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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.
466 used to create lexically obscure code.
467 This use is discouraged.
469 An alias name may be escaped in a command line, so that it is not
470 replaced by its alias value, by using quoting characters within or
471 adjacent to the alias name.
472 This is most often done by prefixing
473 an alias name with a backslash to execute a function, built-in, or
474 normal program with the same name.
479 The shell interprets the words it reads according to a
480 language, the specification of which is outside the scope
481 of this man page (refer to the BNF in the
484 Essentially though, a line is read and if
485 the first word of the line (or after a control operator)
486 is not a keyword, then the shell has recognized a
488 Otherwise, a complex command or some
489 other special construct may have been recognized.
491 If a simple command has been recognized, the shell performs
492 the following actions:
495 Leading words of the form
497 are stripped off and assigned to the environment of
499 Redirection operators and
500 their arguments (as described below) are stripped
501 off and saved for processing.
503 The remaining words are expanded as described in
505 .Sx Word Expansions ,
506 and the first remaining word is considered the command
507 name and the command is located.
509 words are considered the arguments of the command.
510 If no command name resulted, then the
512 variable assignments recognized in 1) affect the
515 Redirections are performed as described in
519 Redirections are used to change where a command reads its input
521 In general, redirections open, close, or
522 duplicate an existing reference to a file.
524 used for redirection is:
526 .D1 Oo Ar n Oc Ar redir-op file
530 is one of the redirection operators mentioned
532 The following gives some examples of how these
533 operators can be used.
534 Note that stdin and stdout are commonly used abbreviations
535 for standard input and standard output respectively.
536 .Bl -tag -width "1234567890XX" -offset indent
537 .It Oo Ar n Oc Ns Li > Ar file
538 redirect stdout (or file descriptor
542 .It Oo Ar n Oc Ns Li >| Ar file
543 same as above, but override the
546 .It Oo Ar n Oc Ns Li >> Ar file
547 append stdout (or file descriptor
551 .It Oo Ar n Oc Ns Li < Ar file
552 redirect stdin (or file descriptor
556 .It Oo Ar n Oc Ns Li <> Ar file
557 redirect stdin (or file descriptor
561 .It Oo Ar n1 Oc Ns Li <& Ns Ar n2
562 duplicate stdin (or file descriptor
566 .It Oo Ar n Oc Ns Li <&-
567 close stdin (or file descriptor
569 .It Oo Ar n1 Oc Ns Li >& Ns Ar n2
570 duplicate stdout (or file descriptor
574 .It Oo Ar n Oc Ns Li >&-
575 close stdout (or file descriptor
579 The following redirection is often called a
581 .Bd -unfilled -offset indent
582 .Oo Ar n Oc Ns Li << Ar delimiter
588 All the text on successive lines up to the delimiter is
589 saved away and made available to the command on standard
590 input, or file descriptor
595 as specified on the initial line is quoted, then the
597 is treated literally, otherwise the text is subjected to
598 parameter expansion, command substitution, and arithmetic
599 expansion (as described in the section on
600 .Sx Word Expansions ) .
609 .Ss Search and Execution
610 There are three types of commands: shell functions,
611 built-in commands, and normal programs.
612 The command is searched for (by name) in that order.
613 The three types of commands are all executed in a different way.
615 When a shell function is executed, all of the shell positional
618 which remains unchanged) are
619 set to the arguments of the shell function.
620 The variables which are explicitly placed in the environment of
621 the command (by placing assignments to them before the
622 function name) are made local to the function and are set
624 Then the command given in the function definition is executed.
625 The positional parameters are restored to their original values
626 when the command completes.
627 This all occurs within the current shell.
629 Shell built-in commands are executed internally to the shell, without
630 spawning a new process.
631 There are two kinds of built-in commands: regular and special.
632 Assignments before special builtins persist after they finish
633 executing and assignment errors, redirection errors and certain
634 operand errors cause a script to be aborted.
635 Both regular and special builtins can affect the shell in ways
636 normal programs cannot.
638 Otherwise, if the command name does not match a function
639 or built-in command, the command is searched for as a normal
640 program in the file system (as described in the next section).
641 When a normal program is executed, the shell runs the program,
642 passing the arguments and the environment to the program.
643 If the program is not a normal executable file
644 (i.e., if it does not begin with the
654 the shell will interpret the program in a subshell.
655 The child shell will reinitialize itself in this case,
656 so that the effect will be
657 as if a new shell had been invoked to handle the ad-hoc shell script,
658 except that the location of hashed commands located in
659 the parent shell will be remembered by the child
660 (see the description of the
662 built-in command below).
664 Note that previous versions of this document
665 and the source code itself misleadingly and sporadically
666 refer to a shell script without a magic number
668 .Dq "shell procedure" .
670 When locating a command, the shell first looks to see if
671 it has a shell function by that name.
