xref: /freebsd/bin/sh/sh.1 (revision 74bf4e164ba5851606a27d4feff27717452583e5)
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4.\" This code is derived from software contributed to Berkeley by
5.\" Kenneth Almquist.
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31.\"	from: @(#)sh.1	8.6 (Berkeley) 5/4/95
32.\" $FreeBSD$
33.\"
34.Dd July 3, 2004
35.Dt SH 1
36.Os
37.Sh NAME
38.Nm sh
39.Nd command interpreter (shell)
40.Sh SYNOPSIS
41.Nm
42.Op Fl /+abCEefIimnPpsTuVvx
43.Op Fl /+o Ar longname
44.Op Fl c Ar string
45.Op Ar arg ...
46.Sh DESCRIPTION
47The
48.Nm
49utility is the standard command interpreter for the system.
50The current version of
51.Nm
52is in the process of being changed to
53conform with the
54.St -p1003.2
55specification for the shell.
56This version has many features which make
57it appear
58similar in some respects to the Korn shell, but it is not a Korn
59shell clone like pdksh.
60Only features
61designated by POSIX, plus a few Berkeley extensions, are being
62incorporated into this shell.
63This man page is not intended to be a tutorial nor a complete
64specification of the shell.
65.Ss Overview
66The shell is a command that reads lines from
67either a file or the terminal, interprets them, and
68generally executes other commands.
69It is the program that is started when a user logs into the system,
70although a user can select a different shell with the
71.Xr chsh 1
72command.
73The shell
74implements a language that has flow control constructs,
75a macro facility that provides a variety of features in
76addition to data storage, along with built-in history and line
77editing capabilities.
78It incorporates many features to
79aid interactive use and has the advantage that the interpretative
80language is common to both interactive and non-interactive
81use (shell scripts).
82That is, commands can be typed directly
83to the running shell or can be put into a file,
84which can be executed directly by the shell.
85.Ss Invocation
86.\"
87.\" XXX This next sentence is incredibly confusing.
88.\"
89If no arguments are present and if the standard input of the shell
90is connected to a terminal
91(or if the
92.Fl i
93option is set),
94the shell is considered an interactive shell.
95An interactive shell
96generally prompts before each command and handles programming
97and command errors differently (as described below).
98When first starting, the shell inspects argument 0, and
99if it begins with a dash
100.Pq Li - ,
101the shell is also considered a login shell.
102This is normally done automatically by the system
103when the user first logs in.
104A login shell first reads commands
105from the files
106.Pa /etc/profile
107and then
108.Pa .profile
109if they exist.
110If the environment variable
111.Ev ENV
112is set on entry to a shell, or is set in the
113.Pa .profile
114of a login shell, the shell then reads commands from the file named in
115.Ev ENV .
116Therefore, a user should place commands that are to be executed only
117at login time in the
118.Pa .profile
119file, and commands that are executed for every shell inside the
120.Ev ENV
121file.
122The user can set the
123.Ev ENV
124variable to some file by placing the following line in the file
125.Pa .profile
126in the home directory,
127substituting for
128.Pa .shinit
129the filename desired:
130.Pp
131.Dl ENV=$HOME/.shinit; export ENV
132.Pp
133The first non-option argument specified on the command line
134will be treated as the
135name of a file from which to read commands (a shell script), and
136the remaining arguments are set as the positional parameters
137of the shell ($1, $2, etc).
138Otherwise, the shell reads commands
139from its standard input.
140.Pp
141Unlike older versions of
142.Nm
143the
144.Ev ENV
145script is only sourced on invocation of interactive shells.
146This
147closes a well-known, and sometimes easily exploitable security
148hole related to poorly thought out
149.Ev ENV
150scripts.
151.Ss Argument List Processing
152All of the single letter options to
153.Nm
154have a corresponding long name,
155with the exception of
156.Fl c
157and
158.Fl /+o .
159These long names are provided next to the single letter options
160in the descriptions below.
161The long name for an option may be specified as an argument to the
162.Fl /+o
163option of
164.Nm .
165Once the shell is running,
166the long name for an option may be specified as an argument to the
167.Fl /+o
168option of the
169.Ic set
170built-in command
171(described later in the section called
172.Sx Built-in Commands ) .
173Introducing an option with a dash
174.Pq Li -
175enables the option,
176while using a plus
177.Pq Li +
178disables the option.
179A
180.Dq Li --
181or plain
182.Dq Li -
183will stop option processing and will force the remaining
184words on the command line to be treated as arguments.
185The
186.Fl /+o
187and
188.Fl c
189options do not have long names.
190They take arguments and are described after the single letter options.
191.Bl -tag -width indent
192.It Fl a Li allexport
193Flag variables for export when assignments are made to them.
194.It Fl b Li notify
195Enable asynchronous notification of background job
196completion.
197(UNIMPLEMENTED)
198.It Fl C Li noclobber
199Do not overwrite existing files with
200.Dq Li > .
201.It Fl E Li emacs
202Enable the built-in
203.Xr emacs 1
204command line editor (disables the
205.Fl V
206option if it has been set).
207.It Fl e Li errexit
208Exit immediately if any untested command fails in non-interactive mode.
209The exit status of a command is considered to be
210explicitly tested if the command is used to control
211an if, elif, while, or until; or if the command is the left
212hand operand of an
213.Dq Li &&
214or
215.Dq Li ||
216operator.
217.It Fl f Li noglob
218Disable pathname expansion.
219.It Fl I Li ignoreeof
220Ignore
221.Dv EOF Ns ' Ns s
222from input when in interactive mode.
223.It Fl i Li interactive
224Force the shell to behave interactively.
225.It Fl m Li monitor
226Turn on job control (set automatically when interactive).
227.It Fl n Li noexec
228If not interactive, read commands but do not
229execute them.
230This is useful for checking the
231syntax of shell scripts.
232.It Fl P Li physical
233Change the default for the
234.Ic cd
235and
236.Ic pwd
237commands from
238.Fl L
239(logical directory layout)
240to
241.Fl P
242(physical directory layout).
243.It Fl p Li privileged
244Turn on privileged mode.
245This mode is enabled on startup
246if either the effective user or group id is not equal to the
247real user or group id.
248Turning this mode off sets the
249effective user and group ids to the real user and group ids.
250When this mode is enabled for interactive shells, the file
251.Pa /etc/suid_profile
252is sourced instead of
253.Pa ~/.profile
254after
255.Pa /etc/profile
256is sourced, and the contents of the
257.Ev ENV
258variable are ignored.
259.It Fl s Li stdin
260Read commands from standard input (set automatically
261if no file arguments are present).
262This option has
263no effect when set after the shell has already started
264running (i.e., when set with the
265.Ic set
266command).
267.It Fl T Li trapsasync
268When waiting for a child, execute traps immediately.
269If this option is not set,
270traps are executed after the child exits,
271as specified in
272.St -p1003.2
273This nonstandard option is useful for putting guarding shells around
274children that block signals.
275The surrounding shell may kill the child
276or it may just return control to the tty and leave the child alone,
277like this:
278.Bd -literal -offset indent
279sh -T -c "trap 'exit 1' 2 ; some-blocking-program"
280.Ed
281.Pp
282.It Fl u Li nounset
283Write a message to standard error when attempting
284to expand a variable that is not set, and if the
285shell is not interactive, exit immediately.
286.It Fl V Li vi
287Enable the built-in
288.Xr vi 1
289command line editor (disables
290.Fl E
291if it has been set).
