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35 .Dd September 27, 2020
40 .Nd command interpreter (shell)
43 .Op Fl /+abCEefhIimnPpTuVvx
44 .Op Fl /+o Ar longname
50 .Op Fl /+abCEefhIimnPpTuVvx
51 .Op Fl /+o Ar longname
58 .Op Fl /+abCEefhIimnPpTuVvx
59 .Op Fl /+o Ar longname
65 utility is the standard command interpreter for the system.
66 The current version of
70 specification for the shell.
71 It only supports features
73 plus a few Berkeley extensions.
74 This man page is not intended to be a tutorial nor a complete
75 specification of the shell.
77 The shell is a command that reads lines from
78 either a file or the terminal, interprets them, and
79 generally executes other commands.
80 It is the program that is started when a user logs into the system,
81 although a user can select a different shell with the
85 implements a language that has flow control constructs,
86 a macro facility that provides a variety of features in
87 addition to data storage, along with built-in history and line
89 It incorporates many features to
90 aid interactive use and has the advantage that the interpretative
91 language is common to both interactive and non-interactive
93 That is, commands can be typed directly
94 to the running shell or can be put into a file,
95 which can be executed directly by the shell.
98 .\" XXX This next sentence is incredibly confusing.
100 If no arguments are present and if the standard input of the shell
101 is connected to a terminal
105 the shell is considered an interactive shell.
107 generally prompts before each command and handles programming
108 and command errors differently (as described below).
109 When first starting, the shell inspects argument 0, and
110 if it begins with a dash
112 the shell is also considered a login shell.
113 This is normally done automatically by the system
114 when the user first logs in.
115 A login shell first reads commands
120 in a user's home directory,
122 If the environment variable
124 is set on entry to a shell, or is set in the
126 of a login shell, the shell then subjects its value to parameter expansion
127 and arithmetic expansion and reads commands from the named file.
128 Therefore, a user should place commands that are to be executed only
131 file, and commands that are executed for every shell inside the
136 variable to some file by placing the following line in the file
138 in the home directory,
141 the filename desired:
143 .Dl "ENV=$HOME/.shrc; export ENV"
145 The first non-option argument specified on the command line
146 will be treated as the
147 name of a file from which to read commands (a shell script), and
148 the remaining arguments are set as the positional parameters
152 Otherwise, the shell reads commands
153 from its standard input.
155 Unlike older versions of
159 script is only sourced on invocation of interactive shells.
161 closes a well-known, and sometimes easily exploitable security
162 hole related to poorly thought out
165 .Ss Argument List Processing
166 All of the single letter options to
168 have a corresponding long name,
169 with the exception of
173 These long names are provided next to the single letter options
174 in the descriptions below.
175 The long name for an option may be specified as an argument to the
179 Once the shell is running,
180 the long name for an option may be specified as an argument to the
185 (described later in the section called
186 .Sx Built-in Commands ) .
187 Introducing an option with a dash
197 will stop option processing and will force the remaining
198 words on the command line to be treated as arguments.
203 options do not have long names.
204 They take arguments and are described after the single letter options.
205 .Bl -tag -width indent
206 .It Fl a Li allexport
207 Flag variables for export when assignments are made to them.
209 Enable asynchronous notification of background job
212 .It Fl C Li noclobber
213 Do not overwrite existing files with
218 command line editor (disables the
220 option if it has been set;
221 set automatically when interactive on terminals).
223 Exit immediately if any untested command fails in non-interactive mode.
224 The exit status of a command is considered to be
225 explicitly tested if the command is part of the list used to control
227 .Ic if , elif , while ,
230 if the command is the left
235 operator; or if the command is a pipeline preceded by the
238 If a shell function is executed and its exit status is explicitly
239 tested, all commands of the function are considered to be tested as
242 It is recommended to check for failures explicitly
243 instead of relying on
245 because it tends to behave in unexpected ways,
246 particularly in larger scripts.
248 Disable pathname expansion.
250 A do-nothing option for POSIX compliance.
251 .It Fl I Li ignoreeof
254 from input when in interactive mode.
255 .It Fl i Li interactive
256 Force the shell to behave interactively.
258 Turn on job control (set automatically when interactive).
259 A new process group is created for each pipeline (called a job).
260 It is possible to suspend jobs or to have them run in the foreground or
262 In a non-interactive shell,
263 this option can be set even if no terminal is available
264 and is useful to place processes in separate process groups.
266 If not interactive, read commands but do not
268 This is useful for checking the
269 syntax of shell scripts.
271 Change the default for the
277 (logical directory layout)
280 (physical directory layout).
281 .It Fl p Li privileged
282 Turn on privileged mode.
283 This mode is enabled on startup
284 if either the effective user or group ID is not equal to the
285 real user or group ID.
286 Turning this mode off sets the
287 effective user and group IDs to the real user and group IDs.
288 When this mode is enabled for interactive shells, the file
289 .Pa /etc/suid_profile
290 is sourced instead of
294 is sourced, and the contents of the
296 variable are ignored.
298 Read commands from standard input (set automatically
299 if no file arguments are present).
301 no effect when set after the shell has already started
302 running (i.e., when set with the
305 .It Fl T Li trapsasync
306 When waiting for a child, execute traps immediately.
307 If this option is not set,
308 traps are executed after the child exits,
311 This nonstandard option is useful for putting guarding shells around
312 children that block signals.
313 The surrounding shell may kill the child
314 or it may just return control to the tty and leave the child alone,
316 .Bd -literal -offset indent
317 sh -T -c "trap 'exit 1' 2 ; some-blocking-program"
320 Write a message to standard error when attempting
321 to expand a variable, a positional parameter or
322 the special parameter
324 that is not set, and if the
325 shell is not interactive, exit immediately.
329 command line editor (disables
333 The shell writes its input to standard error
335 Useful for debugging.
338 (preceded by the value of the
340 variable subjected to parameter expansion and arithmetic expansion)
341 to standard error before it is executed.
342 Useful for debugging.
344 Another do-nothing option for POSIX compliance.
345 It only has a long name.
347 Change the exit status of a pipeline to the last non-zero exit status of
348 any command in the pipeline, if any.
351 counts as a non-zero exit status,
352 this option may cause non-zero exit status for successful pipelines
355 in the pipeline terminates with status 0 without reading its
357 This option only has a long name.
362 option causes the commands to be read from the
364 operand instead of from the standard input.
365 Keep in mind that this option only accepts a single string as its
366 argument, hence multi-word strings must be quoted.
370 option takes as its only argument the long name of an option
371 to be enabled or disabled.
372 For example, the following two invocations of
374 both enable the built-in
377 .Bd -literal -offset indent
382 If used without an argument, the
384 option displays the current option settings in a human-readable format.
387 is used without an argument, the current option settings are output
388 in a format suitable for re-input into the shell.
389 .Ss Lexical Structure
390 The shell reads input in terms of lines from a file and breaks
391 it up into words at whitespace (blanks and tabs), and at
395 which are special to the shell.
396 There are two types of operators: control operators and
397 redirection operators (their meaning is discussed later).