673 built-in command by that name.
674 If a built-in command is not found,
675 one of two things happen:
678 Command names containing a slash are simply executed without
679 performing any searches.
681 The shell searches each entry in the
684 in turn for the command.
687 variable should be a series of
688 entries separated by colons.
689 Each entry consists of a
691 The current directory
692 may be indicated implicitly by an empty directory name,
693 or explicitly by a single period.
695 .Ss Command Exit Status
696 Each command has an exit status that can influence the behavior
697 of other shell commands.
698 The paradigm is that a command exits
699 with zero for normal or success, and non-zero for failure,
700 error, or a false indication.
701 The man page for each command
702 should indicate the various exit codes and what they mean.
703 Additionally, the built-in commands return exit codes, as does
704 an executed shell function.
706 If a command is terminated by a signal, its exit status is 128 plus
708 Signal numbers are defined in the header file
711 Complex commands are combinations of simple commands
712 with control operators or keywords, together creating a larger complex
714 More generally, a command is one of the following:
715 .Bl -item -offset indent
721 list or compound-list
728 Unless otherwise stated, the exit status of a command is
729 that of the last simple command executed by the command.
731 A pipeline is a sequence of one or more commands separated
732 by the control operator
734 The standard output of all but
735 the last command is connected to the standard input
737 The standard output of the last
738 command is inherited from the shell, as usual.
740 The format for a pipeline is:
742 .D1 Oo Li \&! Oc Ar command1 Op Li \&| Ar command2 ...
744 The standard output of
746 is connected to the standard input of
748 The standard input, standard output, or
749 both of a command is considered to be assigned by the
750 pipeline before any redirection specified by redirection
751 operators that are part of the command.
753 Note that unlike some other shells,
755 executes each process in a pipeline with more than one command
756 in a subshell environment and as a child of the
760 If the pipeline is not in the background (discussed later),
761 the shell waits for all commands to complete.
765 does not precede the pipeline, the
766 exit status is the exit status of the last command specified
768 Otherwise, the exit status is the logical
769 NOT of the exit status of the last command.
771 the last command returns zero, the exit status is 1; if
772 the last command returns greater than zero, the exit status
775 Because pipeline assignment of standard input or standard
776 output or both takes place before redirection, it can be
777 modified by redirection.
780 .Dl "command1 2>&1 | command2"
782 sends both the standard output and standard error of
784 to the standard input of
789 or newline terminator causes the preceding
791 (described below in the section called
792 .Sx Short-Circuit List Operators )
793 to be executed sequentially;
796 causes asynchronous execution of the preceding AND-OR-list.
797 .Ss Background Commands (&)
798 If a command is terminated by the control operator ampersand
800 the shell executes the command asynchronously;
801 the shell does not wait for the command to finish
802 before executing the next command.
804 The format for running a command in background is:
806 .D1 Ar command1 Li & Op Ar command2 Li & Ar ...
808 If the shell is not interactive, the standard input of an
809 asynchronous command is set to
811 .Ss Lists (Generally Speaking)
812 A list is a sequence of zero or more commands separated by
813 newlines, semicolons, or ampersands,
814 and optionally terminated by one of these three characters.
816 list are executed in the order they are written.
817 If command is followed by an ampersand, the shell starts the
818 command and immediately proceeds onto the next command;
819 otherwise it waits for the command to terminate before
820 proceeding to the next one.
821 .Ss Short-Circuit List Operators
825 are AND-OR list operators.
827 executes the first command, and then executes the second command
828 if the exit status of the first command is zero.
830 is similar, but executes the second command if the exit
831 status of the first command is nonzero.
835 both have the same priority.
836 .Ss Flow-Control Constructs (if, while, for, case)
840 .Bd -unfilled -offset indent -compact
844 .Ic then Ar list Oc Ar ...
852 .Bd -unfilled -offset indent -compact
858 The two lists are executed repeatedly while the exit status of the
862 command is similar, but has the word
867 repeat until the exit status of the first list is zero.
872 .Bd -unfilled -offset indent -compact
873 .Ic for Ar variable Op Ic in Ar word ...
880 and the following words are omitted,
883 The words are expanded, and then the list is executed
884 repeatedly with the variable set to each word in turn.
889 commands may be replaced with
899 .D1 Ic break Op Ar num
900 .D1 Ic continue Op Ar num
904 command terminates the
913 command continues with the next iteration of the innermost loop.
914 These are implemented as special built-in commands.
919 .Bd -unfilled -offset indent -compact
920 .Ic case Ar word Ic in
921 .Ar pattern Ns Li ) Ar list Li ;;
926 The pattern can actually be one or more patterns
935 command is the exit code of the last command executed in the list or
936 zero if no patterns were matched.
