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32 .\" from: @(#)sh.1 8.6 (Berkeley) 5/4/95
40 .Nd command interpreter (shell)
43 .Op Fl /+abCEefIimnPpTuVvx
44 .Op Fl /+o Ar longname
50 .Op Fl /+abCEefIimnPpTuVvx
51 .Op Fl /+o Ar longname
58 .Op Fl /+abCEefIimnPpTuVvx
59 .Op Fl /+o Ar longname
65 utility is the standard command interpreter for the system.
66 The current version of
68 is in the process of being changed to
71 specification for the shell.
72 This version has many features which make
74 similar in some respects to the Korn shell, but it is not a Korn
80 plus a few Berkeley extensions, are being
81 incorporated into this shell.
82 This man page is not intended to be a tutorial nor a complete
83 specification of the shell.
85 The shell is a command that reads lines from
86 either a file or the terminal, interprets them, and
87 generally executes other commands.
88 It is the program that is started when a user logs into the system,
89 although a user can select a different shell with the
93 implements a language that has flow control constructs,
94 a macro facility that provides a variety of features in
95 addition to data storage, along with built-in history and line
97 It incorporates many features to
98 aid interactive use and has the advantage that the interpretative
99 language is common to both interactive and non-interactive
101 That is, commands can be typed directly
102 to the running shell or can be put into a file,
103 which can be executed directly by the shell.
106 .\" XXX This next sentence is incredibly confusing.
108 If no arguments are present and if the standard input of the shell
109 is connected to a terminal
113 the shell is considered an interactive shell.
115 generally prompts before each command and handles programming
116 and command errors differently (as described below).
117 When first starting, the shell inspects argument 0, and
118 if it begins with a dash
120 the shell is also considered a login shell.
121 This is normally done automatically by the system
122 when the user first logs in.
123 A login shell first reads commands
128 in a user's home directory,
130 If the environment variable
132 is set on entry to a shell, or is set in the
134 of a login shell, the shell then reads commands from the file named in
136 Therefore, a user should place commands that are to be executed only
139 file, and commands that are executed for every shell inside the
144 variable to some file by placing the following line in the file
146 in the home directory,
149 the filename desired:
151 .Dl "ENV=$HOME/.shinit; export ENV"
153 The first non-option argument specified on the command line
154 will be treated as the
155 name of a file from which to read commands (a shell script), and
156 the remaining arguments are set as the positional parameters
160 Otherwise, the shell reads commands
161 from its standard input.
163 Unlike older versions of
167 script is only sourced on invocation of interactive shells.
169 closes a well-known, and sometimes easily exploitable security
170 hole related to poorly thought out
173 .Ss Argument List Processing
174 All of the single letter options to
176 have a corresponding long name,
177 with the exception of
181 These long names are provided next to the single letter options
182 in the descriptions below.
183 The long name for an option may be specified as an argument to the
187 Once the shell is running,
188 the long name for an option may be specified as an argument to the
193 (described later in the section called
194 .Sx Built-in Commands ) .
195 Introducing an option with a dash
205 will stop option processing and will force the remaining
206 words on the command line to be treated as arguments.
211 options do not have long names.
212 They take arguments and are described after the single letter options.
213 .Bl -tag -width indent
214 .It Fl a Li allexport
215 Flag variables for export when assignments are made to them.
217 Enable asynchronous notification of background job
220 .It Fl C Li noclobber
221 Do not overwrite existing files with
226 command line editor (disables the
228 option if it has been set).
230 Exit immediately if any untested command fails in non-interactive mode.
231 The exit status of a command is considered to be
232 explicitly tested if the command is part of the list used to control
234 .Ic if , elif , while ,
237 if the command is the left
242 operator; or if the command is a pipeline preceded by the
245 If a shell function is executed and its exit status is explicitly
246 tested, all commands of the function are considered to be tested as
249 Disable pathname expansion.
250 .It Fl I Li ignoreeof
253 from input when in interactive mode.
254 .It Fl i Li interactive
255 Force the shell to behave interactively.
257 Turn on job control (set automatically when interactive).
259 If not interactive, read commands but do not
261 This is useful for checking the
262 syntax of shell scripts.
264 Change the default for the
270 (logical directory layout)
273 (physical directory layout).
274 .It Fl p Li privileged
275 Turn on privileged mode.
276 This mode is enabled on startup
277 if either the effective user or group ID is not equal to the
278 real user or group ID.
279 Turning this mode off sets the
280 effective user and group IDs to the real user and group IDs.
281 When this mode is enabled for interactive shells, the file
282 .Pa /etc/suid_profile
283 is sourced instead of
287 is sourced, and the contents of the
289 variable are ignored.
291 Read commands from standard input (set automatically
292 if no file arguments are present).
294 no effect when set after the shell has already started
295 running (i.e., when set with the
298 .It Fl T Li trapsasync
299 When waiting for a child, execute traps immediately.
300 If this option is not set,
301 traps are executed after the child exits,
304 This nonstandard option is useful for putting guarding shells around
305 children that block signals.
306 The surrounding shell may kill the child
307 or it may just return control to the tty and leave the child alone,
309 .Bd -literal -offset indent
310 sh -T -c "trap 'exit 1' 2 ; some-blocking-program"
313 Write a message to standard error when attempting
314 to expand a variable, a positional parameter or
315 the special parameter
317 that is not set, and if the
318 shell is not interactive, exit immediately.
322 command line editor (disables
326 The shell writes its input to standard error
328 Useful for debugging.
331 (preceded by the value of the
334 to standard error before it is executed.
335 Useful for debugging.
340 option causes the commands to be read from the
342 operand instead of from the standard input.
343 Keep in mind that this option only accepts a single string as its
344 argument, hence multi-word strings must be quoted.
348 option takes as its only argument the long name of an option
349 to be enabled or disabled.
350 For example, the following two invocations of
352 both enable the built-in
355 .Bd -literal -offset indent
360 If used without an argument, the
362 option displays the current option settings in a human-readable format.
365 is used without an argument, the current option settings are output
366 in a format suitable for re-input into the shell.
367 .Ss Lexical Structure
368 The shell reads input in terms of lines from a file and breaks
369 it up into words at whitespace (blanks and tabs), and at
373 which are special to the shell.