398 The following is a list of valid operators:
399 .Bl -tag -width indent
400 .It Control operators:
401 .Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
402 .It Li & Ta Li && Ta Li \&( Ta Li \&) Ta Li \en
403 .It Li ;; Ta Li ;& Ta Li \&; Ta Li \&| Ta Li ||
405 .It Redirection operators:
406 .Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
407 .It Li < Ta Li > Ta Li << Ta Li >> Ta Li <>
408 .It Li <& Ta Li >& Ta Li <<- Ta Li >| Ta \&
414 introduces a comment if used at the beginning of a word.
415 The word starting with
417 and the rest of the line are ignored.
421 characters (character code 0) are not allowed in shell input.
423 Quoting is used to remove the special meaning of certain characters
424 or words to the shell, such as operators, whitespace, keywords,
427 There are four types of quoting: matched single quotes,
428 dollar-single quotes,
429 matched double quotes, and backslash.
430 .Bl -tag -width indent
432 Enclosing characters in single quotes preserves the literal
433 meaning of all the characters (except single quotes, making
434 it impossible to put single-quotes in a single-quoted string).
435 .It Dollar-Single Quotes
436 Enclosing characters between
440 preserves the literal meaning of all characters
441 except backslashes and single quotes.
442 A backslash introduces a C-style escape sequence:
443 .Bl -tag -width xUnnnnnnnn
445 Alert (ring the terminal bell)
449 The control character denoted by
455 is a backslash, it must be doubled.
457 The ESC character (ASCII 0x1b)
475 The byte whose octal value is
477 (one to three digits)
479 The byte whose hexadecimal value is
481 (one or more digits only the last two of which are used)
483 The Unicode code point
485 (four hexadecimal digits)
486 .It \eU Ns Ar nnnnnnnn
487 The Unicode code point
489 (eight hexadecimal digits)
492 The sequences for Unicode code points are currently only useful with
494 They reject code point 0 and UTF-16 surrogates.
496 If an escape sequence would produce a byte with value 0,
497 that byte and the rest of the string until the matching single-quote
500 Any other string starting with a backslash is an error.
502 Enclosing characters within double quotes preserves the literal
503 meaning of all characters except dollar sign
509 The backslash inside double quotes is historically weird.
510 It remains literal unless it precedes the following characters,
511 which it serves to quote:
513 .Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
514 .It Li $ Ta Li ` Ta Li \&" Ta Li \e Ta Li \en
517 A backslash preserves the literal meaning of the following
518 character, with the exception of the newline character
520 A backslash preceding a newline is treated as a line continuation.
523 Keywords or reserved words are words that have special meaning to the
524 shell and are recognized at the beginning of a line and
525 after a control operator.
526 The following are keywords:
527 .Bl -column "doneXX" "elifXX" "elseXX" "untilXX" "whileX" -offset center
528 .It Li \&! Ta { Ta } Ta Ic case Ta Ic do
529 .It Ic done Ta Ic elif Ta Ic else Ta Ic esac Ta Ic fi
530 .It Ic for Ta Ic if Ta Ic then Ta Ic until Ta Ic while
533 An alias is a name and corresponding value set using the
536 Wherever the command word of a simple command may occur,
537 and after checking for keywords if a keyword may occur, the shell
538 checks the word to see if it matches an alias.
539 If it does, it replaces it in the input stream with its value.
540 For example, if there is an alias called
552 Aliases are also recognized after an alias
553 whose value ends with a space or tab.
554 For example, if there is also an alias called
560 .Dl "nohup lf foobar"
564 .Dl "nohup ls -F foobar"
566 Aliases provide a convenient way for naive users to
567 create shorthands for commands without having to learn how
568 to create functions with arguments.
569 Using aliases in scripts is discouraged
570 because the command that defines them must be executed
571 before the code that uses them is parsed.
572 This is fragile and not portable.
574 An alias name may be escaped in a command line, so that it is not
575 replaced by its alias value, by using quoting characters within or
576 adjacent to the alias name.
577 This is most often done by prefixing
578 an alias name with a backslash to execute a function, built-in, or
579 normal program with the same name.
584 The shell interprets the words it reads according to a
585 language, the specification of which is outside the scope
586 of this man page (refer to the BNF in the
589 Essentially though, a line is read and if
590 the first word of the line (or after a control operator)
591 is not a keyword, then the shell has recognized a
593 Otherwise, a complex command or some
594 other special construct may have been recognized.
596 If a simple command has been recognized, the shell performs
597 the following actions:
600 Leading words of the form
602 are stripped off and assigned to the environment of
604 (they do not affect expansions).
605 Redirection operators and
606 their arguments (as described below) are stripped
607 off and saved for processing.
609 The remaining words are expanded as described in
611 .Sx Word Expansions ,
612 and the first remaining word is considered the command
613 name and the command is located.
615 words are considered the arguments of the command.
616 If no command name resulted, then the
618 variable assignments recognized in 1) affect the
621 Redirections are performed as described in
625 Redirections are used to change where a command reads its input
627 In general, redirections open, close, or
628 duplicate an existing reference to a file.
630 used for redirection is:
632 .D1 Oo Ar n Oc Ar redir-op file
636 is one of the redirection operators mentioned
638 The following gives some examples of how these
639 operators can be used.
640 Note that stdin and stdout are commonly used abbreviations
641 for standard input and standard output respectively.
642 .Bl -tag -width "1234567890XX" -offset indent
643 .It Oo Ar n Oc Ns Li > Ar file
644 redirect stdout (or file descriptor
648 .It Oo Ar n Oc Ns Li >| Ar file
649 same as above, but override the
652 .It Oo Ar n Oc Ns Li >> Ar file
653 append stdout (or file descriptor
657 .It Oo Ar n Oc Ns Li < Ar file
658 redirect stdin (or file descriptor
662 .It Oo Ar n Oc Ns Li <> Ar file
663 redirect stdin (or file descriptor
667 .It Oo Ar n1 Oc Ns Li <& Ns Ar n2
668 duplicate stdin (or file descriptor
672 .It Oo Ar n Oc Ns Li <&-
673 close stdin (or file descriptor
675 .It Oo Ar n1 Oc Ns Li >& Ns Ar n2
676 duplicate stdout (or file descriptor
680 .It Oo Ar n Oc Ns Li >&-
681 close stdout (or file descriptor
685 The following redirection is often called a
687 .Bd -unfilled -offset indent
688 .Oo Ar n Oc Ns Li << Ar delimiter
694 All the text on successive lines up to the delimiter is
695 saved away and made available to the command on standard
696 input, or file descriptor
701 as specified on the initial line is quoted, then the
703 is treated literally, otherwise the text is subjected to
704 parameter expansion, command substitution, and arithmetic
705 expansion (as described in the section on
706 .Sx Word Expansions ) .
715 .Ss Search and Execution
716 There are three types of commands: shell functions,
717 built-in commands, and normal programs.
718 The command is searched for (by name) in that order.
719 The three types of commands are all executed in a different way.
721 When a shell function is executed, all of the shell positional
724 which remains unchanged) are
725 set to the arguments of the shell function.