937 .Ss Grouping Commands Together
938 Commands may be grouped by writing either
940 .D1 Li \&( Ns Ar list Ns Li \%)
944 .D1 Li { Ar list Ns Li \&; }
946 The first form executes the commands in a subshell.
947 Note that built-in commands thus executed do not affect the current shell.
948 The second form does not fork another shell,
949 so it is slightly more efficient.
950 Grouping commands together this way allows the user to
951 redirect their output as though they were one program:
952 .Bd -literal -offset indent
953 { echo -n "hello"; echo " world"; } > greeting
956 The syntax of a function definition is
958 .D1 Ar name Li \&( \&) Ar command
960 A function definition is an executable statement; when
961 executed it installs a function named
973 Variables may be declared to be local to a function by
977 This should appear as the first statement of a function,
980 .D1 Ic local Oo Ar variable ... Oc Op Fl
984 command is implemented as a built-in command.
986 When a variable is made local, it inherits the initial
987 value and exported and readonly flags from the variable
988 with the same name in the surrounding scope, if there is
990 Otherwise, the variable is initially unset.
992 uses dynamic scoping, so that if the variable
994 is made local to function
996 which then calls function
998 references to the variable
1002 will refer to the variable
1006 not to the global variable named
1009 The only special parameter that can be made local is
1013 local causes any shell options that are
1016 command inside the function to be
1017 restored to their original values when the function
1024 .D1 Ic return Op Ar exitstatus
1026 It terminates the current executional scope, returning from the previous
1027 nested function, sourced script, or shell instance, in that order.
1030 command is implemented as a special built-in command.
1031 .Ss Variables and Parameters
1032 The shell maintains a set of parameters.
1034 denoted by a name is called a variable.
1036 the shell turns all the environment variables into shell
1038 New variables can be set using the form
1040 .D1 Ar name Ns = Ns Ar value
1042 Variables set by the user must have a name consisting solely
1043 of alphabetics, numerics, and underscores.
1044 The first letter of a variable name must not be numeric.
1045 A parameter can also be denoted by a number
1046 or a special character as explained below.
1047 .Ss Positional Parameters
1048 A positional parameter is a parameter denoted by a number greater than zero.
1049 The shell sets these initially to the values of its command line
1050 arguments that follow the name of the shell script.
1053 built-in command can also be used to set or reset them.
1054 .Ss Special Parameters
1055 A special parameter is a parameter denoted by a special one-character
1057 The special parameters recognized by the
1061 are shown in the following list, exactly as they would appear in input
1062 typed by the user or in the source of a shell script.
1065 Expands to the positional parameters, starting from one.
1067 the expansion occurs within a double-quoted string
1068 it expands to a single field with the value of each parameter
1069 separated by the first character of the
1076 Expands to the positional parameters, starting from one.
1078 the expansion occurs within double-quotes, each positional
1079 parameter expands as a separate argument.
1080 If there are no positional parameters, the
1083 generates zero arguments, even when
1086 What this basically means, for example, is
1099 .Bd -literal -offset indent
1103 Expands to the number of positional parameters.
1105 Expands to the exit status of the most recent pipeline.
1107 (hyphen) Expands to the current option flags (the single-letter
1108 option names concatenated into a string) as specified on
1111 built-in command, or implicitly
1114 Expands to the process ID of the invoked shell.
1116 retains the same value of
1120 Expands to the process ID of the most recent background
1121 command executed from the current shell.
1123 pipeline, the process ID is that of the last command in the
1126 (zero) Expands to the name of the shell script if passed on the command line,
1129 operand if given (with
1131 or otherwise argument 0 passed to the shell.
1134 This clause describes the various expansions that are
1136 Not all expansions are performed on
1137 every word, as explained later.
1139 Tilde expansions, parameter expansions, command substitutions,
1140 arithmetic expansions, and quote removals that occur within
1141 a single word expand to a single field.
1143 splitting or pathname expansion that can create multiple
1144 fields from a single word.
1145 The single exception to this rule is
1146 the expansion of the special parameter
1148 within double-quotes,
1149 as was described above.
1151 The order of word expansion is:
1154 Tilde Expansion, Parameter Expansion, Command Substitution,
1155 Arithmetic Expansion (these all occur at the same time).
1157 Field Splitting is performed on fields generated by step (1)
1162 Pathname Expansion (unless the
1164 option is in effect).
1171 character is used to introduce parameter expansion, command
1172 substitution, or arithmetic expansion.
1173 .Ss Tilde Expansion (substituting a user's home directory)
1174 A word beginning with an unquoted tilde character
1177 subjected to tilde expansion.
1178 All the characters up to a slash
1180 or the end of the word are treated as a username
1181 and are replaced with the user's home directory.
1183 username is missing (as in
1185 the tilde is replaced with the value of the
1187 variable (the current user's home directory).
