xref: /freebsd/bin/sh/sh.1 (revision 71fe318b852b8dfb3e799cb12ef184750f7f8eac)
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5.\" Kenneth Almquist.
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34.\"
35.\"	from: @(#)sh.1	8.6 (Berkeley) 5/4/95
36.\" $FreeBSD$
37.\"
38.Dd May 5, 1995
39.Dt SH 1
40.Os
41.Sh NAME
42.Nm sh
43.Nd command interpreter (shell)
44.Sh SYNOPSIS
45.Nm
46.Op Fl /+abCEefIimnPpsTuVvx
47.Op Fl /+o Ar longname
48.Op Fl c Ar string
49.Op Ar arg ...
50.Sh DESCRIPTION
51The
52.Nm
53utility is the standard command interpreter for the system.
54The current version of
55.Nm
56is in the process of being changed to
57conform with the
58.St -p1003.2
59specification for the shell.  This version has many features which make
60it appear
61similar in some respects to the Korn shell, but it is not a Korn
62shell clone like
63.Xr pdksh 1 .
64Only features
65designated by POSIX, plus a few Berkeley extensions, are being
66incorporated into this shell.
67This man page is not intended to be a tutorial nor a complete
68specification of the shell.
69.Ss Overview
70The shell is a command that reads lines from
71either a file or the terminal, interprets them, and
72generally executes other commands.
73It is the program that is started when a user logs into the system,
74although a user can select a different shell with the
75.Xr chsh 1
76command.
77The shell
78implements a language that has flow control constructs,
79a macro facility that provides a variety of features in
80addition to data storage, along with built-in history and line
81editing capabilities.  It incorporates many features to
82aid interactive use and has the advantage that the interpretative
83language is common to both interactive and non-interactive
84use (shell scripts).  That is, commands can be typed directly
85to the running shell or can be put into a file,
86which can be executed directly by the shell.
87.Ss Invocation
88.\"
89.\" XXX This next sentence is incredibly confusing.
90.\"
91If no arguments are present and if the standard input of the shell
92is connected to a terminal
93(or if the
94.Fl i
95option is set),
96the shell is considered an interactive shell.  An interactive shell
97generally prompts before each command and handles programming
98and command errors differently (as described below).
99When first starting, the shell inspects argument 0, and
100if it begins with a dash
101.Pq Li - ,
102the shell is also considered a login shell.
103This is normally done automatically by the system
104when the user first logs in.  A login shell first reads commands
105from the files
106.Pa /etc/profile
107and then
108.Pa .profile
109if they exist.  If the environment variable
110.Ev ENV
111is set on entry to a shell, or is set in the
112.Pa .profile
113of a login shell, the shell then reads commands from the file named in
114.Ev ENV .
115Therefore, a user should place commands that are to be executed only
116at login time in the
117.Pa .profile
118file, and commands that are executed for every shell inside the
119.Ev ENV
120file.
121The user can set the
122.Ev ENV
123variable to some file by placing the following line in the file
124.Pa .profile
125in the home directory,
126substituting for
127.Pa .shinit
128the filename desired:
129.Pp
130.Dl ENV=$HOME/.shinit; export ENV
131.Pp
132The first non-option argument specified on the command line
133will be treated as the
134name of a file from which to read commands (a shell script), and
135the remaining arguments are set as the positional parameters
136of the shell ($1, $2, etc).  Otherwise, the shell reads commands
137from its standard input.
138.Pp
139Unlike older versions of
140.Nm
141the
142.Ev ENV
143script is only sourced on invocation of interactive shells.  This
144closes a well-known, and sometimes easily exploitable security
145hole related to poorly thought out
146.Ev ENV
147scripts.
148.Ss Argument List Processing
149All of the single letter options to
150.Nm
151have a corresponding long name,
152with the exception of
153.Fl c
154and
155.Fl /+o .
156These long names are provided next to the single letter options
157in the descriptions below.
158The long name for an option may be specified as an argument to the
159.Fl /+o
160option of
161.Xr sh 1 .
162Once the shell is running,
163the long name for an option may be specified as an argument to the
164.Fl /+o
165option of the
166.Ic set
167built-in command
168(described later in the section called
169.Sx Built-in Commands ) .
170Introducing an option with a dash
171.Pq Li -
172enables the option,
173while using a plus
174.Pq Li +
175disables the option.
176A
177.Dq Li --
178or plain
179.Dq Li -
180will stop option processing and will force the remaining
181words on the command line to be treated as arguments.
182The
183.Fl /+o
184and
185.Fl c
186options do not have long names.
187They take arguments and are described after the single letter options.
188.Bl -tag -width indent
189.It Fl a Li allexport
190Flag variables for export when assignments are made to them.
191.It Fl b Li notify
192Enable asynchronous notification of background job
193completion.
194(UNIMPLEMENTED)
195.It Fl C Li noclobber
196Do not overwrite existing files with
197.Dq Li > .
198.It Fl E Li emacs
199Enable the built-in
200.Xr emacs 1
201command line editor (disables the
202.Fl V
203option if it has been set).
204.It Fl e Li errexit
205Exit immediately if any untested command fails in non-interactive mode.
206The exit status of a command is considered to be
207explicitly tested if the command is used to control
208an if, elif, while, or until; or if the command is the left
209hand operand of an
210.Dq Li &&
211or
212.Dq Li ||
213operator.
214.It Fl f Li noglob
215Disable pathname expansion.
216.It Fl I Li ignoreeof
217Ignore
218.Dv EOF Ns ' Ns s
219from input when in interactive mode.
220.It Fl i Li interactive
221Force the shell to behave interactively.
222.It Fl m Li monitor
223Turn on job control (set automatically when interactive).
224.It Fl n Li noexec
225If not interactive, read commands but do not
226execute them.  This is useful for checking the
227syntax of shell scripts.
228.It Fl P Li physical
229Change the default for the
230.Ic cd
231and
232.Ic pwd
233commands from
234.Fl L
235(logical directory layout)
236to
237.Fl P
238(physical directory layout).
239.It Fl p Li privileged
240Turn on privileged mode.  This mode is enabled on startup
241if either the effective user or group id is not equal to the
242real user or group id.  Turning this mode off sets the
243effective user and group ids to the real user and group ids.
244When this mode is enabled for interactive shells, the file
245.Pa /etc/suid_profile
246is sourced instead of
247.Pa ~/.profile
248after
249.Pa /etc/profile
250is sourced, and the contents of the
251.Ev ENV
252variable are ignored.
253.It Fl s Li stdin
254Read commands from standard input (set automatically
255if no file arguments are present).  This option has
256no effect when set after the shell has already started
257running (i.e. when set with the
258.Ic set
259command).
260.It Fl T Li asynctraps
261When waiting for a child, execute traps immediately.
262If this option is not set,
263traps are executed after the child exits,
264as specified in
265.St -p1003.2
266This nonstandard option is useful for putting guarding shells around
267children that block signals.  The surrounding shell may kill the child
268or it may just return control to the tty and leave the child alone,
269like this:
270.Bd -literal -offset indent
271sh -T -c "trap 'exit 1' 2 ; some-blocking-program"
272.Ed
273.Pp
274.It Fl u Li nounset
275Write a message to standard error when attempting
276to expand a variable that is not set, and if the
277shell is not interactive, exit immediately.
