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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.
1049 Assignments are expanded differently from other words:
1050 tilde expansion is also performed after the equals sign and after any colon
1051 and usernames are also terminated by colons,
1052 and field splitting and pathname expansion are not performed.
1053 .Ss Positional Parameters
1054 A positional parameter is a parameter denoted by a number greater than zero.
1055 The shell sets these initially to the values of its command line
1056 arguments that follow the name of the shell script.
1059 built-in command can also be used to set or reset them.
1060 .Ss Special Parameters
1061 Special parameters are parameters denoted by a single special character
1063 They are shown in the following list, exactly as they would appear in input
1064 typed by the user or in the source of a shell script.
1067 Expands to the positional parameters, starting from one.
1069 the expansion occurs within a double-quoted string
1070 it expands to a single field with the value of each parameter
1071 separated by the first character of the
1078 Expands to the positional parameters, starting from one.
1080 the expansion occurs within double-quotes, each positional
1081 parameter expands as a separate argument.
1082 If there are no positional parameters, the
1085 generates zero arguments, even when
1088 What this basically means, for example, is
1101 .Bd -literal -offset indent
1105 Expands to the number of positional parameters.
1107 Expands to the exit status of the most recent pipeline.
1109 (hyphen) Expands to the current option flags (the single-letter
1110 option names concatenated into a string) as specified on
1113 built-in command, or implicitly
1116 Expands to the process ID of the invoked shell.
1118 retains the same value of
1122 Expands to the process ID of the most recent background
1123 command executed from the current shell.
1125 pipeline, the process ID is that of the last command in the
1128 (zero) Expands to the name of the shell script if passed on the command line,
1131 operand if given (with
1133 or otherwise argument 0 passed to the shell.
1135 .Ss Special Variables
1136 The following variables are set by the shell or
1137 have special meaning to it:
1138 .Bl -tag -width ".Va HISTSIZE"
1140 The search path used with the
1144 The fallback editor used with the
1147 If not set, the default editor is
1150 The default editor used with the
1154 The number of previous commands that are accessible.
1156 The user's home directory,
1157 used in tilde expansion and as a default directory for the
1161 Input Field Separators.
1162 The default value is
1168 This default also applies if
1170 is unset, but not if it is set to the empty string.
1172 .Sx White Space Splitting
1173 section for more details.
1175 The current line number in the script or function.
1177 The name of a mail file, that will be checked for the arrival of new
1184 separated list of file names, for the shell to check for incoming
1186 This variable overrides the
1189 There is a maximum of 10 mailboxes that can be monitored at once.
1191 The default search path for executables.
1194 section for details.
1196 The parent process ID of the invoked shell.
1197 This is set at startup
1198 unless this variable is in the environment.
1199 A later change of parent process ID is not reflected.
1200 A subshell retains the same value of
1203 The primary prompt string, which defaults to
1205 unless you are the superuser, in which case it defaults to
1208 The secondary prompt string, which defaults to
1211 The prefix for the trace output (if
1218 This clause describes the various expansions that are
1220 Not all expansions are performed on
1221 every word, as explained later.
1223 Tilde expansions, parameter expansions, command substitutions,
1224 arithmetic expansions, and quote removals that occur within
1225 a single word expand to a single field.
1227 splitting or pathname expansion that can create multiple
1228 fields from a single word.
1229 The single exception to this rule is
1230 the expansion of the special parameter
1232 within double-quotes,
1233 as was described above.
1235 The order of word expansion is:
1238 Tilde Expansion, Parameter Expansion, Command Substitution,
1239 Arithmetic Expansion (these all occur at the same time).
1241 Field Splitting is performed on fields generated by step (1)
1246 Pathname Expansion (unless the
1248 option is in effect).
1255 character is used to introduce parameter expansion, command
1256 substitution, or arithmetic expansion.
1257 .Ss Tilde Expansion (substituting a user's home directory)
1258 A word beginning with an unquoted tilde character
1261 subjected to tilde expansion.
1262 All the characters up to a slash
1264 or the end of the word are treated as a username
1265 and are replaced with the user's home directory.
1267 username is missing (as in
1269 the tilde is replaced with the value of the
1271 variable (the current user's home directory).