292.It Fl v Li verbose
293The shell writes its input to standard error
294as it is read.
295Useful for debugging.
296.It Fl x Li xtrace
297Write each command
298(preceded by
299.Dq Li +\  )
300to standard error before it is executed.
301Useful for debugging.
302.El
303.Pp
304The
305.Fl c
306option may be used to pass its string argument to the shell
307to be interpreted as input.
308Keep in mind that this option only accepts a single string as its
309argument, hence multi-word strings must be quoted.
310.Pp
311The
312.Fl /+o
313option takes as its only argument the long name of an option
314to be enabled or disabled.
315For example, the following two invocations of
316.Nm
317both enable the built-in
318.Xr emacs 1
319command line editor:
320.Bd -literal -offset indent
321set -E
322set -o emacs
323.Ed
324.Pp
325If used without an argument, the
326.Fl o
327option displays the current option settings in a human-readable format.
328If
329.Cm +o
330is used without an argument, the current option settings are output
331in a format suitable for re-input into the shell.
332.Ss Lexical Structure
333The shell reads input in terms of lines from a file and breaks
334it up into words at whitespace (blanks and tabs), and at
335certain sequences of
336characters called
337.Dq operators ,
338which are special to the shell.
339There are two types of operators: control operators and
340redirection operators (their meaning is discussed later).
341The following is a list of valid operators:
342.Bl -tag -width indent
343.It Control operators:
344.Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
345.It Li & Ta Li && Ta Li ( Ta Li ) Ta Li \en
346.It Li ;; Ta Li ; Ta Li | Ta Li ||
347.El
348.It Redirection operators:
349.Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
350.It Li < Ta Li > Ta Li << Ta Li >> Ta Li <>
351.It Li <& Ta Li >& Ta Li <<- Ta Li >|
352.El
353.El
354.Ss Quoting
355Quoting is used to remove the special meaning of certain characters
356or words to the shell, such as operators, whitespace, or
357keywords.
358There are three types of quoting: matched single quotes,
359matched double quotes, and backslash.
360.Bl -tag -width indent
361.It Single Quotes
362Enclosing characters in single quotes preserves the literal
363meaning of all the characters (except single quotes, making
364it impossible to put single-quotes in a single-quoted string).
365.It Double Quotes
366Enclosing characters within double quotes preserves the literal
367meaning of all characters except dollarsign
368.Pq Li $ ,
369backquote
370.Pq Li ` ,
371and backslash
372.Pq Li \e .
373The backslash inside double quotes is historically weird.
374It remains literal unless it precedes the following characters,
375which it serves to quote:
376.Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
377.It Li $ Ta Li ` Ta Li \&" Ta Li \e\  Ta Li \en
378.El
379.It Backslash
380A backslash preserves the literal meaning of the following
381character, with the exception of the newline character
382.Pq Li \en .
383A backslash preceding a newline is treated as a line continuation.
384.El
385.Ss Reserved Words
386Reserved words are words that have special meaning to the
387shell and are recognized at the beginning of a line and
388after a control operator.
389The following are reserved words:
390.Bl -column "doneXX" "elifXX" "elseXX" "untilXX" "whileX" -offset center
391.It Li \&! Ta { Ta } Ta Ic case Ta Ic do
392.It Ic done Ta Ic elif Ta Ic else Ta Ic esac Ta Ic fi
393.It Ic for Ta Ic if Ta Ic then Ta Ic until Ta Ic while
394.El
395.Ss Aliases
396An alias is a name and corresponding value set using the
397.Ic alias
398built-in command.
399Whenever a reserved word may occur (see above),
400and after checking for reserved words, the shell
401checks the word to see if it matches an alias.
402If it does, it replaces it in the input stream with its value.
403For example, if there is an alias called
404.Dq Li lf
405with the value
406.Dq Li ls -F ,
407then the input
408.Bd -literal -offset indent
409lf foobar
410.Ed
411.Pp
412would become
413.Bd -literal -offset indent
414ls -F foobar
415.Ed
416.Pp
417Aliases provide a convenient way for naive users to
418create shorthands for commands without having to learn how
419to create functions with arguments.
420They can also be
421used to create lexically obscure code.
422This use is discouraged.
423.Ss Commands
424The shell interprets the words it reads according to a
425language, the specification of which is outside the scope
426of this man page (refer to the BNF in the
427.St -p1003.2
428document).
429Essentially though, a line is read and if
430the first word of the line (or after a control operator)
431is not a reserved word, then the shell has recognized a
432simple command.
433Otherwise, a complex command or some
434other special construct may have been recognized.
435.Ss Simple Commands
436If a simple command has been recognized, the shell performs
437the following actions:
438.Bl -enum
439.It
440Leading words of the form
441.Dq Li name=value
442are stripped off and assigned to the environment of
443the simple command.
444Redirection operators and
445their arguments (as described below) are stripped
446off and saved for processing.
447.It
448The remaining words are expanded as described in
449the section called
450.Sx Word Expansions ,
451and the first remaining word is considered the command
452name and the command is located.
453The remaining
454words are considered the arguments of the command.
455If no command name resulted, then the
456.Dq Li name=value
457variable assignments recognized in 1) affect the
458current shell.
459.It
460Redirections are performed as described in
461the next section.
462.El
463.Ss Redirections
464Redirections are used to change where a command reads its input
465or sends its output.
466In general, redirections open, close, or
467duplicate an existing reference to a file.
468The overall format
469used for redirection is:
470.Pp
471.Dl [n] redir-op file
472.Pp
473The
474.Ql redir-op
475is one of the redirection operators mentioned
476previously.
477The following gives some examples of how these
478operators can be used.
479Note that stdin and stdout are commonly used abbreviations
480for standard input and standard output respectively.
481.Bl -tag -width "1234567890XX" -offset indent
482.It Li [n]> file
483redirect stdout (or file descriptor n) to file
484.It Li [n]>| file
485same as above, but override the
486.Fl C
487option
488.It Li [n]>> file
489append stdout (or file descriptor n) to file
490.It Li [n]< file
491redirect stdin (or file descriptor n) from file
492.It Li [n]<> file
493redirect stdin (or file descriptor n) to and from file
494.It Li [n1]<&n2
495duplicate stdin (or file descriptor n1) from file descriptor n2
496.It Li [n]<&-
497close stdin (or file descriptor n)
498.It Li [n1]>&n2
499duplicate stdout (or file descriptor n1) to file descriptor n2
500.It Li [n]>&-
501close stdout (or file descriptor n)
502.El
503.Pp
504The following redirection is often called a
505.Dq here-document .
506.Bd -literal -offset indent
507[n]<< delimiter
508	here-doc-text
509	...
510delimiter
511.Ed
512.Pp
513All the text on successive lines up to the delimiter is
514saved away and made available to the command on standard
515input, or file descriptor n if it is specified.
516If the delimiter
517as specified on the initial line is quoted, then the here-doc-text
518is treated literally, otherwise the text is subjected to
519parameter expansion, command substitution, and arithmetic
520expansion (as described in the section on
521.Sx Word Expansions ) .
522If the operator is
523.Dq Li <<-
524instead of
525.Dq Li << ,
526then leading tabs
527in the here-doc-text are stripped.
528.Ss Search and Execution
529There are three types of commands: shell functions,
530built-in commands, and normal programs.
531The command is searched for (by name) in that order.