374 There are two types of operators: control operators and
375 redirection operators (their meaning is discussed later).
376 The following is a list of valid operators:
377 .Bl -tag -width indent
378 .It Control operators:
379 .Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
380 .It Li & Ta Li && Ta Li ( Ta Li ) Ta Li \en
381 .It Li ;; Ta Li ; Ta Li | Ta Li ||
383 .It Redirection operators:
384 .Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
385 .It Li < Ta Li > Ta Li << Ta Li >> Ta Li <>
386 .It Li <& Ta Li >& Ta Li <<- Ta Li >|
392 introduces a comment if used at the beginning of a word.
393 The word starting with
395 and the rest of the line are ignored.
399 characters (character code 0) are not allowed in shell input.
401 Quoting is used to remove the special meaning of certain characters
402 or words to the shell, such as operators, whitespace, keywords,
405 There are three types of quoting: matched single quotes,
406 matched double quotes, and backslash.
407 .Bl -tag -width indent
409 Enclosing characters in single quotes preserves the literal
410 meaning of all the characters (except single quotes, making
411 it impossible to put single-quotes in a single-quoted string).
413 Enclosing characters within double quotes preserves the literal
414 meaning of all characters except dollar sign
420 The backslash inside double quotes is historically weird.
421 It remains literal unless it precedes the following characters,
422 which it serves to quote:
423 .Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
424 .It Li $ Ta Li ` Ta Li \&" Ta Li \e\ Ta Li \en
427 A backslash preserves the literal meaning of the following
428 character, with the exception of the newline character
430 A backslash preceding a newline is treated as a line continuation.
433 Keywords or reserved words are words that have special meaning to the
434 shell and are recognized at the beginning of a line and
435 after a control operator.
436 The following are keywords:
437 .Bl -column "doneXX" "elifXX" "elseXX" "untilXX" "whileX" -offset center
438 .It Li \&! Ta { Ta } Ta Ic case Ta Ic do
439 .It Ic done Ta Ic elif Ta Ic else Ta Ic esac Ta Ic fi
440 .It Ic for Ta Ic if Ta Ic then Ta Ic until Ta Ic while
443 An alias is a name and corresponding value set using the
446 Whenever a keyword may occur (see above),
447 and after checking for keywords, the shell
448 checks the word to see if it matches an alias.
449 If it does, it replaces it in the input stream with its value.
450 For example, if there is an alias called
462 Aliases provide a convenient way for naive users to
463 create shorthands for commands without having to learn how
464 to create functions with arguments.
465 Using aliases in scripts is discouraged
466 because the command that defines them must be executed
467 before the code that uses them is parsed.
468 This is fragile and not portable.
470 An alias name may be escaped in a command line, so that it is not
471 replaced by its alias value, by using quoting characters within or
472 adjacent to the alias name.
473 This is most often done by prefixing
474 an alias name with a backslash to execute a function, built-in, or
475 normal program with the same name.
480 The shell interprets the words it reads according to a
481 language, the specification of which is outside the scope
482 of this man page (refer to the BNF in the
485 Essentially though, a line is read and if
486 the first word of the line (or after a control operator)
487 is not a keyword, then the shell has recognized a
489 Otherwise, a complex command or some
490 other special construct may have been recognized.
492 If a simple command has been recognized, the shell performs
493 the following actions:
496 Leading words of the form
498 are stripped off and assigned to the environment of
500 Redirection operators and
501 their arguments (as described below) are stripped
502 off and saved for processing.
504 The remaining words are expanded as described in
506 .Sx Word Expansions ,
507 and the first remaining word is considered the command
508 name and the command is located.
510 words are considered the arguments of the command.
511 If no command name resulted, then the
513 variable assignments recognized in 1) affect the
516 Redirections are performed as described in
520 Redirections are used to change where a command reads its input
522 In general, redirections open, close, or
523 duplicate an existing reference to a file.
525 used for redirection is:
527 .D1 Oo Ar n Oc Ar redir-op file
531 is one of the redirection operators mentioned
533 The following gives some examples of how these
534 operators can be used.
535 Note that stdin and stdout are commonly used abbreviations
536 for standard input and standard output respectively.
537 .Bl -tag -width "1234567890XX" -offset indent
538 .It Oo Ar n Oc Ns Li > Ar file
539 redirect stdout (or file descriptor
543 .It Oo Ar n Oc Ns Li >| Ar file
544 same as above, but override the
547 .It Oo Ar n Oc Ns Li >> Ar file
548 append stdout (or file descriptor
552 .It Oo Ar n Oc Ns Li < Ar file
553 redirect stdin (or file descriptor
557 .It Oo Ar n Oc Ns Li <> Ar file
558 redirect stdin (or file descriptor
562 .It Oo Ar n1 Oc Ns Li <& Ns Ar n2
563 duplicate stdin (or file descriptor
567 .It Oo Ar n Oc Ns Li <&-
568 close stdin (or file descriptor
570 .It Oo Ar n1 Oc Ns Li >& Ns Ar n2
571 duplicate stdout (or file descriptor
575 .It Oo Ar n Oc Ns Li >&-
576 close stdout (or file descriptor
580 The following redirection is often called a
582 .Bd -unfilled -offset indent
583 .Oo Ar n Oc Ns Li << Ar delimiter
589 All the text on successive lines up to the delimiter is
590 saved away and made available to the command on standard
591 input, or file descriptor
596 as specified on the initial line is quoted, then the
598 is treated literally, otherwise the text is subjected to
599 parameter expansion, command substitution, and arithmetic
600 expansion (as described in the section on
601 .Sx Word Expansions ) .
610 .Ss Search and Execution
611 There are three types of commands: shell functions,
612 built-in commands, and normal programs.
613 The command is searched for (by name) in that order.
614 The three types of commands are all executed in a different way.
616 When a shell function is executed, all of the shell positional
619 which remains unchanged) are
620 set to the arguments of the shell function.
621 The variables which are explicitly placed in the environment of
622 the command (by placing assignments to them before the
623 function name) are made local to the function and are set
625 Then the command given in the function definition is executed.
626 The positional parameters are restored to their original values
627 when the command completes.
