables filename completion in the command line editor.
 Typing a tab character will extend the current input word to match a
 filename.
 If more than one filename matches it is only extended to be the common prefix.
 Typing a second tab character will list all the matching names.
 One of the editing modes, either
 .Fl E
 or
 .Fl V ,
 must be enabled for this to work.
 .El
 .Ss Lexical Structure
 The shell reads input in terms of lines from a file and breaks it up into
 words at whitespace (blanks and tabs), and at certain sequences of
 characters that are special to the shell called
 .Dq operators .
 There are two types of operators: control operators and redirection
 operators (their meaning is discussed later).
 The following is a list of operators:
 .Bl -ohang -offset indent
 .It "Control operators:"
 .Dl &  &&  \&(  \&)  \&;  ;; ;& | || <newline>
 .It "Redirection operators:"
 .Dl <  >  >|  <<  >>  <&  >&  <<-  <>
 .El
 .Ss Quoting
 Quoting is used to remove the special meaning of certain characters or
 words to the shell, such as operators, whitespace, or keywords.
 There are four types of quoting:
 matched single quotes,
 matched double quotes,
 backslash,
 and
 dollar preceding matched single quotes (enhanced C style strings.)
 .Ss Backslash
 An unquoted backslash preserves the literal meaning of the following
 character, with the exception of
 .Aq newline .
 An unquoted backslash preceding a
 .Aq newline
 is treated as a line continuation, the two characters are simply removed.
 .Ss Single Quotes
 Enclosing characters in single quotes preserves the literal meaning of all
 the characters (except single quotes, making it impossible to put
 single quotes in a single-quoted string).
 .Ss Double Quotes
 Enclosing characters within double quotes preserves the literal
 meaning of all characters except dollar sign
 .Pq $ ,
 backquote
 .Pq ` ,
 and backslash
 .Pq \e .
 The backslash inside double quotes is historically weird, and serves to
 quote only the following characters (and these not in all contexts):
 .Dl $  `  \*q  \e  <newline> ,
 where a backslash newline is a line continuation as above.
 Otherwise it remains literal.
 .Ss Dollar Single Quotes (\&$'...')
 .Bd -filled -offset indent
 .Bf Em
 Note: this form of quoting is still somewhat experimental,
 and yet to be included in the POSIX standard.
 This implementation is based upon the current proposals for
 standardization, and is subject to change should the eventual
 adopted text differ.
 .Ef
 .Ed
 .Pp
 Enclosing characters in a matched pair of single quotes, with the
 first immediately preceded by an unquoted dollar sign
 .Pq \&$
 provides a quoting mechanism similar to single quotes, except
 that within the sequence of characters, any backslash
 .Pq \e ,
 is an escape character, which causes the following character to
 be treated specially.
 Only a subset of the characters that can occur in the string
 are defined after a backslash, others are reserved for future
 definition, and currently generate a syntax error if used.
 The escape sequences are modeled after the similar sequences
 in strings in the C programming language, with some extensions.
 .Pp
 The following characters are treated literally when following
 the escape character (backslash):
 .Dl \e \&' \&"
 The sequence
 .Dq \e\e
 allows the escape character (backslash) to appear in the string literally.
 .Dq \e'
 allows a single quote character into the string, such an
 escaped single quote does not terminate the quoted string.
 .Dq \e"
 is for compatibility with C strings, the double quote has
 no special meaning in a shell C-style string,
 and does not need to be escaped, but may be.
 .Pp
 A newline following the escape character is treated as a line continuation,
 like the same sequence in a double quoted string,
 or when not quoted \(en
 the two characters, escape and newline, are removed from the input string.
 .Pp
 The following characters, when escaped, are converted in a
 manner similar to the way they would be in a string in the C language:
 .Dl a b e f n r t v
 An escaped
 .Sq a
 generates an alert (or
 .Sq BEL )
 character, that is, control-G, or 0x07.
 In a similar way,
 .Sq b
 is backspace (0x08),
 .Sq e
 (an extension to C) is escape (0x1B),
 .Sq f
 is form feed (0x0C),
 .Sq n
 is newline (or line feed, 0x0A),
 .Sq r
 is return (0x0D),
 .Sq t
 is horizontal tab (0x09),
 and
 .Sq v
 is vertical tab (0x13).
 .Pp
 In addition to those there are 5 forms that need additional
 data, which is obtained from the subsequent characters.
 An escape
 .Pq \e
 followed by one, two or three, octal digits
 .Po So 0 Sc Ns \&.. Ns So 7 Sc Ns Pc
 is processed to form an 8 bit character value.
 If only one or two digits are present, the following
 character must be something other than an octal digit.
 It is safest to always use all 3 digits, with leading
 zeros if needed.
 If all three digits are present, the first must be one of
 .So 0 Sc Ns \&.. Ns So 3 Sc .
 .Pp
 An escape followed by
 .Sq x
 (lower case only) can be followed by one or two
 hexadecimal digits
 .Po So 0 Sc Ns \&.. Ns So 9 Sc , So A Sc Ns \&.. Ns So F Sc , or So a Sc Ns \&.. Ns So f Sc . Pc
 As with octal, if only one hex digit is present, the following
 character must be something other than a hex digit,
 so always giving 2 hex digits is best.
 However, unlike octal, it is unspecified in the standard
 how many hex digits can be consumed.
 This
 .Nm
 takes at most two, but other shells will continue consuming
 characters as long as they remain valid hex digits.
 Consequently, users should ensure that the character
 following the hex escape sequence is something other than
 a hex digit.
 One way to achieve this is to end the $'...' string immediately
 after the final hex digit, and then, immediately start
 another, so
 .Dl \&$'\ex33'$'4...'
 always gives the character with value 0x33
 .Pq Sq 3 ,
 followed by the character
 .Sq 4 ,
 whereas
 .Dl \&$'\ex334'
 in some other shells would be the hex value 0x334 (10, or more, bits).
 .Pp
 There are two escape sequences beginning with
 .Sq \eu
 or
 .Sq \eU .
 The former is followed by from 1 to 4 hex digits, the latter by
 from 1 to 8 hex digits.
 Leading zeros can be used to pad the sequences to the maximum
 permitted length, to avoid any possible ambiguity problem with
 the following character, and because there are some shells that
 insist on exactly 4 (or 8) hex digits.
 These sequences are evaluated to form the value of a Unicode code
 point, which is then encoded into UTF-8 form, and entered into the
 string.
 (The code point should be converted to the appropriate
 code point value for the corresponding character in the character
 set given by the current locale, or perhaps the locale in use
 when the shell was started, but is not... currently.)
 Not all values that are possible to write are valid, values that
 specify (known) invalid Unicode code points will be rejected, or
 simply produce
 .Sq \&? .
 .Pp
 Lastly, as another addition to what is available in C, the escape
 character (backslash), followed by
 .Sq c
 (lower case only) followed by one additional character, which must
 be an alphabetic character (a letter), or one of the following:
 .Dl \&@ \&[ \&\e \&] \&^ \&_ \&?
 Other than
 .Sq \ec?
 the value obtained is the least significant 5 bits of the
 ASCII value of the character following the
 .Sq \ec
 escape sequence.
 That is what is commonly known as the
 .Dq control
 character obtained from the given character.
 The escape sequence
 .Sq \ec?
 yields the ASCII DEL character (0x7F).
 Note that to obtain the ASCII FS character (0x1C) this way,
 .Pq "that is control-\e"
 the trailing
 .Sq \e
 must be escaped itself, and so for this one case, the full
 escape sequence is
 .Dq \ec\e\e .
 The sequence
 .Dq \ec\eX
 where
 .Sq X
 is some character other than
 .Sq \e
 is reserved for future use, its meaning is unspecified.
 In this
 .Nm
 an error is generated.
 .Pp
 If any of the preceding escape sequences generate the value
 .Sq \e0
 (a NUL character) that character, and all that follow in the
 same $'...' string, are omitted from the resulting word.
 .Pp
 After the $'...' string has had any included escape sequences
 converted, it is treated as if it had been a single quoted string.
 .Ss Reserved Words
 Reserved words are words that have special meaning to the
 shell and are recognized at the beginning of a line and
 after a control operator.
 The following are reserved words:
 .Bl -column while while while while -offset indent
 .It Ic \&! Ta Ic \&{ Ta Ic \&} Ta Ic case
 .It Ic do Ta Ic done Ta Ic elif Ta Ic else
 .It Ic esac Ta Ic fi Ta Ic for Ta Ic if
 .It Ic in Ta Ic then Ta Ic until Ta Ic while
 .El
 .Pp
 Their meanings are discussed later.
 .Ss Aliases
 An alias is a name and corresponding value set using the
 .Ic alias
 built-in command.
 Whenever a reserved word (see above) may occur,
 and after checking for reserved words, the shell
 checks the word to see if it matches an alias.
 If it does, it replaces it in the input stream with its value.