532The three types of commands are all executed in a different way.
533.Pp
534When a shell function is executed, all of the shell positional
535parameters (except $0, which remains unchanged) are
536set to the arguments of the shell function.
537The variables which are explicitly placed in the environment of
538the command (by placing assignments to them before the
539function name) are made local to the function and are set
540to the values given.
541Then the command given in the function definition is executed.
542The positional parameters are restored to their original values
543when the command completes.
544This all occurs within the current shell.
545.Pp
546Shell built-in commands are executed internally to the shell, without
547spawning a new process.
548.Pp
549Otherwise, if the command name does not match a function
550or built-in command, the command is searched for as a normal
551program in the file system (as described in the next section).
552When a normal program is executed, the shell runs the program,
553passing the arguments and the environment to the program.
554If the program is not a normal executable file
555(i.e., if it does not begin with the
556.Qq magic number
557whose
558.Tn ASCII
559representation is
560.Qq #! ,
561resulting in an
562.Er ENOEXEC
563return value from
564.Xr execve 2 )
565the shell will interpret the program in a subshell.
566The child shell will reinitialize itself in this case,
567so that the effect will be
568as if a new shell had been invoked to handle the ad-hoc shell script,
569except that the location of hashed commands located in
570the parent shell will be remembered by the child.
571.Pp
572Note that previous versions of this document
573and the source code itself misleadingly and sporadically
574refer to a shell script without a magic number
575as a
576.Qq shell procedure .
577.Ss Path Search
578When locating a command, the shell first looks to see if
579it has a shell function by that name.
580Then it looks for a
581built-in command by that name.
582If a built-in command is not found,
583one of two things happen:
584.Bl -enum
585.It
586Command names containing a slash are simply executed without
587performing any searches.
588.It
589The shell searches each entry in
590.Ev PATH
591in turn for the command.
592The value of the
593.Ev PATH
594variable should be a series of
595entries separated by colons.
596Each entry consists of a
597directory name.
598The current directory
599may be indicated implicitly by an empty directory name,
600or explicitly by a single period.
601.El
602.Ss Command Exit Status
603Each command has an exit status that can influence the behavior
604of other shell commands.
605The paradigm is that a command exits
606with zero for normal or success, and non-zero for failure,
607error, or a false indication.
608The man page for each command
609should indicate the various exit codes and what they mean.
610Additionally, the built-in commands return exit codes, as does
611an executed shell function.
612.Pp
613If a command is terminated by a signal, its exit status is 128 plus
614the signal number.
615Signal numbers are defined in the header file
616.In sys/signal.h .
617.Ss Complex Commands
618Complex commands are combinations of simple commands
619with control operators or reserved words, together creating a larger complex
620command.
621More generally, a command is one of the following:
622.Bl -item -offset indent
623.It
624simple command
625.It
626pipeline
627.It
628list or compound-list
629.It
630compound command
631.It
632function definition
633.El
634.Pp
635Unless otherwise stated, the exit status of a command is
636that of the last simple command executed by the command.
637.Ss Pipelines
638A pipeline is a sequence of one or more commands separated
639by the control operator |.
640The standard output of all but
641the last command is connected to the standard input
642of the next command.
643The standard output of the last
644command is inherited from the shell, as usual.
645.Pp
646The format for a pipeline is:
647.Pp
648.Dl [!] command1 [ | command2 ...]
649.Pp
650The standard output of command1 is connected to the standard
651input of command2.
652The standard input, standard output, or
653both of a command is considered to be assigned by the
654pipeline before any redirection specified by redirection
655operators that are part of the command.
656.Pp
657If the pipeline is not in the background (discussed later),
658the shell waits for all commands to complete.
659.Pp
660If the reserved word
661.Ic !\&
662does not precede the pipeline, the
663exit status is the exit status of the last command specified
664in the pipeline.
665Otherwise, the exit status is the logical
666NOT of the exit status of the last command.
667That is, if
668the last command returns zero, the exit status is 1; if
669the last command returns greater than zero, the exit status
670is zero.
671.Pp
672Because pipeline assignment of standard input or standard
673output or both takes place before redirection, it can be
674modified by redirection.
675For example:
676.Pp
677.Dl $ command1 2>&1 | command2
678.Pp
679sends both the standard output and standard error of
680.Ql command1
681to the standard input of
682.Ql command2 .
683.Pp
684A
685.Dq Li \&;
686or newline terminator causes the preceding
687AND-OR-list
688(described below in the section called
689.Sx Short-Circuit List Operators )
690to be executed sequentially;
691an
692.Dq Li &
693causes asynchronous execution of the preceding AND-OR-list.
694.Pp
695Note that unlike some other shells,
696.Nm
697executes each process in the pipeline as a child of the
698.Nm
699process.
700Shell built-in commands are the exception to this rule.
701They are executed in the current shell, although they do not affect its
702environment when used in pipelines.
703.Ss Background Commands (&)
704If a command is terminated by the control operator ampersand
705.Pq Li & ,
706the shell executes the command asynchronously;
707the shell does not wait for the command to finish
708before executing the next command.
709.Pp
710The format for running a command in background is:
711.Bd -literal -offset indent
712command1 & [command2 & ...]
713.Ed
714.Pp
715If the shell is not interactive, the standard input of an
716asynchronous command is set to /dev/null.
717.Ss Lists (Generally Speaking)
718A list is a sequence of zero or more commands separated by
719newlines, semicolons, or ampersands,
720and optionally terminated by one of these three characters.
721The commands in a
722list are executed in the order they are written.
723If command is followed by an ampersand, the shell starts the
724command and immediately proceeds onto the next command;
725otherwise it waits for the command to terminate before
726proceeding to the next one.
727.Ss Short-Circuit List Operators
728.Dq Li &&
729and
730.Dq Li ||
731are AND-OR list operators.
732.Dq Li &&
733executes the first command, and then executes the second command
734if the exit status of the first command is zero.
735.Dq Li ||
736is similar, but executes the second command if the exit
737status of the first command is nonzero.
738.Dq Li &&
739and
740.Dq Li ||
741both have the same priority.
742.Ss Flow-Control Constructs (if, while, for, case)
743The syntax of the
744.Ic if
745command is:
746.\"
747.\" XXX Use .Dl to work around broken handling of .Ic inside .Bd and .Ed .
748.\"
749.Dl Ic if Ar list
750.Dl Ic then Ar list
751.Dl [ Ic elif Ar list
752.Dl Ic then Ar list ] ...
753.Dl [ Ic else Ar list ]
754.Dl Ic fi
755.Pp
756The syntax of the
757.Ic while
758command is:
759.Dl Ic while Ar list
760.Dl Ic do Ar list
761.Dl Ic done
762.Pp
763The two lists are executed repeatedly while the exit status of the
764first list is zero.
765The
766.Ic until
767command is similar, but has the word
768.Ic until
769in place of
770.Ic while ,
771which causes it to
772repeat until the exit status of the first list is zero.
773.Pp
774The syntax of the
775.Ic for
776command is:
777.Dl Ic for Ar variable Ic in Ar word ...
778.Dl Ic do Ar list
779.Dl Ic done
780.Pp
781The words are expanded, and then the list is executed
782repeatedly with the variable set to each word in turn.
783The
784.Ic do
785and
786.Ic done
787commands may be replaced with
788.Dq Li {
789and
790.Dq Li } .