726 The variables which are explicitly placed in the environment of
727 the command (by placing assignments to them before the
728 function name) are made local to the function and are set
730 Then the command given in the function definition is executed.
731 The positional parameters are restored to their original values
732 when the command completes.
733 This all occurs within the current shell.
735 Shell built-in commands are executed internally to the shell, without
736 spawning a new process.
737 There are two kinds of built-in commands: regular and special.
738 Assignments before special builtins persist after they finish
739 executing and assignment errors, redirection errors and certain
740 operand errors cause a script to be aborted.
741 Special builtins cannot be overridden with a function.
742 Both regular and special builtins can affect the shell in ways
743 normal programs cannot.
745 Otherwise, if the command name does not match a function
746 or built-in command, the command is searched for as a normal
747 program in the file system (as described in the next section).
748 When a normal program is executed, the shell runs the program,
749 passing the arguments and the environment to the program.
750 If the program is not a normal executable file
751 (i.e., if it does not begin with the
753 whose ASCII representation is
759 but appears to be a text file,
760 the shell will run a new instance of
764 Note that previous versions of this document
765 and the source code itself misleadingly and sporadically
766 refer to a shell script without a magic number
768 .Dq "shell procedure" .
770 When locating a command, the shell first looks to see if
771 it has a shell function by that name.
773 built-in command by that name.
774 If a built-in command is not found,
775 one of two things happen:
778 Command names containing a slash are simply executed without
779 performing any searches.
781 The shell searches each entry in the
784 in turn for the command.
787 variable should be a series of
788 entries separated by colons.
789 Each entry consists of a
791 The current directory
792 may be indicated implicitly by an empty directory name,
793 or explicitly by a single period.
795 .Ss Command Exit Status
796 Each command has an exit status that can influence the behavior
797 of other shell commands.
798 The paradigm is that a command exits
799 with zero for normal or success, and non-zero for failure,
800 error, or a false indication.
801 The man page for each command
802 should indicate the various exit codes and what they mean.
803 Additionally, the built-in commands return exit codes, as does
804 an executed shell function.
806 If a command is terminated by a signal, its exit status is greater than 128.
807 The signal name can be found by passing the exit status to
810 If there is no command word,
811 the exit status is the exit status of the last command substitution executed,
812 or zero if the command does not contain any command substitutions.
814 Complex commands are combinations of simple commands
815 with control operators or keywords, together creating a larger complex
817 More generally, a command is one of the following:
818 .Bl -item -offset indent
824 list or compound-list
831 Unless otherwise stated, the exit status of a command is
832 that of the last simple command executed by the command,
833 or zero if no simple command was executed.
835 A pipeline is a sequence of one or more commands separated
836 by the control operator
838 The standard output of all but
839 the last command is connected to the standard input
841 The standard output of the last
842 command is inherited from the shell, as usual.
844 The format for a pipeline is:
846 .D1 Oo Li \&! Oc Ar command1 Op Li \&| Ar command2 ...
848 The standard output of
850 is connected to the standard input of
852 The standard input, standard output, or
853 both of a command is considered to be assigned by the
854 pipeline before any redirection specified by redirection
855 operators that are part of the command.
857 Note that unlike some other shells,
859 executes each process in a pipeline with more than one command
860 in a subshell environment and as a child of the
864 If the pipeline is not in the background (discussed later),
865 the shell waits for all commands to complete.
869 does not precede the pipeline, the
870 exit status is the exit status of the last command specified
871 in the pipeline if the
873 option is not set or all commands returned zero,
874 or the last non-zero exit status of any command in the pipeline otherwise.
875 Otherwise, the exit status is the logical
876 NOT of that exit status.
878 that status is zero, the exit status is 1; if
879 that status is greater than zero, the exit status
882 Because pipeline assignment of standard input or standard
883 output or both takes place before redirection, it can be
884 modified by redirection.
887 .Dl "command1 2>&1 | command2"
889 sends both the standard output and standard error of
891 to the standard input of
896 or newline terminator causes the preceding
898 (described below in the section called
899 .Sx Short-Circuit List Operators )
900 to be executed sequentially;
903 causes asynchronous execution of the preceding AND-OR-list.
904 .Ss Background Commands (&)
905 If a command is terminated by the control operator ampersand
907 the shell executes the command in a subshell environment (see
908 .Sx Grouping Commands Together
909 below) and asynchronously;
910 the shell does not wait for the command to finish
911 before executing the next command.
913 The format for running a command in background is:
915 .D1 Ar command1 Li & Op Ar command2 Li & Ar ...
917 If the shell is not interactive, the standard input of an
918 asynchronous command is set to
921 The exit status is zero.
922 .Ss Lists (Generally Speaking)
923 A list is a sequence of zero or more commands separated by
924 newlines, semicolons, or ampersands,
925 and optionally terminated by one of these three characters.
927 list are executed in the order they are written.
928 If command is followed by an ampersand, the shell starts the
929 command and immediately proceeds onto the next command;
930 otherwise it waits for the command to terminate before
931 proceeding to the next one.
932 .Ss Short-Circuit List Operators
936 are AND-OR list operators.
938 executes the first command, and then executes the second command
939 if the exit status of the first command is zero.
941 is similar, but executes the second command if the exit
942 status of the first command is nonzero.
946 both have the same priority.
947 .Ss Flow-Control Constructs (if, while, for, case)
951 .Bd -unfilled -offset indent -compact
955 .Ic then Ar list Oc Ar ...
960 The exit status is that of selected
965 or zero if no list was selected.
970 .Bd -unfilled -offset indent -compact
976 The two lists are executed repeatedly while the exit status of the
980 command is similar, but has the word
985 repeat until the exit status of the first list is zero.
987 The exit status is that of the last execution of the second list,
988 or zero if it was never executed.
993 .Bd -unfilled -offset indent -compact
994 .Ic for Ar variable Op Ic in Ar word ...
1001 and the following words are omitted,
1004 The words are expanded, and then the list is executed
1005 repeatedly with the variable set to each word in turn.
1010 commands may be replaced with
1020 .D1 Ic break Op Ar num
1021 .D1 Ic continue Op Ar num
1025 command terminates the
1034 command continues with the next iteration of the innermost loop.
1035 These are implemented as special built-in commands.
1040 .Bd -unfilled -offset indent -compact
1041 .Ic case Ar word Ic in
1042 .Ar pattern ) Ar list Li ;;
1047 The pattern can actually be one or more patterns
1054 Tilde expansion, parameter expansion, command substitution,
1055 arithmetic expansion and quote removal are applied to the word.
1056 Then, each pattern is expanded in turn using tilde expansion,
1057 parameter expansion, command substitution and arithmetic expansion and
1058 the expanded form of the word is checked against it.
1059 If a match is found, the corresponding list is executed.
1060 If the selected list is terminated by the control operator
1064 execution continues with the next list,
1065 continuing until a list terminated with
1070 .Ss Grouping Commands Together
1071 Commands may be grouped by writing either
1073 .Bd -literal -offset -ident
1079 .Bd -literal -offset -ident
1083 The first form executes the commands in a subshell environment.
1084 A subshell environment has its own copy of:
1087 The current working directory as set by
1090 The file creation mask as set by
1093 Resource limits as set by
1096 References to open files.
1103 Positional parameters and variables.
1112 These are copied from the parent shell environment,
1113 except that trapped (but not ignored) signals are reset to the default action
1114 and known jobs are cleared.