 For example, if there is an alias called
 .Dq lf
 with the value
 .Dq "ls -F" ,
 then the input:
 .Pp
 .Dl lf foobar Aq return
 .Pp
 would become
 .Pp
 .Dl ls -F foobar Aq return
 .Pp
 Aliases provide a convenient way for naive users to create shorthands for
 commands without having to learn how to create functions with arguments.
 They can also be used to create lexically obscure code.
 This use is strongly discouraged.
 .Ss Commands
 The shell interprets the words it reads according to a language, the
 specification of which is outside the scope of this man page (refer to the
 BNF in the POSIX 1003.2 document).
 Essentially though, a line is read and if the first
 word of the line (or after a control operator) is not a reserved word,
 then the shell has recognized a simple command.
 Otherwise, a complex
 command or some other special construct may have been recognized.
 .Ss Simple Commands
 If a simple command has been recognized, the shell performs
 the following actions:
 .Bl -enum -offset indent
 .It
 Leading words of the form
 .Dq name=value
 are stripped off, the value is expanded, as described below,
 and the results are assigned to the environment of the simple command.
 Redirection operators and their arguments (as described below) are
 stripped off and saved for processing in step 3 below.
 .It
 The remaining words are expanded as described in the
 .Sx Word Expansions
 section below.
 The first remaining word is considered the command name and the
 command is located.
 Any remaining words are considered the arguments of the command.
 If no command name resulted, then the
 .Dq name=value
 variable assignments recognized in item 1 affect the current shell.
 .It
 Redirections are performed, from first to last, in the order given,
 as described in the next section.
 .El
 .Ss Redirections
 Redirections are used to change where a command reads its input or sends
 its output.
 In general, redirections open, close, or duplicate an
 existing reference to a file.
 The overall format used for redirection is:
 .Pp
 .Dl [n] Ns Va redir-op Ar file
 .Pp
 where
 .Va redir-op
 is one of the redirection operators mentioned previously.
 The following is a list of the possible redirections.
 The
 .Bq n
 is an optional number, as in
 .Sq 3
 (not
 .Sq Bq 3 ) ,
 that refers to a file descriptor.
 If present it must occur immediately before the redirection
 operator, with no intervening white space, and becomes a
 part of that operator.
 .Bl -tag -width aaabsfiles -offset indent
 .It Oo Ar n Oc Ns > Ar file
 Redirect standard output (or n) to
 .Ar file .
 .It Oo Ar n Oc Ns >| file
 The same, but override the
 .Fl C
 option.
 .It Oo Ar n Oc Ns >> Ar file
 Append standard output (or n) to
 .Ar file .
 .It Oo Ar n Oc Ns < Ar file
 Redirect standard input (or
 .Ar n )
 from
 .Ar file .
 .It Oo Ar n1 Oc Ns <& Ns Ar n2
 Duplicate standard input (or
 .Ar n1 )
 from file descriptor
 .Ar n2 .
 .Ar n2
 is expanded if not a digit string, the result must be a number.
 .It Oo Ar n Oc Ns <&-
 Close standard input (or
 .Ar n ) .
 .It Oo Ar n1 Oc Ns >& Ns Ar n2
 Duplicate standard output (or
 .Ar n1 )
 to
 .Ar n2 .
 .It Oo Ar n Oc Ns >&-
 Close standard output (or n).
 .It Oo Ar n Oc Ns <> Ar file
 Open
 .Ar file
 for reading and writing on standard input (or
 .Ar n ) .
 .El
 .Pp
 The following redirection is often called a
 .Dq here-document .
 .Bl -item -offset indent
 .It
 .Li [n]<< delimiter
 .Dl here-doc-text ...
 .Li delimiter
 .El
 .Pp
 The
 .Dq here-doc-text
 starts immediately after the next unquoted newline character following
 the here-doc redirection operator.
 If there is more than one here-document redirection on the same
 line, then the text for the first (from left to right) is read
 first, and subsequent here-doc-text for later here-doc redirections
 follows immediately after, until all such redirections have been
 processed.
 .Pp
 All the text on successive lines up to the delimiter,
 which must appear on a line by itself, with nothing other
 than an immediately following newline, is
 saved away and made available to the command on standard input, or file
 descriptor n if it is specified.
 If the delimiter as specified on the initial line is
 quoted, then the here-doc-text is treated literally; otherwise, the text is
 treated much like a double quoted string, except that
 .Sq \&"
 characters have no special meaning, and are not escaped by
 .Sq \&\e ,
 and is subjected to parameter expansion, command substitution, and arithmetic
 expansion as described in the
 .Sx Word Expansions
 section below.
 If the operator is
 .Dq <<\(mi
 instead of
 .Dq << ,
 then leading tabs in all lines in the here-doc-text, including before the
 end delimiter, are stripped.
 If the delimiter is not quoted, lines in here-doc-text that end with
 an unquoted \e are joined to the following line, the \e and following
 newline are simply removed while reading the here-doc, which thus guarantees
 that neither of those lines can be the end delimiter.
 .Pp
 It is a syntax error for the end of the input file (or string) to be
 reached before the delimiter is encountered.
 .Ss Search and Execution
 There are three types of commands: shell functions, built-in commands, and
 normal programs -- and the command is searched for (by name) in that order.
 A command that contains a slash
 .Sq \&/
 in its name is always a normal program.
 They each are executed in a different way.
 .Pp
 When a shell function is executed, all of the shell positional parameters
 (except $0, which remains unchanged) are set to the arguments of the shell
 function.
 The variables which are explicitly placed in the environment of
 the command (by placing assignments to them before the function name) are
 made local to the function and are set to the values given,
 and exported for the benefit of programs executed with the function.
 Then the command given in the function definition is executed.
 The positional parameters, and local variables, are restored to
 their original values when the command completes.
 This all occurs within the current shell, and the function
 can alter variables, or other settings, of the shell.
 .Pp
 Shell built-ins are executed internally to the shell, without spawning a
 new process.
 .Pp
 Otherwise, if the command name doesn't match a function or built-in, the
 command is searched for as a normal program in the file system (as
 described in the next section).
 When a normal program is executed, the shell runs the program,
 passing the arguments and the environment to the program.
 If the program is not a normal executable file, and if it does
 not begin with the "magic number" whose ASCII representation is "#!", so
 .Xr execve 2
 returns
 .Er ENOEXEC
 then) the shell will interpret the program in a sub-shell.
 The child shell will reinitialize itself in this case,
 so that the effect will be as if a
 new shell had been invoked to handle the ad-hoc shell script, except that
 the location of hashed commands located in the parent shell will be
 remembered by the child.
 .Pp
 Note that previous versions of this document and the source code itself
 misleadingly and sporadically refer to a shell script without a magic
 number as a "shell procedure".
 .Ss Path Search
 When locating a command, the shell first looks to see if it has a shell
 function by that name.
 Then it looks for a built-in command by that name.
 If a built-in command is not found, one of two things happen:
 .Bl -enum
 .It
 Command names containing a slash are simply executed without performing
 any searches.
 .It
 Otherwise, the shell searches each entry in
 .Ev PATH
 in turn for the command.
 The value of the
 .Ev PATH
 variable should be a series of entries separated by colons.
 Each entry consists of a directory name.
 The current directory may be indicated
 implicitly by an empty directory name, or explicitly by a single period.
 If a directory searched contains an executable file with the same
 name as the command given,
 the search terminates, and that program is executed.
 .El
 .Ss Command Exit Status
 Each command has an exit status that can influence the behavior
 of other shell commands.
 The paradigm is that a command exits
 with zero in normal cases, or to indicate success,
 and non-zero for failure,
 error, or a false indication.
 The man page for each command
 should indicate the various exit codes and what they mean.
 Additionally, the built-in commands return exit codes, as does
 an executed shell function.
 .Pp
 If a command consists entirely of variable assignments then the
 exit status of the command is that of the last command substitution
 if any, otherwise 0.
 .Pp
 If redirections are present, and any fail to be correctly performed,
 any command present is not executed, and an exit status of 2
 is returned.
 .Ss Complex Commands
 Complex commands are combinations of simple commands with control
 operators or reserved words, together creating a larger complex command.
 Overall, a shell program is a:
 .Bl -tag -width XpipelineX
 .It list
 Which is a sequence of one or more AND-OR lists.
 .It "AND-OR list"
 is a sequence of one or more pipelines.
 .It pipeline
 is a sequence of one or more commands.
 .It command
 is one of a simple command, a compound command, or a function definition.
 .It "simple command"
 has been explained above, and is the basic building block.
 .It "compound command"
 provides mechanisms to group lists to achieve different effects.
 .It "function definition"
 allows new simple commands to be created as groupings of existing commands.
 .El
 .Pp
 Unless otherwise stated, the exit status of a list
 is that of the last simple command executed by the list.
 .Ss Pipelines
 A pipeline is a sequence of one or more commands separated
 by the control operator
 .Sq \&| ,
 and optionally preceded by the
 .Dq \&!
 reserved word.
 Note that
 .Sq \&|
 is an operator, and so is recognized anywhere it appears unquoted,
 it does not require surrounding white space or other syntax elements.