1272 .Ss Parameter Expansion
1273 The format for parameter expansion is as follows:
1275 .D1 Li ${ Ns Ar expression Ns Li }
1279 consists of all characters until the matching
1283 escaped by a backslash or within a quoted string, and characters in
1284 embedded arithmetic expansions, command substitutions, and variable
1285 expansions, are not examined in determining the matching
1288 The simplest form for parameter expansion is:
1290 .D1 Li ${ Ns Ar parameter Ns Li }
1292 The value, if any, of
1296 The parameter name or symbol can be enclosed in braces, which are
1297 optional except for positional parameters with more than one digit or
1298 when parameter is followed by a character that could be interpreted as
1300 If a parameter expansion occurs inside double-quotes:
1303 Field splitting is not performed on the results of the
1304 expansion, with the exception of the special parameter
1307 Pathname expansion is not performed on the results of the
1311 In addition, a parameter expansion can be modified by using one of the
1313 .Bl -tag -width indent
1314 .It Li ${ Ns Ar parameter Ns Li :- Ns Ar word Ns Li }
1318 is unset or null, the expansion of
1320 is substituted; otherwise, the value of
1323 .It Li ${ Ns Ar parameter Ns Li := Ns Ar word Ns Li }
1324 Assign Default Values.
1327 is unset or null, the expansion of
1337 does not prevent field splitting or pathname expansion.
1338 Only variables, not positional
1339 parameters or special parameters, can be
1340 assigned in this way.
1341 .It Li ${ Ns Ar parameter Ns Li :? Ns Oo Ar word Oc Ns Li }
1342 Indicate Error if Null or Unset.
1345 is unset or null, the expansion of
1347 (or a message indicating it is unset if
1349 is omitted) is written to standard
1350 error and the shell exits with a nonzero
1352 Otherwise, the value of
1356 interactive shell need not exit.
1357 .It Li ${ Ns Ar parameter Ns Li :+ Ns Ar word Ns Li }
1358 Use Alternate Value.
1361 is unset or null, null is substituted;
1362 otherwise, the expansion of
1367 In the parameter expansions shown previously, use of the colon in the
1368 format results in a test for a parameter that is unset or null; omission
1369 of the colon results in a test for a parameter that is only unset.
1370 .Bl -tag -width indent
1371 .It Li ${# Ns Ar parameter Ns Li }
1373 The length in characters of
1378 The following four varieties of parameter expansion provide for substring
1380 In each case, pattern matching notation
1382 .Sx Shell Patterns ) ,
1383 rather than regular expression notation,
1384 is used to evaluate the patterns.
1385 If parameter is one of the special parameters
1389 the result of the expansion is unspecified.
1390 Enclosing the full parameter expansion string in double-quotes does not
1391 cause the following four varieties of pattern characters to be quoted,
1392 whereas quoting characters within the braces has this effect.
1393 .Bl -tag -width indent
1394 .It Li ${ Ns Ar parameter Ns Li % Ns Ar word Ns Li }
1395 Remove Smallest Suffix Pattern.
1398 is expanded to produce a pattern.
1400 parameter expansion then results in
1402 with the smallest portion of the
1403 suffix matched by the pattern deleted.
1404 .It Li ${ Ns Ar parameter Ns Li %% Ns Ar word Ns Li }
1405 Remove Largest Suffix Pattern.
1408 is expanded to produce a pattern.
1410 parameter expansion then results in
1412 with the largest portion of the
1413 suffix matched by the pattern deleted.
1414 .It Li ${ Ns Ar parameter Ns Li # Ns Ar word Ns Li }
1415 Remove Smallest Prefix Pattern.
1418 is expanded to produce a pattern.
1420 parameter expansion then results in
1422 with the smallest portion of the
1423 prefix matched by the pattern deleted.
1424 .It Li ${ Ns Ar parameter Ns Li ## Ns Ar word Ns Li }
1425 Remove Largest Prefix Pattern.
1428 is expanded to produce a pattern.
1430 parameter expansion then results in
1432 with the largest portion of the
1433 prefix matched by the pattern deleted.
1435 .Ss Command Substitution
1436 Command substitution allows the output of a command to be substituted in
1437 place of the command name itself.