1188 .Ss Parameter Expansion
1189 The format for parameter expansion is as follows:
1191 .D1 Li ${ Ns Ar expression Ns Li }
1195 consists of all characters until the matching
1199 escaped by a backslash or within a quoted string, and characters in
1200 embedded arithmetic expansions, command substitutions, and variable
1201 expansions, are not examined in determining the matching
1204 The simplest form for parameter expansion is:
1206 .D1 Li ${ Ns Ar parameter Ns Li }
1208 The value, if any, of
1212 The parameter name or symbol can be enclosed in braces, which are
1213 optional except for positional parameters with more than one digit or
1214 when parameter is followed by a character that could be interpreted as
1216 If a parameter expansion occurs inside double-quotes:
1219 Pathname expansion is not performed on the results of the
1222 Field splitting is not performed on the results of the
1223 expansion, with the exception of the special parameter
1227 In addition, a parameter expansion can be modified by using one of the
1229 .Bl -tag -width indent
1230 .It Li ${ Ns Ar parameter Ns Li :- Ns Ar word Ns Li }
1234 is unset or null, the expansion of
1236 is substituted; otherwise, the value of
1239 .It Li ${ Ns Ar parameter Ns Li := Ns Ar word Ns Li }
1240 Assign Default Values.
1243 is unset or null, the expansion of
1253 does not prevent field splitting or pathname expansion.
1254 Only variables, not positional
1255 parameters or special parameters, can be
1256 assigned in this way.
1257 .It Li ${ Ns Ar parameter Ns Li :? Ns Oo Ar word Oc Ns Li }
1258 Indicate Error if Null or Unset.
1261 is unset or null, the expansion of
1263 (or a message indicating it is unset if
1265 is omitted) is written to standard
1266 error and the shell exits with a nonzero
1268 Otherwise, the value of
1272 interactive shell need not exit.
1273 .It Li ${ Ns Ar parameter Ns Li :+ Ns Ar word Ns Li }
1274 Use Alternate Value.
1277 is unset or null, null is substituted;
1278 otherwise, the expansion of
1283 In the parameter expansions shown previously, use of the colon in the
1284 format results in a test for a parameter that is unset or null; omission
1285 of the colon results in a test for a parameter that is only unset.
1286 .Bl -tag -width indent
1287 .It Li ${# Ns Ar parameter Ns Li }
1289 The length in characters of
1294 The following four varieties of parameter expansion provide for substring
1296 In each case, pattern matching notation
1298 .Sx Shell Patterns ) ,
1299 rather than regular expression notation,
1300 is used to evaluate the patterns.
1301 If parameter is one of the special parameters
1305 the result of the expansion is unspecified.
1306 Enclosing the full parameter expansion string in double-quotes does not
1307 cause the following four varieties of pattern characters to be quoted,
1308 whereas quoting characters within the braces has this effect.
1309 .Bl -tag -width indent
1310 .It Li ${ Ns Ar parameter Ns Li % Ns Ar word Ns Li }
1311 Remove Smallest Suffix Pattern.
1314 is expanded to produce a pattern.
1316 parameter expansion then results in
1318 with the smallest portion of the
1319 suffix matched by the pattern deleted.
1320 .It Li ${ Ns Ar parameter Ns Li %% Ns Ar word Ns Li }
1321 Remove Largest Suffix Pattern.
1324 is expanded to produce a pattern.
1326 parameter expansion then results in
1328 with the largest portion of the
1329 suffix matched by the pattern deleted.
1330 .It Li ${ Ns Ar parameter Ns Li # Ns Ar word Ns Li }
1331 Remove Smallest Prefix Pattern.
1334 is expanded to produce a pattern.
1336 parameter expansion then results in
1338 with the smallest portion of the
1339 prefix matched by the pattern deleted.
1340 .It Li ${ Ns Ar parameter Ns Li ## Ns Ar word Ns Li }
1341 Remove Largest Prefix Pattern.
1344 is expanded to produce a pattern.
1346 parameter expansion then results in
1348 with the largest portion of the
1349 prefix matched by the pattern deleted.
1351 .Ss Command Substitution
1352 Command substitution allows the output of a command to be substituted in
1353 place of the command name itself.
1354 Command substitution occurs when
1355 the command is enclosed as follows:
1357 .D1 Li $( Ns Ar command Ns Li )\&
1359 or the backquoted version:
1361 .D1 Li ` Ns Ar command Ns Li `
1363 The shell expands the command substitution by executing command in a
1364 subshell environment and replacing the command substitution
1365 with the standard output of the command,
1366 removing sequences of one or more newlines at the end of the substitution.
1367 Embedded newlines before the end of the output are not removed;
1368 however, during field splitting, they may be translated into spaces
1369 depending on the value of
1371 and the quoting that is in effect.
1372 .Ss Arithmetic Expansion
1373 Arithmetic expansion provides a mechanism for evaluating an arithmetic
1374 expression and substituting its value.