 On the other hand
 .Dq \&!
 being a reserved word, must be separated from adjacent words by
 white space (or other operators, perhaps redirects) and is only
 recognized as the reserved word when it appears in a command word
 position (such as at the beginning of a pipeline.)
 .Pp
 The standard output of all but
 the last command in the sequence is connected to the standard input
 of the next command.
 The standard output of the last
 command is inherited from the shell, as usual,
 as is the standard input of the first command.
 .Pp
 The format for a pipeline is:
 .Pp
 .Dl [!] command1 [ | command2 ...]
 .Pp
 The standard output of command1 is connected to the standard input of
 command2.
 The standard input, standard output, or both of each command is
 considered to be assigned by the pipeline before any redirection specified
 by redirection operators that are part of the command are performed.
 .Pp
 If the pipeline is not in the background (discussed later), the shell
 waits for all commands to complete.
 .Pp
 The commands in a pipeline can either be simple commands,
 or one of the compound commands described below.
 The simplest case of a pipeline is a single simple command.
 .Pp
 If the
 .Ic pipefail
 option is set when the pipeline completes and its status is
 collected, the pipeline status is the status of
 the last (rightmost) command in the pipeline to exit with non-zero exit
 status, or zero, if, and only if, all commands in the pipeline
 exited with a status of zero.
 If the
 .Ic pipefail
 option is not set, which is the default state,
 the pipeline status is the exit
 status of the last command in the pipeline,
 and the exit status of any other commands in the pipeline is ignored.
 .Pp
 If the reserved word ! precedes the pipeline, the exit status
 becomes the logical NOT of the pipeline status as determined above.
 That is, if the pipeline status is zero, the exit status is 1;
 if the pipeline status is other than zero, the exit status is zero.
 If there is no ! reserved word, the pipeline status becomes the exit status.
 .Pp
 Because pipeline assignment of standard input or standard output or both
 takes place before redirection, it can be modified by redirection.
 For example:
 .Pp
 .Dl $ command1 2>&1 | command2
 .Pp
 sends both the standard output and standard error of command1
 to the standard input of command2.
 .Pp
 Note that unlike some other shells, each process in the pipeline is a
 child of the invoking shell (unless it is a shell built-in, in which case
 it executes in the current shell -- but any effect it has on the
 environment is wiped).
 .Pp
 A pipeline is a simple case of an AND-OR-list (described below.)
 A ; or
 .Aq newline
 terminator causes the preceding pipeline, or more generally,
 the preceding AND-OR-list to be executed sequentially;
 that is, the shell executes the commands, and waits for them
 to finish before proceeding to following commands.
 An & terminator causes asynchronous (background) execution
 of the preceding AND-OR-list (see the next paragraph below).
 The exit status of an asynchronous AND-OR-list is zero.
 The actual status of the commands,
 after they have completed,
 can be obtained using the
 .Ic wait
 built-in command described later.
 .Ss Background Commands -- &
 If a command, pipeline, or AND-OR-list
 is terminated by the control operator ampersand (&), the
 shell executes the command asynchronously -- that is, the shell does not
 wait for the command to finish before executing the next command.
 .Pp
 The format for running a command in background is:
 .Pp
 .Dl command1 & [command2 & ...]
 .Pp
 If the shell is not interactive, the standard input of an asynchronous
 command is set to
 .Pa /dev/null .
 The process identifier of the most recent command started in the
 background can be obtained from the value of the special parameter
 .Dq \&!
 (see
 .Sx Special Parameters )
 provided it is accessed before the next asynchronous command is started.
 .Ss Lists -- Generally Speaking
 A list is a sequence of one or more commands separated by newlines,
 semicolons, or ampersands, and optionally terminated by one of these three
 characters.
 A shell program, which includes the commands given to an
 interactive shell, is a list.
 Each command in such a list is executed when it is fully parsed.
 Another use of a list is as a complete-command,
 which is parsed in its entirety, and then later the commands in
 the list are executed only if there were no parsing errors.
 .Pp
 The commands in a list are executed in the order they are written.
 If command is followed by an ampersand, the shell starts the
 command and immediately proceeds to the next command; otherwise it waits
 for the command to terminate before proceeding to the next one.
 A newline is equivalent to a
 .Sq \&;
 when no other operator is present, and the command being input
 could syntactically correctly be terminated at the point where
 the newline is encountered, otherwise it is just whitespace.
 .Ss AND-OR Lists (Short-Circuit List Operators)
 .Dq &&
 and
 .Dq ||
 are AND-OR list operators.
 After executing the commands that precede the
 .Dq &&
 the subsequent command is executed
 if and only if the exit status of the preceding command(s) is zero.
 .Dq ||
 is similar, but executes the subsequent command if and only if the exit status
 of the preceding command is nonzero.
 If a command is not executed, the exit status remains unchanged
 and the following AND-OR list operator (if any) uses that status.
 .Dq &&
 and
 .Dq ||
 both have the same priority.
 Note that these operators are left-associative, so
 .Dl true || echo bar && echo baz
 writes
 .Dq baz
 and nothing else.
 This is not the way it works in C.
 .Ss Flow-Control Constructs -- if, while, until, for, case
 These commands are instances of compound commands.
 The syntax of the
 .Ic if
 command is
 .Bd -literal -offset indent
 if list
 then list
 [ elif list
 then    list ] ...
 [ else list ]
 fi
 .Ed
 The first list is executed, and if the exit status of that list is zero,
 the list following the
 .Ic then
 is executed.
 Otherwise the list after an
 .Ic elif
 (if any) is executed and the process repeats.
 When no more
 .Ic elif
 reserved words, and accompanying lists, appear,
 the list after the
 .Ic else
 reserved word, if any, is executed.
 .Pp
 The syntax of the
 .Ic while
 command is
 .Bd -literal -offset indent
 while list
 do   list
 done
 .Ed
 .Pp
 The two lists are executed repeatedly while the exit status of the
 first list is zero.
 The
 .Ic until
 command is similar, but has the word
 .Ic until
 in place of
 .Ic while ,
 which causes it to repeat until the exit status of the first list is zero.
 .Pp
 The syntax of the
 .Ic for
 command is
 .Bd -literal -offset indent
 for variable [ in word ... ]
 do   list
 done
 .Ed
 .Pp
 The words are expanded, or "$@" if
 .Dq in
 (and the following words) is not present,
 and then the list is executed repeatedly with the
 variable set to each word in turn.
 If
 .Dq in
 appears after the variable, but no words are
 present, the list is not executed, and the exit status is zero.
 .Ic do
 and
 .Ic done
 may be replaced with
 .Sq Ic \&{
 and
 .Sq Ic \&} ,
 but doing so is non-standard and not recommended.
 .Pp
 The syntax of the
 .Ic break
 and
 .Ic continue
 commands is
 .Bd -literal -offset indent
 break [ num ]
 continue [ num ]
 .Ed
 .Pp
 .Ic break
 terminates the
 .Ar num
 innermost
 .Ic for , while ,
 or
 .Ic until
 loops.
 .Ic continue
 breaks execution of the
 .Ar num\-1
 innermost
 .Ic for , while ,
 or
 .Ic until
 loops, and then continues with the next iteration of the enclosing loop.
 These are implemented as special built-in commands.
 The parameter
 .Ar num ,
 if given, must be an unsigned positive integer (greater than zero).
 If not given, 1 is used.
 .Pp
 The syntax of the
 .Ic case
 command is
 .Bd -literal -offset indent
 case word in
 [(] pattern ) [ list ] ;&
 [(] pattern ) [ list ] ;;
 \&...
 esac
 .Ed
 .Pp
 The pattern can actually be one or more patterns (see
 .Sx Shell Patterns
 described later), separated by
 .Dq \*(Ba
 characters.
 .Pp
 Word is expanded and matched against each pattern in turn,
 from first to last,
 with each pattern being expanded just before the match is attempted.
 When a match is found, pattern comparisons cease, and the associated
 .Dq list ,
 if given,
 is evaluated.
 If the list is terminated with
 .Dq \&;&
 execution then falls through to the following list, if any,
 without evaluating its pattern, or attempting a match.
 When a list terminated with
 .Dq \&;;
 has been executed, or when
 .Ic esac
 is reached, execution of the
 .Ic case
 statement is complete.
 The exit status is that of the last command executed
 from the last list evaluated, if any, or zero otherwise.
 .Ss Grouping Commands Together
 Commands may be grouped by writing either
 .Dl (list)
 or
 .Dl { list; }
 These also form compound commands.
 .Pp
 Note that while parentheses are operators, and do not require
 any extra syntax, braces are reserved words, so the opening brace
 must be followed by white space (or some other operator), and the
 closing brace must occur in a position where a new command word might
 otherwise appear.
 .Pp
 The first of these executes the commands in a sub-shell.
 Built-in commands grouped into a (list) will not affect the current shell.
 The second form does not fork another shell so is slightly more efficient,
 and allows for commands which do affect the current shell.