1438 Command substitution occurs when
1439 the command is enclosed as follows:
1441 .D1 Li $( Ns Ar command Ns Li )\&
1443 or the backquoted version:
1445 .D1 Li ` Ns Ar command Ns Li `
1447 The shell expands the command substitution by executing command in a
1448 subshell environment and replacing the command substitution
1449 with the standard output of the command,
1450 removing sequences of one or more newlines at the end of the substitution.
1451 Embedded newlines before the end of the output are not removed;
1452 however, during field splitting, they may be translated into spaces
1453 depending on the value of
1455 and the quoting that is in effect.
1456 .Ss Arithmetic Expansion
1457 Arithmetic expansion provides a mechanism for evaluating an arithmetic
1458 expression and substituting its value.
1459 The format for arithmetic expansion is as follows:
1461 .D1 Li $(( Ns Ar expression Ns Li ))
1465 is treated as if it were in double-quotes, except
1466 that a double-quote inside the expression is not treated specially.
1468 shell expands all tokens in the
1470 for parameter expansion,
1471 command substitution,
1472 arithmetic expansion
1475 The allowed expressions are a subset of C expressions,
1477 .Bl -tag -width "Variables" -offset indent
1479 All values are of type
1482 Decimal, octal (starting with
1484 and hexadecimal (starting with
1488 Shell variables can be read and written
1489 and contain integer constants.
1492 .It Binary operators
1493 .Li "* / % + - << >> < <= > >= == != & ^ | && ||"
1494 .It Assignment operators
1495 .Li "= += -= *= /= %= <<= >>= &= ^= |="
1496 .It Short-circuit evaluation
1501 operators always evaluate both sides.
1505 The result of the expression is substituted in decimal.
1506 .Ss White Space Splitting (Field Splitting)
1507 In certain contexts,
1508 after parameter expansion, command substitution, and
1509 arithmetic expansion the shell scans the results of
1510 expansions and substitutions that did not occur in double-quotes for
1511 field splitting and multiple fields can result.
1522 are treated differently from other characters in
1527 at the beginning or end of a word is discarded.
1529 Subsequently, a field is delimited by either
1532 a non-whitespace character in
1534 with any whitespace in
1538 one or more whitespace characters in
1542 If a word ends with a non-whitespace character in
1544 there is no empty field after this character.
1546 If no field is delimited, the word is discarded.
1547 In particular, if a word consists solely of an unquoted substitution
1548 and the result of the substitution is null,
1549 it is removed by field splitting even if
1552 .Ss Pathname Expansion (File Name Generation)
1556 file name generation is performed
1557 after word splitting is complete.
1559 viewed as a series of patterns, separated by slashes.
1561 process of expansion replaces the word with the names of
1562 all existing files whose names can be formed by replacing
1563 each pattern with a string that matches the specified pattern.
1564 There are two restrictions on this: first, a pattern cannot match
1565 a string containing a slash, and second,
1566 a pattern cannot match a string starting with a period
1567 unless the first character of the pattern is a period.
1568 The next section describes the patterns used for
1570 the four varieties of parameter expansion for substring processing and the
1574 A pattern consists of normal characters, which match themselves,
1575 and meta-characters.
1576 The meta-characters are
1581 These characters lose their special meanings if they are quoted.
1582 When command or variable substitution is performed and the dollar sign
1583 or back quotes are not double-quoted, the value of the
1584 variable or the output of the command is scanned for these
1585 characters and they are turned into meta-characters.
1589 matches any string of characters.
1592 matches any single character.
1595 introduces a character class.
1596 The end of the character class is indicated by a
1604 rather than introducing a character class.
1605 A character class matches any of the characters between the square brackets.
1606 A locale-dependent range of characters may be specified using a minus sign.
1607 The character class may be complemented by making an exclamation point
1609 the first character of the character class.
1612 has the same effect but is non-standard.
1616 in a character class, make it the first character listed
1624 make it the first or last character listed.
1625 .Ss Built-in Commands
1626 This section lists the built-in commands.
1627 .Bl -tag -width indent
1629 A null command that returns a 0 (true) exit value.
1631 The commands in the specified file are read and executed by the shell.
1634 command may be used to return to the
1641 characters, it is used as is.
1642 Otherwise, the shell searches the
1645 If it is not found in the
1647 it is sought in the current working directory.