1375 The format for arithmetic expansion is as follows:
1377 .D1 Li $(( Ns Ar expression Ns Li ))
1381 is treated as if it were in double-quotes, except
1382 that a double-quote inside the expression is not treated specially.
1384 shell expands all tokens in the
1386 for parameter expansion,
1387 command substitution,
1388 arithmetic expansion
1391 The allowed expressions are a subset of C expressions,
1393 .Bl -tag -width "Variables" -offset indent
1395 All values are of type
1398 Decimal, octal (starting with
1400 and hexadecimal (starting with
1404 Shell variables can be read and written
1405 and contain integer constants.
1408 .It Binary operators
1409 .Li "* / % + - << >> < <= > >= == != & ^ | && ||"
1410 .It Assignment operators
1411 .Li "= += -= *= /= %= <<= >>= &= ^= |="
1412 .It Short-circuit evaluation
1417 operators always evaluate both sides.
1421 The result of the expression is substituted in decimal.
1422 .Ss White Space Splitting (Field Splitting)
1423 After parameter expansion, command substitution, and
1424 arithmetic expansion the shell scans the results of
1425 expansions and substitutions that did not occur in double-quotes for
1426 field splitting and multiple fields can result.
1428 The shell treats each character of the
1430 variable as a delimiter and uses
1431 the delimiters to split the results of parameter expansion and command
1432 substitution into fields.
1433 .Ss Pathname Expansion (File Name Generation)
1437 file name generation is performed
1438 after word splitting is complete.
1440 viewed as a series of patterns, separated by slashes.
1442 process of expansion replaces the word with the names of
1443 all existing files whose names can be formed by replacing
1444 each pattern with a string that matches the specified pattern.
1445 There are two restrictions on this: first, a pattern cannot match
1446 a string containing a slash, and second,
1447 a pattern cannot match a string starting with a period
1448 unless the first character of the pattern is a period.
1449 The next section describes the patterns used for both
1450 Pathname Expansion and the
1454 A pattern consists of normal characters, which match themselves,
1455 and meta-characters.
1456 The meta-characters are
1462 These characters lose their special meanings if they are quoted.
1463 When command or variable substitution is performed and the dollar sign
1464 or back quotes are not double-quoted, the value of the
1465 variable or the output of the command is scanned for these
1466 characters and they are turned into meta-characters.
1470 matches any string of characters.
1473 matches any single character.
1476 introduces a character class.
1477 The end of the character class is indicated by a
1485 rather than introducing a character class.
1486 A character class matches any of the characters between the square brackets.
1487 A range of characters may be specified using a minus sign.
1488 The character class may be complemented by making an exclamation point
1490 the first character of the character class.
1494 in a character class, make it the first character listed
1500 make it the first or last character listed.
1501 .Ss Built-in Commands
1502 This section lists the commands which
1503 are built-in because they need to perform some operation
1504 that cannot be performed by a separate process.
1506 these, built-in versions of essential utilities
1507 are provided for efficiency.
1508 .Bl -tag -width indent
1510 A null command that returns a 0 (true) exit value.
1512 The commands in the specified file are read and executed by the shell.
1515 command may be used to return to the
1522 characters, it is used as is.
1523 Otherwise, the shell searches the
1526 If it is not found in the
1528 it is sought in the current working directory.
1530 A built-in equivalent of
1532 .It Ic alias Oo Ar name Ns Oo = Ns Ar string Oc ... Oc
1534 .Ar name Ns = Ns Ar string
1535 is specified, the shell defines the alias
1541 is specified, the value of the alias
1544 With no arguments, the
1546 built-in command prints the names and values of all defined aliases
1549 Alias values are written with appropriate quoting so that they are
1550 suitable for re-input to the shell.
1554 .It Ic bg Op Ar job ...
1555 Continue the specified jobs
1556 (or the current job if no jobs are given)
1558 .It Ic builtin Ar cmd Op Ar arg ...
1559 Execute the specified built-in command,
1561 This is useful when the user wishes to override a shell function
1562 with the same name as a built-in command.
1563 .It Ic bind Oo Fl aeklrsv Oc Oo Ar key Oo Ar command Oc Oc
1564 List or alter key bindings for the line editor.
1565 This command is documented in
1567 .It Ic cd Oo Fl L | P Oc Op Ar directory
1568 Switch to the specified
1570 or to the directory specified in the
1572 environment variable if no
1581 then the directories listed in the
1584 searched for the specified
1588 is unset, the current directory is searched.
1591 is the same as that of
1593 In an interactive shell,
1596 command will print out the name of the directory
1597 that it actually switched to
1598 if this is different from the name that the user gave.
1599 These may be different either because the
1601 mechanism was used or because a symbolic link was crossed.
1605 option is specified,
1607 is handled physically and symbolic links are resolved before
1609 components are processed.
1612 option is specified,
1614 is handled logically.