 Grouping commands together this way allows you to redirect
 their output as though they were one program:
 .Bd -literal -offset indent
 { echo -n \*qhello \*q ; echo \*qworld\*q ; } > greeting
 .Ed
 .Pp
 Note that
 .Dq }
 must follow a control operator (here,
 .Dq \&; )
 so that it is recognized as a reserved word and not as another command argument.
 .Ss Functions
 The syntax of a function definition is
 .Pp
 .Dl name ( ) command [ redirect... ]
 .Pp
 A function definition is an executable statement; when executed it
 installs a function named name and returns an exit status of zero.
 The command is normally a list enclosed between
 .Dq {
 and
 .Dq } .
 The standard syntax also allows the command to be any of the other
 compound commands, including a sub-shell, all of which are supported.
 As an extension, this shell also allows a simple command
 (or even another function definition) to be
 used, though users should be aware this is non-standard syntax.
 This means that
 .Dl l() ls "$@"
 works to make
 .Dq l
 an alternative name for the
 .Ic ls
 command.
 .Pp
 If the optional redirect, (see
 .Sx Redirections ) ,
 which may be of any of the normal forms,
 is given, it is applied each time the
 function is called.
 This means that a simple
 .Dq Hello World
 function might be written (in the extended syntax) as:
 .Dl hello() cat <<EOF
 .Dl Hello World!
 .Dl EOF
 To be correctly standards conforming this should be re-written as:
 .Dl hello() { cat; } <<EOF
 .Dl Hello World!
 .Dl EOF
 Note the distinction between those forms, and
 .Dl hello() { cat <<EOF
 .Dl Hello World!
 .Dl EOF
 .Dl \&}
 which reads and processes the
 .Ic here document
 each time the shell executes the function, and which applies
 that input only to the cat command, not to any other commands
 that might appear in the function.
 .Pp
 Variables may be declared to be local to a function by using the
 .Ic local
 command.
 This should usually appear as the first statement of a function,
 though
 .Ic local
 is an executable command which can be used anywhere in a function.
 See
 .Sx Built-ins
 below for its definition.
 .Pp
 The function completes after having executed
 .Ar command
 with exit status set to the status returned by
 @@ -2531,1501 +2531,1501 @@ With the
 .Fl p
 flag, the output contains only the process identifier of the lead
 process.
 .Pp
 In an interactive shell, each job shown as completed in the output
 from the jobs command is implicitly waited for, and is removed from
 the jobs table, never to be seen again.
 In an interactive shell, when a background job terminates, the
 .Ic jobs
 command (with that job as an argument) is implicitly run just
 before outputting the next PS1 command prompt, after the job
 terminated.
 This indicates that the job finished, shows its status,
 and cleans up the job table entry for that job.
 Non-interactive shells need to execute
 .Ic wait
 commands to clean up terminated background jobs.
 .It local Oo Fl INx Oc Oo Ar variable | \- Oc ...
 Define local variables for a function.
 Local variables have their attributes, and values,
 as they were before the
 .Ic local
 declaration, restored when the function terminates.
 .Pp
 With the
 .Fl N
 flag, variables made local, are unset initially inside
 the function.
 Unless the
 .Fl x
 flag is also given, such variables are also unexported.
 The
 .Fl I
 flag, which is the default in this shell, causes
 the initial value and exported attribute
 of local variables
 to be inherited from the variable
 with the same name in the surrounding
 scope, if there is one.
 If there is not, the variable is initially unset,
 and not exported.
 The
 .Fl N
 and
 .Fl I
 flags are mutually exclusive, if both are given, the last specified applies.
 The read-only and unexportable attributes are always
 inherited, if a variable with the same name already exists.
 .Pp
 The
 .Fl x
 flag (lower case) causes the local variable to be exported,
 while the function runs, unless it has the unexportable attribute.
 This can also be accomplished by using the
 .Ic export
 command, giving the same
 .Ar variable
 names, after the
 .Ic local
 command.
 .Pp
 Making an existing read-only variable local is possible,
 but pointless.
 If an attempt is made to assign an initial value to such
 a variable, the
 .Ic local
 command fails, as does any later attempted assignment.
 If the
 .Ic readonly
 command is applied to a variable that has been declared local,
 the variable cannot be (further) modified within the function,
 or any other functions it calls, however when the function returns,
 the previous status (and value) of the variable is returned.
 .Pp
 Values may be given to local variables on the
 .Ic local
 command line in a similar fashion as used for
 .Ic export
 and
 .Ic readonly .
 These values are assigned immediately after the initialization
 described above.
 Note that any variable references on the command line will have
 been expanded before
 .Ic local
 is executed, so expressions like
 .Dl "local -N X=${X}"
 are well defined, first $X is expanded, and then the command run is
 .Dl "local -N X=old-value-of-X"
 After arranging to preserve the old value and attributes, of X
 .Dq ( old-value-of X )
 .Ic local
 unsets
 .Ev X ,
 unexports it, and then assigns the
 .Dq old-value-of-X
 to
 .Ev X .
 .Pp
 The shell uses dynamic scoping, so that if you make the variable x local to
 function f, which then calls function g, references to the variable x made
 inside g will refer to the variable x declared inside f, not to the global
 variable named x.
 .Pp
 Another way to view this, is as if the shell just has one flat, global,
 namespace, in which all variables exist.
 The
 .Ic local
 command conceptually copies the variable(s) named to unnamed temporary
 variables, and when the function ends, copies them back again.
 All references to the variables reference the same global variables,
 but while the function is active, after the
 .Ic local
 command has run, the values and attributes of the variables might
 be altered, and later, when the function completes, be restored.
 .Pp
 Note that the parameters $1, $2, ... (see
 .Sx Positional Parameters ) ,
 and $#, $* and $@ (see
 .Sx Special Parameters ) ,
 are always made local in all functions, and are reset inside the
 function to represent the options and arguments passed to the function.
 Note that $0 however retains the value it had outside the function,
 as do all the other special parameters.
 .Pp
 The only other special parameter that can be made local is
 .Dq \- .
 Making
 .Dq \-
 local causes any shell options that are changed via the set command inside the
 function to be restored to their original values when the function
 returns.
 If
 .Fl X
 option is altered after
 .Dq \-
 has been made local, then when the function returns, the previous
 destination for
 .Ic xtrace
 output (as of the time of the
 .Ic local
 command) will also be restored.
 .Pp
 It is an error to use
 .Ic local
 outside the scope of a function definition.
 When used inside a function, it exits with status 0,
 unless an undefined option is used, or an attempt is made to
 assign a value to a read-only variable.
 .Pp
 Note that either
 .Fl I
 or
 .Fl N
 should always be used, or variables made local should always
 be given a value, or explicitly unset, as the default behavior
 (inheriting the earlier value, or starting unset after
 .Ic local )
 differs amongst shell implementations.
 Using
 .Dq local \&\-
 is an extension not implemented by most shells.
 .Pp
 See the section
 .Sx LINENO
 below for details of the effects of making the variable
 .Ev LINENO
 local.
 .It pwd Op Fl \&LP
 Print the current directory.
 If
 .Fl L
 is specified the cached value (initially set from
 .Ev PWD )
 is checked to see if it refers to the current directory; if it does
 the value is printed.
 Otherwise the current directory name is found using
 .Xr getcwd 3 .
 The environment variable
 .Ev PWD
 is set to the printed value.
 .Pp
 The default is
 .Ic pwd
 .Fl L ,
 but note that the built-in
 .Ic cd
 command doesn't support the
 .Fl L
 option and will cache (almost) the absolute path.
 If
 .Ic cd
 is changed (as unlikely as that is),
 .Ic pwd
 may be changed to default to
 .Ic pwd
 .Fl P .
 .Pp
 If the current directory is renamed and replaced by a symlink to the
 same directory, or the initial
 .Ev PWD
 value followed a symbolic link, then the cached value may not
 be the absolute path.
 .Pp
 The built-in command may differ from the program of the same name because
 the program will use
 .Ev PWD
 and the built-in uses a separately cached value.
 .It read Oo Fl p Ar prompt Oc Oo Fl r Oc Ar variable Oo Ar ... Oc
 The prompt is printed if the
 .Fl p
 option is specified and the standard input is a terminal.
 Then a line is read from the standard input.
 The trailing newline is deleted from the
 line and the line is split as described in the field splitting section of the
 .Sx Word Expansions
 section above, and the pieces are assigned to the variables in order.
 If there are more pieces than variables, the remaining pieces
 (along with the characters in
 .Ev IFS
 that separated them) are assigned to the last variable.
 If there are more variables than pieces,
 the remaining variables are assigned the null string.
 The
 .Ic read
 built-in will indicate success unless EOF is encountered on input, in
 which case failure is returned.
 .Pp
 By default, unless the
 .Fl r
 option is specified, the backslash
 .Dq \e
 acts as an escape character, causing the following character to be treated
 literally.
 If a backslash is followed by a newline, the backslash and the
 newline will be deleted.
 .It readonly Ar name ...
 .It readonly Oo Fl p Oc
 The specified names are marked as read only, so that they cannot be
 subsequently modified or unset.