1649 A built-in equivalent of
1651 .It Ic alias Oo Ar name Ns Oo = Ns Ar string Oc ... Oc
1653 .Ar name Ns = Ns Ar string
1654 is specified, the shell defines the alias
1660 is specified, the value of the alias
1663 With no arguments, the
1665 built-in command prints the names and values of all defined aliases
1668 Alias values are written with appropriate quoting so that they are
1669 suitable for re-input to the shell.
1673 .It Ic bg Op Ar job ...
1674 Continue the specified jobs
1675 (or the current job if no jobs are given)
1677 .It Ic bind Oo Fl aeklrsv Oc Oo Ar key Oo Ar command Oc Oc
1678 List or alter key bindings for the line editor.
1679 This command is documented in
1681 .It Ic break Op Ar num
1683 .Sx Flow-Control Constructs
1685 .It Ic builtin Ar cmd Op Ar arg ...
1686 Execute the specified built-in command,
1688 This is useful when the user wishes to override a shell function
1689 with the same name as a built-in command.
1690 .It Ic cd Oo Fl L | P Oc Op Ar directory
1691 Switch to the specified
1693 or to the directory specified in the
1695 environment variable if no
1704 then the directories listed in the
1707 searched for the specified
1711 is unset, the current directory is searched.
1714 is the same as that of
1716 In an interactive shell,
1719 command will print out the name of the directory
1720 that it actually switched to
1721 if this is different from the name that the user gave.
1722 These may be different either because the
1724 mechanism was used or because a symbolic link was crossed.
1728 option is specified,
1730 is handled physically and symbolic links are resolved before
1732 components are processed.
1735 option is specified,
1737 is handled logically.
1738 This is the default.
1743 .It Ic command Oo Fl p Oc Op Ar utility Op Ar argument ...
1744 .It Ic command Oo Fl v | V Oc Op Ar utility
1745 The first form of invocation executes the specified
1747 as a simple command (see the
1753 option is specified, the command search is performed using a
1756 that is guaranteed to find all of the standard utilities.
1760 option is specified,
1762 is not executed but a description of its interpretation by the shell is
1764 For ordinary commands the output is the path name; for shell built-in
1765 commands, shell functions and keywords only the name is written.
1766 Aliases are printed as
1767 .Dq Ic alias Ar name Ns = Ns Ar value .
1771 option is identical to
1773 except for the output.
1775 .Dq Ar utility Ic is Ar description
1781 a special shell builtin,
1788 .It Ic continue Op Ar num
1790 .Sx Flow-Control Constructs
1792 .It Ic echo Oo Fl e | n Oc Op Ar string ...
1793 Print a space-separated list of the arguments to the standard output
1794 and append a newline character.
1795 .Bl -tag -width indent
1797 Suppress the output of the trailing newline.
1799 Process C-style backslash escape sequences.
1802 command understands the following character escapes:
1803 .Bl -tag -width indent
1805 Alert (ring the terminal bell)
1809 Suppress the trailing newline (this has the side-effect of truncating the
1810 line if it is not the last character)
1828 (Zero) The character whose octal value is
1834 is not enclosed in quotes then the backslash itself must be escaped
1835 with a backslash to protect it from the shell.
1837 .Bd -literal -offset indent
1846 $ echo -e a\e\e\e\eb
1855 options may be specified.
1856 .It Ic eval Ar string ...
1857 Concatenate all the arguments with spaces.
1858 Then re-parse and execute the command.
1859 .It Ic exec Op Ar command Op arg ...
1863 the shell process is replaced with the specified program
1864 (which must be a real program, not a shell built-in command or function).
1865 Any redirections on the
1867 command are marked as permanent,
1868 so that they are not undone when the
1871 .It Ic exit Op Ar exitstatus
1872 Terminate the shell process.
1876 it is used as the exit status of the shell;
1877 otherwise the exit status of the preceding command is used.
1878 The exit status should be an integer between 0 and 255.
1879 .It Ic export Ar name ...
1880 .It Ic export Op Fl p
1881 The specified names are exported so that they will
1882 appear in the environment of subsequent commands.
1883 The only way to un-export a variable is to
1886 The shell allows the value of a variable to be set
1887 at the same time as it is exported by writing
1889 .D1 Ic export Ar name Ns = Ns Ar value
1891 With no arguments the
1893 command lists the names
1894 of all exported variables.
1897 option is specified, the exported variables are printed as
1898 .Dq Ic export Ar name Ns = Ns Ar value
1899 lines, suitable for re-input to the shell.