1115 Any changes do not affect the parent shell environment.
1117 A subshell environment may be implemented as a child process or differently.
1118 If job control is enabled in an interactive shell,
1119 commands grouped in parentheses can be suspended and continued as a unit.
1121 For compatibility with other shells,
1122 two open parentheses in sequence should be separated by whitespace.
1124 The second form never forks another shell,
1125 so it is slightly more efficient.
1126 Grouping commands together this way allows the user to
1127 redirect their output as though they were one program:
1128 .Bd -literal -offset indent
1129 { echo -n "hello"; echo " world"; } > greeting
1132 The syntax of a function definition is
1134 .D1 Ar name Li \&( \&) Ar command
1136 A function definition is an executable statement; when
1137 executed it installs a function named
1140 exit status of zero.
1149 Variables may be declared to be local to a function by
1153 This should appear as the first statement of a function,
1156 .D1 Ic local Oo Ar variable ... Oc Op Fl
1160 command is implemented as a built-in command.
1161 The exit status is zero
1162 unless the command is not in a function or a variable name is invalid.
1164 When a variable is made local, it inherits the initial
1165 value and exported and readonly flags from the variable
1166 with the same name in the surrounding scope, if there is
1168 Otherwise, the variable is initially unset.
1170 uses dynamic scoping, so that if the variable
1172 is made local to function
1174 which then calls function
1176 references to the variable
1180 will refer to the variable
1184 not to the global variable named
1187 The only special parameter that can be made local is
1191 local causes any shell options
1192 (including those that only have long names)
1196 command inside the function to be
1197 restored to their original values when the function
1204 .D1 Ic return Op Ar exitstatus
1206 It terminates the current executional scope, returning from the closest
1207 nested function or sourced script;
1208 if no function or sourced script is being executed,
1209 it exits the shell instance.
1212 command is implemented as a special built-in command.
1213 .Ss Variables and Parameters
1214 The shell maintains a set of parameters.
1218 of alphabetics, numerics, and underscores,
1219 and starting with an alphabetic or an underscore)
1220 is called a variable.
1222 the shell turns all environment variables with valid names into shell
1224 New variables can be set using the form
1226 .D1 Ar name Ns = Ns Ar value
1228 A parameter can also be denoted by a number
1229 or a special character as explained below.
1231 Assignments are expanded differently from other words:
1232 tilde expansion is also performed after the equals sign and after any colon
1233 and usernames are also terminated by colons,
1234 and field splitting and pathname expansion are not performed.
1236 This special expansion applies not only to assignments that form a simple
1237 command by themselves or precede a command word,
1238 but also to words passed to the
1243 built-in commands that have this form.
1244 For this, the builtin's name must be literal
1245 (not the result of an expansion)
1246 and may optionally be preceded by one or more literal instances of
1249 .Ss Positional Parameters
1250 A positional parameter is a parameter denoted by a number greater than zero.
1251 The shell sets these initially to the values of its command line
1252 arguments that follow the name of the shell script.
1255 built-in command can also be used to set or reset them.
1256 .Ss Special Parameters
1257 Special parameters are parameters denoted by a single special character
1259 They are shown in the following list, exactly as they would appear in input
1260 typed by the user or in the source of a shell script.
1263 Expands to the positional parameters, starting from one.
1265 the expansion occurs within a double-quoted string
1266 it expands to a single field with the value of each parameter
1267 separated by the first character of the
1274 Expands to the positional parameters, starting from one.
1276 the expansion occurs within double-quotes, each positional
1277 parameter expands as a separate argument.
1278 If there are no positional parameters, the
1281 generates zero arguments, even when
1284 What this basically means, for example, is
1297 .Bd -literal -offset indent
1301 Expands to the number of positional parameters.
1303 Expands to the exit status of the most recent pipeline.
1305 (hyphen) Expands to the current option flags (the single-letter
1306 option names concatenated into a string) as specified on
1309 built-in command, or implicitly
1312 Expands to the process ID of the invoked shell.
1314 retains the same value of
1318 Expands to the process ID of the most recent background
1319 command executed from the current shell.
1321 pipeline, the process ID is that of the last command in the
1323 If this parameter is referenced, the shell will remember
1324 the process ID and its exit status until the
1326 built-in command reports completion of the process.
1328 (zero) Expands to the name of the shell script if passed on the command line,
1331 operand if given (with
1333 or otherwise argument 0 passed to the shell.
1335 .Ss Special Variables
1336 The following variables are set by the shell or
1337 have special meaning to it:
1338 .Bl -tag -width ".Va HISTSIZE"
1340 The search path used with the
1344 The fallback editor used with the
1347 If not set, the default editor is
1350 The default editor used with the
1354 The number of previous commands that are accessible.
1356 The user's home directory,
1357 used in tilde expansion and as a default directory for the
1361 Input Field Separators.
1362 This is initialized at startup to
1368 This value also applies if
1370 is unset, but not if it is set to the empty string.
1372 .Sx White Space Splitting
1373 section for more details.
1375 The current line number in the script or function.
1377 The name of a mail file, that will be checked for the arrival of new
1384 separated list of file names, for the shell to check for incoming
1386 This variable overrides the
1389 There is a maximum of 10 mailboxes that can be monitored at once.
1391 The index of the next argument to be processed by
1393 This is initialized to 1 at startup.
1395 The default search path for executables.
1398 section for details.
1400 The parent process ID of the invoked shell.
1401 This is set at startup
1402 unless this variable is in the environment.
1403 A later change of parent process ID is not reflected.
1404 A subshell retains the same value of
1407 The primary prompt string, which defaults to
1409 unless you are the superuser, in which case it defaults to
1412 may include any of the following formatting sequences,
1413 which are replaced by the given information:
1414 .Bl -tag -width indent
1416 This system's fully-qualified hostname (FQDN).
1418 This system's hostname.
1422 The final component of the current working directory.
1424 The entire path of the current working directory.
1428 for normal users and
1432 A literal backslash.
1435 The secondary prompt string, which defaults to
1438 may include any of the formatting sequences from
1441 The prefix for the trace output (if
1448 This clause describes the various expansions that are
1450 Not all expansions are performed on
1451 every word, as explained later.
1453 Tilde expansions, parameter expansions, command substitutions,
1454 arithmetic expansions, and quote removals that occur within
1455 a single word expand to a single field.
1457 splitting or pathname expansion that can create multiple
1458 fields from a single word.
1459 The single exception to this rule is
1460 the expansion of the special parameter
1462 within double-quotes,
1463 as was described above.
1465 The order of word expansion is:
1468 Tilde Expansion, Parameter Expansion, Command Substitution,
1469 Arithmetic Expansion (these all occur at the same time).
1471 Field Splitting is performed on fields generated by step (1)
1476 Pathname Expansion (unless the
1478 option is in effect).