 The shell allows the value of a variable
 to be set at the same time it is marked read only by writing
 .Pp
 .Dl readonly name=value
 .Pp
 With no arguments the readonly command lists the names of all read only
 variables.
 With the
 .Fl p
 option specified the output will be formatted suitably for non-interactive use.
 .It return Op Ar n
 Stop executing the current function or a dot command with return value of
 .Ar n
 or the value of the last executed command, if not specified.
 For portability,
 .Ar n
 should be in the range from 0 to 255.
 .Pp
 The POSIX standard says that the results of
 .Sq return
 outside a function or a dot command are unspecified.
 This implementation treats such a return as a no-op with a return value of 0
 (success, true).
 Use the exit command instead, if you want to return from a script or exit
 your shell.
 .It set Oo { Fl options | Cm +options | Cm \-- } Oc Ar arg ...
 The
 .Ic set
 command performs four different functions.
 .Pp
 With no arguments, it lists the values of all shell variables.
 .Pp
 With a single option of either
 .Sq Fl o
 or
 .Sq Cm +o
 .Ic set
 outputs the current values of the options.
 In the
 .Fl o
 form, all options are listed, with their current values.
 In the
 .Cm +o
 form, the shell outputs a string that can later be used
 as a command to reset all options to their current values.
 .Pp
 If options are given, it sets the specified option
 flags, or clears them as described in the
 .Sx Argument List Processing
 section.
 In addition to the options listed there,
 when the
 .Dq "option name"
 given to
 .Ic set Fl o
 is
 .Cm default
 all of the options are reset to the values they had immediately
 after
 .Nm
 initialization, before any startup scripts, or other input, had been processed.
 While this may be of use to users or scripts, its primary purpose
 is for use in the output of
 .Dq Ic set Cm +o ,
 to avoid that command needing to list every available option.
 There is no
 .Cm +o default .
 .Pp
 The fourth use of the set command is to set the values of the shell's
 positional parameters to the specified arguments.
 To change the positional
 parameters without changing any options, use
 .Dq -\|-
 as the first argument to set.
 If no following arguments are present, the set command
 will clear all the positional parameters (equivalent to executing
 .Dq shift $# . )
 Otherwise the following arguments become
 .Dq \&$1 ,
 .Dq \&$2 ,
 \&...,
 and
 .Dq \&$#
 is set to the number of arguments present.
 .It setvar Ar variable Ar value
 Assigns value to variable.
 (In general it is better to write
 variable=value rather than using
 .Ic setvar .
 .Ic setvar
 is intended to be used in
 functions that assign values to variables whose names are passed as
 parameters.)
 .It shift Op Ar n
 Shift the positional parameters n times.
 If n is omitted, 1 is assumed.
 Each
 .Ic shift
 sets the value of
 .Va $1
 to the previous value of
 .Va $2 ,
 the value of
 .Va $2
 to the previous value of
 .Va $3 ,
 and so on, decreasing
 the value of
 .Va $#
 by one.
 The shift count must be less than or equal to the number of
 positional parameters (
 .Dq $# )
 before the shift.
 .It times
 Prints two lines to standard output.
 Each line contains two accumulated time values, expressed
 in minutes and seconds (including fractions of a second.)
 The first value gives the user time consumed, the second the system time.
 .Pp
 The first output line gives the CPU and system times consumed by the
 shell itself.
 The second line gives the accumulated times for children of this
 shell (and their descendants) which have exited, and then been
 successfully waited for by the relevant parent.
 See
 .Xr times 3
 for more information.
 .Pp
 .Ic times
 has no parameters, and exits with an exit status of 0 unless
 an attempt is made to give it an option.
 .It trap Ar action signal ...
 .It trap \-
 .It trap Oo Fl l Oc
 .It trap Oo Fl p Oc signal ...
 .It trap Ar N signal ...
 .Pp
 Cause the shell to parse and execute action when any of the specified
 signals are received.
 The signals are specified by signal number or as the name of the signal.
 If
 .Ar signal
 is
 .Li 0
 or its equivalent, EXIT,
 the action is executed when the shell exits.
 The
 .Ar action
 may be a null (empty) string,
 which causes the specified signals to be ignored.
 With
 .Ar action
 set to
 .Sq -
 the specified signals are set to their default actions.
 If the first
 .Ar signal
 is specified in its numeric form, then
 .Ar action
 can be omitted to achieve the same effect.
 This archaic,
 but still standard,
 form should not be relied upon, use the explicit
 .Sq -
 action.
 If no signals are specified with an action of
 .Sq - ,
 all signals are reset.
 .Pp
 When the shell forks off a sub-shell, it resets trapped (but not ignored)
 signals to the default action.
 On non-interactive shells, the
 .Ic trap
 command has no effect on signals that were
 ignored on entry to the shell.
 On interactive shells, the
 .Ic trap
 command will catch or reset signals ignored on entry.
 .Pp
 Issuing
 .Ic trap
 with option
 .Fl l
 will print a list of valid signal names.
 .Ic trap
 without any arguments causes it to write a list of signals and their
 associated non-default actions to the standard output in a format
 that is suitable as an input to the shell that achieves the same
 trapping results.
 With the
 .Fl p
 flag, trap prints the same information for the signals specified,
 or if none are given, for all signals, including those where the
 action is the default.
 .Pp
 Examples:
 .Pp
 .Dl trap
 .Pp
 List trapped signals and their corresponding actions.
 .Pp
 .Dl trap -l
 .Pp
 Print a list of valid signals.
 .Pp
 .Dl trap '' INT QUIT tstp 30
 .Pp
 Ignore signals INT QUIT TSTP USR1.
 .Pp
 .Dl trap date INT
 .Pp
 Run the
 .Dq date
 command (print the date) upon receiving signal INT.
 .Pp
 .Dl trap HUP INT
 .Pp
 Run the
 .Dq HUP
 command, or function, upon receiving signal INT.
 .Pp
 .Dl trap 1 2
 .Pp
 Reset the actions for signals 1 (HUP) and 2 (INT) to their defaults.
 .Bd -literal -offset indent
 traps=$(trap -p)
    # more commands ...
 trap 'action' SIG
    # more commands ...
 eval "$traps"
 .Ed
 .Pp
 Save the trap status, execute commands, changing some traps,
 and then reset all traps to their values at the start of the sequence.
 The
 .Fl p
 option is required in the first command here,
 or any signals that were previously
 untrapped (in their default states)
 and which were altered during the intermediate code,
 would not be reset by the final
 .Dq eval .
 .It type Op Ar name ...
 Interpret each name as a command and print the resolution of the command
 search.
 Possible resolutions are:
 shell keyword, alias, shell built-in,
 command, tracked alias and not found.
 For aliases the alias expansion is
 printed; for commands and tracked aliases the complete pathname of the
 command is printed.
 .It ulimit Oo Fl H \*(Ba Fl S Oc Oo Fl a \*(Ba Fl btfdscmlrpnv Oo Ar value Oc Oc
 Inquire about or set the hard or soft limits on processes or set new
 limits.
 The choice between hard limit (which no process is allowed to
 violate, and which may not be raised once it has been lowered) and soft
 limit (which causes processes to be signaled but not necessarily killed,
 and which may be raised) is made with these flags:
 .Bl -tag -width Fl
 .It Fl H
 set or inquire about hard limits
 .It Fl S
 set or inquire about soft limits.
 If neither
 .Fl H
 nor
 .Fl S
 is specified, the soft limit is displayed or both limits are set.
 If both are specified, the last one wins.
 .El
 .Pp
 The limit to be interrogated or set, then, is chosen by specifying
 any one of these flags:
 .Bl -tag -width Fl
 .It Fl a
 show all the current limits
 .It Fl b
 show or set the limit on the socket buffer size of a process (in bytes)
 .It Fl c
 show or set the limit on the largest core dump size that can be produced
 (in 512-byte blocks)
 .It Fl d
 show or set the limit on the data segment size of a process (in kilobytes)
 .It Fl f
 show or set the limit on the largest file that can be created
 (in 512-byte blocks)
 .It Fl l
 show or set the limit on how much memory a process can lock with
 .Xr mlock 2
 (in kilobytes)
 .It Fl m
 show or set the limit on the total physical memory that can be
 in use by a process (in kilobytes)
 .It Fl n
 show or set the limit on the number of files a process can have open at once
 .It Fl p
 show or set the limit on the number of processes this user can
 have at one time
 .It Fl r
 show or set the limit on the number of threads this user can
 have at one time
 .It Fl s
 show or set the limit on the stack size of a process (in kilobytes)
 .It Fl t
 show or set the limit on CPU time (in seconds)
 .It Fl v
 show or set the limit on how large a process address space can be
 .El
 .Pp
 If none of these is specified, it is the limit on file size that is shown
 or set.
 If value is specified, the limit is set to that number; otherwise
 the current limit is displayed.
 .Pp
 Limits of an arbitrary process can be displayed or set using the
 .Xr sysctl 8
 utility.
 .It umask Oo Fl S Oc  Op Ar mask
 Set the value of umask (see
 .Xr umask 2 )
 to the specified octal value.