278.It Fl V Li vi
279Enable the built-in
280.Xr vi 1
281command line editor (disables
282.Fl E
283if it has been set).
284.It Fl v Li verbose
285The shell writes its input to standard error
286as it is read.  Useful for debugging.
287.It Fl x Li xtrace
288Write each command
289(preceded by
290.Dq Li +\  )
291to standard error before it is executed.
292Useful for debugging.
293.El
294.Pp
295The
296.Fl c
297option may be used to pass its string argument to the shell
298to be interpreted as input.
299Keep in mind that this option only accepts a single string as its
300argument, hence multi-word strings must be quoted.
301.Pp
302The
303.Fl /+o
304option takes as its only argument the long name of an option
305to be enabled or disabled.
306For example, the following two invocations of
307.Nm
308both enable the built-in
309.Xr emacs 1
310command line editor:
311.Bd -literal -offset indent
312set -E
313set -o emacs
314.Ed
315.Pp
316If used without an argument, the
317.Fl o
318option displays the current option settings in a human-readable format.
319If
320.Cm +o
321is used without an argument, the current option settings are output
322in a format suitable for re-input into the shell.
323.Ss Lexical Structure
324The shell reads input in terms of lines from a file and breaks
325it up into words at whitespace (blanks and tabs), and at
326certain sequences of
327characters called
328.Dq operators ,
329which are special to the shell.
330There are two types of operators: control operators and
331redirection operators (their meaning is discussed later).
332The following is a list of valid operators:
333.Bl -tag -width indent
334.It Control operators:
335.Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
336.It Li & Ta Li && Ta Li ( Ta Li ) Ta Li \en
337.It Li ;; Ta Li ; Ta Li | Ta Li ||
338.El
339.It Redirection operators:
340.Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
341.It Li < Ta Li > Ta Li << Ta Li >> Ta Li <>
342.It Li <& Ta Li >& Ta Li <<- Ta Li >|
343.El
344.El
345.Ss Quoting
346Quoting is used to remove the special meaning of certain characters
347or words to the shell, such as operators, whitespace, or
348keywords.  There are three types of quoting: matched single quotes,
349matched double quotes, and backslash.
350.Bl -tag -width indent
351.It Single Quotes
352Enclosing characters in single quotes preserves the literal
353meaning of all the characters (except single quotes, making
354it impossible to put single-quotes in a single-quoted string).
355.It Double Quotes
356Enclosing characters within double quotes preserves the literal
357meaning of all characters except dollarsign
358.Pq Li $ ,
359backquote
360.Pq Li ` ,
361and backslash
362.Pq Li \e .
363The backslash inside double quotes is historically weird.
364It remains literal unless it precedes the following characters,
365which it serves to quote:
366.Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
367.It Li $ Ta Li ` Ta Li \&" Ta Li \e\  Ta Li \en
368.El
369.It Backslash
370A backslash preserves the literal meaning of the following
371character, with the exception of the newline character
372.Pq Li \en .
373A backslash preceding a newline is treated as a line continuation.
374.El
375.Ss Reserved Words
376Reserved words are words that have special meaning to the
377shell and are recognized at the beginning of a line and
378after a control operator.  The following are reserved words:
379.Bl -column "doneXX" "elifXX" "elseXX" "untilXX" "whileX" -offset center
380.It Li \&! Ta { Ta } Ta Ic case Ta Ic do
381.It Ic done Ta Ic elif Ta Ic else Ta Ic esac Ta Ic fi
382.It Ic for Ta Ic if Ta Ic then Ta Ic until Ta Ic while
383.El
384.Ss Aliases
385An alias is a name and corresponding value set using the
386.Ic alias
387built-in command.  Whenever a reserved word may occur (see above),
388and after checking for reserved words, the shell
389checks the word to see if it matches an alias.
390If it does, it replaces it in the input stream with its value.
391For example, if there is an alias called
392.Dq Li lf
393with the value
394.Dq Li ls -F ,
395then the input
396.Bd -literal -offset indent
397lf foobar
398.Ed
399.Pp
400would become
401.Bd -literal -offset indent
402ls -F foobar
403.Ed
404.Pp
405Aliases provide a convenient way for naive users to
406create shorthands for commands without having to learn how
407to create functions with arguments.  They can also be
408used to create lexically obscure code.  This use is discouraged.
409.Ss Commands
410The shell interprets the words it reads according to a
411language, the specification of which is outside the scope
412of this man page (refer to the BNF in the
413.St -p1003.2
414document).  Essentially though, a line is read and if
415the first word of the line (or after a control operator)
416is not a reserved word, then the shell has recognized a
417simple command.  Otherwise, a complex command or some
418other special construct may have been recognized.
419.Ss Simple Commands
420If a simple command has been recognized, the shell performs
421the following actions:
422.Bl -enum
423.It
424Leading words of the form
425.Dq Li name=value
426are stripped off and assigned to the environment of
427the simple command.  Redirection operators and
428their arguments (as described below) are stripped
429off and saved for processing.
430.It
431The remaining words are expanded as described in
432the section called
433.Sx Word Expansions ,
434and the first remaining word is considered the command
435name and the command is located.  The remaining
436words are considered the arguments of the command.
437If no command name resulted, then the
438.Dq Li name=value
439variable assignments recognized in 1) affect the
440current shell.
441.It
442Redirections are performed as described in
443the next section.
444.El
445.Ss Redirections
446Redirections are used to change where a command reads its input
447or sends its output.  In general, redirections open, close, or
448duplicate an existing reference to a file.  The overall format
449used for redirection is:
450.Pp
451.Dl [n] redir-op file
452.Pp
453The
454.Ql redir-op
455is one of the redirection operators mentioned
456previously.  The following gives some examples of how these
457operators can be used.
458Note that stdin and stdout are commonly used abbreviations
459for standard input and standard output respectively.
460.Bl -tag -width "1234567890XX" -offset indent
461.It Li [n]> file
462redirect stdout (or file descriptor n) to file
463.It Li [n]>| file
464same as above, but override the
465.Fl C
466option
467.It Li [n]>> file
468append stdout (or file descriptor n) to file
469.It Li [n]< file
470redirect stdin (or file descriptor n) from file
471.It Li [n]<> file
472redirect stdin (or file descriptor n) to and from file
473.It Li [n1]<&n2
474duplicate stdin (or file descriptor n1) from file descriptor n2
475.It Li [n]<&-
476close stdin (or file descriptor n)
477.It Li [n1]>&n2
478duplicate stdout (or file descriptor n1) to file descriptor n2
479.It Li [n]>&-
480close stdout (or file descriptor n)
481.El
482.Pp
483The following redirection is often called a
484.Dq here-document .
485.Bd -literal -offset indent
486[n]<< delimiter
487	here-doc-text
488	...
489delimiter
490.Ed
491.Pp
492All the text on successive lines up to the delimiter is
493saved away and made available to the command on standard
494input, or file descriptor n if it is specified.  If the delimiter
495as specified on the initial line is quoted, then the here-doc-text
496is treated literally, otherwise the text is subjected to
497parameter expansion, command substitution, and arithmetic
498expansion (as described in the section on
499.Sx Word Expansions ) .
500If the operator is
501.Dq Li <<-
502instead of
503.Dq Li << ,
504then leading tabs
505in the here-doc-text are stripped.