1615 This is the default.
1620 .It Ic command Oo Fl p Oc Op Ar utility Op Ar argument ...
1621 .It Ic command Oo Fl v | V Oc Op Ar utility
1622 The first form of invocation executes the specified
1624 as a simple command (see the
1630 option is specified, the command search is performed using a
1633 that is guaranteed to find all of the standard utilities.
1637 option is specified,
1639 is not executed but a description of its interpretation by the shell is
1641 For ordinary commands the output is the path name; for shell built-in
1642 commands, shell functions and keywords only the name is written.
1643 Aliases are printed as
1644 .Dq Ic alias Ar name Ns = Ns Ar value .
1648 option is identical to
1650 except for the output.
1652 .Dq Ar utility Ic is Ar description
1658 a special shell builtin,
1665 .It Ic echo Oo Fl e | n Oc Op Ar string ...
1666 Print a space-separated list of the arguments to the standard output
1667 and append a newline character.
1668 .Bl -tag -width indent
1670 Suppress the output of the trailing newline.
1672 Process C-style backslash escape sequences.
1675 command understands the following character escapes:
1676 .Bl -tag -width indent
1678 Alert (ring the terminal bell)
1682 Suppress the trailing newline (this has the side-effect of truncating the
1683 line if it is not the last character)
1701 (Zero) The character whose octal value is
1707 is not enclosed in quotes then the backslash itself must be escaped
1708 with a backslash to protect it from the shell.
1710 .Bd -literal -offset indent
1719 $ echo -e a\e\e\e\eb
1728 options may be specified.
1729 .It Ic eval Ar string ...
1730 Concatenate all the arguments with spaces.
1731 Then re-parse and execute the command.
1732 .It Ic exec Op Ar command Op arg ...
1736 the shell process is replaced with the specified program
1737 (which must be a real program, not a shell built-in command or function).
1738 Any redirections on the
1740 command are marked as permanent,
1741 so that they are not undone when the
1744 .It Ic exit Op Ar exitstatus
1745 Terminate the shell process.
1749 it is used as the exit status of the shell;
1750 otherwise the exit status of the preceding command is used.
1751 The exit status should be an integer between 0 and 255.
1752 .It Ic export Ar name ...
1753 .It Ic export Op Fl p
1754 The specified names are exported so that they will
1755 appear in the environment of subsequent commands.
1756 The only way to un-export a variable is to
1759 The shell allows the value of a variable to be set
1760 at the same time as it is exported by writing
1762 .D1 Ic export Ar name Ns = Ns Ar value
1764 With no arguments the
1766 command lists the names
1767 of all exported variables.
1770 option is specified, the exported variables are printed as
1771 .Dq Ic export Ar name Ns = Ns Ar value
1772 lines, suitable for re-input to the shell.
1774 A null command that returns a non-zero (false) exit value.
1775 .It Ic fc Oo Fl e Ar editor Oc Op Ar first Op Ar last
1776 .It Ic fc Fl l Oo Fl nr Oc Op Ar first Op Ar last
1777 .It Ic fc Fl s Oo Ar old Ns = Ns Ar new Oc Op Ar first
1780 built-in command lists, or edits and re-executes,
1781 commands previously entered to an interactive shell.
1782 .Bl -tag -width indent
1784 Use the editor named by
1786 to edit the commands.
1789 string is a command name,
1790 subject to search via the
1795 variable is used as a default when
1800 is null or unset, the value of the
1807 is used as the editor.
1809 List the commands rather than invoking
1811 The commands are written in the
1812 sequence indicated by the
1816 operands, as affected by
1818 with each command preceded by the command number.
1820 Suppress command numbers when listing with
1823 Reverse the order of the commands listed
1832 Re-execute the command without invoking an editor.
1835 Select the commands to list or edit.
1836 The number of previous commands that can be accessed
1837 are determined by the value of the
1844 or both are one of the following:
1845 .Bl -tag -width indent
1846 .It Oo Cm + Oc Ns Ar num
1847 A positive number representing a command number;
1848 command numbers can be displayed with the
1852 A negative decimal number representing the
1853 command that was executed
1856 commands previously.
1857 For example, \-1 is the immediately previous command.
1859 A string indicating the most recently entered command
1860 that begins with that string.
1862 .Ar old Ns = Ns Ar new
1863 operand is not also specified with
1865 the string form of the first operand cannot contain an embedded equal sign.
1869 The following environment variables affect the execution of
1871 .Bl -tag -width ".Ev HISTSIZE"
1873 Name of the editor to use for history editing.
1875 The number of previous commands that are accessible.
1880 or the current job to the foreground.
1881 .It Ic getopts Ar optstring var
1888 command deprecates the older
1891 The first argument should be a series of letters, each possibly
1892 followed by a colon which indicates that the option takes an argument.