791.Pp
792The syntax of the
793.Ic break
794and
795.Ic continue
796commands is:
797.Dl Ic break Op Ar num
798.Dl Ic continue Op Ar num
799.Pp
800The
801.Ic break
802command terminates the
803.Ar num
804innermost
805.Ic for
806or
807.Ic while
808loops.
809The
810.Ic continue
811command continues with the next iteration of the innermost loop.
812These are implemented as built-in commands.
813.Pp
814The syntax of the
815.Ic case
816command is
817.Dl Ic case Ar word Ic in
818.Dl pattern) list ;;
819.Dl ...
820.Dl Ic esac
821.Pp
822The pattern can actually be one or more patterns
823(see
824.Sx Shell Patterns
825described later),
826separated by
827.Dq Li \&|
828characters.
829.Ss Grouping Commands Together
830Commands may be grouped by writing either
831.Bd -literal -offset indent
832(list)
833.Ed
834.Pp
835or
836.Bd -literal -offset indent
837{ list; }
838.Ed
839.Pp
840The first form executes the commands in a subshell.
841Note that built-in commands thus executed do not affect the current shell.
842The second form does not fork another shell,
843so it is slightly more efficient.
844Grouping commands together this way allows the user to
845redirect their output as though they were one program:
846.Bd -literal -offset indent
847{ echo -n "hello"; echo " world"; } > greeting
848.Ed
849.Ss Functions
850The syntax of a function definition is
851.Bd -literal -offset indent
852name ( ) command
853.Ed
854.Pp
855A function definition is an executable statement; when
856executed it installs a function named name and returns an
857exit status of zero.
858The command is normally a list
859enclosed between
860.Dq Li {
861and
862.Dq Li } .
863.Pp
864Variables may be declared to be local to a function by
865using the
866.Ic local
867command.
868This should appear as the first statement of a function,
869and the syntax is:
870.Bd -ragged -offset indent
871.Ic local
872.Op Ar variable ...
873.Op Ar -
874.Ed
875.Pp
876The
877.Ic local
878command is implemented as a built-in command.
879.Pp
880When a variable is made local, it inherits the initial
881value and exported and readonly flags from the variable
882with the same name in the surrounding scope, if there is
883one.
884Otherwise, the variable is initially unset.
885The shell
886uses dynamic scoping, so that if the variable
887.Em x
888is made local to function
889.Em f ,
890which then calls function
891.Em g ,
892references to the variable
893.Em x
894made inside
895.Em g
896will refer to the variable
897.Em x
898declared inside
899.Em f ,
900not to the global variable named
901.Em x .
902.Pp
903The only special parameter than can be made local is
904.Dq Li - .
905Making
906.Dq Li -
907local causes any shell options that are
908changed via the set command inside the function to be
909restored to their original values when the function
910returns.
911.Pp
912The syntax of the
913.Ic return
914command is
915.Bd -ragged -offset indent
916.Ic return
917.Op Ar exitstatus
918.Ed
919.Pp
920It terminates the current executional scope, returning from the previous
921nested function, sourced script, or shell instance, in that order.
922The
923.Ic return
924command is implemented as a built-in command.
925.Ss Variables and Parameters
926The shell maintains a set of parameters.
927A parameter
928denoted by a name is called a variable.
929When starting up,
930the shell turns all the environment variables into shell
931variables.
932New variables can be set using the form
933.Bd -literal -offset indent
934name=value
935.Ed
936.Pp
937Variables set by the user must have a name consisting solely
938of alphabetics, numerics, and underscores.
939The first letter of a variable name must not be numeric.
940A parameter can also be denoted by a number
941or a special character as explained below.
942.Ss Positional Parameters
943A positional parameter is a parameter denoted by a number greater than zero.
944The shell sets these initially to the values of its command line
945arguments that follow the name of the shell script.
946The
947.Ic set
948built-in command can also be used to set or reset them.
949.Ss Special Parameters
950A special parameter is a parameter denoted by one of the following
951special characters.
952The value of the parameter is listed
953next to its character.
954.Bl -hang
955.It Li *
956Expands to the positional parameters, starting from one.
957When
958the expansion occurs within a double-quoted string
959it expands to a single field with the value of each parameter
960separated by the first character of the
961.Ev IFS
962variable,
963or by a
964.Aq space
965if
966.Ev IFS
967is unset.
968.It Li @
969Expands to the positional parameters, starting from one.
970When
971the expansion occurs within double-quotes, each positional
972parameter expands as a separate argument.
973If there are no positional parameters, the
974expansion of
975.Li @
976generates zero arguments, even when
977.Li @
978is double-quoted.
979What this basically means, for example, is
980if $1 is
981.Dq abc
982and $2 is
983.Dq def ghi ,
984then
985.Qq Li $@
986expands to
987the two arguments:
988.Bd -literal -offset indent
989"abc"   "def ghi"
990.Ed
991.It Li #
992Expands to the number of positional parameters.
993.It Li \&?
994Expands to the exit status of the most recent pipeline.
995.It Li -
996(hyphen) Expands to the current option flags (the single-letter
997option names concatenated into a string) as specified on
998invocation, by the set built-in command, or implicitly
999by the shell.
1000.It Li $
1001Expands to the process ID of the invoked shell.
1002A subshell
1003retains the same value of $ as its parent.
1004.It Li \&!
1005Expands to the process ID of the most recent background
1006command executed from the current shell.
1007For a
1008pipeline, the process ID is that of the last command in the
1009pipeline.
1010.It Li 0
1011(zero) Expands to the name of the shell or shell script.
1012.El
1013.Ss Word Expansions
1014This clause describes the various expansions that are
1015performed on words.
1016Not all expansions are performed on
1017every word, as explained later.
1018.Pp
1019Tilde expansions, parameter expansions, command substitutions,
1020arithmetic expansions, and quote removals that occur within
1021a single word expand to a single field.
1022It is only field
1023splitting or pathname expansion that can create multiple
1024fields from a single word.
1025The single exception to this rule is
1026the expansion of the special parameter
1027.Li @
1028within double-quotes,
1029as was described above.
1030.Pp
1031The order of word expansion is:
1032.Bl -enum
1033.It
1034Tilde Expansion, Parameter Expansion, Command Substitution,
1035Arithmetic Expansion (these all occur at the same time).
1036.It
1037Field Splitting is performed on fields generated by step (1)
1038unless the
1039.Ev IFS
1040variable is null.
1041.It
1042Pathname Expansion (unless the
1043.Fl f
1044option is in effect).
1045.It
1046Quote Removal.
1047.El
1048.Pp
1049The
1050.Dq Li $
1051character is used to introduce parameter expansion, command
1052substitution, or arithmetic evaluation.
1053.Ss Tilde Expansion (substituting a user's home directory)
1054A word beginning with an unquoted tilde character
1055.Pq Li ~
1056is
1057subjected to tilde expansion.
1058All the characters up to a slash
1059.Pq Li /
1060or the end of the word are treated as a username
1061and are replaced with the user's home directory.
1062If the
1063username is missing (as in ~/foobar), the tilde is replaced
1064with the value of the HOME variable (the current user's
1065home directory).
1066.Ss Parameter Expansion
1067The format for parameter expansion is as follows:
1068.Bd -literal -offset indent
1069${expression}
1070.Ed
1071.Pp
1072where expression consists of all characters until the matching
1073.Dq Li } .