506.Ss Search and Execution
507There are three types of commands: shell functions,
508built-in commands, and normal programs.
509The command is searched for (by name) in that order.
510The three types of commands are all executed in a different way.
511.Pp
512When a shell function is executed, all of the shell positional
513parameters (except $0, which remains unchanged) are
514set to the arguments of the shell function.
515The variables which are explicitly placed in the environment of
516the command (by placing assignments to them before the
517function name) are made local to the function and are set
518to the values given.
519Then the command given in the function definition is executed.
520The positional parameters are restored to their original values
521when the command completes.
522This all occurs within the current shell.
523.Pp
524Shell built-in commands are executed internally to the shell, without
525spawning a new process.
526.Pp
527Otherwise, if the command name does not match a function
528or built-in command, the command is searched for as a normal
529program in the file system (as described in the next section).
530When a normal program is executed, the shell runs the program,
531passing the arguments and the environment to the program.
532If the program is not a normal executable file
533(i.e. if it does not begin with the
534.Qq magic number
535whose
536.Tn ASCII
537representation is
538.Qq #! ,
539resulting in an
540.Er ENOEXEC
541return value from
542.Xr execve 2 )
543the shell will interpret the program in a subshell.
544The child shell will reinitialize itself in this case,
545so that the effect will be
546as if a new shell had been invoked to handle the ad-hoc shell script,
547except that the location of hashed commands located in
548the parent shell will be remembered by the child.
549.Pp
550Note that previous versions of this document
551and the source code itself misleadingly and sporadically
552refer to a shell script without a magic number
553as a
554.Qq shell procedure .
555.Ss Path Search
556When locating a command, the shell first looks to see if
557it has a shell function by that name.  Then it looks for a
558built-in command by that name.  If a built-in command is not found,
559one of two things happen:
560.Bl -enum
561.It
562Command names containing a slash are simply executed without
563performing any searches.
564.It
565The shell searches each entry in
566.Ev PATH
567in turn for the command.  The value of the
568.Ev PATH
569variable should be a series of
570entries separated by colons.  Each entry consists of a
571directory name.
572The current directory
573may be indicated implicitly by an empty directory name,
574or explicitly by a single period.
575.El
576.Ss Command Exit Status
577Each command has an exit status that can influence the behavior
578of other shell commands.  The paradigm is that a command exits
579with zero for normal or success, and non-zero for failure,
580error, or a false indication.  The man page for each command
581should indicate the various exit codes and what they mean.
582Additionally, the built-in commands return exit codes, as does
583an executed shell function.
584.Pp
585If a command is terminated by a signal, its exit status is 128 plus
586the signal number.  Signal numbers are defined in the header file
587.Aq Pa sys/signal.h .
588.Ss Complex Commands
589Complex commands are combinations of simple commands
590with control operators or reserved words, together creating a larger complex
591command.  More generally, a command is one of the following:
592.Bl -item -offset indent
593.It
594simple command
595.It
596pipeline
597.It
598list or compound-list
599.It
600compound command
601.It
602function definition
603.El
604.Pp
605Unless otherwise stated, the exit status of a command is
606that of the last simple command executed by the command.
607.Ss Pipelines
608A pipeline is a sequence of one or more commands separated
609by the control operator |.  The standard output of all but
610the last command is connected to the standard input
611of the next command.  The standard output of the last
612command is inherited from the shell, as usual.
613.Pp
614The format for a pipeline is:
615.Pp
616.Dl [!] command1 [ | command2 ...]
617.Pp
618The standard output of command1 is connected to the standard
619input of command2.  The standard input, standard output, or
620both of a command is considered to be assigned by the
621pipeline before any redirection specified by redirection
622operators that are part of the command.
623.Pp
624If the pipeline is not in the background (discussed later),
625the shell waits for all commands to complete.
626.Pp
627If the reserved word ! does not precede the pipeline, the
628exit status is the exit status of the last command specified
629in the pipeline.  Otherwise, the exit status is the logical
630NOT of the exit status of the last command.  That is, if
631the last command returns zero, the exit status is 1; if
632the last command returns greater than zero, the exit status
633is zero.
634.Pp
635Because pipeline assignment of standard input or standard
636output or both takes place before redirection, it can be
637modified by redirection.  For example:
638.Pp
639.Dl $ command1 2>&1 | command2
640.Pp
641sends both the standard output and standard error of
642.Ql command1
643to the standard input of
644.Ql command2 .
645.Pp
646A
647.Dq Li \&;
648or newline terminator causes the preceding
649AND-OR-list
650(described below in the section called
651.Sx Short-Circuit List Operators )
652to be executed sequentially;
653an
654.Dq Li &
655causes asynchronous execution of the preceding AND-OR-list.
656.Pp
657Note that unlike some other shells,
658.Nm
659executes each process in the pipeline as a child of the
660.Nm
661process.
662Shell built-in commands are the exception to this rule.
663They are executed in the current shell, although they do not affect its
664environment when used in pipelines.
665.Ss Background Commands (&)
666If a command is terminated by the control operator ampersand
667.Pq Li & ,
668the shell executes the command asynchronously;
669the shell does not wait for the command to finish
670before executing the next command.
671.Pp
672The format for running a command in background is:
673.Bd -literal -offset indent
674command1 & [command2 & ...]
675.Ed
676.Pp
677If the shell is not interactive, the standard input of an
678asynchronous command is set to /dev/null.
679.Ss Lists (Generally Speaking)
680A list is a sequence of zero or more commands separated by
681newlines, semicolons, or ampersands,
682and optionally terminated by one of these three characters.
683The commands in a
684list are executed in the order they are written.
685If command is followed by an ampersand, the shell starts the
686command and immediately proceeds onto the next command;
687otherwise it waits for the command to terminate before
688proceeding to the next one.
689.Ss Short-Circuit List Operators
690.Dq Li &&
691and
692.Dq Li ||
693are AND-OR list operators.
694.Dq Li &&
695executes the first command, and then executes the second command
696if the exit status of the first command is zero.
697.Dq Li ||
698is similar, but executes the second command if the exit
699status of the first command is nonzero.
700.Dq Li &&
701and
702.Dq Li ||
703both have the same priority.
704.Ss Flow-Control Constructs (if, while, for, case)
705The syntax of the
706.Ic if
707command is:
708.\"
709.\" XXX Use .Dl to work around broken handling of .Ic inside .Bd and .Ed .
710.\"
711.Dl Ic if Ar list
712.Dl Ic then Ar list
713.Dl [ Ic elif Ar list
714.Dl Ic then Ar list ] ...
715.Dl [ Ic else Ar list ]
716.Dl Ic fi
717.Pp
718The syntax of the
719.Ic while
720command is:
721.Dl Ic while Ar list
722.Dl Ic do Ar list
723.Dl Ic done
724.Pp
725The two lists are executed repeatedly while the exit status of the
726first list is zero.
727The
728.Ic until
729command is similar, but has the word
730.Ic until
731in place of
732.Ic while ,
733which causes it to
734repeat until the exit status of the first list is zero.
735.Pp
736The syntax of the
737.Ic for
738command is:
739.Dl Ic for Ar variable Ic in Ar word ...
740.Dl Ic do Ar list
741.Dl Ic done
742.Pp
743The words are expanded, and then the list is executed
744repeatedly with the variable set to each word in turn.