1901 A null command that returns a non-zero (false) exit value.
1902 .It Ic fc Oo Fl e Ar editor Oc Op Ar first Op Ar last
1903 .It Ic fc Fl l Oo Fl nr Oc Op Ar first Op Ar last
1904 .It Ic fc Fl s Oo Ar old Ns = Ns Ar new Oc Op Ar first
1907 built-in command lists, or edits and re-executes,
1908 commands previously entered to an interactive shell.
1909 .Bl -tag -width indent
1911 Use the editor named by
1913 to edit the commands.
1916 string is a command name,
1917 subject to search via the
1922 variable is used as a default when
1927 is null or unset, the value of the
1934 is used as the editor.
1936 List the commands rather than invoking
1938 The commands are written in the
1939 sequence indicated by the
1943 operands, as affected by
1945 with each command preceded by the command number.
1947 Suppress command numbers when listing with
1950 Reverse the order of the commands listed
1959 Re-execute the command without invoking an editor.
1962 Select the commands to list or edit.
1963 The number of previous commands that can be accessed
1964 are determined by the value of the
1971 or both are one of the following:
1972 .Bl -tag -width indent
1973 .It Oo Cm + Oc Ns Ar num
1974 A positive number representing a command number;
1975 command numbers can be displayed with the
1979 A negative decimal number representing the
1980 command that was executed
1983 commands previously.
1984 For example, \-1 is the immediately previous command.
1986 A string indicating the most recently entered command
1987 that begins with that string.
1989 .Ar old Ns = Ns Ar new
1990 operand is not also specified with
1992 the string form of the first operand cannot contain an embedded equal sign.
1996 The following variables affect the execution of
1998 .Bl -tag -width ".Va HISTSIZE"
2000 Name of the editor to use for history editing.
2002 The number of previous commands that are accessible.
2007 or the current job to the foreground.
2008 .It Ic getopts Ar optstring var
2015 command deprecates the older
2018 The first argument should be a series of letters, each possibly
2019 followed by a colon which indicates that the option takes an argument.
2020 The specified variable is set to the parsed option.
2022 the next argument is placed into the shell variable
2024 If an option takes an argument, it is placed into the shell variable
2026 If an invalid option is encountered,
2030 It returns a false value (1) when it encounters the end of the options.
2031 .It Ic hash Oo Fl rv Oc Op Ar command ...
2032 The shell maintains a hash table which remembers the locations of commands.
2033 With no arguments whatsoever, the
2035 command prints out the contents of this table.
2036 Entries which have not been looked at since the last
2038 command are marked with an asterisk;
2039 it is possible for these entries to be invalid.
2043 command removes each specified
2045 from the hash table (unless they are functions) and then locates it.
2050 prints the locations of the commands as it finds them.
2055 command to delete all the entries in the hash table except for functions.
2056 .It Ic jobid Op Ar job
2057 Print the process IDs of the processes in the specified
2061 argument is omitted, use the current job.
2062 .It Ic jobs Oo Fl lps Oc Op Ar job ...
2063 Print information about the specified jobs, or all jobs if no
2066 The information printed includes job ID, status and command name.
2070 option is specified, the PID of each job is also printed.
2073 option is specified, only the process IDs for the process group leaders
2074 are printed, one per line.
2077 option is specified, only the PIDs of the job commands are printed, one per
2079 .It Ic local Oo Ar variable ... Oc Op Fl
2083 .It Ic pwd Op Fl L | P
2084 Print the path of the current directory.
2085 The built-in command may
2086 differ from the program of the same name because the
2087 built-in command remembers what the current directory
2088 is rather than recomputing it each time.
2091 However, if the current directory is
2093 the built-in version of
2095 will continue to print the old name for the directory.
2099 option is specified, symbolic links are resolved.
2102 option is specified, the shell's notion of the current directory
2103 is printed (symbolic links are not resolved).
2104 This is the default.
2105 .It Ic read Oo Fl p Ar prompt Oc Oo
2106 .Fl t Ar timeout Oc Oo Fl er Oc Ar variable ...
2112 and the standard input is a terminal.
2114 read from the standard input.
2115 The trailing newline
2116 is deleted from the line and the line is split as
2117 described in the section on
2118 .Sx White Space Splitting (Field Splitting)
2120 the pieces are assigned to the variables in order.