1485 character is used to introduce parameter expansion, command
1486 substitution, or arithmetic expansion.
1487 .Ss Tilde Expansion (substituting a user's home directory)
1488 A word beginning with an unquoted tilde character
1491 subjected to tilde expansion.
1492 All the characters up to a slash
1494 or the end of the word are treated as a username
1495 and are replaced with the user's home directory.
1497 username is missing (as in
1499 the tilde is replaced with the value of the
1501 variable (the current user's home directory).
1502 .Ss Parameter Expansion
1503 The format for parameter expansion is as follows:
1505 .D1 Li ${ Ns Ar expression Ns Li }
1509 consists of all characters until the matching
1513 escaped by a backslash or within a single-quoted or double-quoted
1514 string, and characters in
1515 embedded arithmetic expansions, command substitutions, and variable
1516 expansions, are not examined in determining the matching
1518 If the variants with
1524 occur within a double-quoted string,
1525 as an extension there may be unquoted parts
1526 (via double-quotes inside the expansion);
1528 within such parts are also not examined in determining the matching
1531 The simplest form for parameter expansion is:
1533 .D1 Li ${ Ns Ar parameter Ns Li }
1535 The value, if any, of
1539 The parameter name or symbol can be enclosed in braces, which are
1540 optional except for positional parameters with more than one digit or
1541 when parameter is followed by a character that could be interpreted as
1543 If a parameter expansion occurs inside double-quotes:
1546 Field splitting is not performed on the results of the
1547 expansion, with the exception of the special parameter
1550 Pathname expansion is not performed on the results of the
1554 In addition, a parameter expansion can be modified by using one of the
1556 .Bl -tag -width indent
1557 .It Li ${ Ns Ar parameter Ns Li :- Ns Ar word Ns Li }
1561 is unset or null, the expansion of
1563 is substituted; otherwise, the value of
1566 .It Li ${ Ns Ar parameter Ns Li := Ns Ar word Ns Li }
1567 Assign Default Values.
1570 is unset or null, the expansion of
1580 does not prevent field splitting or pathname expansion.
1581 Only variables, not positional
1582 parameters or special parameters, can be
1583 assigned in this way.
1584 .It Li ${ Ns Ar parameter Ns Li :? Ns Oo Ar word Oc Ns Li }
1585 Indicate Error if Null or Unset.
1588 is unset or null, the expansion of
1590 (or a message indicating it is unset if
1592 is omitted) is written to standard
1593 error and the shell exits with a nonzero
1595 Otherwise, the value of
1599 interactive shell need not exit.
1600 .It Li ${ Ns Ar parameter Ns Li :+ Ns Ar word Ns Li }
1601 Use Alternate Value.
1604 is unset or null, null is substituted;
1605 otherwise, the expansion of
1610 In the parameter expansions shown previously, use of the colon in the
1611 format results in a test for a parameter that is unset or null; omission
1612 of the colon results in a test for a parameter that is only unset.
1616 inherits the type of quoting
1617 (unquoted, double-quoted or here-document)
1618 from the surroundings,
1619 with the exception that a backslash that quotes a closing brace is removed
1620 during quote removal.
1621 .Bl -tag -width indent
1622 .It Li ${# Ns Ar parameter Ns Li }
1624 The length in characters of
1629 The following four varieties of parameter expansion provide for substring
1631 In each case, pattern matching notation
1633 .Sx Shell Patterns ) ,
1634 rather than regular expression notation,
1635 is used to evaluate the patterns.
1636 If parameter is one of the special parameters
1640 the result of the expansion is unspecified.
1641 Enclosing the full parameter expansion string in double-quotes does not
1642 cause the following four varieties of pattern characters to be quoted,
1643 whereas quoting characters within the braces has this effect.
1644 .Bl -tag -width indent
1645 .It Li ${ Ns Ar parameter Ns Li % Ns Ar word Ns Li }
1646 Remove Smallest Suffix Pattern.
1649 is expanded to produce a pattern.
1651 parameter expansion then results in
1653 with the smallest portion of the
1654 suffix matched by the pattern deleted.
1655 .It Li ${ Ns Ar parameter Ns Li %% Ns Ar word Ns Li }
1656 Remove Largest Suffix Pattern.
1659 is expanded to produce a pattern.
1661 parameter expansion then results in
1663 with the largest portion of the
1664 suffix matched by the pattern deleted.
1665 .It Li ${ Ns Ar parameter Ns Li # Ns Ar word Ns Li }
1666 Remove Smallest Prefix Pattern.
1669 is expanded to produce a pattern.
1671 parameter expansion then results in
1673 with the smallest portion of the
1674 prefix matched by the pattern deleted.
1675 .It Li ${ Ns Ar parameter Ns Li ## Ns Ar word Ns Li }
1676 Remove Largest Prefix Pattern.
1679 is expanded to produce a pattern.
1681 parameter expansion then results in
1683 with the largest portion of the
1684 prefix matched by the pattern deleted.
1686 .Ss Command Substitution
1687 Command substitution allows the output of a command to be substituted in
1688 place of the command name itself.
1689 Command substitution occurs when
1690 the command is enclosed as follows:
1692 .D1 Li $( Ns Ar command Ns Li )\&
1694 or the backquoted version:
1696 .D1 Li ` Ns Ar command Ns Li `
1698 The shell expands the command substitution by executing command
1699 and replacing the command substitution
1700 with the standard output of the command,
1701 removing sequences of one or more newlines at the end of the substitution.
1702 Embedded newlines before the end of the output are not removed;
1703 however, during field splitting, they may be translated into spaces
1704 depending on the value of
1706 and the quoting that is in effect.
1707 The command is executed in a subshell environment,
1708 except that the built-in commands
1713 return information about the parent shell environment
1716 returns information about the same process
1717 if they are the only command in a command substitution.
1719 If a command substitution of the
1721 form begins with a subshell,
1726 must be separated by whitespace
1727 to avoid ambiguity with arithmetic expansion.
1728 .Ss Arithmetic Expansion
1729 Arithmetic expansion provides a mechanism for evaluating an arithmetic
1730 expression and substituting its value.
1731 The format for arithmetic expansion is as follows:
1733 .D1 Li $(( Ns Ar expression Ns Li ))
1737 is treated as if it were in double-quotes, except
1738 that a double-quote inside the expression is not treated specially.
1740 shell expands all tokens in the
1742 for parameter expansion,
1743 command substitution,
1744 arithmetic expansion
1747 The allowed expressions are a subset of C expressions,
1749 .Bl -tag -width "Variables" -offset indent
1751 All values are of type
1754 Decimal, octal (starting with
1756 and hexadecimal (starting with
1760 Shell variables can be read and written
1761 and contain integer constants.
1764 .It Binary operators
1765 .Li "* / % + - << >> < <= > >= == != & ^ | && ||"\&
1766 .It Assignment operators
1767 .Li "= += -= *= /= %= <<= >>= &= ^= |="
1768 .It Conditional operator
1772 The result of the expression is substituted in decimal.
1773 .Ss White Space Splitting (Field Splitting)
1774 In certain contexts,
1775 after parameter expansion, command substitution, and
1776 arithmetic expansion the shell scans the results of
1777 expansions and substitutions that did not occur in double-quotes for
1778 field splitting and multiple fields can result.