1074Any
1075.Dq Li }
1076escaped by a backslash or within a quoted string, and characters in
1077embedded arithmetic expansions, command substitutions, and variable
1078expansions, are not examined in determining the matching
1079.Dq Li } .
1080.Pp
1081The simplest form for parameter expansion is:
1082.Bd -literal -offset indent
1083${parameter}
1084.Ed
1085.Pp
1086The value, if any, of parameter is substituted.
1087.Pp
1088The parameter name or symbol can be enclosed in braces, which are
1089optional except for positional parameters with more than one digit or
1090when parameter is followed by a character that could be interpreted as
1091part of the name.
1092If a parameter expansion occurs inside double-quotes:
1093.Bl -enum
1094.It
1095Pathname expansion is not performed on the results of the
1096expansion.
1097.It
1098Field splitting is not performed on the results of the
1099expansion, with the exception of the special parameter
1100.Li @ .
1101.El
1102.Pp
1103In addition, a parameter expansion can be modified by using one of the
1104following formats.
1105.Bl -tag -width indent
1106.It Li ${parameter:-word}
1107Use Default Values.
1108If parameter is unset or
1109null, the expansion of word is
1110substituted; otherwise, the value of
1111parameter is substituted.
1112.It Li ${parameter:=word}
1113Assign Default Values.
1114If parameter is unset
1115or null, the expansion of word is
1116assigned to parameter.
1117In all cases, the
1118final value of parameter is
1119substituted.
1120Only variables, not positional
1121parameters or special parameters, can be
1122assigned in this way.
1123.It Li ${parameter:?[word]}
1124Indicate Error if Null or Unset.
1125If
1126parameter is unset or null, the expansion of
1127word (or a message indicating it is unset if
1128word is omitted) is written to standard
1129error and the shell exits with a nonzero
1130exit status.
1131Otherwise, the value of
1132parameter is substituted.
1133An
1134interactive shell need not exit.
1135.It Li ${parameter:+word}
1136Use Alternate Value.
1137If parameter is unset
1138or null, null is substituted;
1139otherwise, the expansion of word is
1140substituted.
1141.El
1142.Pp
1143In the parameter expansions shown previously, use of the colon in the
1144format results in a test for a parameter that is unset or null; omission
1145of the colon results in a test for a parameter that is only unset.
1146.Bl -tag -width indent
1147.It Li ${#parameter}
1148String Length.
1149The length in characters of
1150the value of parameter.
1151.El
1152.Pp
1153The following four varieties of parameter expansion provide for substring
1154processing.
1155In each case, pattern matching notation
1156(see
1157.Sx Shell Patterns ) ,
1158rather than regular expression notation,
1159is used to evaluate the patterns.
1160If parameter is one of the special parameters
1161.Li *
1162or
1163.Li @ ,
1164the result of the expansion is unspecified.
1165Enclosing the full parameter expansion string in double-quotes does not
1166cause the following four varieties of pattern characters to be quoted,
1167whereas quoting characters within the braces has this effect.
1168.Bl -tag -width indent
1169.It Li ${parameter%word}
1170Remove Smallest Suffix Pattern.
1171The word
1172is expanded to produce a pattern.
1173The
1174parameter expansion then results in
1175parameter, with the smallest portion of the
1176suffix matched by the pattern deleted.
1177.It Li ${parameter%%word}
1178Remove Largest Suffix Pattern.
1179The word
1180is expanded to produce a pattern.
1181The
1182parameter expansion then results in
1183parameter, with the largest portion of the
1184suffix matched by the pattern deleted.
1185.It Li ${parameter#word}
1186Remove Smallest Prefix Pattern.
1187The word
1188is expanded to produce a pattern.
1189The
1190parameter expansion then results in
1191parameter, with the smallest portion of the
1192prefix matched by the pattern deleted.
1193.It Li ${parameter##word}
1194Remove Largest Prefix Pattern.
1195The word
1196is expanded to produce a pattern.
1197The
1198parameter expansion then results in
1199parameter, with the largest portion of the
1200prefix matched by the pattern deleted.
1201.El
1202.Ss Command Substitution
1203Command substitution allows the output of a command to be substituted in
1204place of the command name itself.
1205Command substitution occurs when
1206the command is enclosed as follows:
1207.Bd -literal -offset indent
1208$(command)
1209.Ed
1210.Pp
1211or the backquoted version:
1212.Bd -literal -offset indent
1213`command`
1214.Ed
1215.Pp
1216The shell expands the command substitution by executing command in a
1217subshell environment and replacing the command substitution
1218with the standard output of the command,
1219removing sequences of one or more newlines at the end of the substitution.
1220Embedded newlines before the end of the output are not removed;
1221however, during field splitting, they may be translated into spaces
1222depending on the value of
1223.Ev IFS
1224and the quoting that is in effect.
1225.Ss Arithmetic Expansion
1226Arithmetic expansion provides a mechanism for evaluating an arithmetic
1227expression and substituting its value.
1228The format for arithmetic expansion is as follows:
1229.Bd -literal -offset indent
1230$((expression))
1231.Ed
1232.Pp
1233The expression is treated as if it were in double-quotes, except
1234that a double-quote inside the expression is not treated specially.
1235The
1236shell expands all tokens in the expression for parameter expansion,
1237command substitution, and quote removal.
1238.Pp
1239Next, the shell treats this as an arithmetic expression and
1240substitutes the value of the expression.
1241.Ss White Space Splitting (Field Splitting)
1242After parameter expansion, command substitution, and
1243arithmetic expansion the shell scans the results of
1244expansions and substitutions that did not occur in double-quotes for
1245field splitting and multiple fields can result.
1246.Pp
1247The shell treats each character of the
1248.Ev IFS
1249as a delimiter and uses
1250the delimiters to split the results of parameter expansion and command
1251substitution into fields.
1252.Ss Pathname Expansion (File Name Generation)
1253Unless the
1254.Fl f
1255option is set,
1256file name generation is performed
1257after word splitting is complete.
1258Each word is
1259viewed as a series of patterns, separated by slashes.
1260The
1261process of expansion replaces the word with the names of
1262all existing files whose names can be formed by replacing
1263each pattern with a string that matches the specified pattern.
1264There are two restrictions on this: first, a pattern cannot match
1265a string containing a slash, and second,
1266a pattern cannot match a string starting with a period
1267unless the first character of the pattern is a period.
1268The next section describes the patterns used for both
1269Pathname Expansion and the
1270.Ic case
1271command.
1272.Ss Shell Patterns
1273A pattern consists of normal characters, which match themselves,
1274and meta-characters.
1275The meta-characters are
1276.Dq Li \&! ,
1277.Dq Li * ,
1278.Dq Li \&? ,
1279and
1280.Dq Li [ .
1281These characters lose their special meanings if they are quoted.
1282When command or variable substitution is performed and the dollar sign
1283or back quotes are not double-quoted, the value of the
1284variable or the output of the command is scanned for these
1285characters and they are turned into meta-characters.
1286.Pp
1287An asterisk
1288.Pq Li *
1289matches any string of characters.
1290A question mark
1291.Pq Li \&?
1292matches any single character.
1293A left bracket
1294.Pq Li [
1295introduces a character class.
1296The end of the character class is indicated by a
1297.Dq Li \&] ;
1298if the
1299.Dq Li \&]
1300is missing then the
1301.Dq Li [
1302matches a
1303.Dq Li [
1304rather than introducing a character class.
1305A character class matches any of the characters between the square brackets.