 If the argument is omitted, the umask value is printed.
 With
 .Fl S
 a symbolic form is used instead of an octal number.
 .It unalias Oo Fl a Oc Oo Ar name Oc
 If
 .Ar name
 is specified, the shell removes that alias.
 If
 .Fl a
 is specified, all aliases are removed.
 .It unset Oo Fl efvx Oc Ar name ...
 If
 .Fl v
 is specified, the specified variables are unset and unexported.
 Readonly variables cannot be unset.
 If
 .Fl f
 is specified, the specified functions are undefined.
 If
 .Fl e
 is given, the specified variables are unexported, but otherwise unchanged,
 alternatively, if
 .Fl x
 is given, the exported status of the variable will be retained,
 even after it is unset.
 .Pp
 If no flags are provided
 .Fl v
 is assumed.
 If
 .Fl f
 is given with one of the other flags,
 then the named variables will be unset, or unexported, and functions
 of the same names will be undefined.
 The
 .Fl e
 and
 .Fl x
 flags both imply
 .Fl v .
 If
 .Fl e
 is given, the
 .Fl x
 flag is ignored.
 .Pp
 The exit status is 0, unless an attempt was made to unset
 a readonly variable, in which case the exit status is 1.
 It is not an error to unset (or undefine) a variable (or function)
 that is not currently set (or defined.)
 .It wait Oo Fl n Oc Oo Fl p Ar var Oc Op Ar job ...
 Wait for the specified jobs to complete
 and return the exit status of the last job in the parameter list,
 or 127 if that job is not a current child of the shell.
 .Pp
 If no
 .Ar job
 arguments are given, wait for all jobs to
 complete and then return an exit status of zero
 (including when there were no jobs, and so nothing exited.)
 .Pp
 With the
 .Fl n
 option, wait instead for any one of the given
 .Ar job Ns s,
 or if none are given, any job, to complete, and
 return the exit status of that job.
 If none of the given
 .Ar job
 arguments is a current child of the shell,
 or if no
 .Ar job
 arguments are given and the shell has no unwaited for children,
 then the exit status will be 127.
 .Pp
 The
 .Fl p Ar var
 option allows the process (or job) identifier of the
 job for which the exit status is returned to be obtained.
 The variable named (which must not be readonly) will be
 unset initially, then if a job has exited and its status is
 being returned, set to the identifier from the
 arg list (if given) of that job,
 or the lead process identifier of the job to exit when used with
 .Fl n
 and no job arguments.
 Note that
 .Fl p
 with neither
 .Fl n
 nor
 .Ar job
 arguments is useless, as in that case no job status is
 returned, the variable named is simply unset.
 .Pp
 If the wait is interrupted by a signal,
 its exit status will be greater than 128,
 and
 .Ar var ,
 if given, will remain unset.
 .Pp
 Once waited upon, by specific process number or job-id,
 or by a
 .Ic wait
 with no arguments,
 knowledge of the child is removed from the system,
 and it cannot be waited upon again.
 .Pp
 Note than when a list of jobs are given, more that
 one argument might refer to the same job.
 In that case, if the final argument represents a job
 that is also given earlier in the list, it is not
 defined whether the status returned will be the
 exit status of the job, or 127 indicating that
 the child no longer existed when the wait command
 reached the later argument in the list.
 In this
 .Nm
 the exit status will be that from the job.
 .Nm
 waits for each job exactly once, regardless of
 how many times (or how many different ways) it
 is listed in the arguments to
 .Ic wait .
 That is
 .Bd -literal -compact
 	wait 100 100 100
 .Ed
 is identical to
 .Bd -literal -compact
 	wait 100
 .Ed
 .El
 .Ss Job Control
 Each process (or set of processes) started by
 .Nm
 is created as a
 .Dq job
 and added to the jobs table.
 When enabled by the
 .Fl m
 option
 .Pq aka Fl o Ar monitor
 when the job is created,
 .Nm
 places each job (if run from the top level shell)
 into a process group of its own, which allows control
 of the process(es), and its/their descendants, as a unit.
 When the
 .Fl m
 option is off, or when started from a sub-shell environment,
 jobs share the same process group as the parent shell.
 The
 .Fl m
 option is enabled by default in interactive shells with
 a terminal as standard input and standard error.
 .Pp
 Jobs with separate process groups may be stopped, and then later
 resumed in the foreground (with access to the terminal)
 or in the background (where attempting to read from the
 terminal will result in the job stopping.)
 A list of current jobs can be obtained using the
 .Ic jobs
 built-in command.
 Jobs are identified using either the process identifier
 of the lead process of the job (the value available in
 the special parameter
 .Sq \&$!
 if the job is started in the background), or using percent
 notation.
 Each job is given a
 .Dq job number
 which is a small integer, starting from 1, and can be
 referenced as
 .Sq \&%n
 where n is that number.
 Note that this applies to jobs both with and without their own process groups.
 Job numbers are shown in the output from the
 .Ic jobs
 command enclosed in brackets
 .Po
 .Sq \&[
 and
 .Sq \&]
 .Pc .
 Whenever the job table becomes empty, the numbers begin at one again.
 In addition, there is the concept of a current, and a previous job,
 identified by
 .Sq \&%+
 .Po
 or
 .Sq \&%%
 or even just
 .Sq \&%
 .Pc ,
 and a previous job, identified by
 .Sq \&%\- .
 Whenever a background job is started,
 or a job is resumed in the background,
 it becomes the current job.
 The job that was the current job
 (prepare for a big surprise here, drum roll..., wait for it...\&)
 becomes the previous job.
 When the current job terminates, the previous job is
 promoted to be the current job.
 In addition the form
 .Dq %string
 finds the job for which the command starts with
 .Dq string
 and the form
 .Dq %?string
 finds the job which contains the
 .Dq string
 in its command somewhere.
 Both forms require the result to be unambiguous.
 For this purpose the
 .Dq command
 is that shown in the output from the
 .Ic jobs
 command, not the original command line.
 .Pp
 The
 .Ic bg ,
 .Ic fg ,
 .Ic jobid ,
 .Ic jobs ,
 .Ic kill ,
 and
 .Ic wait
 commands all accept job identifiers as arguments, in addition to
 process identifiers (larger integers).
 See the
 .Sx Built-ins
 section above, and
 .Xr kill 1 ,
 for more details of those commands.
 In addition, a job identifier
 .Pq using one of the Sq \&% forms
 issued as a command, without arguments, is interpreted as
 if it had been given as the argument to the
 .Ic fg
 command.
 .Pp
 To cause a foreground process to stop, enter the terminal's
 .Ic stop
 character (usually control-Z).
 To cause a background process to stop, send it a
 .Dv STOP
 signal, using the kill command.
 A useful function to define is
 .Bd -literal -compact
         stop() { kill -s STOP "${@:-%%}"; }
 .Ed
 .Pp
 The
 .Ic fg
 command resumes a stopped job, placing it in the foreground,
 and
 .Ic bg
 resumes a stopped job in the background.
 The
 .Ic jobid
 command provides information about process identifiers, job identifiers,
 and the process group identifier, for a job.
 .Pp
 Whenever a sub-shell is created, the jobs table becomes invalid
 (the sub-shell has no children.)
 However, to enable uses like
 .Bd -literal -compact
         PID=$(jobid -p %1)
 .Ed
 the table is only actually cleared in a sub-shell when needed to
 create the first job there (built-in commands run in the foreground
 do not create jobs.)
 Note that in this environment, there is no useful current job
 (%% actually refers to the sub-shell itself, but is not accessible)
 but the job which is the current job in the parent can be accessed as %\-.
 .Ss Command Line Editing
 When
 .Nm
 is being used interactively from a terminal, the current command
 and the command history (see
 .Ic fc
 in the
 .Sx Built-ins
 section)
 can be edited using emacs-mode or vi-mode command-line editing.
 The command
 .Ql set -o emacs
 enables emacs-mode editing.
 The command
 .Ql set -o vi
 enables vi-mode editing and places the current shell process into
 .Ar vi
 insert mode.
 (See the
 .Sx Argument List Processing
 section above.)
 .Pp
 The
 .Ar vi
 mode uses commands similar to a subset of those described in the
 .Xr vi 1
 man page.
 With vi-mode
 enabled,
 .Nm sh
 can be switched between insert mode and command mode.
 It's similar to
 .Xr vi 1 :
 pressing the
 .Aq ESC
 key will throw you into command VI command mode.
 Pressing the
 .Aq return
 key while in command mode will pass the line to the shell.
 .Pp
 The
 .Ar emacs
 mode uses commands similar to a subset available in the
 .Ic emacs
 editor.
 With emacs-mode enabled, special keys can be used to modify the text
 in the buffer using the control key.
 .Pp
 .Nm
 uses the
 .Xr editline 3
 library.
 See
 .Xr editline 7
 for a list of the possible command bindings,
 and the default settings in
 .Ar emacs
 and
 .Ar vi
 modes.