1789 are treated differently from other characters in
1794 at the beginning or end of a word is discarded.
1796 Subsequently, a field is delimited by either
1799 a non-whitespace character in
1801 with any whitespace in
1805 one or more whitespace characters in
1809 If a word ends with a non-whitespace character in
1811 there is no empty field after this character.
1813 If no field is delimited, the word is discarded.
1814 In particular, if a word consists solely of an unquoted substitution
1815 and the result of the substitution is null,
1816 it is removed by field splitting even if
1819 .Ss Pathname Expansion (File Name Generation)
1823 file name generation is performed
1824 after word splitting is complete.
1826 viewed as a series of patterns, separated by slashes.
1828 process of expansion replaces the word with the names of
1829 all existing files whose names can be formed by replacing
1830 each pattern with a string that matches the specified pattern.
1831 There are two restrictions on this: first, a pattern cannot match
1832 a string containing a slash, and second,
1833 a pattern cannot match a string starting with a period
1834 unless the first character of the pattern is a period.
1835 The next section describes the patterns used for
1837 the four varieties of parameter expansion for substring processing and the
1841 A pattern consists of normal characters, which match themselves,
1842 and meta-characters.
1843 The meta-characters are
1848 These characters lose their special meanings if they are quoted.
1849 When command or variable substitution is performed and the dollar sign
1850 or back quotes are not double-quoted, the value of the
1851 variable or the output of the command is scanned for these
1852 characters and they are turned into meta-characters.
1856 matches any string of characters.
1859 matches any single character.
1862 introduces a character class.
1863 The end of the character class is indicated by a
1871 rather than introducing a character class.
1872 A character class matches any of the characters between the square brackets.
1873 A locale-dependent range of characters may be specified using a minus sign.
1874 A named class of characters (see
1876 may be specified by surrounding the name with
1881 .Ql \&[\&[:alpha:\&]\&]
1882 is a shell pattern that matches a single letter.
1883 The character class may be complemented by making an exclamation point
1885 the first character of the character class.
1888 has the same effect but is non-standard.
1892 in a character class, make it the first character listed
1900 make it the first or last character listed.
1901 .Ss Built-in Commands
1902 This section lists the built-in commands.
1903 .Bl -tag -width indent
1905 A null command that returns a 0 (true) exit value.
1907 The commands in the specified file are read and executed by the shell.
1910 command may be used to return to the
1917 characters, it is used as is.
1918 Otherwise, the shell searches the
1921 If it is not found in the
1923 it is sought in the current working directory.
1925 A built-in equivalent of
1927 .It Ic alias Oo Ar name Ns Oo = Ns Ar string Oc ... Oc
1929 .Ar name Ns = Ns Ar string
1930 is specified, the shell defines the alias
1936 is specified, the value of the alias
1939 With no arguments, the
1941 built-in command prints the names and values of all defined aliases
1944 Alias values are written with appropriate quoting so that they are
1945 suitable for re-input to the shell.
1949 .It Ic bg Op Ar job ...
1950 Continue the specified jobs
1951 (or the current job if no jobs are given)
1953 .It Ic bind Oo Fl aeklrsv Oc Oo Ar key Oo Ar command Oc Oc
1954 List or alter key bindings for the line editor.
1955 This command is documented in
1957 .It Ic break Op Ar num
1959 .Sx Flow-Control Constructs
1961 .It Ic builtin Ar cmd Op Ar arg ...
1962 Execute the specified built-in command,
1964 This is useful when the user wishes to override a shell function
1965 with the same name as a built-in command.
1966 .It Ic cd Oo Fl L | P Oc Oo Fl e Oc Op Ar directory
1968 Switch to the specified
1970 to the directory specified in the
1972 environment variable if no
1975 to the directory specified in the
1977 environment variable if
1987 then the directories listed in the
1990 searched for the specified
1994 is unset, the current directory is searched.
1997 is the same as that of
1999 In an interactive shell,
2002 command will print out the name of the directory
2003 that it actually switched to
2006 mechanism was used or if
2013 option is specified,
2015 is handled physically and symbolic links are resolved before
2017 components are processed.
2020 option is specified,
2022 is handled logically.
2023 This is the default.
2029 to return exit status 1 if the full pathname of the new directory
2030 cannot be determined reliably or at all.
2031 Normally this is not considered an error,
2032 although a warning is printed.
2034 If changing the directory fails, the exit status is greater than 1.
2035 If the directory is changed, the exit status is 0, or also 1 if
2042 .It Ic command Oo Fl p Oc Op Ar utility Op Ar argument ...
2043 .It Ic command Oo Fl p Oc Fl v Ar utility
2044 .It Ic command Oo Fl p Oc Fl V Ar utility
2045 The first form of invocation executes the specified
2047 ignoring shell functions in the search.
2050 is a special builtin,
2051 it is executed as if it were a regular builtin.
2055 option is specified, the command search is performed using a
2058 that is guaranteed to find all of the standard utilities.
2062 option is specified,
2064 is not executed but a description of its interpretation by the shell is
2066 For ordinary commands the output is the path name; for shell built-in
2067 commands, shell functions and keywords only the name is written.
2068 Aliases are printed as
2069 .Dq Ic alias Ar name Ns = Ns Ar value .
2073 option is identical to
2075 except for the output.
2077 .Dq Ar utility Ic is Ar description
2083 a special shell builtin,
2090 .It Ic continue Op Ar num
2092 .Sx Flow-Control Constructs
2094 .It Ic echo Oo Fl e | n Oc Op Ar string ...
2095 Print a space-separated list of the arguments to the standard output
2096 and append a newline character.
2097 .Bl -tag -width indent
2099 Suppress the output of the trailing newline.
2101 Process C-style backslash escape sequences.
2104 command understands the following character escapes:
2105 .Bl -tag -width indent
2107 Alert (ring the terminal bell)
2111 Suppress the trailing newline (this has the side-effect of truncating the
2112 line if it is not the last character)
2114 The ESC character (ASCII 0x1b)
2128 (Zero) The character whose octal value is
2134 is not enclosed in quotes then the backslash itself must be escaped
2135 with a backslash to protect it from the shell.
2137 .Bd -literal -offset indent
2146 $ echo -e a\e\e\e\eb
2155 options may be specified.
2156 .It Ic eval Ar string ...
2157 Concatenate all the arguments with spaces.
2158 Then re-parse and execute the command.
2159 .It Ic exec Op Ar command Op arg ...
2163 the shell process is replaced with the specified program
2164 (which must be a real program, not a shell built-in command or function).
2165 Any redirections on the
2167 command are marked as permanent,
2168 so that they are not undone when the
2171 .It Ic exit Op Ar exitstatus
2172 Terminate the shell process.
2176 it is used as the exit status of the shell.
2177 Otherwise, if the shell is executing an
2179 trap, the exit status of the last command before the trap is used;
2180 if the shell is executing a trap for a signal,
2181 the shell exits by resending the signal to itself.