628 This all occurs within the current shell.
630 Shell built-in commands are executed internally to the shell, without
631 spawning a new process.
632 There are two kinds of built-in commands: regular and special.
633 Assignments before special builtins persist after they finish
634 executing and assignment errors, redirection errors and certain
635 operand errors cause a script to be aborted.
636 Both regular and special builtins can affect the shell in ways
637 normal programs cannot.
639 Otherwise, if the command name does not match a function
640 or built-in command, the command is searched for as a normal
641 program in the file system (as described in the next section).
642 When a normal program is executed, the shell runs the program,
643 passing the arguments and the environment to the program.
644 If the program is not a normal executable file
645 (i.e., if it does not begin with the
655 the shell will interpret the program in a subshell.
656 The child shell will reinitialize itself in this case,
657 so that the effect will be
658 as if a new shell had been invoked to handle the ad-hoc shell script,
659 except that the location of hashed commands located in
660 the parent shell will be remembered by the child
661 (see the description of the
663 built-in command below).
665 Note that previous versions of this document
666 and the source code itself misleadingly and sporadically
667 refer to a shell script without a magic number
669 .Dq "shell procedure" .
671 When locating a command, the shell first looks to see if
672 it has a shell function by that name.
674 built-in command by that name.
675 If a built-in command is not found,
676 one of two things happen:
679 Command names containing a slash are simply executed without
680 performing any searches.
682 The shell searches each entry in the
685 in turn for the command.
688 variable should be a series of
689 entries separated by colons.
690 Each entry consists of a
692 The current directory
693 may be indicated implicitly by an empty directory name,
694 or explicitly by a single period.
696 .Ss Command Exit Status
697 Each command has an exit status that can influence the behavior
698 of other shell commands.
699 The paradigm is that a command exits
700 with zero for normal or success, and non-zero for failure,
701 error, or a false indication.
702 The man page for each command
703 should indicate the various exit codes and what they mean.
704 Additionally, the built-in commands return exit codes, as does
705 an executed shell function.
707 If a command is terminated by a signal, its exit status is 128 plus
709 Signal numbers are defined in the header file
712 Complex commands are combinations of simple commands
713 with control operators or keywords, together creating a larger complex
715 More generally, a command is one of the following:
716 .Bl -item -offset indent
722 list or compound-list
729 Unless otherwise stated, the exit status of a command is
730 that of the last simple command executed by the command.
732 A pipeline is a sequence of one or more commands separated
733 by the control operator
735 The standard output of all but
736 the last command is connected to the standard input
738 The standard output of the last
739 command is inherited from the shell, as usual.
741 The format for a pipeline is:
743 .D1 Oo Li \&! Oc Ar command1 Op Li \&| Ar command2 ...
745 The standard output of
747 is connected to the standard input of
749 The standard input, standard output, or
750 both of a command is considered to be assigned by the
751 pipeline before any redirection specified by redirection
752 operators that are part of the command.
754 Note that unlike some other shells,
756 executes each process in a pipeline with more than one command
757 in a subshell environment and as a child of the
761 If the pipeline is not in the background (discussed later),
762 the shell waits for all commands to complete.
766 does not precede the pipeline, the
767 exit status is the exit status of the last command specified
769 Otherwise, the exit status is the logical
770 NOT of the exit status of the last command.
772 the last command returns zero, the exit status is 1; if
773 the last command returns greater than zero, the exit status
776 Because pipeline assignment of standard input or standard
777 output or both takes place before redirection, it can be
778 modified by redirection.
781 .Dl "command1 2>&1 | command2"
783 sends both the standard output and standard error of
785 to the standard input of
790 or newline terminator causes the preceding
792 (described below in the section called
793 .Sx Short-Circuit List Operators )
794 to be executed sequentially;
797 causes asynchronous execution of the preceding AND-OR-list.
798 .Ss Background Commands (&)
799 If a command is terminated by the control operator ampersand
801 the shell executes the command asynchronously;
802 the shell does not wait for the command to finish
803 before executing the next command.
805 The format for running a command in background is:
807 .D1 Ar command1 Li & Op Ar command2 Li & Ar ...
809 If the shell is not interactive, the standard input of an
810 asynchronous command is set to
812 .Ss Lists (Generally Speaking)
813 A list is a sequence of zero or more commands separated by
814 newlines, semicolons, or ampersands,
815 and optionally terminated by one of these three characters.
817 list are executed in the order they are written.
818 If command is followed by an ampersand, the shell starts the
819 command and immediately proceeds onto the next command;
820 otherwise it waits for the command to terminate before
821 proceeding to the next one.
822 .Ss Short-Circuit List Operators
826 are AND-OR list operators.
828 executes the first command, and then executes the second command
829 if the exit status of the first command is zero.
831 is similar, but executes the second command if the exit
832 status of the first command is nonzero.
836 both have the same priority.
837 .Ss Flow-Control Constructs (if, while, for, case)
841 .Bd -unfilled -offset indent -compact
845 .Ic then Ar list Oc Ar ...
853 .Bd -unfilled -offset indent -compact
859 The two lists are executed repeatedly while the exit status of the
863 command is similar, but has the word
868 repeat until the exit status of the first list is zero.
873 .Bd -unfilled -offset indent -compact
874 .Ic for Ar variable Op Ic in Ar word ...
881 and the following words are omitted,
884 The words are expanded, and then the list is executed
885 repeatedly with the variable set to each word in turn.
890 commands may be replaced with
900 .D1 Ic break Op Ar num
901 .D1 Ic continue Op Ar num
905 command terminates the
914 command continues with the next iteration of the innermost loop.
915 These are implemented as special built-in commands.
920 .Bd -unfilled -offset indent -compact
921 .Ic case Ar word Ic in
922 .Ar pattern Ns Li ) Ar list Li ;;
927 The pattern can actually be one or more patterns
936 command is the exit code of the last command executed in the list or
937 zero if no patterns were matched.
938 .Ss Grouping Commands Together
939 Commands may be grouped by writing either
941 .D1 Li \&( Ns Ar list Ns Li \%)
945 .D1 Li { Ar list Ns Li \&; }
947 The first form executes the commands in a subshell.
948 Note that built-in commands thus executed do not affect the current shell.
949 The second form does not fork another shell,
950 so it is slightly more efficient.
