 @@ -1,2122 +1,2122 @@
-.\"	$NetBSD: sh.1,v 1.178 2018/03/13 19:43:52 uwe Exp $
+.\"	$NetBSD: sh.1,v 1.179 2018/03/13 20:08:11 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 -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>
+.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 ...]
+.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
+.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:
 .Bd -literal -offset indent
 hello() cat <<EOF
 Hello World!
 EOF
 .Ed
 .Pp
 To be correctly standards conforming this should be re-written as:
 .Bd -literal -offset indent
 hello() { cat; } <<EOF
 Hello World!
 EOF
 .Ed
 .Pp
 Note the distinction between those forms, and
 .Bd -literal -offset indent
 hello() { cat <<EOF
 Hello World!
 EOF
 \&}
 .Ed
 .Pp
 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
 .Ar command .
 If
 .Ar command
 is a compound-command
 it can use the
 .Ic return
 command (see
 .Sx Built-ins
 below)
 to finish before completing all of
 .Ar command .
 .Ss Variables and Parameters
 The shell maintains a set of parameters.
 A parameter denoted by a name is called a variable.
 When starting up, the shell turns all the environment
 variables into shell variables, and exports them.
 New variables can be set using the form
 .Pp
 .Dl name=value
 .Pp
 Variables set by the user must have a name consisting solely of
 alphabetics, numerics, and underscores - the first of which must not be
 numeric.
 A parameter can also be denoted by a number or a special
 character as explained below.
 .Ss Positional Parameters
 A positional parameter is a parameter denoted by a number (n > 0).
 The shell sets these initially to the values of its command line arguments
 that follow the name of the shell script.
 The
 .Ic set
 built-in can also be used to set or reset them, and
 .Ic shift
 can be used to manipulate the list.
 .Pp
 To refer to the 10th (and later) positional parameters,
 the form ${n} must be used.
 Without the braces, a digit following
 .Dq $
 can only refer to one of the first 9 positional parameters,
 or the special parameter
 .Dq 0 .
 The word
 .Dq $10
 is treated identically to
 .Dq ${1}0 .
 .Ss Special Parameters
 A special parameter is a parameter denoted by one of the following special
 characters.
 The value of the parameter is listed next to its character.
 .Bl -tag -width thinhyphena
 .It *
 Expands to the positional parameters, starting from one.
 When the
 expansion occurs within a double-quoted string it expands to a single
 field with the value of each parameter separated by the first character of
 the
 .Ev IFS
 variable, or by a
 .Aq space
 if
 .Ev IFS
 is unset.
 .It @
 Expands to the positional parameters, starting from one.
 When the expansion occurs within double quotes, each positional
 parameter expands as a separate argument.
 If there are no positional parameters, the
 expansion of @ generates zero arguments, even when @ is
 double-quoted.
 What this basically means, for example, is
 if $1 is
 .Dq abc
 and $2 is
 .Dq def ghi ,
 then
 .Qq $@
 expands to
 the two arguments:
 .Pp
 .Sm off
 .Dl \*q abc \*q \  \*q def\ ghi \*q
 .Sm on
 .It #
 Expands to the number of positional parameters.
 .It \&?
 Expands to the exit status of the most recent pipeline.
 .It \- (Hyphen, or minus.)
 Expands to the current option flags (the single-letter
 option names concatenated into a string) as specified on
 invocation, by the set built-in command, or implicitly
 by the shell.
 .It $
 Expands to the process ID of the invoked shell.
 A sub-shell retains the same value of $ as its parent.
 .It \&!
 Expands to the process ID of the most recent background
 command executed from the current shell.
 For a pipeline, the process ID is that of the last command in the pipeline.
 If no background commands have yet been started by the shell, then
 .Dq \&!
 will be unset.
 Once set, the value of
 .Dq \&!
 will be retained until another background command is started.
 .It 0 (Zero.)
 Expands to the name of the shell or shell script.
 .El
 .Ss Word Expansions
 This section describes the various expansions that are performed on words.
 Not all expansions are performed on every word, as explained later.
 .Pp
 Tilde expansions, parameter expansions, command substitutions, arithmetic
 expansions, and quote removals that occur within a single word expand to a
 single field.
 It is only field splitting or pathname expansion that can
 create multiple fields from a single word.