1893 The specified variable is set to the parsed option.
1895 the next argument is placed into the shell variable
1897 If an option takes an argument, it is placed into the shell variable
1899 If an invalid option is encountered,
1903 It returns a false value (1) when it encounters the end of the options.
1904 .It Ic hash Oo Fl rv Oc Op Ar command ...
1905 The shell maintains a hash table which remembers the locations of commands.
1906 With no arguments whatsoever, the
1908 command prints out the contents of this table.
1909 Entries which have not been looked at since the last
1911 command are marked with an asterisk;
1912 it is possible for these entries to be invalid.
1916 command removes each specified
1918 from the hash table (unless they are functions) and then locates it.
1923 prints the locations of the commands as it finds them.
1928 command to delete all the entries in the hash table except for functions.
1929 .It Ic jobid Op Ar job
1930 Print the process IDs of the processes in the specified
1934 argument is omitted, use the current job.
1935 .It Ic jobs Oo Fl lps Oc Op Ar job ...
1936 Print information about the specified jobs, or all jobs if no
1939 The information printed includes job ID, status and command name.
1943 option is specified, the PID of each job is also printed.
1946 option is specified, only the process IDs for the process group leaders
1947 are printed, one per line.
1950 option is specified, only the PIDs of the job commands are printed, one per
1952 .It Ic local Oo Ar variable ... Oc Op Fl
1956 .It Ic pwd Op Fl L | P
1957 Print the path of the current directory.
1958 The built-in command may
1959 differ from the program of the same name because the
1960 built-in command remembers what the current directory
1961 is rather than recomputing it each time.
1964 However, if the current directory is
1966 the built-in version of
1968 will continue to print the old name for the directory.
1972 option is specified, symbolic links are resolved.
1975 option is specified, the shell's notion of the current directory
1976 is printed (symbolic links are not resolved).
1977 This is the default.
1978 .It Ic read Oo Fl p Ar prompt Oc Oo
1979 .Fl t Ar timeout Oc Oo Fl er Oc Ar variable ...
1985 and the standard input is a terminal.
1987 read from the standard input.
1988 The trailing newline
1989 is deleted from the line and the line is split as
1990 described in the section on
1991 .Sx White Space Splitting (Field Splitting)
1993 the pieces are assigned to the variables in order.
1994 If there are more pieces than variables, the remaining
1995 pieces (along with the characters in
1997 that separated them)
1998 are assigned to the last variable.
1999 If there are more variables than pieces, the remaining
2000 variables are assigned the null string.
2002 Backslashes are treated specially, unless the
2006 If a backslash is followed by
2007 a newline, the backslash and the newline will be
2009 If a backslash is followed by any other
2010 character, the backslash will be deleted and the following
2011 character will be treated as though it were not in
2017 option is specified and the
2019 elapses before a complete line of input is supplied,
2022 command will return an exit status of 1 without assigning any values.
2025 value may optionally be followed by one of
2030 to explicitly specify seconds, minutes or hours.
2031 If none is supplied,
2037 option exists only for backward compatibility with older scripts.
2038 .It Ic readonly Oo Fl p Oc Op Ar name ...
2041 is marked as read only,
2042 so that it cannot be subsequently modified or unset.
2043 The shell allows the value of a variable to be set
2044 at the same time as it is marked read only
2045 by using the following form:
2047 .D1 Ic readonly Ar name Ns = Ns Ar value
2049 With no arguments the
2051 command lists the names of all read only variables.
2054 option is specified, the read-only variables are printed as
2055 .Dq Ic readonly Ar name Ns = Ns Ar value
2056 lines, suitable for re-input to the shell.
2057 .It Ic return Op Ar exitstatus
2061 .It Ic set Oo Fl /+abCEefIimnpTuVvx Oc Oo Fl /+o Ar longname Oc Oo
2062 .Fl c Ar string Oc Op Fl - Ar arg ...
2065 command performs three different functions:
2068 With no arguments, it lists the values of all shell variables.
2070 If options are given,
2071 either in short form or using the long
2072 .Dq Fl /+o Ar longname
2074 it sets or clears the specified options as described in the section called
2075 .Sx Argument List Processing .
2079 option is specified,
2081 will replace the shell's positional parameters with the subsequent
2083 If no arguments follow the
2086 all the positional parameters will be cleared,
2087 which is equivalent to executing the command
2091 flag may be omitted when specifying arguments to be used
2092 as positional replacement parameters.
2093 This is not recommended,
2094 because the first argument may begin with a dash
2100 command will interpret as a request to enable or disable options.
2102 .It Ic setvar Ar variable value
2103 Assigns the specified
2109 command is intended to be used in functions that
2110 assign values to variables whose names are passed as parameters.