1306A range of characters may be specified using a minus sign.
1307The character class may be complemented by making an exclamation point
1308.Pq Li !\&
1309the first character of the character class.
1310.Pp
1311To include a
1312.Dq Li \&]
1313in a character class, make it the first character listed
1314(after the
1315.Dq Li \&! ,
1316if any).
1317To include a
1318.Dq Li - ,
1319make it the first or last character listed.
1320.Ss Built-in Commands
1321This section lists the commands which
1322are built-in because they need to perform some operation
1323that cannot be performed by a separate process.
1324In addition to
1325these, a built-in version of the
1326.Xr test 1
1327command is provided for efficiency.
1328.Bl -tag -width indent
1329.It Ic \&:
1330A null command that returns a 0 (true) exit value.
1331.It Ic \&. Ar file
1332The commands in the specified file are read and executed by the shell.
1333The
1334.Ic return
1335command may be used to return to the
1336.Ic \&.
1337command's caller.
1338If
1339.Ar file
1340contains any
1341.Dq /
1342characters, it is used as is.
1343Otherwise, the shell searches the
1344.Ev PATH
1345for the file.
1346If it is not found in the
1347.Ev PATH ,
1348it is sought in the current working directory.
1349.It Ic alias Op Ar name ...
1350.It Ic alias Op Ar name Ns = Ns Ar string ...
1351If
1352.Ar name Ns = Ns Ar string
1353is specified, the shell defines the alias
1354.Ar name
1355with value
1356.Ar string .
1357If just
1358.Ar name
1359is specified, the value of the alias
1360.Ar name
1361is printed.
1362With no arguments, the
1363.Ic alias
1364built-in command prints the names and values of all defined aliases
1365(see
1366.Ic unalias ) .
1367Alias values are written with appropriate quoting so that they are
1368suitable for re-input to the shell.
1369.It Ic bg Op Ar job ...
1370Continue the specified jobs
1371(or the current job if no jobs are given)
1372in the background.
1373.It Ic builtin Ar cmd Op Ar arg ...
1374Execute the specified built-in command,
1375.Ar cmd .
1376This is useful when the user wishes to override a shell function
1377with the same name as a built-in command.
1378.It Ic bind Oo Fl aeklrsv Oc Oo Ar key Oo Ar command Oc Oc
1379List or alter key bindings for the line editor.
1380This command is documented in
1381.Xr editrc 5 .
1382.It Ic cd Oo Fl L | P Oc Op Ar directory
1383Switch to the specified
1384.Ar directory ,
1385or to the directory specified in the
1386.Ev HOME
1387environment variable if no
1388.Ar directory
1389is specified.
1390If
1391.Ar directory
1392does not begin with
1393.Pa / , \&. ,
1394or
1395.Pa .. ,
1396then the directories listed in the
1397.Ev CDPATH
1398variable will be
1399searched for the specified
1400.Ar directory .
1401If
1402.Ev CDPATH
1403is unset, the current directory is searched.
1404The format of
1405.Ar CDPATH
1406is the same as that of
1407.Ev PATH .
1408In an interactive shell,
1409the
1410.Ic cd
1411command will print out the name of the directory
1412that it actually switched to
1413if this is different from the name that the user gave.
1414These may be different either because the
1415.Ev CDPATH
1416mechanism was used or because a symbolic link was crossed.
1417.Pp
1418If the
1419.Fl P
1420option is specified,
1421.Pa ..
1422is handled physically and symbolic links are resolved before
1423.Pa ..
1424components are processed.
1425If the
1426.Fl L
1427option is specified,
1428.Pa ..
1429is handled logically.
1430This is the default.
1431.It Ic chdir
1432A synonym for the
1433.Ic cd
1434built-in command.
1435.It Ic command Oo Fl p Oc Op Ar utility Op Ar argument ...
1436Execute the specified
1437.Ar utility
1438as a simple command (see the
1439.Sx Simple Commands
1440section).
1441.Pp
1442If the
1443.Fl p
1444option is specified, the command search is performed using a
1445default value of
1446.Ev PATH
1447that is guaranteed to find all of the standard utilities.
1448.It Ic echo Oo Fl e | n Oc Op Ar string
1449Print
1450.Ar string
1451to the standard output with a newline appended.
1452.Bl -tag -width indent
1453.It Fl n
1454Suppress the output of the trailing newline.
1455.It Fl e
1456Process C-style backslash escape sequences.
1457.Ic echo
1458understands the following character escapes:
1459.Bl -tag -width indent
1460.It \ea
1461Alert (ring the terminal bell)
1462.It \eb
1463Backspace
1464.It \ec
1465Suppress the trailing newline (this has the side-effect of truncating the
1466line if it is not the last character)
1467.It \ee
1468The ESC character (ASCII 0x1b)
1469.It \ef
1470Formfeed
1471.It \en
1472Newline
1473.It \er
1474Carriage return
1475.It \et
1476Horizontal tab
1477.It \ev
1478Vertical tab
1479.It \e\e
1480Literal backslash
1481.It \e0nnn
1482(Zero) The character whose octal value is nnn
1483.El
1484.Pp
1485If
1486.Ar string
1487is not enclosed in quotes then the backslash itself must be escaped
1488with a backslash to protect it from the shell.
1489For example
1490.Bd -literal -offset indent
1491$ echo -e "a\evb"
1492a
1493 b
1494$ echo -e a\e\evb
1495a
1496 b
1497$ echo -e "a\e\eb"
1498a\eb
1499$ echo -e a\e\e\e\eb
1500a\eb
1501.Ed
1502.El
1503.Pp
1504Only one of the
1505.Fl e
1506and
1507.Fl n
1508options may be specified.
1509.It Ic eval Ar string ...
1510Concatenate all the arguments with spaces.
1511Then re-parse and execute the command.
1512.It Ic exec Op Ar command Op arg ...
1513Unless
1514.Ar command
1515is omitted,
1516the shell process is replaced with the specified program
1517(which must be a real program, not a shell built-in command or function).
1518Any redirections on the
1519.Ic exec
1520command are marked as permanent,
1521so that they are not undone when the
1522.Ic exec
1523command finishes.
1524.It Ic exit Op Ar exitstatus
1525Terminate the shell process.
1526If
1527.Ar exitstatus
1528is given
1529it is used as the exit status of the shell;
1530otherwise the exit status of the preceding command is used.
1531.It Ic export Oo Fl p Oc Op Ar name ...
1532The specified names are exported so that they will
1533appear in the environment of subsequent commands.
1534The only way to un-export a variable is to
1535.Ic unset
1536it.
1537The shell allows the value of a variable to be set
1538at the same time as it is exported by writing
1539.Bd -literal -offset indent
1540export name=value
1541.Ed
1542.Pp
1543With no arguments the export command lists the names
1544of all exported variables.
1545If the
1546.Fl p
1547option is specified, the exported variables are printed as
1548.Dq Ic export Ar name Ns = Ns Ar value
1549lines, suitable for re-input to the shell.
1550.It Ic fc Oo Fl e Ar editor Oc Op Ar first Op Ar last
1551.It Ic fc Fl l Oo Fl nr Oc Op Ar first Op Ar last
1552.It Ic fc Fl s Oo Ar old Ns = Ns Ar new Oc Op Ar first
1553The
1554.Ic fc
1555built-in command lists, or edits and re-executes,
1556commands previously entered to an interactive shell.