745The
746.Ic do
747and
748.Ic done
749commands may be replaced with
750.Dq Li {
751and
752.Dq Li } .
753.Pp
754The syntax of the
755.Ic break
756and
757.Ic continue
758commands is:
759.Dl Ic break Op Ar num
760.Dl Ic continue Op Ar num
761.Pp
762The
763.Ic break
764command terminates the
765.Ar num
766innermost
767.Ic for
768or
769.Ic while
770loops.
771The
772.Ic continue
773command continues with the next iteration of the innermost loop.
774These are implemented as built-in commands.
775.Pp
776The syntax of the
777.Ic case
778command is
779.Dl Ic case Ar word Ic in
780.Dl pattern) list ;;
781.Dl ...
782.Dl Ic esac
783.Pp
784The pattern can actually be one or more patterns
785(see
786.Sx Shell Patterns
787described later),
788separated by
789.Dq Li \&|
790characters.
791.Ss Grouping Commands Together
792Commands may be grouped by writing either
793.Bd -literal -offset indent
794(list)
795.Ed
796.Pp
797or
798.Bd -literal -offset indent
799{ list; }
800.Ed
801.Pp
802The first form executes the commands in a subshell.
803Note that built-in commands thus executed do not affect the current shell.
804The second form does not fork another shell,
805so it is slightly more efficient.
806Grouping commands together this way allows the user to
807redirect their output as though they were one program:
808.Bd -literal -offset indent
809{ echo -n "hello"; echo " world"; } > greeting
810.Ed
811.Ss Functions
812The syntax of a function definition is
813.Bd -literal -offset indent
814name ( ) command
815.Ed
816.Pp
817A function definition is an executable statement; when
818executed it installs a function named name and returns an
819exit status of zero.  The command is normally a list
820enclosed between
821.Dq Li {
822and
823.Dq Li } .
824.Pp
825Variables may be declared to be local to a function by
826using the
827.Ic local
828command.
829This should appear as the first statement of a function,
830and the syntax is:
831.Bd -ragged -offset indent
832.Ic local
833.Op Ar variable ...
834.Op Ar -
835.Ed
836.Pp
837The
838.Ic local
839command is implemented as a built-in command.
840.Pp
841When a variable is made local, it inherits the initial
842value and exported and readonly flags from the variable
843with the same name in the surrounding scope, if there is
844one.  Otherwise, the variable is initially unset.  The shell
845uses dynamic scoping, so that if the variable
846.Em x
847is made local to function
848.Em f ,
849which then calls function
850.Em g ,
851references to the variable
852.Em x
853made inside
854.Em g
855will refer to the variable
856.Em x
857declared inside
858.Em f ,
859not to the global variable named
860.Em x .
861.Pp
862The only special parameter than can be made local is
863.Dq Li - .
864Making
865.Dq Li -
866local causes any shell options that are
867changed via the set command inside the function to be
868restored to their original values when the function
869returns.
870.Pp
871The syntax of the
872.Ic return
873command is
874.Bd -ragged -offset indent
875.Ic return
876.Op Ar exitstatus
877.Ed
878.Pp
879It terminates the currently executing function.
880The
881.Ic return
882command is implemented as a built-in command.
883.Ss Variables and Parameters
884The shell maintains a set of parameters.  A parameter
885denoted by a name is called a variable.  When starting up,
886the shell turns all the environment variables into shell
887variables.  New variables can be set using the form
888.Bd -literal -offset indent
889name=value
890.Ed
891.Pp
892Variables set by the user must have a name consisting solely
893of alphabetics, numerics, and underscores.
894The first letter of a variable name must not be numeric.
895A parameter can also be denoted by a number
896or a special character as explained below.
897.Ss Positional Parameters
898A positional parameter is a parameter denoted by a number greater than zero.
899The shell sets these initially to the values of its command line
900arguments that follow the name of the shell script.  The
901.Ic set
902built-in command can also be used to set or reset them.
903.Ss Special Parameters
904A special parameter is a parameter denoted by one of the following
905special characters.  The value of the parameter is listed
906next to its character.
907.Bl -hang
908.It Li *
909Expands to the positional parameters, starting from one.  When
910the expansion occurs within a double-quoted string
911it expands to a single field with the value of each parameter
912separated by the first character of the
913.Ev IFS
914variable,
915or by a
916.Aq space
917if
918.Ev IFS
919is unset.
920.It Li @
921Expands to the positional parameters, starting from one.  When
922the expansion occurs within double-quotes, each positional
923parameter expands as a separate argument.
924If there are no positional parameters, the
925expansion of
926.Li @
927generates zero arguments, even when
928.Li @
929is double-quoted.  What this basically means, for example, is
930if $1 is
931.Dq abc
932and $2 is
933.Dq def ghi ,
934then
935.Qq Li $@
936expands to
937the two arguments:
938.Bd -literal -offset indent
939"abc"   "def ghi"
940.Ed
941.It Li #
942Expands to the number of positional parameters.
943.It Li \&?
944Expands to the exit status of the most recent pipeline.
945.It Li -
946(hyphen) Expands to the current option flags (the single-letter
947option names concatenated into a string) as specified on
948invocation, by the set built-in command, or implicitly
949by the shell.
950.It Li $
951Expands to the process ID of the invoked shell.  A subshell
952retains the same value of $ as its parent.
953.It Li \&!
954Expands to the process ID of the most recent background
955command executed from the current shell.  For a
956pipeline, the process ID is that of the last command in the
957pipeline.
958.It Li 0
959(zero) Expands to the name of the shell or shell script.
960.El
961.Ss Word Expansions
962This clause describes the various expansions that are
963performed on words.  Not all expansions are performed on
964every word, as explained later.
965.Pp
966Tilde expansions, parameter expansions, command substitutions,
967arithmetic expansions, and quote removals that occur within
968a single word expand to a single field.  It is only field
969splitting or pathname expansion that can create multiple
970fields from a single word.
971The single exception to this rule is
972the expansion of the special parameter
973.Li @
974within double-quotes,
975as was described above.
976.Pp
977The order of word expansion is:
978.Bl -enum
979.It
980Tilde Expansion, Parameter Expansion, Command Substitution,
981Arithmetic Expansion (these all occur at the same time).
982.It
983Field Splitting is performed on fields generated by step (1)
984unless the
985.Ev IFS
986variable is null.
987.It
988Pathname Expansion (unless the
989.Fl f
990option is in effect).
991.It
992Quote Removal.
993.El
994.Pp
995The
996.Dq Li $
997character is used to introduce parameter expansion, command
998substitution, or arithmetic evaluation.
999.Ss Tilde Expansion (substituting a user's home directory)
1000A word beginning with an unquoted tilde character
1001.Pq Li ~
1002is
1003subjected to tilde expansion.
1004All the characters up to a slash
1005.Pq Li /
1006or the end of the word are treated as a username
1007and are replaced with the user's home directory.  If the
1008username is missing (as in ~/foobar), the tilde is replaced
1009with the value of the HOME variable (the current user's
1010home directory).
1011.Ss Parameter Expansion
1012The format for parameter expansion is as follows:
1013.Bd -literal -offset indent
1014${expression}
1015.Ed
1016.Pp
1017where expression consists of all characters until the matching
1018.Dq Li } .