951 Grouping commands together this way allows the user to
952 redirect their output as though they were one program:
953 .Bd -literal -offset indent
954 { echo -n "hello"; echo " world"; } > greeting
957 The syntax of a function definition is
959 .D1 Ar name Li \&( \&) Ar command
961 A function definition is an executable statement; when
962 executed it installs a function named
974 Variables may be declared to be local to a function by
978 This should appear as the first statement of a function,
981 .D1 Ic local Oo Ar variable ... Oc Op Fl
985 command is implemented as a built-in command.
987 When a variable is made local, it inherits the initial
988 value and exported and readonly flags from the variable
989 with the same name in the surrounding scope, if there is
991 Otherwise, the variable is initially unset.
993 uses dynamic scoping, so that if the variable
995 is made local to function
997 which then calls function
999 references to the variable
1003 will refer to the variable
1007 not to the global variable named
1010 The only special parameter that can be made local is
1014 local causes any shell options that are
1017 command inside the function to be
1018 restored to their original values when the function
1025 .D1 Ic return Op Ar exitstatus
1027 It terminates the current executional scope, returning from the previous
1028 nested function, sourced script, or shell instance, in that order.
1031 command is implemented as a special built-in command.
1032 .Ss Variables and Parameters
1033 The shell maintains a set of parameters.
1035 denoted by a name is called a variable.
1037 the shell turns all the environment variables into shell
1039 New variables can be set using the form
1041 .D1 Ar name Ns = Ns Ar value
1043 Variables set by the user must have a name consisting solely
1044 of alphabetics, numerics, and underscores.
1045 The first letter of a variable name must not be numeric.
1046 A parameter can also be denoted by a number
1047 or a special character as explained below.
1048 .Ss Positional Parameters
1049 A positional parameter is a parameter denoted by a number greater than zero.
1050 The shell sets these initially to the values of its command line
1051 arguments that follow the name of the shell script.
1054 built-in command can also be used to set or reset them.
1055 .Ss Special Parameters
1056 Special parameters are parameters denoted by a single special character
1058 They are shown in the following list, exactly as they would appear in input
1059 typed by the user or in the source of a shell script.
1062 Expands to the positional parameters, starting from one.
1064 the expansion occurs within a double-quoted string
1065 it expands to a single field with the value of each parameter
1066 separated by the first character of the
1073 Expands to the positional parameters, starting from one.
1075 the expansion occurs within double-quotes, each positional
1076 parameter expands as a separate argument.
1077 If there are no positional parameters, the
1080 generates zero arguments, even when
1083 What this basically means, for example, is
1096 .Bd -literal -offset indent
1100 Expands to the number of positional parameters.
1102 Expands to the exit status of the most recent pipeline.
1104 (hyphen) Expands to the current option flags (the single-letter
1105 option names concatenated into a string) as specified on
1108 built-in command, or implicitly
1111 Expands to the process ID of the invoked shell.
1113 retains the same value of
1117 Expands to the process ID of the most recent background
1118 command executed from the current shell.
1120 pipeline, the process ID is that of the last command in the
1123 (zero) Expands to the name of the shell script if passed on the command line,
1126 operand if given (with
1128 or otherwise argument 0 passed to the shell.
1130 .Ss Special Variables
1131 The following variables are set by the shell or
1132 have special meaning to it:
1133 .Bl -tag -width ".Va HISTSIZE"
1135 The search path used with the
1139 The fallback editor used with the
1142 If not set, the default editor is
1145 The default editor used with the
1149 The number of previous commands that are accessible.
1151 The user's home directory,
1152 used in tilde expansion and as a default directory for the
1156 Input Field Separators.
1157 This is normally set to
1163 .Sx White Space Splitting
1164 section for more details.
1166 The current line number in the script or function.
1168 The name of a mail file, that will be checked for the arrival of new
1175 separated list of file names, for the shell to check for incoming
1177 This variable overrides the
1180 There is a maximum of 10 mailboxes that can be monitored at once.
1182 The default search path for executables.
1185 section for details.
1187 The parent process ID of the invoked shell.
1188 This is set at startup
1189 unless this variable is in the environment.
1190 A later change of parent process ID is not reflected.
1191 A subshell retains the same value of
1194 The primary prompt string, which defaults to
1196 unless you are the superuser, in which case it defaults to
1199 The secondary prompt string, which defaults to
1202 The prefix for the trace output (if
1209 This clause describes the various expansions that are
1211 Not all expansions are performed on
1212 every word, as explained later.
1214 Tilde expansions, parameter expansions, command substitutions,
1215 arithmetic expansions, and quote removals that occur within
1216 a single word expand to a single field.
1218 splitting or pathname expansion that can create multiple
1219 fields from a single word.
1220 The single exception to this rule is
1221 the expansion of the special parameter
1223 within double-quotes,
1224 as was described above.
1226 The order of word expansion is:
1229 Tilde Expansion, Parameter Expansion, Command Substitution,
1230 Arithmetic Expansion (these all occur at the same time).
1232 Field Splitting is performed on fields generated by step (1)
1237 Pathname Expansion (unless the
1239 option is in effect).
1246 character is used to introduce parameter expansion, command
1247 substitution, or arithmetic expansion.
1248 .Ss Tilde Expansion (substituting a user's home directory)
1249 A word beginning with an unquoted tilde character
1252 subjected to tilde expansion.
1253 All the characters up to a slash
1255 or the end of the word are treated as a username
1256 and are replaced with the user's home directory.
1258 username is missing (as in
1260 the tilde is replaced with the value of the
1262 variable (the current user's home directory).