 The single exception to this
 rule is the expansion of the special parameter @ within double quotes, as
 was described above.
 .Pp
 The order of word expansion is:
 .Bl -enum
 .It
 Tilde Expansion, Parameter Expansion, Command Substitution,
 Arithmetic Expansion (these all occur at the same time).
 .It
 Field Splitting is performed on fields
 generated by step (1) unless the
 .Ev IFS
 variable is null.
 .It
 Pathname Expansion (unless set
 .Fl f
 is in effect).
 .It
 Quote Removal.
 .El
 .Pp
 The $ character is used to introduce parameter expansion, command
 substitution, or arithmetic evaluation.
 .Ss Tilde Expansion (substituting a user's home directory)
 A word beginning with an unquoted tilde character (~) is
 subjected to tilde expansion.
 Provided all of the subsequent characters in the word are unquoted
 up to an unquoted slash (/)
 or when in an assignment or not in posix mode, an unquoted colon (:),
 or if neither of those appear, the end of the word,
 they are treated as a user name
 and are replaced with the pathname of the named user's home directory.
 If the user name is missing (as in
 .Pa ~/foobar ) ,
 the tilde is replaced with the value of the
 .Va HOME
 variable (the current user's home directory).
 .Pp
 In variable assignments,
 an unquoted tilde immediately after the assignment operator (=), and
 each unquoted tilde immediately after an unquoted colon in the value
 to be assigned is also subject to tilde expansion as just stated.
 .Ss Parameter Expansion
 The format for parameter expansion is as follows:
 .Pp
 .Dl ${expression}
 .Pp
 where expression consists of all characters until the matching
 .Dq } .
 Any
 .Dq }
 escaped by a backslash or within a quoted string, and characters in
 embedded arithmetic expansions, command substitutions, and variable
 expansions, are not examined in determining the matching
 .Dq } .
 .Pp
 The simplest form for parameter expansion is:
 .Pp
 .Dl ${parameter}
 .Pp
 The value, if any, of parameter is substituted.
 .Pp
 The parameter name or symbol can be enclosed in braces,
 which are optional in this simple case,
 except for positional parameters with more than one digit or
 when parameter is followed by a character that could be interpreted as
 part of the name.
 If a parameter expansion occurs inside double quotes:
 .Bl -enum
 .It
 Pathname expansion is not performed on the results of the expansion.
 .It
 Field splitting is not performed on the results of the
 expansion, with the exception of the special rules for @.
 .El
 .Pp
 In addition, a parameter expansion where braces are used,
 can be modified by using one of the following formats.
 If the
 .Dq Dv \&:
 is omitted in the following modifiers, then the test in the expansion
 applies only to unset parameters, not null ones.
 .Bl -tag -width aaparameterwordaaaaa
 .It ${parameter:\(miword}
 Use Default Values.
 If parameter is unset or null, the expansion of word
 is substituted; otherwise, the value of parameter is substituted.
 .It ${parameter:=word}
 Assign Default Values.
 If parameter is unset or null, the expansion of
 word is assigned to parameter.
 In all cases, the final value of parameter is substituted.
 Only variables, not positional parameters or special
 parameters, can be assigned in this way.
 .It ${parameter:?[word]}
 Indicate Error if Null or Unset.
 If parameter is unset or null, the
 expansion of word (or a message indicating it is unset if word is omitted)
 is written to standard error and a non-interactive shell exits with
 a nonzero exit status.
 An interactive shell will not exit, but any associated command(s) will
 not be executed.
 If the parameter is set, its value is substituted.
 .It ${parameter:+word}
 Use Alternative Value.
 If parameter is unset or null, null is
 substituted; otherwise, the expansion of word is substituted.
 The value of parameter is not used in this expansion.
 .It ${#parameter}
 String Length.
 The length in characters of the value of parameter.
 .El
 .Pp
 The following four varieties of parameter expansion provide for substring
 processing.
 In each case, pattern matching notation (see
 .Sx Shell Patterns ) ,
 rather than regular expression notation, is used to evaluate the patterns.
 If parameter is * or @, the result of the expansion is unspecified.
 Enclosing the full parameter expansion string in double quotes does not
 cause the following four varieties of pattern characters to be quoted,
 whereas quoting characters within the braces has this effect.