2121 If there are more pieces than variables, the remaining
2122 pieces (along with the characters in
2124 that separated them)
2125 are assigned to the last variable.
2126 If there are more variables than pieces, the remaining
2127 variables are assigned the null string.
2129 Backslashes are treated specially, unless the
2133 If a backslash is followed by
2134 a newline, the backslash and the newline will be
2136 If a backslash is followed by any other
2137 character, the backslash will be deleted and the following
2138 character will be treated as though it were not in
2144 option is specified and the
2146 elapses before a complete line of input is supplied,
2149 command will return an exit status of 1 without assigning any values.
2152 value may optionally be followed by one of
2157 to explicitly specify seconds, minutes or hours.
2158 If none is supplied,
2164 option exists only for backward compatibility with older scripts.
2165 .It Ic readonly Oo Fl p Oc Op Ar name ...
2168 is marked as read only,
2169 so that it cannot be subsequently modified or unset.
2170 The shell allows the value of a variable to be set
2171 at the same time as it is marked read only
2172 by using the following form:
2174 .D1 Ic readonly Ar name Ns = Ns Ar value
2176 With no arguments the
2178 command lists the names of all read only variables.
2181 option is specified, the read-only variables are printed as
2182 .Dq Ic readonly Ar name Ns = Ns Ar value
2183 lines, suitable for re-input to the shell.
2184 .It Ic return Op Ar exitstatus
2188 .It Ic set Oo Fl /+abCEefIimnpTuVvx Oc Oo Fl /+o Ar longname Oc Oo
2189 .Fl c Ar string Oc Op Fl - Ar arg ...
2192 command performs three different functions:
2195 With no arguments, it lists the values of all shell variables.
2197 If options are given,
2198 either in short form or using the long
2199 .Dq Fl /+o Ar longname
2201 it sets or clears the specified options as described in the section called
2202 .Sx Argument List Processing .
2206 option is specified,
2208 will replace the shell's positional parameters with the subsequent
2210 If no arguments follow the
2213 all the positional parameters will be cleared,
2214 which is equivalent to executing the command
2218 flag may be omitted when specifying arguments to be used
2219 as positional replacement parameters.
2220 This is not recommended,
2221 because the first argument may begin with a dash
2227 command will interpret as a request to enable or disable options.
2229 .It Ic setvar Ar variable value
2230 Assigns the specified
2236 command is intended to be used in functions that
2237 assign values to variables whose names are passed as parameters.
2238 In general it is better to write
2239 .Dq Ar variable Ns = Ns Ar value
2242 .It Ic shift Op Ar n
2243 Shift the positional parameters
2248 A shift sets the value of
2257 decreasing the value of
2260 If there are zero positional parameters, shifting does not do anything.
2262 A built-in equivalent of
2265 Print the amount of time spent executing the shell and its children.
2266 The first output line shows the user and system times for the shell
2267 itself, the second one contains the user and system times for the
2269 .It Ic trap Oo Ar action Oc Ar signal ...
2271 Cause the shell to parse and execute
2276 The signals are specified by name or number.
2277 In addition, the pseudo-signal
2279 may be used to specify an
2281 that is performed when the shell terminates.
2284 may be an empty string or a dash
2286 the former causes the specified signal to be ignored
2287 and the latter causes the default action to be taken.
2290 is another way to request the default action, for compatibility reasons this
2291 usage is not recommended though.
2292 When the shell forks off a subshell,
2293 it resets trapped (but not ignored) signals to the default action.
2296 command has no effect on signals that were ignored on entry to the shell.
2302 command to display a list of valid signal names.
2304 A null command that returns a 0 (true) exit value.
2305 .It Ic type Op Ar name ...
2308 as a command and print the resolution of the command search.
2309 Possible resolutions are:
2310 shell keyword, alias, special shell builtin, shell builtin, command,
2313 For aliases the alias expansion is printed;
2314 for commands and tracked aliases
2315 the complete pathname of the command is printed.
2316 .It Ic ulimit Oo Fl HSabcdflmnpstuvw Oc Op Ar limit
2317 Set or display resource limits (see
2321 is specified, the named resource will be set;
2322 otherwise the current resource value will be displayed.
2326 is specified, the hard limits will be set or displayed.