1263 .Ss Parameter Expansion
1264 The format for parameter expansion is as follows:
1266 .D1 Li ${ Ns Ar expression Ns Li }
1270 consists of all characters until the matching
1274 escaped by a backslash or within a quoted string, and characters in
1275 embedded arithmetic expansions, command substitutions, and variable
1276 expansions, are not examined in determining the matching
1279 The simplest form for parameter expansion is:
1281 .D1 Li ${ Ns Ar parameter Ns Li }
1283 The value, if any, of
1287 The parameter name or symbol can be enclosed in braces, which are
1288 optional except for positional parameters with more than one digit or
1289 when parameter is followed by a character that could be interpreted as
1291 If a parameter expansion occurs inside double-quotes:
1294 Pathname expansion is not performed on the results of the
1297 Field splitting is not performed on the results of the
1298 expansion, with the exception of the special parameter
1302 In addition, a parameter expansion can be modified by using one of the
1304 .Bl -tag -width indent
1305 .It Li ${ Ns Ar parameter Ns Li :- Ns Ar word Ns Li }
1309 is unset or null, the expansion of
1311 is substituted; otherwise, the value of
1314 .It Li ${ Ns Ar parameter Ns Li := Ns Ar word Ns Li }
1315 Assign Default Values.
1318 is unset or null, the expansion of
1328 does not prevent field splitting or pathname expansion.
1329 Only variables, not positional
1330 parameters or special parameters, can be
1331 assigned in this way.
1332 .It Li ${ Ns Ar parameter Ns Li :? Ns Oo Ar word Oc Ns Li }
1333 Indicate Error if Null or Unset.
1336 is unset or null, the expansion of
1338 (or a message indicating it is unset if
1340 is omitted) is written to standard
1341 error and the shell exits with a nonzero
1343 Otherwise, the value of
1347 interactive shell need not exit.
1348 .It Li ${ Ns Ar parameter Ns Li :+ Ns Ar word Ns Li }
1349 Use Alternate Value.
1352 is unset or null, null is substituted;
1353 otherwise, the expansion of
1358 In the parameter expansions shown previously, use of the colon in the
1359 format results in a test for a parameter that is unset or null; omission
1360 of the colon results in a test for a parameter that is only unset.
1361 .Bl -tag -width indent
1362 .It Li ${# Ns Ar parameter Ns Li }
1364 The length in characters of
1369 The following four varieties of parameter expansion provide for substring
1371 In each case, pattern matching notation
1373 .Sx Shell Patterns ) ,
1374 rather than regular expression notation,
1375 is used to evaluate the patterns.
1376 If parameter is one of the special parameters
1380 the result of the expansion is unspecified.
1381 Enclosing the full parameter expansion string in double-quotes does not
1382 cause the following four varieties of pattern characters to be quoted,
1383 whereas quoting characters within the braces has this effect.
1384 .Bl -tag -width indent
1385 .It Li ${ Ns Ar parameter Ns Li % Ns Ar word Ns Li }
1386 Remove Smallest Suffix Pattern.
1389 is expanded to produce a pattern.
1391 parameter expansion then results in
1393 with the smallest portion of the
1394 suffix matched by the pattern deleted.
1395 .It Li ${ Ns Ar parameter Ns Li %% Ns Ar word Ns Li }
1396 Remove Largest Suffix Pattern.
1399 is expanded to produce a pattern.
1401 parameter expansion then results in
1403 with the largest portion of the
1404 suffix matched by the pattern deleted.
1405 .It Li ${ Ns Ar parameter Ns Li # Ns Ar word Ns Li }
1406 Remove Smallest Prefix Pattern.
1409 is expanded to produce a pattern.
1411 parameter expansion then results in
1413 with the smallest portion of the
1414 prefix matched by the pattern deleted.
1415 .It Li ${ Ns Ar parameter Ns Li ## Ns Ar word Ns Li }
1416 Remove Largest Prefix Pattern.
1419 is expanded to produce a pattern.
1421 parameter expansion then results in
1423 with the largest portion of the
1424 prefix matched by the pattern deleted.
1426 .Ss Command Substitution
1427 Command substitution allows the output of a command to be substituted in
1428 place of the command name itself.
1429 Command substitution occurs when
1430 the command is enclosed as follows:
1432 .D1 Li $( Ns Ar command Ns Li )\&
1434 or the backquoted version:
1436 .D1 Li ` Ns Ar command Ns Li `
1438 The shell expands the command substitution by executing command in a
1439 subshell environment and replacing the command substitution
1440 with the standard output of the command,
1441 removing sequences of one or more newlines at the end of the substitution.
1442 Embedded newlines before the end of the output are not removed;
1443 however, during field splitting, they may be translated into spaces
1444 depending on the value of
1446 and the quoting that is in effect.
1447 .Ss Arithmetic Expansion
1448 Arithmetic expansion provides a mechanism for evaluating an arithmetic
1449 expression and substituting its value.
1450 The format for arithmetic expansion is as follows:
1452 .D1 Li $(( Ns Ar expression Ns Li ))
1456 is treated as if it were in double-quotes, except
1457 that a double-quote inside the expression is not treated specially.
1459 shell expands all tokens in the
1461 for parameter expansion,
1462 command substitution,
1463 arithmetic expansion
1466 The allowed expressions are a subset of C expressions,
1468 .Bl -tag -width "Variables" -offset indent
1470 All values are of type
1473 Decimal, octal (starting with
1475 and hexadecimal (starting with
1479 Shell variables can be read and written
1480 and contain integer constants.
1483 .It Binary operators
1484 .Li "* / % + - << >> < <= > >= == != & ^ | && ||"
1485 .It Assignment operators
1486 .Li "= += -= *= /= %= <<= >>= &= ^= |="
1487 .It Short-circuit evaluation
1492 operators always evaluate both sides.
1496 The result of the expression is substituted in decimal.
1497 .Ss White Space Splitting (Field Splitting)
1498 After parameter expansion, command substitution, and
1499 arithmetic expansion the shell scans the results of
1500 expansions and substitutions that did not occur in double-quotes for
1501 field splitting and multiple fields can result.
1503 The shell treats each character of the
1505 variable as a delimiter and uses
1506 the delimiters to split the results of parameter expansion and command
1507 substitution into fields.
1508 .Ss Pathname Expansion (File Name Generation)
1512 file name generation is performed
1513 after word splitting is complete.
1515 viewed as a series of patterns, separated by slashes.
1517 process of expansion replaces the word with the names of
1518 all existing files whose names can be formed by replacing
1519 each pattern with a string that matches the specified pattern.