 .Bl -tag -width aaparameterwordaaaaa
 .It ${parameter%word}
 Remove Smallest Suffix Pattern.
 The word is expanded to produce a pattern.
 The parameter expansion then results in parameter, with the
 smallest portion of the suffix matched by the pattern deleted.
 If the word is to start with a
 .Sq \&%
 character, it must be quoted.
 .It ${parameter%%word}
 Remove Largest Suffix Pattern.
 The word is expanded to produce a pattern.
 The parameter expansion then results in parameter, with the largest
 portion of the suffix matched by the pattern deleted.
 The
 .Dq %%
 pattern operator only produces different results from the
 .Dq \&%
 operator when the pattern contains at least one unquoted
 .Sq \&* .
 .It ${parameter#word}
 Remove Smallest Prefix Pattern.
 The word is expanded to produce a pattern.
 The parameter expansion then results in parameter, with the
 smallest portion of the prefix matched by the pattern deleted.
 If the word is to start with a
 .Sq \&#
 character, it must be quoted.
 .It ${parameter##word}
 Remove Largest Prefix Pattern.
 The word is expanded to produce a pattern.
 The parameter expansion then results in parameter, with the largest
 portion of the prefix matched by the pattern deleted.
 This has the same relationship with the
 .Sq \&#
 pattern operator as
 .Dq %%
 has with
 .Dq \&% .
 .El
 .Ss Command Substitution
 Command substitution allows the output of a command to be substituted in
 place of the command (and surrounding syntax).
 Command substitution occurs when the command is enclosed as follows:
 .Pp
 .Dl $(command)
 .Pp
 or
 .Po
 .Dq backquoted
 version
 .Pc :
 .Pp
 .Dl `command`
 .Pp
 The shell expands the command substitution by executing command in a
 sub-shell environment and replacing the command substitution with the
 standard output of the command, removing sequences of one or more
 .Ao newline Ac Ns s
 at the end of the substitution.
 (Embedded
 .Ao newline Ac Ns s
 before
 the end of the output are not removed; however, during field splitting,
 they may be translated into
 .Ao space Ac Ns s ,
 depending on the value of
 .Ev IFS
 and any quoting that is in effect.)
 .Ss Arithmetic Expansion
 Arithmetic expansion provides a mechanism for evaluating an arithmetic
 expression and substituting its value.
 The format for arithmetic expansion is as follows:
 .Pp
 .Dl $((expression))
 .Pp
 The expression in an arithmetic expansion is treated as if it were in
 double quotes, except that a double quote character inside the expression
 is just a normal character (it quotes nothing.)
 The shell expands all tokens in the expression for parameter expansion,
 command substitution, and quote removal (the only quoting character is
 the backslash
 .Sq \&\e ,
 and only when followed by another
 .Sq \&\e ,
 a dollar sign
 .Sq \&$ ,
 a backquote
 .Sq \&`
 or a newline.)
 .Pp
 Next, the shell evaluates the expanded result as an arithmetic expression
 and substitutes the calculated value of that expression.
 .Pp
 Arithmetic expressions use a syntax similar to that
 of the C language, and are evaluated using the
 .Ql intmax_t
 data type (this is an extension to POSIX, which requires only
 .Ql long
 arithmetic.)
 Shell variables may be referenced by name inside an arithmetic
 expression, without needing a
 .Dq \&$
 sign.
 Variables that are not set, or which have an empty (null string) value,
 used this way evaluate as zero (that is,
 .Dq x
 in arithmetic, as an R-Value, is evaluated as
 .Dq ${x:-0} )
 unless the
 .Nm
 .Fl u
 flag is set, in which case a reference to an unset variable is an error.
 Note that unset variables used in the ${var} form expand to a null
 string, which might result in syntax errors.
 Referencing the value of a variable which is not numeric is an error.
 .Pp
 All of the C expression operators applicable to integers are supported,
 and operate as they would in a C expression.
 Use white space, or parentheses, to disambiguate confusing syntax,
 otherwise, as in C, the longest sequence of consecutive characters
 which make a valid token (operator, variable name, or number) is taken
 to be that token, even if the token designated cannot be used
 and a different interpretation could produce a successful parse.
 This means, as an example, that
 .Dq a+++++b
 is parsed as the gibberish sequence
 .Dq "a ++ ++ + b" ,
 rather than as the valid alternative
 .Dq "a ++ + ++ b" .