1019Any
1020.Dq Li }
1021escaped by a backslash or within a quoted string, and characters in
1022embedded arithmetic expansions, command substitutions, and variable
1023expansions, are not examined in determining the matching
1024.Dq Li } .
1025.Pp
1026The simplest form for parameter expansion is:
1027.Bd -literal -offset indent
1028${parameter}
1029.Ed
1030.Pp
1031The value, if any, of parameter is substituted.
1032.Pp
1033The parameter name or symbol can be enclosed in braces, which are
1034optional except for positional parameters with more than one digit or
1035when parameter is followed by a character that could be interpreted as
1036part of the name.
1037If a parameter expansion occurs inside double-quotes:
1038.Bl -enum
1039.It
1040Pathname expansion is not performed on the results of the
1041expansion.
1042.It
1043Field splitting is not performed on the results of the
1044expansion, with the exception of the special parameter
1045.Li @ .
1046.El
1047.Pp
1048In addition, a parameter expansion can be modified by using one of the
1049following formats.
1050.Bl -tag -width indent
1051.It Li ${parameter:-word}
1052Use Default Values.  If parameter is unset or
1053null, the expansion of word is
1054substituted; otherwise, the value of
1055parameter is substituted.
1056.It Li ${parameter:=word}
1057Assign Default Values.  If parameter is unset
1058or null, the expansion of word is
1059assigned to parameter.  In all cases, the
1060final value of parameter is
1061substituted.  Only variables, not positional
1062parameters or special parameters, can be
1063assigned in this way.
1064.It Li ${parameter:?[word]}
1065Indicate Error if Null or Unset.  If
1066parameter is unset or null, the expansion of
1067word (or a message indicating it is unset if
1068word is omitted) is written to standard
1069error and the shell exits with a nonzero
1070exit status.
1071Otherwise, the value of
1072parameter is substituted.  An
1073interactive shell need not exit.
1074.It Li ${parameter:+word}
1075Use Alternate Value.  If parameter is unset
1076or null, null is substituted;
1077otherwise, the expansion of word is
1078substituted.
1079.Pp
1080In the parameter expansions shown previously, use of the colon in the
1081format results in a test for a parameter that is unset or null; omission
1082of the colon results in a test for a parameter that is only unset.
1083.It Li ${#parameter}
1084String Length.  The length in characters of
1085the value of parameter.
1086.El
1087.Pp
1088The following four varieties of parameter expansion provide for substring
1089processing.
1090In each case, pattern matching notation
1091(see
1092.Sx Shell Patterns ) ,
1093rather than regular expression notation,
1094is used to evaluate the patterns.
1095If parameter is one of the special parameters
1096.Li *
1097or
1098.Li @ ,
1099the result of the expansion is unspecified.
1100Enclosing the full parameter expansion string in double-quotes does not
1101cause the following four varieties of pattern characters to be quoted,
1102whereas quoting characters within the braces has this effect.
1103.Bl -tag -width indent
1104.It Li ${parameter%word}
1105Remove Smallest Suffix Pattern.  The word
1106is expanded to produce a pattern.  The
1107parameter expansion then results in
1108parameter, with the smallest portion of the
1109suffix matched by the pattern deleted.
1110.It Li ${parameter%%word}
1111Remove Largest Suffix Pattern.  The word
1112is expanded to produce a pattern.  The
1113parameter expansion then results in
1114parameter, with the largest portion of the
1115suffix matched by the pattern deleted.
1116.It Li ${parameter#word}
1117Remove Smallest Prefix Pattern.  The word
1118is expanded to produce a pattern.  The
1119parameter expansion then results in
1120parameter, with the smallest portion of the
1121prefix matched by the pattern deleted.
1122.It Li ${parameter##word}
1123Remove Largest Prefix Pattern.  The word
1124is expanded to produce a pattern.  The
1125parameter expansion then results in
1126parameter, with the largest portion of the
1127prefix matched by the pattern deleted.
1128.El
1129.Ss Command Substitution
1130Command substitution allows the output of a command to be substituted in
1131place of the command name itself.  Command substitution occurs when
1132the command is enclosed as follows:
1133.Bd -literal -offset indent
1134$(command)
1135.Ed
1136.Pp
1137or the backquoted version:
1138.Bd -literal -offset indent
1139`command`
1140.Ed
1141.Pp
1142The shell expands the command substitution by executing command in a
1143subshell environment and replacing the command substitution
1144with the standard output of the command,
1145removing sequences of one or more newlines at the end of the substitution.
1146Embedded newlines before the end of the output are not removed;
1147however, during field splitting, they may be translated into spaces
1148depending on the value of
1149.Ev IFS
1150and the quoting that is in effect.
1151.Ss Arithmetic Expansion
1152Arithmetic expansion provides a mechanism for evaluating an arithmetic
1153expression and substituting its value.
1154The format for arithmetic expansion is as follows:
1155.Bd -literal -offset indent
1156$((expression))
1157.Ed
1158.Pp
1159The expression is treated as if it were in double-quotes, except
1160that a double-quote inside the expression is not treated specially.  The
1161shell expands all tokens in the expression for parameter expansion,
1162command substitution, and quote removal.
1163.Pp
1164Next, the shell treats this as an arithmetic expression and
1165substitutes the value of the expression.
1166.Ss White Space Splitting (Field Splitting)
1167After parameter expansion, command substitution, and
1168arithmetic expansion the shell scans the results of
1169expansions and substitutions that did not occur in double-quotes for
1170field splitting and multiple fields can result.
1171.Pp
1172The shell treats each character of the
1173.Ev IFS
1174as a delimiter and uses
1175the delimiters to split the results of parameter expansion and command
1176substitution into fields.
1177.Ss Pathname Expansion (File Name Generation)
1178Unless the
1179.Fl f
1180option is set,
1181file name generation is performed
1182after word splitting is complete.  Each word is
1183viewed as a series of patterns, separated by slashes.  The
1184process of expansion replaces the word with the names of
1185all existing files whose names can be formed by replacing
1186each pattern with a string that matches the specified pattern.
1187There are two restrictions on this: first, a pattern cannot match
1188a string containing a slash, and second,
1189a pattern cannot match a string starting with a period
1190unless the first character of the pattern is a period.
1191The next section describes the patterns used for both
1192Pathname Expansion and the
1193.Ic case
1194command.
1195.Ss Shell Patterns
1196A pattern consists of normal characters, which match themselves,
1197and meta-characters.
1198The meta-characters are
1199.Dq Li \&! ,
1200.Dq Li * ,
1201.Dq Li \&? ,
1202and
1203.Dq Li [ .
1204These characters lose their special meanings if they are quoted.
1205When command or variable substitution is performed and the dollar sign
1206or back quotes are not double-quoted, the value of the
1207variable or the output of the command is scanned for these
1208characters and they are turned into meta-characters.
1209.Pp
1210An asterisk
1211.Pq Li *
1212matches any string of characters.
1213A question mark
1214.Pq Li \&?
1215matches any single character.
1216A left bracket
1217.Pq Li [
1218introduces a character class.
1219The end of the character class is indicated by a
1220.Dq Li \&] ;
1221if the
1222.Dq Li \&]
1223is missing then the
1224.Dq Li [
1225matches a
1226.Dq Li [
1227rather than introducing a character class.