1520 There are two restrictions on this: first, a pattern cannot match
1521 a string containing a slash, and second,
1522 a pattern cannot match a string starting with a period
1523 unless the first character of the pattern is a period.
1524 The next section describes the patterns used for both
1525 Pathname Expansion and the
1529 A pattern consists of normal characters, which match themselves,
1530 and meta-characters.
1531 The meta-characters are
1537 These characters lose their special meanings if they are quoted.
1538 When command or variable substitution is performed and the dollar sign
1539 or back quotes are not double-quoted, the value of the
1540 variable or the output of the command is scanned for these
1541 characters and they are turned into meta-characters.
1545 matches any string of characters.
1548 matches any single character.
1551 introduces a character class.
1552 The end of the character class is indicated by a
1560 rather than introducing a character class.
1561 A character class matches any of the characters between the square brackets.
1562 A range of characters may be specified using a minus sign.
1563 The character class may be complemented by making an exclamation point
1565 the first character of the character class.
1569 in a character class, make it the first character listed
1575 make it the first or last character listed.
1576 .Ss Built-in Commands
1577 This section lists the built-in commands.
1578 .Bl -tag -width indent
1580 A null command that returns a 0 (true) exit value.
1582 The commands in the specified file are read and executed by the shell.
1585 command may be used to return to the
1592 characters, it is used as is.
1593 Otherwise, the shell searches the
1596 If it is not found in the
1598 it is sought in the current working directory.
1600 A built-in equivalent of
1602 .It Ic alias Oo Ar name Ns Oo = Ns Ar string Oc ... Oc
1604 .Ar name Ns = Ns Ar string
1605 is specified, the shell defines the alias
1611 is specified, the value of the alias
1614 With no arguments, the
1616 built-in command prints the names and values of all defined aliases
1619 Alias values are written with appropriate quoting so that they are
1620 suitable for re-input to the shell.
1624 .It Ic bg Op Ar job ...
1625 Continue the specified jobs
1626 (or the current job if no jobs are given)
1628 .It Ic bind Oo Fl aeklrsv Oc Oo Ar key Oo Ar command Oc Oc
1629 List or alter key bindings for the line editor.
1630 This command is documented in
1632 .It Ic break Op Ar num
1634 .Sx Flow-Control Constructs
1636 .It Ic builtin Ar cmd Op Ar arg ...
1637 Execute the specified built-in command,
1639 This is useful when the user wishes to override a shell function
1640 with the same name as a built-in command.
1641 .It Ic cd Oo Fl L | P Oc Op Ar directory
1642 Switch to the specified
1644 or to the directory specified in the
1646 environment variable if no
1655 then the directories listed in the
1658 searched for the specified
1662 is unset, the current directory is searched.
1665 is the same as that of
1667 In an interactive shell,
1670 command will print out the name of the directory
1671 that it actually switched to
1672 if this is different from the name that the user gave.
1673 These may be different either because the
1675 mechanism was used or because a symbolic link was crossed.
1679 option is specified,
1681 is handled physically and symbolic links are resolved before
1683 components are processed.
1686 option is specified,
1688 is handled logically.
1689 This is the default.
1694 .It Ic command Oo Fl p Oc Op Ar utility Op Ar argument ...
1695 .It Ic command Oo Fl v | V Oc Op Ar utility
1696 The first form of invocation executes the specified
1698 as a simple command (see the
1704 option is specified, the command search is performed using a
1707 that is guaranteed to find all of the standard utilities.
1711 option is specified,
1713 is not executed but a description of its interpretation by the shell is
1715 For ordinary commands the output is the path name; for shell built-in
1716 commands, shell functions and keywords only the name is written.
1717 Aliases are printed as
1718 .Dq Ic alias Ar name Ns = Ns Ar value .
1722 option is identical to
1724 except for the output.
1726 .Dq Ar utility Ic is Ar description
1732 a special shell builtin,
1739 .It Ic continue Op Ar num
1741 .Sx Flow-Control Constructs
1743 .It Ic echo Oo Fl e | n Oc Op Ar string ...
1744 Print a space-separated list of the arguments to the standard output
1745 and append a newline character.
1746 .Bl -tag -width indent
1748 Suppress the output of the trailing newline.
1750 Process C-style backslash escape sequences.
1753 command understands the following character escapes:
1754 .Bl -tag -width indent
1756 Alert (ring the terminal bell)
1760 Suppress the trailing newline (this has the side-effect of truncating the
1761 line if it is not the last character)
1779 (Zero) The character whose octal value is
1785 is not enclosed in quotes then the backslash itself must be escaped
1786 with a backslash to protect it from the shell.
1788 .Bd -literal -offset indent
1797 $ echo -e a\e\e\e\eb
1806 options may be specified.
1807 .It Ic eval Ar string ...
1808 Concatenate all the arguments with spaces.
1809 Then re-parse and execute the command.
1810 .It Ic exec Op Ar command Op arg ...
1814 the shell process is replaced with the specified program
1815 (which must be a real program, not a shell built-in command or function).
1816 Any redirections on the
1818 command are marked as permanent,
1819 so that they are not undone when the
1822 .It Ic exit Op Ar exitstatus
1823 Terminate the shell process.
1827 it is used as the exit status of the shell;
1828 otherwise the exit status of the preceding command is used.
1829 The exit status should be an integer between 0 and 255.
1830 .It Ic export Ar name ...
1831 .It Ic export Op Fl p
1832 The specified names are exported so that they will
1833 appear in the environment of subsequent commands.
1834 The only way to un-export a variable is to
1837 The shell allows the value of a variable to be set
1838 at the same time as it is exported by writing
1840 .D1 Ic export Ar name Ns = Ns Ar value
1842 With no arguments the
1844 command lists the names
1845 of all exported variables.
1848 option is specified, the exported variables are printed as
1849 .Dq Ic export Ar name Ns = Ns Ar value
1850 lines, suitable for re-input to the shell.
1852 A null command that returns a non-zero (false) exit value.
1853 .It Ic fc Oo Fl e Ar editor Oc Op Ar first Op Ar last
1854 .It Ic fc Fl l Oo Fl nr Oc Op Ar first Op Ar last
1855 .It Ic fc Fl s Oo Ar old Ns = Ns Ar new Oc Op Ar first
1858 built-in command lists, or edits and re-executes,
1859 commands previously entered to an interactive shell.