2182 Otherwise, the exit status of the preceding command is used.
2183 The exit status should be an integer between 0 and 255.
2184 .It Ic export Ar name ...
2185 .It Ic export Op Fl p
2186 The specified names are exported so that they will
2187 appear in the environment of subsequent commands.
2188 The only way to un-export a variable is to
2191 The shell allows the value of a variable to be set
2192 at the same time as it is exported by writing
2194 .D1 Ic export Ar name Ns = Ns Ar value
2196 With no arguments the
2198 command lists the names
2199 of all exported variables.
2202 option is specified, the exported variables are printed as
2203 .Dq Ic export Ar name Ns = Ns Ar value
2204 lines, suitable for re-input to the shell.
2206 A null command that returns a non-zero (false) exit value.
2207 .It Ic fc Oo Fl e Ar editor Oc Op Ar first Op Ar last
2208 .It Ic fc Fl l Oo Fl nr Oc Op Ar first Op Ar last
2209 .It Ic fc Fl s Oo Ar old Ns = Ns Ar new Oc Op Ar first
2212 built-in command lists, or edits and re-executes,
2213 commands previously entered to an interactive shell.
2214 .Bl -tag -width indent
2216 Use the editor named by
2218 to edit the commands.
2221 string is a command name,
2222 subject to search via the
2227 variable is used as a default when
2232 is null or unset, the value of the
2239 is used as the editor.
2241 List the commands rather than invoking
2243 The commands are written in the
2244 sequence indicated by the
2248 operands, as affected by
2250 with each command preceded by the command number.
2252 Suppress command numbers when listing with
2255 Reverse the order of the commands listed
2264 Re-execute the command without invoking an editor.
2267 Select the commands to list or edit.
2268 The number of previous commands that can be accessed
2269 are determined by the value of the
2276 or both are one of the following:
2277 .Bl -tag -width indent
2278 .It Oo Cm + Oc Ns Ar num
2279 A positive number representing a command number;
2280 command numbers can be displayed with the
2284 A negative decimal number representing the
2285 command that was executed
2288 commands previously.
2289 For example, \-1 is the immediately previous command.
2291 A string indicating the most recently entered command
2292 that begins with that string.
2294 .Ar old Ns = Ns Ar new
2295 operand is not also specified with
2297 the string form of the first operand cannot contain an embedded equal sign.
2301 The following variables affect the execution of
2303 .Bl -tag -width ".Va HISTSIZE"
2305 Name of the editor to use for history editing.
2307 The number of previous commands that are accessible.
2312 or the current job to the foreground.
2313 .It Ic getopts Ar optstring var
2319 command deprecates the older
2322 The first argument should be a series of letters, each possibly
2323 followed by a colon which indicates that the option takes an argument.
2324 The specified variable is set to the parsed option.
2326 the next argument is placed into the shell variable
2328 If an option takes an argument, it is placed into the shell variable
2330 If an invalid option is encountered,
2334 It returns a false value (1) when it encounters the end of the options.
2335 A new set of arguments may be parsed by assigning
2337 .It Ic hash Oo Fl rv Oc Op Ar command ...
2338 The shell maintains a hash table which remembers the locations of commands.
2339 With no arguments whatsoever, the
2341 command prints out the contents of this table.
2345 command removes each specified
2347 from the hash table (unless they are functions) and then locates it.
2352 prints the locations of the commands as it finds them.
2357 command to delete all the entries in the hash table except for functions.
2358 .It Ic jobid Op Ar job
2359 Print the process IDs of the processes in the specified
2363 argument is omitted, use the current job.
2364 .It Ic jobs Oo Fl lps Oc Op Ar job ...
2365 Print information about the specified jobs, or all jobs if no
2368 The information printed includes job ID, status and command name.
2372 option is specified, the PID of each job is also printed.
2375 option is specified, only the process IDs for the process group leaders
2376 are printed, one per line.
2379 option is specified, only the PIDs of the job commands are printed, one per
2382 A built-in equivalent of
2384 that additionally supports sending signals to jobs.
2385 .It Ic local Oo Ar variable ... Oc Op Fl
2390 A built-in equivalent of
2392 .It Ic pwd Op Fl L | P
2393 Print the path of the current directory.
2394 The built-in command may
2395 differ from the program of the same name because the
2396 built-in command remembers what the current directory
2397 is rather than recomputing it each time.
2400 However, if the current directory is
2402 the built-in version of
2404 will continue to print the old name for the directory.
2408 option is specified, symbolic links are resolved.
2411 option is specified, the shell's notion of the current directory
2412 is printed (symbolic links are not resolved).
2413 This is the default.
2414 .It Ic read Oo Fl p Ar prompt Oc Oo
2415 .Fl t Ar timeout Oc Oo Fl er Oc Ar variable ...
2421 and the standard input is a terminal.
2423 read from the standard input.
2424 The trailing newline
2425 is deleted from the line and the line is split as
2426 described in the section on
2427 .Sx White Space Splitting (Field Splitting)\&
2429 the pieces are assigned to the variables in order.
2430 If there are more pieces than variables, the remaining
2431 pieces (along with the characters in
2433 that separated them)
2434 are assigned to the last variable.
2435 If there are more variables than pieces, the remaining
2436 variables are assigned the null string.
2438 Backslashes are treated specially, unless the
2442 If a backslash is followed by
2443 a newline, the backslash and the newline will be
2445 If a backslash is followed by any other
2446 character, the backslash will be deleted and the following
2447 character will be treated as though it were not in
2453 option is specified and the
2455 elapses before a complete line of input is supplied,
2458 command will return an exit status as if terminated by
2460 without assigning any values.
2463 value may optionally be followed by one of
2468 to explicitly specify seconds, minutes or hours.
2469 If none is supplied,
2475 option exists only for backward compatibility with older scripts.
2477 The exit status is 0 on success, 1 on end of file,
2478 between 2 and 128 if an error occurs
2479 and greater than 128 if a trapped signal interrupts
2481 .It Ic readonly Oo Fl p Oc Op Ar name ...
2484 is marked as read only,
2485 so that it cannot be subsequently modified or unset.
2486 The shell allows the value of a variable to be set
2487 at the same time as it is marked read only
2488 by using the following form:
2490 .D1 Ic readonly Ar name Ns = Ns Ar value
2492 With no arguments the
2494 command lists the names of all read only variables.
2497 option is specified, the read-only variables are printed as
2498 .Dq Ic readonly Ar name Ns = Ns Ar value
2499 lines, suitable for re-input to the shell.
2500 .It Ic return Op Ar exitstatus
2504 .It Ic set Oo Fl /+abCEefIimnpTuVvx Oc Oo Fl /+o Ar longname
2505 .Oc Op Fl - Ar arg ...
2508 command performs three different functions:
2511 With no arguments, it lists the values of all shell variables.
2513 If options are given,
2514 either in short form or using the long
2515 .Dq Fl /+o Ar longname
2517 it sets or clears the specified options as described in the section called
2518 .Sx Argument List Processing .