1557.Bl -tag -width indent
1558.It Fl e Ar editor
1559Use the editor named by
1560.Ar editor
1561to edit the commands.
1562The editor string is a command name,
1563subject to search via the
1564.Ev PATH
1565variable.
1566The value in the
1567.Ev FCEDIT
1568variable is used as a default when
1569.Fl e
1570is not specified.
1571If
1572.Ev FCEDIT
1573is null or unset, the value of the
1574.Ev EDITOR
1575variable is used.
1576If
1577.Ev EDITOR
1578is null or unset,
1579.Xr ed 1
1580is used as the editor.
1581.It Fl l No (ell)
1582List the commands rather than invoking
1583an editor on them.
1584The commands are written in the
1585sequence indicated by the first and last operands, as
1586affected by
1587.Fl r ,
1588with each command preceded by the command number.
1589.It Fl n
1590Suppress command numbers when listing with
1591.Fl l .
1592.It Fl r
1593Reverse the order of the commands listed
1594(with
1595.Fl l )
1596or edited
1597(with neither
1598.Fl l
1599nor
1600.Fl s ) .
1601.It Fl s
1602Re-execute the command without invoking an editor.
1603.It Ar first
1604.It Ar last
1605Select the commands to list or edit.
1606The number of previous commands that can be accessed
1607are determined by the value of the
1608.Ev HISTSIZE
1609variable.
1610The value of
1611.Ar first
1612or
1613.Ar last
1614or both are one of the following:
1615.Bl -tag -width indent
1616.It Ar [+]num
1617A positive number representing a command number;
1618command numbers can be displayed with the
1619.Fl l
1620option.
1621.It Ar -num
1622A negative decimal number representing the
1623command that was executed
1624.Ar num
1625of
1626commands previously.
1627For example, -1 is the immediately previous command.
1628.It Ar string
1629A string indicating the most recently entered command
1630that begins with that string.
1631If the
1632.Ar old=new
1633operand is not also specified with
1634.Fl s ,
1635the string form of the first operand cannot contain an embedded equal sign.
1636.El
1637.El
1638.Pp
1639The following environment variables affect the execution of
1640.Ic fc :
1641.Bl -tag -width indent
1642.It Ev FCEDIT
1643Name of the editor to use.
1644.It Ev HISTSIZE
1645The number of previous commands that are accessible.
1646.El
1647.It Ic fg Op Ar job
1648Move the specified
1649.Ar job
1650or the current job to the foreground.
1651.It Ic getopts Ar optstring Ar var
1652The POSIX
1653.Ic getopts
1654command.
1655The
1656.Ic getopts
1657command deprecates the older
1658.Xr getopt 1
1659command.
1660The first argument should be a series of letters, each possibly
1661followed by a colon which indicates that the option takes an argument.
1662The specified variable is set to the parsed option.
1663The index of
1664the next argument is placed into the shell variable
1665.Ev OPTIND .
1666If an option takes an argument, it is placed into the shell variable
1667.Ev OPTARG .
1668If an invalid option is encountered,
1669.Ev var
1670is set to
1671.Dq Li \&? .
1672It returns a false value (1) when it encounters the end of the options.
1673.It Ic hash Oo Fl rv Oc Op Ar command ...
1674The shell maintains a hash table which remembers the locations of commands.
1675With no arguments whatsoever, the
1676.Ic hash
1677command prints out the contents of this table.
1678Entries which have not been looked at since the last
1679.Ic cd
1680command are marked with an asterisk;
1681it is possible for these entries to be invalid.
1682.Pp
1683With arguments, the
1684.Ic hash
1685command removes each specified
1686.Ar command
1687from the hash table (unless they are functions) and then locates it.
1688With the
1689.Fl v
1690option,
1691.Ic hash
1692prints the locations of the commands as it finds them.
1693The
1694.Fl r
1695option causes the
1696.Ic hash
1697command to delete all the entries in the hash table except for functions.
1698.It Ic jobid Op Ar job
1699Print the process id's of the processes in the specified
1700.Ar job .
1701If the
1702.Ar job
1703argument is omitted, use the current job.
1704.It Ic jobs Oo Fl ls Oc Op Ar job ...
1705Print information about the specified jobs, or all jobs if no
1706.Ar job
1707argument is given.
1708The information printed includes job ID, status and command name.
1709.Pp
1710If the
1711.Fl l
1712option is specified, the PID of each job is also printed.
1713If the
1714.Fl s
1715option is specified, only the PIDs of the jobs are printed, one per line.
1716.It Ic pwd Op Fl L | P
1717Print the path of the current directory.
1718The built-in command may
1719differ from the program of the same name because the
1720built-in command remembers what the current directory
1721is rather than recomputing it each time.
1722This makes
1723it faster.
1724However, if the current directory is
1725renamed,
1726the built-in version of
1727.Xr pwd 1
1728will continue to print the old name for the directory.
1729.Pp
1730If the
1731.Fl P
1732option is specified, symbolic links are resolved.
1733If the
1734.Fl L
1735option is specified, the shell's notion of the current directory
1736is printed (symbolic links are not resolved).
1737This is the default.
1738.It Ic read Oo Fl p Ar prompt Oc Oo Fl t Ar timeout Oc Oo Fl er Oc Ar variable ...
1739The
1740.Ar prompt
1741is printed if the
1742.Fl p
1743option is specified
1744and the standard input is a terminal.
1745Then a line is
1746read from the standard input.
1747The trailing newline
1748is deleted from the line and the line is split as
1749described in the section on
1750.Sx White Space Splitting (Field Splitting)
1751above, and
1752the pieces are assigned to the variables in order.
1753If there are more pieces than variables, the remaining
1754pieces (along with the characters in
1755.Ev IFS
1756that separated them)
1757are assigned to the last variable.
1758If there are more variables than pieces, the remaining
1759variables are assigned the null string.
1760.Pp
1761Backslashes are treated specially, unless the
1762.Fl r
1763option is
1764specified.
1765If a backslash is followed by
1766a newline, the backslash and the newline will be
1767deleted.
1768If a backslash is followed by any other
1769character, the backslash will be deleted and the following
1770character will be treated as though it were not in
1771.Ev IFS ,
1772even if it is.
1773.Pp
1774If the
1775.Fl t
1776option is specified and the
1777.Ar timeout
1778elapses before any input is supplied,
1779the
1780.Ic read
1781command will return without assigning any values.
1782The
1783.Ar timeout
1784value may optionally be followed by one of
1785.Dq s ,
1786.Dq m
1787or
1788.Dq h
1789to explicitly specify seconds, minutes or hours.
1790If none is supplied,
1791.Dq s
1792is assumed.
1793.Pp
1794The
1795.Fl e
1796option exists only for backward compatibility with older scripts.
1797.It Ic readonly Oo Fl p Oc Op Ar name ...
1798Each specified
1799.Ar name
1800is marked as read only,
1801so that it cannot be subsequently modified or unset.
1802The shell allows the value of a variable to be set
1803at the same time as it is marked read only
1804by using the following form:
1805.Bd -literal -offset indent
1806readonly name=value
1807.Ed
1808.Pp
1809With no arguments the
1810.Ic readonly
1811command lists the names of all read only variables.
1812If the
1813.Fl p
1814option is specified, the read-only variables are printed as
1815.Dq Ic readonly Ar name Ns = Ns Ar value
1816lines, suitable for re-input to the shell.