 Also see
 .Xr editrc 5
 for the commands that can be given to configure
 .Xr editline 7
 in the file named by the
 .Ev EDITRC
 parameter,
 or a file used with the
 .Ic inputrc
 built-in command,
 or using
 .Xr editline 7 Ap s
 configuration command line.
 .Pp
 When command line editing is enabled, the
 .Xr editline 7
 functions control printing of the
 .Ev PS1
 and
 .Ev PS2
 prompts when required.
 As, in this mode, the command line editor needs to
 keep track of what characters are in what position on
 the command line, care needs to be taken when setting
 the prompts.
 Normal printing characters are handled automatically,
 however mode setting sequences, which do not actually display
 on the terminal, need to be identified to
 .Xr editline 7 .
 This is done, when needed, by choosing a character that
 is not needed anywhere in the prompt, including in the mode
 setting sequences, any single character is acceptable,
 and assigning it to the shell parameter
 .Ev PSlit .
 Then that character should be used, in pairs, in the
 prompt string.
 Between each pair of
 .Ev PSlit
 characters are mode setting sequences, which affect the printing
 attributes of the following (normal) characters of the prompt,
 but do not themselves appear visibly, nor change the terminal's
 cursor position.
 .Pp
 Each such sequence, that is
 .Ev PSlit
 character, mode setting character sequence, and another
 .Ev PSlit
 character, must currently be followed by at least one following
 normal prompt character, or it will be ignored.
 That is, a
 .Ev PSlit
 character cannot be the final character of
 .Ev PS1
 or
 .Ev PS2 ,
 nor may two
 .Ev PSlit
 delimited sequences appear adjacent to each other.
 Each sequence can contain as many mode altering sequences as are
 required however.
 Only the first character from
 .Ev PSlit
 will be used.
 When set
 .Ev PSlit
 should usually be set to a string containing just one
 character, then it can simply be embedded in
 .Ev PS1
 (or
 .Ev PS2 )
 as in
 .Dl PS1="${PSlit}mset${PSlit}XYZ${PSlit}mclr${PSlit}ABC"
 The prompt visible will be
 .Dq XYZABC
 with the
 .Dq XYZ
 part shown according as defined by the mode setting characters
 .Dq mset ,
 and then cleared again by
 .Dq mclr .
 See
 .Xr tput 1
 for one method to generate appropriate mode sequences.
 Note that both parts, XYZ and ABC, must each contain at least one
 character.
 .Pp
 If
 .Ev PSlit
 is unset, which is its initial state, or set to a null string,
 no literal character will be defined,
 and all characters of the prompt strings will be assumed
 to be visible characters (which includes spaces etc.)
 To allow smooth use of prompts, without needing redefinition, when
 .Xr editline 7
 is disabled, the character chosen should be one which will be
 ignored by the terminal if received, as when
 .Xr editline 7
 is not in use, the prompt strings are simply written to the terminal.
 For example, setting:
-.Bd -compact -literal -offset left
+.Bd -literal -offset left -compact
   PSlit="$(printf\ '\e1')"
   PS1="${PSlit}$(tput\ bold\ blink)${PSlit}\e$${PSlit}$(tput\ sgr0)${PSlit}\ "
 .Ed
 will arrange for the primary prompt to be a bold blinking dollar sign,
 if supported by the current terminal, followed by an (ordinary) space,
 and, as the SOH (Control-A) character ('\e1') will not normally affect
 a terminal, this same prompt will usually work with
 .Xr editline 7
 enabled or disabled.
 .Sh ENVIRONMENT
 .Bl -tag -width MAILCHECK
 .It Ev CDPATH
 The search path used with the
 .Ic cd
 built-in.
 .It Ev EDITRC
 Gives the name of the file containing commands for
 .Xr editline 7 .
 See
 .Xr editrc 5
 for possible content and format.
 The file is processed, when in interactive mode with
 command line editing enabled, whenever
 .Ev EDITRC
 is set (even with no actual value change,)
 and if command line editing changes from disabled to enabled,
 or the editor style used is changed.
 (See the
 .Fl E
 and
 .Fl V
 options of the
 .Ic set
 built-in command, described in
 .Sx Built-ins
 above, which are documented further above in
 .Sx Argument List Processing . )
 If unset
 .Dq $HOME/.editrc
 is used.
 .It Ev HISTSIZE
 The number of lines in the history buffer for the shell.
 .It Ev HOME
 Set automatically by
 .Xr login 1
 from the user's login directory in the password file
 .Pq Xr passwd 5 .
 This environment variable also functions as the default argument for the
 .Ic cd
 built-in.
 .It Ev HOSTNAME
 Set to the current hostname of the system, as returned by
 .Xr gethostname 3 .
 This is obtained each time
 .Ev HOSTNAME
 is expanded, so changes to the system's name are reflected
 without further action.
 If unset, it returns nothing.
 Setting it does nothing except reverse the effect of an earlier
 .Ic unset .
 .It Ev IFS
 Input Field Separators.
 This is normally set to
 .Aq space ,
 .Aq tab ,
 and
 .Aq newline .
 See the
 .Sx White Space Splitting
 section for more details.
 .It Ev LANG
 The string used to specify localization information that allows users
 to work with different culture-specific and language conventions.
 See
 .Xr nls 7 .
 .It Ev LINENO
 The current line number in the script or function.
 See the section
 .Sx LINENO
 below for more details.
 .It Ev MAIL
 The name of a mail file, that will be checked for the arrival of new mail.
 Overridden by
 .Ev MAILPATH .
 The check occurs just before
 .Ev PS1
 is written, immediately after reporting jobs which have changed status,
 in interactive shells only.
 New mail is considered to have arrived if the monitored file
 has increased in size since the last check.
 .\" .It Ev MAILCHECK
 .\" The frequency in seconds that the shell checks for the arrival of mail
 .\" in the files specified by the
 .\" .Ev MAILPATH
 .\" or the
 .\" .Ev MAIL
 .\" file.
 .\" If set to 0, the check will occur at each prompt.
 .It Ev MAILPATH
 A colon
 .Dq \&:
 separated list of file names, for the shell to check for incoming mail.
 This environment setting overrides the
 .Ev MAIL
 setting.
 There is a maximum of 10 mailboxes that can be monitored at once.
 .It Ev PATH
 The default search path for executables.
 See the
 .Sx Path Search
 section above.
 .It Ev PPID
 The process identified of the parent process of the
 current shell.
 This value is set at shell startup, ignoring
 any value in the environment, and then made readonly.
 .It Ev PS1
 The primary prompt string, which defaults to
 .Dq $ \  ,
 unless you are the superuser, in which case it defaults to
 .Dq # \  .
 This string is subject to parameter, arithmetic, and if
 enabled by setting the
 .Ic promptcmds
 option, command substitution before being output.
 During execution of commands used by command substitution,
 execution tracing, the
 .Ic xtrace
 .Ic ( set Fl x )
 option is temporarily disabled.
 If
 .Ic promptcmds
 is not set and the prompt string uses command substitution,
 the prompt used will be an appropriate error string.
 For other expansion errors, a message will be output,
 and the unexpanded string will then be used as the prompt.
 .It Ev PS2
 The secondary prompt string, which defaults to
 .Dq > \  .
 After expansion (as for
 .Ev PS1 )
 it is written whenever more input is required to complete the
 current command.
 .It Ev PS4
 Output, after expansion like
 .Ev PS1 ,
 before each line when execution trace
 .Ic ( set Fl x )
 is enabled.
 .Ev PS4
 defaults to
 .Dq + \  .
 .It Ev PSc
 Initialized by the shell, ignoring any value from the environment,
 to a single character string, either
 .Sq \&#
 or
 .Sq \&$ ,
 depending upon whether the current user is the superuser or not.
 This is intended for use when building a custom
 .Ev PS1 .
 .It Ev PSlit
 Defines the character which may be embedded in pairs, in
 .Ev PS1
 or
 .Ev PS2
 to indicate to
 .Xr editline 7
 that the characters between each pair of occurrences of the
 .Ev PSlit
 character will not appear in the visible prompt, and will not
 cause the terminal's cursor to change position, but rather set terminal
 attributes for the following prompt character(s) at least one of
 which must be present.
 See
 .Sx Command Line Editing
 above for more information.
 .It Ev RANDOM
 Returns a different pseudo-random integer,
 in the range [0,32767] each time it is accessed.
 .Ev RANDOM
 can be assigned an integer value to seed the PRNG.
 If the value assigned is a constant, then the
 sequence of values produces on subsequent references of
 .Ev RANDOM
 will repeat after the next time the same constant is assigned.
 Note, this is not guaranteed to remain constant from one version
 of the shell to another \(en the PRNG algorithm, or seeding
 method is subject to change.
 If
 .Ev RANDOM
 is assigned an empty value (null string) then the next time
 .Ev RANDOM
 is accessed, it will be seeded from a more genuinely random source.
 The sequence of pseudo-random numbers generated will not be able to
 be generated again (except by luck, whether good or bad, depends!)