 Similarly, separate the
 .Sq \&,
 operator from numbers with white space to avoid the possibility
 of confusion with the decimal indicator in some locales (though
 fractional, or floating-point, numbers are not supported in this
 implementation.)
 .Pp
 It should not be necessary to state that the C operators which
 operate on, or produce, pointer types, are not supported.
 Those include unary
 .Dq \&*
 and
 .Dq \&&
 and the struct and array referencing binary operators:
 .Dq \&. ,
 .Dq \&->
 and
 .Dq \&[ .
 .Ss White Space Splitting (Field Splitting)
 After parameter expansion, command substitution, and
 arithmetic expansion the shell scans the results of
 expansions and substitutions that did not occur in double quotes,
 and
 .Dq $@
 even if it did,
 for field splitting and multiple fields can result.
 .Pp
 The shell treats each character of the
 .Ev IFS
 as a delimiter and uses the delimiters to split the results of parameter
 expansion and command substitution into fields.
 .Pp
 Non-whitespace characters in
 .Ev IFS
 are treated strictly as parameter separators.
 So adjacent non-whitespace
 .Ev IFS
 characters will produce empty parameters.
 On the other hand, any sequence of whitespace
 characters that occur in
 .Ev IFS
 (known as
 .Ev IFS
 whitespace)
 can occur, leading and trailing
 .Ev IFS
 whitespace, and any
 .Ev IFS
 whitespace surrounding a non whitespace
 .Ev IFS
 delimiter, is removed.
 Any sequence of
 .Ev IFS
 whitespace characters without a non-whitespace
 .Ev IFS
 delimiter acts as a single field separator.
 .Pp
 If
 .Ev IFS
 is unset it is assumed to contain space, tab, and newline,
 all of which are
 .Ev IFS
 whitespace characters.
 If
 .Ev IFS
 is set to a null string, there are no delimiters,
 and no field splitting occurs.
 .Ss Pathname Expansion (File Name Generation)
 Unless the
 .Fl f
 flag is set, file name generation is performed after word splitting is
 complete.
 Each word is viewed as a series of patterns, separated by slashes.
 The process of expansion replaces the word with the names of all
 existing files whose names can be formed by replacing each pattern with a
 string that matches the specified pattern.
 There are two restrictions on
 this: first, a pattern cannot match a string containing a slash, and
 second, a pattern cannot match a string starting with a period unless the
 first character of the pattern is a period.
 The next section describes the
 patterns used for both Pathname Expansion and the
 .Ic case
 command.
 .Ss Shell Patterns
 A pattern consists of normal characters, which match themselves,
 and meta-characters.
 The meta-characters are
 .Dq \&! ,
 .Dq * ,
 .Dq \&? ,
 and
 .Dq \&[ .
 These characters lose their special meanings if they are quoted.
 When command or variable substitution is performed
 and the dollar sign or backquotes are not double-quoted,
 the value of the variable or the output of
 the command is scanned for these characters and they are turned into
 meta-characters.
 .Pp
 An asterisk
 .Pq Dq *
 matches any string of characters.
 A question mark
 .Pq Dq \&?
 matches any single character.
 A left bracket
 .Pq Dq \&[
 introduces a character class.
 The end of the character class is indicated by a right bracket
 .Pq Dq \&] ;
 if this
 .Dq \&]
 is missing then the
 .Dq \&[
 matches a
 .Dq \&[
 rather than introducing a character class.
 A character class matches any of the characters between the square brackets.
 A named class of characters (see
 .Xr wctype 3 )
 may be specified by surrounding the name with
 .Pq Dq [:
 and
 .Pq Dq :] .
 For example,
 .Pq Dq [[:alpha:]]
 is a shell pattern that matches a single letter.
 A range of characters may be specified using a minus sign
 .Pq Dq \(mi .
 The character class may be complemented
 by making an exclamation mark
 .Pq Dq \&!
 the first character of the character class.
 .Pp
 To include a
 .Dq \&]
 in a character class, make it the first character listed (after the
 .Dq \&! ,
 if any).
 To include a
 .Dq \(mi ,
 make it the first (after !) or last character listed.