2522 option is specified,
2524 will replace the shell's positional parameters with the subsequent
2526 If no arguments follow the
2529 all the positional parameters will be cleared,
2530 which is equivalent to executing the command
2534 flag may be omitted when specifying arguments to be used
2535 as positional replacement parameters.
2536 This is not recommended,
2537 because the first argument may begin with a dash
2543 command will interpret as a request to enable or disable options.
2545 .It Ic setvar Ar variable value
2546 Assigns the specified
2552 command is intended to be used in functions that
2553 assign values to variables whose names are passed as parameters.
2554 In general it is better to write
2555 .Dq Ar variable Ns = Ns Ar value
2558 .It Ic shift Op Ar n
2559 Shift the positional parameters
2564 A shift sets the value of
2573 decreasing the value of
2576 For portability, shifting if there are zero positional parameters
2577 should be avoided, since the shell may abort.
2579 A built-in equivalent of
2582 Print the amount of time spent executing the shell process and its children.
2583 The first output line shows the user and system times for the shell process
2584 itself, the second one contains the user and system times for the
2586 .It Ic trap Oo Ar action Oc Ar signal ...
2588 Cause the shell to parse and execute
2593 The signals are specified by name or number.
2594 In addition, the pseudo-signal
2596 may be used to specify an
2598 that is performed when the shell terminates.
2601 may be an empty string or a dash
2603 the former causes the specified signal to be ignored
2604 and the latter causes the default action to be taken.
2607 and using only signal numbers is another way to request the default action.
2608 In a subshell or utility environment,
2609 the shell resets trapped (but not ignored) signals to the default action.
2612 command has no effect on signals that were ignored on entry to the shell.
2618 command to display a list of valid signal names.
2620 A null command that returns a 0 (true) exit value.
2621 .It Ic type Op Ar name ...
2624 as a command and print the resolution of the command search.
2625 Possible resolutions are:
2626 shell keyword, alias, special shell builtin, shell builtin, command,
2629 For aliases the alias expansion is printed;
2630 for commands and tracked aliases
2631 the complete pathname of the command is printed.
2632 .It Ic ulimit Oo Fl HSabcdfklmnopstuvw Oc Op Ar limit
2633 Set or display resource limits (see
2637 is specified, the named resource will be set;
2638 otherwise the current resource value will be displayed.
2642 is specified, the hard limits will be set or displayed.
2643 While everybody is allowed to reduce a hard limit,
2644 only the superuser can increase it.
2648 specifies the soft limits instead.
2649 When displaying limits,
2655 The default is to display the soft limits,
2656 and to set both the hard and the soft limits.
2662 command to display all resources.
2665 is not acceptable in this mode.
2667 The remaining options specify which resource value is to be
2668 displayed or modified.
2669 They are mutually exclusive.
2670 .Bl -tag -width indent
2672 The maximum size of socket buffer usage, in bytes.
2673 .It Fl c Ar coredumpsize
2674 The maximal size of core dump files, in 512-byte blocks.
2677 to 0 prevents core dump files from being created.
2678 .It Fl d Ar datasize
2679 The maximal size of the data segment of a process, in kilobytes.
2680 .It Fl f Ar filesize
2681 The maximal size of a file, in 512-byte blocks.
2683 The maximal number of kqueues
2687 .It Fl l Ar lockedmem
2688 The maximal size of memory that can be locked by a process, in
2690 .It Fl m Ar memoryuse
2691 The maximal resident set size of a process, in kilobytes.
2693 The maximal number of descriptors that could be opened by a process.
2695 The maximal number of process-shared locks
2699 .It Fl p Ar pseudoterminals
2700 The maximal number of pseudo-terminals for this user ID.
2701 .It Fl s Ar stacksize
2702 The maximal size of the stack segment, in kilobytes.
2704 The maximal amount of CPU time to be used by each process, in seconds.
2705 .It Fl u Ar userproc
2706 The maximal number of simultaneous processes for this user ID.
2707 .It Fl v Ar virtualmem
2708 The maximal virtual size of a process, in kilobytes.
2710 The maximum amount of swap space reserved or used for this user ID,
2713 .It Ic umask Oo Fl S Oc Op Ar mask
2714 Set the file creation mask (see
2716 to the octal or symbolic (see
2720 If the argument is omitted, the current mask value is printed.
2723 option is specified, the output is symbolic, otherwise the output is octal.
2724 .It Ic unalias Oo Fl a Oc Op Ar name ...
2725 The specified alias names are removed.
2728 is specified, all aliases are removed.
2729 .It Ic unset Oo Fl fv Oc Ar name ...
2730 The specified variables or functions are unset and unexported.
2733 option is specified or no options are given, the
2735 arguments are treated as variable names.
2738 option is specified, the
2740 arguments are treated as function names.
2741 .It Ic wait Op Ar job ...
2742 Wait for each specified
2744 to complete and return the exit status of the last process in the
2749 specified is unknown to the shell, it is treated as if it
2750 were a known job that exited with exit status 127.
2751 If no operands are given, wait for all jobs to complete
2752 and return an exit status of zero.
2754 .Ss Command Line Editing
2757 is being used interactively from a terminal, the current command
2758 and the command history
2762 .Sx Built-in Commands )
2765 command line editing.
2766 This mode uses commands similar
2767 to a subset of those described in the
2785 can be switched between insert mode and command mode by typing
2789 while in command mode will pass the line to the shell.
2792 .Dq Li "set -o emacs"
2795 command can be used to enable a subset of
2797 command line editing features.
2799 The following environment variables affect the execution of
2801 .Bl -tag -width ".Ev LANGXXXXXX"
2803 Initialization file for interactive shells.
2804 .It Ev LANG , Ev LC_*
2806 These are inherited by children of the shell,
2807 and is used in a limited manner by the shell itself.
2809 The previous current directory.
2810 This is used and updated by
2813 An absolute pathname for the current directory,
2814 possibly containing symbolic links.
2815 This is used and updated by the shell.
2817 The default terminal setting for the shell.
2818 This is inherited by children of the shell, and is used in the history
2822 Additionally, environment variables are turned into shell variables
2824 which may affect the shell as described under
2825 .Sx Special Variables .
2827 .Bl -tag -width "/etc/suid_profileXX" -compact
2829 User's login profile.
2831 System login profile.
2834 .It Pa /etc/suid_profile
2835 Privileged shell profile.
2840 cannot be found, the exit status will be 127;
2841 if it cannot be opened for another reason, the exit status will be 126.
2842 Other errors that are detected by the shell, such as a syntax error, will
2843 cause the shell to exit with a non-zero exit status.
2844 If the shell is not an interactive shell, the execution of the shell
2845 file will be aborted.
2846 Otherwise the shell will return the exit status of the last command
2849 builtin is used with a numeric argument, it
2850 will return the argument.
2871 command, the Thompson shell, appeared in
2873 It was superseded in
2875 by the Bourne shell, which inherited the name
2880 was rewritten in 1989 under the
2882 license after the Bourne shell from
2887 was originally written by
2888 .An Kenneth Almquist .
2892 utility does not recognize multibyte characters other than UTF-8.
2895 does not recognize multibyte characters.