1817.It Ic set Oo Fl /+abCEefIimnpTuVvx Oc Oo Fl /+o Ar longname Oc Oo
1818.Fl c Ar string Oc Op Fl - Ar arg ...
1819The
1820.Ic set
1821command performs three different functions:
1822.Bl -item
1823.It
1824With no arguments, it lists the values of all shell variables.
1825.It
1826If options are given,
1827either in short form or using the long
1828.Dq Fl /+o Ar longname
1829form,
1830it sets or clears the specified options as described in the section called
1831.Sx Argument List Processing .
1832.It
1833If the
1834.Dq Fl -
1835option is specified,
1836.Ic set
1837will replace the shell's positional parameters with the subsequent
1838arguments.
1839If no arguments follow the
1840.Dq Fl -
1841option,
1842all the positional parameters will be cleared,
1843which is equivalent to executing the command
1844.Dq Li shift $# .
1845The
1846.Dq Fl -
1847flag may be omitted when specifying arguments to be used
1848as positional replacement parameters.
1849This is not recommended,
1850because the first argument may begin with a dash
1851.Pq Li -
1852or a plus
1853.Pq Li + ,
1854which the
1855.Ic set
1856command will interpret as a request to enable or disable options.
1857.El
1858.It Ic setvar Ar variable Ar value
1859Assigns the specified
1860.Ar value
1861to the specified
1862.Ar variable .
1863.Ic Setvar
1864is intended to be used in functions that
1865assign values to variables whose names are passed as parameters.
1866In general it is better to write
1867.Bd -literal -offset indent
1868variable=value
1869.Ed
1870rather than using
1871.Ic setvar .
1872.It Ic shift Op Ar n
1873Shift the positional parameters
1874.Ar n
1875times, or once if
1876.Ar n
1877is not specified.
1878A shift sets the value of $1 to the value of $2,
1879the value of $2 to the value of $3, and so on,
1880decreasing the value of $# by one.
1881If there are zero positional parameters, shifting does not do anything.
1882.It Ic trap Oo Ar action Oc Ar signal ...
1883Cause the shell to parse and execute
1884.Ar action
1885when any specified
1886.Ar signal
1887is received.
1888The signals are specified by name or number.
1889In addition, the pseudo-signal
1890.Cm EXIT
1891may be used to specify an action that is performed when the shell terminates.
1892The
1893.Ar action
1894may be null or omitted;
1895the former causes the specified signal to be ignored
1896and the latter causes the default action to be taken.
1897When the shell forks off a subshell,
1898it resets trapped (but not ignored) signals to the default action.
1899The
1900.Ic trap
1901command has no effect on signals that were ignored on entry to the shell.
1902.It Ic type Op Ar name ...
1903Interpret each
1904.Ar name
1905as a command and print the resolution of the command search.
1906Possible resolutions are:
1907shell keyword, alias, shell built-in command, command, tracked alias
1908and not found.
1909For aliases the alias expansion is printed;
1910for commands and tracked aliases
1911the complete pathname of the command is printed.
1912.It Ic ulimit Oo Fl HSabcdflmnstuv Oc Op Ar limit
1913Set or display resource limits (see
1914.Xr getrlimit 2 ) .
1915If
1916.Ar limit
1917is specified, the named resource will be set;
1918otherwise the current resource value will be displayed.
1919.Pp
1920If
1921.Fl H
1922is specified, the hard limits will be set or displayed.
1923While everybody is allowed to reduce a hard limit,
1924only the superuser can increase it.
1925The
1926.Fl S
1927option
1928specifies the soft limits instead.
1929When displaying limits,
1930only one of
1931.Fl S
1932or
1933.Fl H
1934can be given.
1935The default is to display the soft limits,
1936and to set both the hard and the soft limits.
1937.Pp
1938Option
1939.Fl a
1940causes the
1941.Ic ulimit
1942command to display all resources.
1943The parameter
1944.Ar limit
1945is not acceptable in this mode.
1946.Pp
1947The remaining options specify which resource value is to be
1948displayed or modified.
1949They are mutually exclusive.
1950.Bl -tag -width indent
1951.It Fl b Ar sbsize
1952The maximum size of socket buffer usage, in bytes.
1953.It Fl c Ar coredumpsize
1954The maximal size of core dump files, in 512-byte blocks.
1955.It Fl d Ar datasize
1956The maximal size of the data segment of a process, in kilobytes.
1957.It Fl f Ar filesize
1958The maximal size of a file, in 512-byte blocks.
1959.It Fl l Ar lockedmem
1960The maximal size of memory that can be locked by a process, in
1961kilobytes.
1962.It Fl m Ar memoryuse
1963The maximal resident set size of a process, in kilobytes.
1964.It Fl n Ar nofiles
1965The maximal number of descriptors that could be opened by a process.
1966.It Fl s Ar stacksize
1967The maximal size of the stack segment, in kilobytes.
1968.It Fl t Ar time
1969The maximal amount of CPU time to be used by each process, in seconds.
1970.It Fl u Ar userproc
1971The maximal number of simultaneous processes for this user ID.
1972.It Fl v Ar virtualmem
1973The maximal virtual size of a process, in kilobytes.
1974.El
1975.It Ic umask Op Ar mask
1976Set the file creation mask (see
1977.Xr umask 2 )
1978to the octal value specified by
1979.Ar mask .
1980If the argument is omitted, the current mask value is printed.
1981.It Ic unalias Oo Fl a Oc Op Ar name
1982If
1983.Ar name
1984is specified, the shell removes that alias.
1985If
1986.Fl a
1987is specified, all aliases are removed.
1988.It Ic unset Oo Fl fv Oc Ar name ...
1989The specified variables or functions are unset and unexported.
1990If the
1991.Fl v
1992option is specified or no options are given, the
1993.Ar name
1994arguments are treated as variable names.
1995If the
1996.Fl f
1997option is specified, the
1998.Ar name
1999arguments are treated as function names.
2000.It Ic wait Op Ar job
2001Wait for the specified
2002.Ar job
2003to complete and return the exit status of the last process in the
2004.Ar job .
2005If the argument is omitted, wait for all jobs to complete
2006and return an exit status of zero.
2007.El
2008.Ss Commandline Editing
2009When
2010.Nm
2011is being used interactively from a terminal, the current command
2012and the command history
2013(see
2014.Ic fc
2015in
2016.Sx Built-in Commands )
2017can be edited using vi-mode command line editing.
2018This mode uses commands similar
2019to a subset of those described in the vi man page.
2020The command
2021.Dq Li set -o vi
2022(or
2023.Dq Li set -V )
2024enables vi-mode editing and places
2025.Nm
2026into vi insert mode.
2027With vi-mode enabled,
2028.Nm
2029can be switched between insert mode and command mode by typing
2030.Aq ESC .
2031Hitting
2032.Aq return
2033while in command mode will pass the line to the shell.
2034.Pp
2035Similarly, the
2036.Dq Li set -o emacs
2037(or
2038.Dq Li set -E )
2039command can be used to enable a subset of
2040emacs-style command line editing features.
2041.Sh SEE ALSO
2042.Xr builtin 1 ,
2043.Xr echo 1 ,
2044.Xr expr 1 ,
2045.Xr pwd 1 ,
2046.Xr test 1
2047.Sh HISTORY
2048A
2049.Nm
2050command appeared in
2051.At v1 .
2052.Sh BUGS
2053The
2054.Nm
2055utility does not recognize multibyte characters.
2056