1860 .Bl -tag -width indent
1862 Use the editor named by
1864 to edit the commands.
1867 string is a command name,
1868 subject to search via the
1873 variable is used as a default when
1878 is null or unset, the value of the
1885 is used as the editor.
1887 List the commands rather than invoking
1889 The commands are written in the
1890 sequence indicated by the
1894 operands, as affected by
1896 with each command preceded by the command number.
1898 Suppress command numbers when listing with
1901 Reverse the order of the commands listed
1910 Re-execute the command without invoking an editor.
1913 Select the commands to list or edit.
1914 The number of previous commands that can be accessed
1915 are determined by the value of the
1922 or both are one of the following:
1923 .Bl -tag -width indent
1924 .It Oo Cm + Oc Ns Ar num
1925 A positive number representing a command number;
1926 command numbers can be displayed with the
1930 A negative decimal number representing the
1931 command that was executed
1934 commands previously.
1935 For example, \-1 is the immediately previous command.
1937 A string indicating the most recently entered command
1938 that begins with that string.
1940 .Ar old Ns = Ns Ar new
1941 operand is not also specified with
1943 the string form of the first operand cannot contain an embedded equal sign.
1947 The following variables affect the execution of
1949 .Bl -tag -width ".Va HISTSIZE"
1951 Name of the editor to use for history editing.
1953 The number of previous commands that are accessible.
1958 or the current job to the foreground.
1959 .It Ic getopts Ar optstring var
1966 command deprecates the older
1969 The first argument should be a series of letters, each possibly
1970 followed by a colon which indicates that the option takes an argument.
1971 The specified variable is set to the parsed option.
1973 the next argument is placed into the shell variable
1975 If an option takes an argument, it is placed into the shell variable
1977 If an invalid option is encountered,
1981 It returns a false value (1) when it encounters the end of the options.
1982 .It Ic hash Oo Fl rv Oc Op Ar command ...
1983 The shell maintains a hash table which remembers the locations of commands.
1984 With no arguments whatsoever, the
1986 command prints out the contents of this table.
1987 Entries which have not been looked at since the last
1989 command are marked with an asterisk;
1990 it is possible for these entries to be invalid.
1994 command removes each specified
1996 from the hash table (unless they are functions) and then locates it.
2001 prints the locations of the commands as it finds them.
2006 command to delete all the entries in the hash table except for functions.
2007 .It Ic jobid Op Ar job
2008 Print the process IDs of the processes in the specified
2012 argument is omitted, use the current job.
2013 .It Ic jobs Oo Fl lps Oc Op Ar job ...
2014 Print information about the specified jobs, or all jobs if no
2017 The information printed includes job ID, status and command name.
2021 option is specified, the PID of each job is also printed.
2024 option is specified, only the process IDs for the process group leaders
2025 are printed, one per line.
2028 option is specified, only the PIDs of the job commands are printed, one per
2030 .It Ic local Oo Ar variable ... Oc Op Fl
2034 .It Ic pwd Op Fl L | P
2035 Print the path of the current directory.
2036 The built-in command may
2037 differ from the program of the same name because the
2038 built-in command remembers what the current directory
2039 is rather than recomputing it each time.
2042 However, if the current directory is
2044 the built-in version of
2046 will continue to print the old name for the directory.
2050 option is specified, symbolic links are resolved.
2053 option is specified, the shell's notion of the current directory
2054 is printed (symbolic links are not resolved).
2055 This is the default.
2056 .It Ic read Oo Fl p Ar prompt Oc Oo
2057 .Fl t Ar timeout Oc Oo Fl er Oc Ar variable ...
2063 and the standard input is a terminal.
2065 read from the standard input.
2066 The trailing newline
2067 is deleted from the line and the line is split as
2068 described in the section on
2069 .Sx White Space Splitting (Field Splitting)
2071 the pieces are assigned to the variables in order.
2072 If there are more pieces than variables, the remaining
2073 pieces (along with the characters in
2075 that separated them)
2076 are assigned to the last variable.
2077 If there are more variables than pieces, the remaining
2078 variables are assigned the null string.
2080 Backslashes are treated specially, unless the
2084 If a backslash is followed by
2085 a newline, the backslash and the newline will be
2087 If a backslash is followed by any other
2088 character, the backslash will be deleted and the following
2089 character will be treated as though it were not in
2095 option is specified and the
2097 elapses before a complete line of input is supplied,
2100 command will return an exit status of 1 without assigning any values.
2103 value may optionally be followed by one of
2108 to explicitly specify seconds, minutes or hours.
2109 If none is supplied,
2115 option exists only for backward compatibility with older scripts.
2116 .It Ic readonly Oo Fl p Oc Op Ar name ...
2119 is marked as read only,
2120 so that it cannot be subsequently modified or unset.
2121 The shell allows the value of a variable to be set
2122 at the same time as it is marked read only
2123 by using the following form:
2125 .D1 Ic readonly Ar name Ns = Ns Ar value
2127 With no arguments the
2129 command lists the names of all read only variables.
2132 option is specified, the read-only variables are printed as
2133 .Dq Ic readonly Ar name Ns = Ns Ar value
2134 lines, suitable for re-input to the shell.
2135 .It Ic return Op Ar exitstatus
2139 .It Ic set Oo Fl /+abCEefIimnpTuVvx Oc Oo Fl /+o Ar longname Oc Oo
2140 .Fl c Ar string Oc Op Fl - Ar arg ...
2143 command performs three different functions:
2146 With no arguments, it lists the values of all shell variables.
2148 If options are given,
2149 either in short form or using the long
2150 .Dq Fl /+o Ar longname
2152 it sets or clears the specified options as described in the section called
2153 .Sx Argument List Processing .
2157 option is specified,
2159 will replace the shell's positional parameters with the subsequent
2161 If no arguments follow the
2164 all the positional parameters will be cleared,
2165 which is equivalent to executing the command
2169 flag may be omitted when specifying arguments to be used
2170 as positional replacement parameters.
2171 This is not recommended,
2172 because the first argument may begin with a dash
2178 command will interpret as a request to enable or disable options.