1228A character class matches any of the characters between the square brackets.
1229A range of characters may be specified using a minus sign.
1230The character class may be complemented by making an exclamation point
1231.Pq Li !\&
1232the first character of the character class.
1233.Pp
1234To include a
1235.Dq Li \&]
1236in a character class, make it the first character listed
1237(after the
1238.Dq Li \&! ,
1239if any).
1240To include a
1241.Dq Li - ,
1242make it the first or last character listed.
1243.Ss Built-in Commands
1244This section lists the commands which
1245are built-in because they need to perform some operation
1246that cannot be performed by a separate process.  In addition to
1247these, a built-in version of the
1248.Xr test 1
1249command is provided for efficiency.
1250.Bl -tag -width indent
1251.It Ic \&:
1252A null command that returns a 0 (true) exit value.
1253.It Ic \&. Ar file
1254The commands in the specified file are read and executed by the shell.
1255If
1256.Ar file
1257contains any
1258.Dq /
1259characters, it is used as is.  Otherwise, the shell searches the
1260.Ev PATH
1261for the file.  If it is not found in the
1262.Ev PATH ,
1263it is sought in the current working directory.
1264.It Ic alias Op Ar name ...
1265.It Ic alias Op Ar name Ns = Ns Ar string ...
1266If
1267.Ar name Ns = Ns Ar string
1268is specified, the shell defines the alias
1269.Ar name
1270with value
1271.Ar string .
1272If just
1273.Ar name
1274is specified, the value of the alias
1275.Ar name
1276is printed.
1277With no arguments, the
1278.Ic alias
1279built-in command prints the names and values of all defined aliases
1280(see
1281.Ic unalias ) .
1282Alias values are written with appropriate quoting so that they are
1283suitable for re-input to the shell.
1284.It Ic bg Op Ar job ...
1285Continue the specified jobs
1286(or the current job if no jobs are given)
1287in the background.
1288.It Ic builtin Ar cmd Op Ar arg ...
1289Execute the specified built-in command,
1290.Ar cmd .
1291This is useful when the user wishes to override a shell function
1292with the same name as a built-in command.
1293.It Ic bind Oo Fl aeklrsv Oc Oo Ar key Oo Ar command Oc Oc
1294List or alter key bindings for the line editor.
1295This command is documented in
1296.Xr editrc 5 .
1297.It Ic cd Oo Fl LP Oc Op Ar directory
1298Switch to the specified
1299.Ar directory ,
1300or to the directory specified in the
1301.Ev HOME
1302environment variable if no
1303.Ar directory
1304is specified.
1305If
1306.Ar directory
1307does not begin with
1308.Pa / , \&. ,
1309or
1310.Pa .. ,
1311then the directories listed in the
1312.Ev CDPATH
1313variable will be
1314searched for the specified
1315.Ar directory .
1316If
1317.Ev CDPATH
1318is unset, the current directory is searched.
1319The format of
1320.Ar CDPATH
1321is the same as that of
1322.Ev PATH .
1323In an interactive shell,
1324the
1325.Ic cd
1326command will print out the name of the directory
1327that it actually switched to
1328if this is different from the name that the user gave.
1329These may be different either because the
1330.Ev CDPATH
1331mechanism was used or because a symbolic link was crossed.
1332.Pp
1333If the
1334.Fl P
1335option is specified,
1336.Pa ..
1337is handled physically and symbolic links are resolved before
1338.Pa ..
1339components are processed.
1340If the
1341.Fl L
1342option is specified,
1343.Pa ..
1344is handled logically.
1345This is the default.
1346.It Ic chdir
1347A synonym for the
1348.Ic cd
1349built-in command.
1350.It Ic command Oo Fl p Oc Op Ar utility Op Ar argument ...
1351Execute the specified
1352.Ar utility
1353as a simple command (see the
1354.Sx Simple Commands
1355section).
1356.Pp
1357If the
1358.Fl p
1359option is specified, the command search is performed using a
1360default value of
1361.Ev PATH
1362that is guaranteed to find all of the standard utilities.
1363.It Ic echo Oo Fl e | n Oc Op Ar string
1364Print
1365.Ar string
1366to the standard output with a newline appended.
1367.Bl -tag -width indent
1368.It Fl n
1369Suppress the output of the trailing newline.
1370.It Fl e
1371Process C-style backslash escape sequences.
1372.Ic echo
1373understands the following character escapes:
1374.Bl -tag -width indent
1375.It \ea
1376Alert (ring the terminal bell)
1377.It \eb
1378Backspace
1379.It \ec
1380Suppress the trailing newline (this has the side-effect of truncating the
1381line if it is not the last character)
1382.It \ee
1383The ESC character (ASCII 0x1b)
1384.It \ef
1385Formfeed
1386.It \en
1387Newline
1388.It \er
1389Carriage return
1390.It \et
1391Horizontal tab
1392.It \ev
1393Vertical tab
1394.It \e\e
1395Literal backslash
1396.It \e0nnn
1397(Zero) The character whose octal value is nnn
1398.El
1399.Pp
1400If
1401.Ar string
1402is not enclosed in quotes then the backslash itself must be escaped
1403with a backslash to protect it from the shell. For example
1404.Bd -literal -offset indent
1405$ echo -e "a\evb"
1406a
1407 b
1408$ echo -e a\e\evb
1409a
1410 b
1411$ echo -e "a\e\eb"
1412a\eb
1413$ echo -e a\e\e\e\eb
1414a\eb
1415.Ed
1416.El
1417.Pp
1418Only one of the
1419.Fl e
1420and
1421.Fl n
1422options may be specified.
1423.It Ic eval Ar string ...
1424Concatenate all the arguments with spaces.
1425Then re-parse and execute the command.
1426.It Ic exec Op Ar command Op arg ...
1427Unless
1428.Ar command
1429is omitted,
1430the shell process is replaced with the specified program
1431(which must be a real program, not a shell built-in command or function).
1432Any redirections on the
1433.Ic exec
1434command are marked as permanent,
1435so that they are not undone when the
1436.Ic exec
1437command finishes.
1438.It Ic exit Op Ar exitstatus
1439Terminate the shell process.
1440If
1441.Ar exitstatus
1442is given
1443it is used as the exit status of the shell;
1444otherwise the exit status of the preceding command is used.
1445.It Ic export Oo Fl p Oc Op Ar name ...
1446The specified names are exported so that they will
1447appear in the environment of subsequent commands.
1448The only way to un-export a variable is to
1449.Ic unset
1450it.
1451The shell allows the value of a variable to be set
1452at the same time as it is exported by writing
1453.Bd -literal -offset indent
1454export name=value
1455.Ed
1456.Pp
1457With no arguments the export command lists the names
1458of all exported variables.
1459If the
1460.Fl p
1461option is specified, the exported variables are printed as
1462.Dq Ic export Ar name Ns = Ns Ar value
1463lines, suitable for re-input to the shell.
1464.It Ic fc Oo Fl e Ar editor Oc Op Ar first Op Ar last
1465.It Ic fc Fl l Oo Fl nr Oc Op Ar first Op Ar last
1466.It Ic fc Fl s Oo Ar old Ns = Ns Ar new Oc Op Ar first
1467The
1468.Ic fc
1469built-in command lists, or edits and re-executes,
1470commands previously entered to an interactive shell.