2327 While everybody is allowed to reduce a hard limit,
2328 only the superuser can increase it.
2332 specifies the soft limits instead.
2333 When displaying limits,
2339 The default is to display the soft limits,
2340 and to set both the hard and the soft limits.
2346 command to display all resources.
2349 is not acceptable in this mode.
2351 The remaining options specify which resource value is to be
2352 displayed or modified.
2353 They are mutually exclusive.
2354 .Bl -tag -width indent
2356 The maximum size of socket buffer usage, in bytes.
2357 .It Fl c Ar coredumpsize
2358 The maximal size of core dump files, in 512-byte blocks.
2359 .It Fl d Ar datasize
2360 The maximal size of the data segment of a process, in kilobytes.
2361 .It Fl f Ar filesize
2362 The maximal size of a file, in 512-byte blocks.
2363 .It Fl l Ar lockedmem
2364 The maximal size of memory that can be locked by a process, in
2366 .It Fl m Ar memoryuse
2367 The maximal resident set size of a process, in kilobytes.
2369 The maximal number of descriptors that could be opened by a process.
2370 .It Fl p Ar pseudoterminals
2371 The maximal number of pseudo-terminals for this user ID.
2372 .It Fl s Ar stacksize
2373 The maximal size of the stack segment, in kilobytes.
2375 The maximal amount of CPU time to be used by each process, in seconds.
2376 .It Fl u Ar userproc
2377 The maximal number of simultaneous processes for this user ID.
2378 .It Fl v Ar virtualmem
2379 The maximal virtual size of a process, in kilobytes.
2381 The maximum amount of swap space reserved or used for this user ID,
2384 .It Ic umask Oo Fl S Oc Op Ar mask
2385 Set the file creation mask (see
2387 to the octal or symbolic (see
2391 If the argument is omitted, the current mask value is printed.
2394 option is specified, the output is symbolic, otherwise the output is octal.
2395 .It Ic unalias Oo Fl a Oc Op Ar name ...
2396 The specified alias names are removed.
2399 is specified, all aliases are removed.
2400 .It Ic unset Oo Fl fv Oc Ar name ...
2401 The specified variables or functions are unset and unexported.
2404 option is specified or no options are given, the
2406 arguments are treated as variable names.
2409 option is specified, the
2411 arguments are treated as function names.
2412 .It Ic wait Op Ar job
2413 Wait for the specified
2415 to complete and return the exit status of the last process in the
2417 If the argument is omitted, wait for all jobs to complete
2418 and return an exit status of zero.
2420 .Ss Commandline Editing
2423 is being used interactively from a terminal, the current command
2424 and the command history
2428 .Sx Built-in Commands )
2431 command line editing.
2432 This mode uses commands similar
2433 to a subset of those described in the
2451 can be switched between insert mode and command mode by typing
2455 while in command mode will pass the line to the shell.
2458 .Dq Li "set -o emacs"
2461 command can be used to enable a subset of
2463 command line editing features.
2465 The following environment variables affect the execution of
2467 .Bl -tag -width ".Ev LANGXXXXXX"
2469 Initialization file for interactive shells.
2470 .It Ev LANG , Ev LC_*
2472 These are inherited by children of the shell,
2473 and is used in a limited manner by the shell itself.
2475 An absolute pathname for the current directory,
2476 possibly containing symbolic links.
2477 This is used and updated by the shell.
2479 The default terminal setting for the shell.
2480 This is inherited by children of the shell, and is used in the history
2484 Additionally, all environment variables are turned into shell variables
2486 which may affect the shell as described under
2487 .Sx Special Variables .
2489 Errors that are detected by the shell, such as a syntax error, will
2490 cause the shell to exit with a non-zero exit status.
2491 If the shell is not an interactive shell, the execution of the shell
2492 file will be aborted.
2493 Otherwise the shell will return the exit status of the last command
2496 builtin is used with a numeric argument, it
2497 will return the argument.
2514 command, the Thompson shell, appeared in
2516 It was superseded in
2518 by the Bourne shell, which inherited the name
2523 was rewritten in 1989 under the
2525 license after the Bourne shell from
2530 was originally written by
2531 .An Kenneth Almquist .
2535 utility does not recognize multibyte characters.
projects / FreeBSD / stable / 8.git / blob