2180 .It Ic setvar Ar variable value
2181 Assigns the specified
2187 command is intended to be used in functions that
2188 assign values to variables whose names are passed as parameters.
2189 In general it is better to write
2190 .Dq Ar variable Ns = Ns Ar value
2193 .It Ic shift Op Ar n
2194 Shift the positional parameters
2199 A shift sets the value of
2208 decreasing the value of
2211 If there are zero positional parameters, shifting does not do anything.
2213 A built-in equivalent of
2216 Print the amount of time spent executing the shell and its children.
2217 The first output line shows the user and system times for the shell
2218 itself, the second one contains the user and system times for the
2220 .It Ic trap Oo Ar action Oc Ar signal ...
2222 Cause the shell to parse and execute
2227 The signals are specified by name or number.
2228 In addition, the pseudo-signal
2230 may be used to specify an
2232 that is performed when the shell terminates.
2235 may be an empty string or a dash
2237 the former causes the specified signal to be ignored
2238 and the latter causes the default action to be taken.
2241 is another way to request the default action, for compatibility reasons this
2242 usage is not recommended though.
2243 When the shell forks off a subshell,
2244 it resets trapped (but not ignored) signals to the default action.
2247 command has no effect on signals that were ignored on entry to the shell.
2253 command to display a list of valid signal names.
2255 A null command that returns a 0 (true) exit value.
2256 .It Ic type Op Ar name ...
2259 as a command and print the resolution of the command search.
2260 Possible resolutions are:
2261 shell keyword, alias, special shell builtin, shell builtin, command,
2264 For aliases the alias expansion is printed;
2265 for commands and tracked aliases
2266 the complete pathname of the command is printed.
2267 .It Ic ulimit Oo Fl HSabcdflmnpstuvw Oc Op Ar limit
2268 Set or display resource limits (see
2272 is specified, the named resource will be set;
2273 otherwise the current resource value will be displayed.
2277 is specified, the hard limits will be set or displayed.
2278 While everybody is allowed to reduce a hard limit,
2279 only the superuser can increase it.
2283 specifies the soft limits instead.
2284 When displaying limits,
2290 The default is to display the soft limits,
2291 and to set both the hard and the soft limits.
2297 command to display all resources.
2300 is not acceptable in this mode.
2302 The remaining options specify which resource value is to be
2303 displayed or modified.
2304 They are mutually exclusive.
2305 .Bl -tag -width indent
2307 The maximum size of socket buffer usage, in bytes.
2308 .It Fl c Ar coredumpsize
2309 The maximal size of core dump files, in 512-byte blocks.
2310 .It Fl d Ar datasize
2311 The maximal size of the data segment of a process, in kilobytes.
2312 .It Fl f Ar filesize
2313 The maximal size of a file, in 512-byte blocks.
2314 .It Fl l Ar lockedmem
2315 The maximal size of memory that can be locked by a process, in
2317 .It Fl m Ar memoryuse
2318 The maximal resident set size of a process, in kilobytes.
2320 The maximal number of descriptors that could be opened by a process.
2321 .It Fl p Ar pseudoterminals
2322 The maximal number of pseudo-terminals for this user ID.
2323 .It Fl s Ar stacksize
2324 The maximal size of the stack segment, in kilobytes.
2326 The maximal amount of CPU time to be used by each process, in seconds.
2327 .It Fl u Ar userproc
2328 The maximal number of simultaneous processes for this user ID.
2329 .It Fl v Ar virtualmem
2330 The maximal virtual size of a process, in kilobytes.
2332 The maximum amount of swap space reserved or used for this user ID,
2335 .It Ic umask Oo Fl S Oc Op Ar mask
2336 Set the file creation mask (see
2338 to the octal or symbolic (see
2342 If the argument is omitted, the current mask value is printed.
2345 option is specified, the output is symbolic, otherwise the output is octal.
2346 .It Ic unalias Oo Fl a Oc Op Ar name ...
2347 The specified alias names are removed.
2350 is specified, all aliases are removed.
2351 .It Ic unset Oo Fl fv Oc Ar name ...
2352 The specified variables or functions are unset and unexported.
2355 option is specified or no options are given, the
2357 arguments are treated as variable names.
2360 option is specified, the
2362 arguments are treated as function names.
2363 .It Ic wait Op Ar job
2364 Wait for the specified
2366 to complete and return the exit status of the last process in the
2368 If the argument is omitted, wait for all jobs to complete
2369 and return an exit status of zero.
2371 .Ss Commandline Editing
2374 is being used interactively from a terminal, the current command
2375 and the command history
2379 .Sx Built-in Commands )
2382 command line editing.
2383 This mode uses commands similar
2384 to a subset of those described in the
2402 can be switched between insert mode and command mode by typing
2406 while in command mode will pass the line to the shell.
2409 .Dq Li "set -o emacs"
2412 command can be used to enable a subset of
2414 command line editing features.
2416 The following environment variables affect the execution of
2418 .Bl -tag -width ".Ev LANGXXXXXX"
2420 Initialization file for interactive shells.
2421 .It Ev LANG , Ev LC_*
2423 These are inherited by children of the shell,
2424 and is used in a limited manner by the shell itself.
2426 An absolute pathname for the current directory,
2427 possibly containing symbolic links.
2428 This is used and updated by the shell.
2430 The default terminal setting for the shell.
2431 This is inherited by children of the shell, and is used in the history
2435 Additionally, all environment variables are turned into shell variables
2437 which may affect the shell as described under
2438 .Sx Special Variables .
2440 Errors that are detected by the shell, such as a syntax error, will
2441 cause the shell to exit with a non-zero exit status.
2442 If the shell is not an interactive shell, the execution of the shell
2443 file will be aborted.
2444 Otherwise the shell will return the exit status of the last command
2447 builtin is used with a numeric argument, it
2448 will return the argument.
2465 command, the Thompson shell, appeared in
2467 It was superseded in
2469 by the Bourne shell, which inherited the name
2474 was rewritten in 1989 under the
2476 license after the Bourne shell from
2481 was originally written by
2482 .An Kenneth Almquist .
2486 utility does not recognize multibyte characters.
projects / FreeBSD / stable / 8.git / blob