1471.Bl -tag -width indent
1472.It Fl e Ar editor
1473Use the editor named by
1474.Ar editor
1475to edit the commands.
1476The editor string is a command name,
1477subject to search via the
1478.Ev PATH
1479variable.
1480The value in the
1481.Ev FCEDIT
1482variable is used as a default when
1483.Fl e
1484is not specified.
1485If
1486.Ev FCEDIT
1487is null or unset, the value of the
1488.Ev EDITOR
1489variable is used.
1490If
1491.Ev EDITOR
1492is null or unset,
1493.Xr ed 1
1494is used as the editor.
1495.It Fl l No (ell)
1496List the commands rather than invoking
1497an editor on them.  The commands are written in the
1498sequence indicated by the first and last operands, as
1499affected by
1500.Fl r ,
1501with each command preceded by the command number.
1502.It Fl n
1503Suppress command numbers when listing with
1504.Fl l .
1505.It Fl r
1506Reverse the order of the commands listed
1507(with
1508.Fl l )
1509or edited
1510(with neither
1511.Fl l
1512nor
1513.Fl s ) .
1514.It Fl s
1515Re-execute the command without invoking an editor.
1516.It Ar first
1517.It Ar last
1518Select the commands to list or edit.
1519The number of previous commands that can be accessed
1520are determined by the value of the
1521.Ev HISTSIZE
1522variable.
1523The value of
1524.Ar first
1525or
1526.Ar last
1527or both are one of the following:
1528.Bl -tag -width indent
1529.It Ar [+]num
1530A positive number representing a command number;
1531command numbers can be displayed with the
1532.Fl l
1533option.
1534.It Ar -num
1535A negative decimal number representing the
1536command that was executed
1537.Ar num
1538of
1539commands previously.
1540For example, -1 is the immediately previous command.
1541.It Ar string
1542A string indicating the most recently entered command
1543that begins with that string.
1544If the
1545.Ar old=new
1546operand is not also specified with
1547.Fl s ,
1548the string form of the first operand cannot contain an embedded equal sign.
1549.El
1550.El
1551.Pp
1552The following environment variables affect the execution of
1553.Ic fc :
1554.Bl -tag -width indent
1555.It Ev FCEDIT
1556Name of the editor to use.
1557.It Ev HISTSIZE
1558The number of previous commands that are accessible.
1559.El
1560.It Ic fg Op Ar job
1561Move the specified
1562.Ar job
1563or the current job to the foreground.
1564.It Ic getopts Ar optstring Ar var
1565The POSIX
1566.Ic getopts
1567command.
1568The
1569.Ic getopts
1570command deprecates the older
1571.Xr getopt 1
1572command.
1573The first argument should be a series of letters, each possibly
1574followed by a colon which indicates that the option takes an argument.
1575The specified variable is set to the parsed option.  The index of
1576the next argument is placed into the shell variable
1577.Ev OPTIND .
1578If an option takes an argument, it is placed into the shell variable
1579.Ev OPTARG .
1580If an invalid option is encountered,
1581.Ev var
1582is set to
1583.Dq Li \&? .
1584It returns a false value (1) when it encounters the end of the options.
1585.It Ic hash Oo Fl rv Oc Op Ar command ...
1586The shell maintains a hash table which remembers the locations of commands.
1587With no arguments whatsoever, the
1588.Ic hash
1589command prints out the contents of this table.
1590Entries which have not been looked at since the last
1591.Ic cd
1592command are marked with an asterisk;
1593it is possible for these entries to be invalid.
1594.Pp
1595With arguments, the
1596.Ic hash
1597command removes each specified
1598.Ar command
1599from the hash table (unless they are functions) and then locates it.
1600With the
1601.Fl v
1602option,
1603.Ic hash
1604prints the locations of the commands as it finds them.
1605The
1606.Fl r
1607option causes the
1608.Ic hash
1609command to delete all the entries in the hash table except for functions.
1610.It Ic jobid Op Ar job
1611Print the process id's of the processes in the specified
1612.Ar job .
1613If the
1614.Ar job
1615argument is omitted, use the current job.
1616.It Ic jobs Oo Fl ls Oc Op Ar job ...
1617Print information about the specified jobs, or all jobs if no
1618.Ar job
1619argument is given.
1620The information printed includes job ID, status and command name.
1621.Pp
1622If the
1623.Fl l
1624option is specified, the PID of each job is also printed.
1625If the
1626.Fl s
1627option is specified, only the PIDs of the jobs are printed, one per line.
1628.It Ic pwd Op Fl LP
1629Print the path of the current directory.  The built-in command may
1630differ from the program of the same name because the
1631built-in command remembers what the current directory
1632is rather than recomputing it each time.  This makes
1633it faster.  However, if the current directory is
1634renamed,
1635the built-in version of
1636.Xr pwd 1
1637will continue to print the old name for the directory.
1638.Pp
1639If the
1640.Fl P
1641option is specified, symbolic links are resolved.
1642If the
1643.Fl L
1644option is specified, the shell's notion of the current directory
1645is printed (symbolic links are not resolved).
1646This is the default.
1647.It Ic read Oo Fl p Ar prompt Oc Oo Fl t Ar timeout Oc Oo Fl er Oc Ar variable ...
1648The
1649.Ar prompt
1650is printed if the
1651.Fl p
1652option is specified
1653and the standard input is a terminal.  Then a line is
1654read from the standard input.  The trailing newline
1655is deleted from the line and the line is split as
1656described in the section on
1657.Sx White Space Splitting (Field Splitting)
1658above, and
1659the pieces are assigned to the variables in order.
1660If there are more pieces than variables, the remaining
1661pieces (along with the characters in
1662.Ev IFS
1663that separated them)
1664are assigned to the last variable.
1665If there are more variables than pieces, the remaining
1666variables are assigned the null string.
1667.Pp
1668Backslashes are treated specially, unless the
1669.Fl r
1670option is
1671specified.  If a backslash is followed by
1672a newline, the backslash and the newline will be
1673deleted.  If a backslash is followed by any other
1674character, the backslash will be deleted and the following
1675character will be treated as though it were not in
1676.Ev IFS ,
1677even if it is.
1678.Pp
1679If the
1680.Fl t
1681option is specified and the
1682.Ar timeout
1683elapses before any input is supplied,
1684the
1685.Ic read
1686command will return without assigning any values.
1687The
1688.Ar timeout
1689value may optionally be followed by one of
1690.Dq s ,
1691.Dq m
1692or
1693.Dq h
1694to explicitly specify seconds, minutes or hours.
1695If none is supplied,
1696.Dq s
1697is assumed.
1698.Pp
1699The
1700.Fl e
1701option exists only for backward compatibility with older scripts.
1702.It Ic readonly Oo Fl p Oc Op Ar name ...
1703Each specified
1704.Ar name
1705is marked as read only,
1706so that it cannot be subsequently modified or unset.
1707The shell allows the value of a variable to be set
1708at the same time as it is marked read only
1709by using the following form:
1710.Bd -literal -offset indent
1711readonly name=value
1712.Ed
1713.Pp
1714With no arguments the
1715.Ic readonly
1716command lists the names of all read only variables.
1717If the
1718.Fl p
1719option is specified, the read-only variables are printed as
1720.Dq Ic readonly Ar name Ns = Ns Ar value
1721lines, suitable for re-input to the shell.