 This is also how the initial seed is generated, if none has been
 assigned before
 .Ev RANDOM
 is first accessed after shell initialization.
 .It Ev SECONDS
 Returns the number of seconds since the current shell was started.
 Attempts to set this variable are ignored.
 If unset, it remains unset, and returns nothing, unless set again.
 .It Ev START_TIME
 Initialized by the shell to the number of seconds since the Epoch
 (see
 .Xr localtime 3 )
 when the shell was started.
 The value of
 .Dl $(( Ns Ev START_TIME + Ev SECONDS Ns ))
 represents the current time, if
 .Ev START_TIME
 has not been modified, and
 .Ev SECONDS
 is not unset.
 .It Ev TERM
 The default terminal setting for the shell.
 This is inherited by
 children of the shell, and is used in the history editing modes.
 .\" This is explicitly last, not in sort order - please leave!
 .It Ev ToD
 When referenced, uses the value of
 .Ev ToD_FORMAT
 (or
 .Dq \&%T
 if
 .Ev ToD_FORMAT
 is unset) as the format argument to
 .Xr strftime 3
 to encode the current time of day, in the time zone
 defined by
 .Ev TZ
 if set, or current local time if not, and returns the result.
 If unset
 .Ev ToD
 returns nothing.
 Setting it has no effect, other than to reverse the effect of an earlier
 .Ic unset .
 .It Ev ToD_FORMAT
 Can be set to the
 .Xr strftime 3
 format string to be used when expanding
 .Ev ToD .
 Initially unset.
 .It Ev TZ
 If set, gives the time zone
 (see
 .Xr localtime 3 ,
 .Xr environ 7 )
 to use when formatting
 .Ev ToD
 and if exported, other utilities that deal with times.
 If unset, the system's local wall clock time zone is used.
 .It Ev NETBSD_SHELL
 Unlike the variables previously mentioned,
 this variable is somewhat strange,
 in that it cannot be set,
 inherited from the environment,
 modified, or exported from the shell.
 If set, by the shell, it indicates that the shell is the
 .Ic sh
 defined by this manual page, and gives its version information.
 It can also give information in additional space separated words,
 after the version string.
 If the shell was built as part of a reproducible build,
 the relevant date that was used for that build will be included.
 Finally, any non-standard compilation options,
 which may affect features available,
 that were used when building the shell will be listed.
 .Ev NETBSD_SHELL
 behaves like any other variable that has the read-only
 and un-exportable attributes set.
 .El
 .Ss Ev LINENO
 .Ev LINENO
 is in many respects a normal shell variable, containing an
 integer value. and can be expanded using any of the forms
 mentioned above which can be used for any other variable.
 .Pp
 .Ev LINENO
 can be exported, made readonly, or unset, as with any other
 variable, with similar effects.
 Note that while being readonly prevents later attempts to
 set, or unset,
 .Ev LINENO ,
 it does not prevent its value changing.
 References to
 .Ev LINENO
 .Pq "when not unset"
 always obtain the current line number.
 However,
 .Ev LINENO
 should normally not ever be set or unset.
 In this shell setting
 .Ev LINENO
 reverses the effect of an earlier
 .Ic unset ,
 but does not otherwise affect the value obtained.
 If unset,
 .Ev LINENO
 should not normally be set again, doing so is not portable.
 If
 .Ev LINENO
 is set or unset, different shells act differently.
 The value of
 .Ev LINENO
 is never imported from the environment when the shell is
 started, though if present there, as with any other variable,
 .Ev LINENO
 will be exported by this shell.
 .Pp
 .Ev LINENO
 is set automatically by the shell to be the number of the source
 line on which it occurs.
 When exported,
 .Ev LINENO
 is exported with its value set to the line number it would have
 had had it been referenced on the command line of the command to
 which it is exported.
 Line numbers are counted from 1, which is the first line the shell
 reads from any particular file.
 For this shell, standard input, including in an interactive shell,
 the user's terminal, is just another file and lines are counted
 there as well.
 However note that not all shells count interactive
 lines this way, it is not wise to rely upon
 .Ev LINENO
 having a useful value, except in a script, or a function.
 .Pp
 The role of
 .Ev LINENO
 in functions is less clear.
 In some shells,
 .Ev LINENO
 continues to refer to the line number in the script which defines
 the function,
 in others lines count from one within the function, always (and
 resume counting normally once the function definition is complete)
 and others count in functions from one if the function is defined
 interactively, but otherwise just reference the line number in the
 script in which the function is defined.
 This shell gives the user the option to choose.
 If the
 .Fl L
 flag (the
 .Ic local_lineno
 option, see
 .Sx Argument List Processing )
 is set, when the function is defined, then the function
 defaults to counting lines with one being the first line of the
 function.
 When the
 .Fl L
 flag is not set, the shell counts lines in a function definition
 in the same continuous sequence as the lines that surround the
 function definition.
 Further, if
 .Ev LINENO
 is made local
 (see
 .Sx Built-ins
 above)
 inside the function, the function can decide which
 behavior it prefers.
 If
 .Ev LINENO
 is made local and inherited, and not given a value, as in
 .Dl local Fl I Ev LINENO
 then from that point in the function,
 .Ev LINENO
 will give the line number as if lines are counted in sequence
 with the lines that surround the function definition (and
 any other function definitions in which this is nested.)
 If
 .Ev LINENO
 is made local, and in that same command, given a value, as
 .Dl local Oo Fl I Ns | Ns Fl N Oc Ev LINENO Ns = Ns Ar value
 then
 .Ev LINENO
 will give the line number as if lines are counted from one
 from the beginning of the function.
 The value nominally assigned in this case is irrelevant, and ignored.
 For completeness, if lineno is made local and unset, as in
 .Dl local Fl N Ev LINENO
 then
 .Ev LINENO
 is simply unset inside the function, and gives no value at all.
 .Pp
 Now for some technical details.
 The line on which
 .Ev LINENO
 occurs in a parameter expansion, is the line that contains the
 .Sq \&$
 that begins the expansion of
 .Ev LINENO .
 In the case of nested expansions, that
 .Sq \&$
 is the one that actually has
 .Ev LINENO
 as its parameter.
 In an arithmetic expansion, where no
 .Sq \&$
 is used to evaluate
 .Ev LINENO
 but
 .Ev LINENO
 is simply referenced as a variable, then the value is the
 line number of the line that contains the
 .Sq L
 of
 .Ev LINENO .
 For functions line one of the function definition (when relevant)
 is the line that contains the first character of the
 function name in the definition.
 When exported, the line number of the command is the line number
 where the first character of the word which becomes the command name occurs.
 .Pp
 When the shell opens a new file, for any reason,
 it counts lines from one in that file,
 and then resumes its original counting once it resumes reading the
 previous input stream.
 When handling a string passed to
 .Ic eval
 the line number starts at the line on which the string starts,
 and then if the string contains internal newline characters,
 those characters increase the line number.
 This means that references to
 .Ev LINENO
 in such a case can produce values larger than would be
 produced by a reference on the line after the
 .Ic eval .
 .Sh FILES
 .Bl -item
 .It
 .Pa $HOME/.profile
 .It
 .Pa /etc/profile
 .El
 .Sh EXIT STATUS
 Errors that are detected by the shell, such as a syntax error, will cause the
 shell to exit with a non-zero exit status.
 If the shell is not an
 interactive shell, the execution of the shell file will be aborted.
 Otherwise
 the shell will return the exit status of the last command executed, or
 if the exit built-in is used with a numeric argument, it will return the
 argument.
 .Sh SEE ALSO
 .Xr csh 1 ,
 .Xr echo 1 ,
 .Xr getopt 1 ,
 .Xr ksh 1 ,
 .Xr login 1 ,
 .Xr printf 1 ,
 .Xr test 1 ,
 .Xr editline 3 ,
 .Xr getopt 3 ,
 .\" .Xr profile 4 ,
 .Xr editrc 5 ,
 .Xr passwd 5 ,
 .Xr editline 7 ,
 .Xr environ 7 ,
 .Xr nls 7 ,
 .Xr sysctl 8
 .Sh HISTORY
+A
 .Nm
 command appeared in
 .At v1 .
 It was replaced in
 .At v7
 with a version that introduced the basis of the current syntax.
 That was, however, unmaintainable so we wrote this one.
 .Sh BUGS
 Setuid shell scripts should be avoided at all costs, as they are a
 significant security risk.
 .Pp
 The characters generated by filename completion should probably be quoted
 to ensure that the filename is still valid after the input line has been
 processed.
 .Pp
 The
 .Ic trap
 command cannot usefully be used, yet, within a command substitution,
 to obtain the current trap values,
 as all command substitutions are currently executed within a
 sub-shell environment,
 and in sub-shells all non-ignored, non-default, traps are reset.
 As a workaround, it is possible to redirect output from
 .Dq trap
 or
 .Dq trap -p
 to a file, and then read the file later using the
 .Dq \&.
 command.
 .Pp
 Job control of compound statements (loops, etc) is a complete mess.
 .Pp
 Many, many, more.
 (But less than there were...)