 If both
 .Dq \&]
 and
 .Dq \(mi
 are to be included, the
 .Dq \&]
 must be first (after !)
 and the
 .Dq \(mi
 last, in the character class.
 .Ss Built-ins
 This section lists the built-in commands which are built-in because they
 need to perform some operation that can't be performed by a separate
 process.
 Or just because they traditionally are.
 In addition to these, there are several other commands that may
 be built in for efficiency (e.g.
 .Xr printf 1 ,
 .Xr echo 1 ,
 .Xr test 1 ,
 etc).
 .Bl -tag -width 5n
 .It : [ Ar arg ... ]
 A null command that returns a 0 (true) exit value.
 Any arguments or redirects are evaluated, then ignored.
 .It \&. file
 The dot command reads and executes the commands from the specified
 .Ar file
 in the current shell environment.
 The file does not need to be executable and is looked up from the directories
 listed in the
 .Ev PATH
 variable if its name does not contain a directory separator
 .Pq Sq / .
 The return command
 (see
 .Sx Built-ins
 below)
 can be used for a premature return from the sourced file.
 .Pp
 The POSIX standard has been unclear on how loop control keywords (break
 and continue) behave across a dot command boundary.
 This implementation allows them to control loops surrounding the dot command,
 but obviously such behavior should not be relied on.
 It is now permitted by the standard, but not required.
 .It alias Op Ar name Ns Op Ar "=string ..."
 If
 .Ar name=string
 is specified, the shell defines the alias
 .Ar name
 with value
 .Ar string .
 If just
 .Ar name
 is specified, the value of the alias
 .Ar name
 is printed.
 With no arguments, the
 .Ic alias
 built-in prints the
 names and values of all defined aliases (see
 .Ic unalias ) .
 .It bg [ Ar job ] ...
 Continue the specified jobs (or the current job if no
 jobs are given) in the background.
 .It command Oo Fl p Oc Oo Fl v Oc Oo Fl V Oc Ar command Oo Ar arg ... Oc
 Execute the specified command but ignore shell functions when searching
 for it.
 (This is useful when you
 have a shell function with the same name as a command.)
 .Bl -tag -width 5n
 .It Fl p
 search for command using a
 .Ev PATH
 that guarantees to find all the standard utilities.
 .It Fl V
 Do not execute the command but
 search for the command and print the resolution of the
 command search.
 This is the same as the
 .Ic type
 built-in.
 .It Fl v
 Do not execute the command but
 search for the command and print the absolute pathname
 of utilities, the name for built-ins or the expansion of aliases.
 .El
 .It cd Oo Fl P Oc Op Ar directory Op Ar replace
 Switch to the specified directory (default
 .Ev $HOME ) .
 If
 .Ar replace
 is specified, then the new directory name is generated by replacing
 the first occurrence of the string
 .Ar directory
 in the current directory name with
 .Ar replace .
 Otherwise if
 .Ar directory
 is
 .Sq - ,
 then the current working directory is changed to the previous current
 working directory as set in
 .Ev OLDPWD .
 Otherwise if an entry for
 .Ev CDPATH
 appears in the environment of the
 .Ic cd
 command or the shell variable
 .Ev CDPATH
 is set and the directory name does not begin with a slash,
 and its first (or only) component isn't dot or dot dot,
 then the directories listed in
 .Ev CDPATH
 will be searched for the specified directory.
 The format of
 .Ev CDPATH
 is the same as that of
 .Ev PATH .
 .Pp
 The
 .Fl P
 option instructs the shell to update
 .Ev PWD
 with the specified physical directory path and change to that directory.
 This is the default.
 .Pp
 When the directory changes, the variable
 .Ev OLDPWD
 is set to the working directory before the change.
 .Pp
 Some shells also support a
 .Fl L
 option, which instructs the shell to update
 .Ev PWD
 with the logical path and to change the current directory
 accordingly.
 This is not supported.
 .Pp
 In an interactive shell, the
 .Ic cd
 command will print out the name of the
 directory that it actually switched to if this is different from the name
 that the user gave,
 or always if the
 .Ic cdprint
 option is set.
 The destination may be different either because the
 .Ev CDPATH
 mechanism was used
 .\" or because a symbolic link was crossed.
 or if the
 .Ar replace
 argument was used.
 .It eval Ar string ...
 Concatenate all the arguments with spaces.