1722.It Ic set Oo Fl /+abCEefIimnpTuVvx Oc Oo Fl /+o Ar longname Oc Oo
1723.Fl c Ar string Oc Op Fl - Ar arg ...
1724The
1725.Ic set
1726command performs three different functions:
1727.Bl -item
1728.It
1729With no arguments, it lists the values of all shell variables.
1730.It
1731If options are given,
1732either in short form or using the long
1733.Dq Fl /+o Ar longname
1734form,
1735it sets or clears the specified options as described in the section called
1736.Sx Argument List Processing .
1737.It
1738If the
1739.Dq Fl -
1740option is specified,
1741.Ic set
1742will replace the shell's positional parameters with the subsequent
1743arguments.
1744If no arguments follow the
1745.Dq Fl -
1746option,
1747all the positional parameters will be cleared,
1748which is equivalent to executing the command
1749.Dq Li shift $# .
1750The
1751.Dq Fl -
1752flag may be omitted when specifying arguments to be used
1753as positional replacement parameters.
1754This is not recommended,
1755because the first argument may begin with a dash
1756.Pq Li -
1757or a plus
1758.Pq Li + ,
1759which the
1760.Ic set
1761command will interpret as a request to enable or disable options.
1762.El
1763.It Ic setvar Ar variable Ar value
1764Assigns the specified
1765.Ar value
1766to the specified
1767.Ar variable .
1768.Ic Setvar
1769is intended to be used in functions that
1770assign values to variables whose names are passed as parameters.
1771In general it is better to write
1772.Bd -literal -offset indent
1773variable=value
1774.Ed
1775rather than using
1776.Ic setvar .
1777.It Ic shift Op Ar n
1778Shift the positional parameters
1779.Ar n
1780times, or once if
1781.Ar n
1782is not specified.
1783A shift sets the value of $1 to the value of $2,
1784the value of $2 to the value of $3, and so on,
1785decreasing the value of $# by one.
1786If there are zero positional parameters, shifting does not do anything.
1787.It Ic trap Oo Ar action Oc Ar signal ...
1788Cause the shell to parse and execute
1789.Ar action
1790when any specified
1791.Ar signal
1792is received.
1793The signals are specified by signal number.
1794The
1795.Ar action
1796may be null or omitted;
1797the former causes the specified signal to be ignored
1798and the latter causes the default action to be taken.
1799When the shell forks off a subshell,
1800it resets trapped (but not ignored) signals to the default action.
1801The
1802.Ic trap
1803command has no effect on signals that were ignored on entry to the shell.
1804.It Ic type Op Ar name ...
1805Interpret each
1806.Ar name
1807as a command and print the resolution of the command search.
1808Possible resolutions are:
1809shell keyword, alias, shell built-in command, command, tracked alias
1810and not found.
1811For aliases the alias expansion is printed;
1812for commands and tracked aliases
1813the complete pathname of the command is printed.
1814.It Ic ulimit Oo Fl HSabcdflmnstuv Oc Op Ar limit
1815Set or display resource limits (see
1816.Xr getrlimit 2 ) .
1817If
1818.Ar limit
1819is specified, the named resource will be set;
1820otherwise the current resource value will be displayed.
1821.Pp
1822If
1823.Fl H
1824is specified, the hard limits will be set or displayed.
1825While everybody is allowed to reduce a hard limit,
1826only the superuser can increase it.
1827The
1828.Fl S
1829option
1830specifies the soft limits instead.  When displaying limits,
1831only one of
1832.Fl S
1833or
1834.Fl H
1835can be given.
1836The default is to display the soft limits,
1837and to set both the hard and the soft limits.
1838.Pp
1839Option
1840.Fl a
1841causes the
1842.Ic ulimit
1843command to display all resources.
1844The parameter
1845.Ar limit
1846is not acceptable in this mode.
1847.Pp
1848The remaining options specify which resource value is to be
1849displayed or modified.
1850They are mutually exclusive.
1851.Bl -tag -width indent
1852.It Fl b Ar sbsize
1853The maximum size of socket buffer usage, in bytes.
1854.It Fl c Ar coredumpsize
1855The maximal size of core dump files, in 512-byte blocks.
1856.It Fl d Ar datasize
1857The maximal size of the data segment of a process, in kilobytes.
1858.It Fl f Ar filesize
1859The maximal size of a file, in 512-byte blocks.
1860.It Fl l Ar lockedmem
1861The maximal size of memory that can be locked by a process, in
1862kilobytes.
1863.It Fl m Ar memoryuse
1864The maximal resident set size of a process, in kilobytes.
1865.It Fl n Ar nofiles
1866The maximal number of descriptors that could be opened by a process.
1867.It Fl s Ar stacksize
1868The maximal size of the stack segment, in kilobytes.
1869.It Fl t Ar time
1870The maximal amount of CPU time to be used by each process, in seconds.
1871.It Fl u Ar userproc
1872The maximal number of simultaneous processes for this user ID.
1873.It Fl v Ar virtualmem
1874The maximal virtual size of a process, in kilobytes.
1875.El
1876.It Ic umask Op Ar mask
1877Set the file creation mask (see
1878.Xr umask 2 )
1879to the octal value specified by
1880.Ar mask .
1881If the argument is omitted, the current mask value is printed.
1882.It Ic unalias Oo Fl a Oc Op Ar name
1883If
1884.Ar name
1885is specified, the shell removes that alias.
1886If
1887.Fl a
1888is specified, all aliases are removed.
1889.It Ic unset Oo Fl fv Oc Ar name ...
1890The specified variables or functions are unset and unexported.
1891If the
1892.Fl v
1893option is specified or no options are given, the
1894.Ar name
1895arguments are treated as variable names.
1896If the
1897.Fl f
1898option is specified, the
1899.Ar name
1900arguments are treated as function names.
1901.It Ic wait Op Ar job
1902Wait for the specified
1903.Ar job
1904to complete and return the exit status of the last process in the
1905.Ar job .
1906If the argument is omitted, wait for all jobs to complete
1907and return an exit status of zero.
1908.El
1909.Ss Commandline Editing
1910When
1911.Nm
1912is being used interactively from a terminal, the current command
1913and the command history
1914(see
1915.Ic fc
1916in
1917.Sx Built-in Commands )
1918can be edited using vi-mode command line editing.
1919This mode uses commands similar
1920to a subset of those described in the vi man page.
1921The command
1922.Dq Li set -o vi
1923(or
1924.Dq Li set -V )
1925enables vi-mode editing and places
1926.Nm
1927into vi insert mode.  With vi-mode enabled,
1928.Nm
1929can be switched between insert mode and command mode by typing
1930.Aq ESC .
1931Hitting
1932.Aq return
1933while in command mode will pass the line to the shell.
1934.Pp
1935Similarly, the
1936.Dq Li set -o emacs
1937(or
1938.Dq Li set -E )
1939command can be used to enable a subset of
1940emacs-style command line editing features.
1941.Sh SEE ALSO
1942.Xr builtin 1 ,
1943.Xr echo 1 ,
1944.Xr expr 1 ,
1945.Xr pwd 1 ,
1946.Xr test 1
1947.Sh HISTORY
1948A
1949.Nm
1950command appeared in
1951.At v1 .
1952