2111 In general it is better to write
2112 .Dq Ar variable Ns = Ns Ar value
2115 .It Ic shift Op Ar n
2116 Shift the positional parameters
2121 A shift sets the value of
2130 decreasing the value of
2133 If there are zero positional parameters, shifting does not do anything.
2135 A built-in equivalent of
2138 Print the amount of time spent executing the shell and its children.
2139 The first output line shows the user and system times for the shell
2140 itself, the second one contains the user and system times for the
2142 .It Ic trap Oo Ar action Oc Ar signal ...
2144 Cause the shell to parse and execute
2149 The signals are specified by name or number.
2150 In addition, the pseudo-signal
2152 may be used to specify an
2154 that is performed when the shell terminates.
2157 may be an empty string or a dash
2159 the former causes the specified signal to be ignored
2160 and the latter causes the default action to be taken.
2163 is another way to request the default action, for compatibility reasons this
2164 usage is not recommended though.
2165 When the shell forks off a subshell,
2166 it resets trapped (but not ignored) signals to the default action.
2169 command has no effect on signals that were ignored on entry to the shell.
2175 command to display a list of valid signal names.
2177 A null command that returns a 0 (true) exit value.
2178 .It Ic type Op Ar name ...
2181 as a command and print the resolution of the command search.
2182 Possible resolutions are:
2183 shell keyword, alias, special shell builtin, shell builtin, command,
2186 For aliases the alias expansion is printed;
2187 for commands and tracked aliases
2188 the complete pathname of the command is printed.
2189 .It Ic ulimit Oo Fl HSabcdflmnpstuvw Oc Op Ar limit
2190 Set or display resource limits (see
2194 is specified, the named resource will be set;
2195 otherwise the current resource value will be displayed.
2199 is specified, the hard limits will be set or displayed.
2200 While everybody is allowed to reduce a hard limit,
2201 only the superuser can increase it.
2205 specifies the soft limits instead.
2206 When displaying limits,
2212 The default is to display the soft limits,
2213 and to set both the hard and the soft limits.
2219 command to display all resources.
2222 is not acceptable in this mode.
2224 The remaining options specify which resource value is to be
2225 displayed or modified.
2226 They are mutually exclusive.
2227 .Bl -tag -width indent
2229 The maximum size of socket buffer usage, in bytes.
2230 .It Fl c Ar coredumpsize
2231 The maximal size of core dump files, in 512-byte blocks.
2232 .It Fl d Ar datasize
2233 The maximal size of the data segment of a process, in kilobytes.
2234 .It Fl f Ar filesize
2235 The maximal size of a file, in 512-byte blocks.
2236 .It Fl l Ar lockedmem
2237 The maximal size of memory that can be locked by a process, in
2239 .It Fl m Ar memoryuse
2240 The maximal resident set size of a process, in kilobytes.
2242 The maximal number of descriptors that could be opened by a process.
2243 .It Fl p Ar pseudoterminals
2244 The maximal number of pseudo-terminals for this user ID.
2245 .It Fl s Ar stacksize
2246 The maximal size of the stack segment, in kilobytes.
2248 The maximal amount of CPU time to be used by each process, in seconds.
2249 .It Fl u Ar userproc
2250 The maximal number of simultaneous processes for this user ID.
2251 .It Fl v Ar virtualmem
2252 The maximal virtual size of a process, in kilobytes.
2254 The maximum amount of swap space reserved or used for this user ID,
2257 .It Ic umask Oo Fl S Oc Op Ar mask
2258 Set the file creation mask (see
2260 to the octal or symbolic (see
2264 If the argument is omitted, the current mask value is printed.
2267 option is specified, the output is symbolic, otherwise the output is octal.
2268 .It Ic unalias Oo Fl a Oc Op Ar name ...
2269 The specified alias names are removed.
2272 is specified, all aliases are removed.
2273 .It Ic unset Oo Fl fv Oc Ar name ...
2274 The specified variables or functions are unset and unexported.
2277 option is specified or no options are given, the
2279 arguments are treated as variable names.
2282 option is specified, the
2284 arguments are treated as function names.
2285 .It Ic wait Op Ar job
2286 Wait for the specified
2288 to complete and return the exit status of the last process in the
2290 If the argument is omitted, wait for all jobs to complete
2291 and return an exit status of zero.
2293 .Ss Commandline Editing
2296 is being used interactively from a terminal, the current command
2297 and the command history
2301 .Sx Built-in Commands )
2304 command line editing.
2305 This mode uses commands similar
2306 to a subset of those described in the
2324 can be switched between insert mode and command mode by typing
2328 while in command mode will pass the line to the shell.
2331 .Dq Li "set -o emacs"
2334 command can be used to enable a subset of
2336 command line editing features.
2338 The following environment variables affect the execution of
2340 .Bl -tag -width ".Ev HISTSIZE"
2342 The search path used with the
2346 The fallback editor used with the
2349 If not set, the default editor is
2352 The default editor used with the
2356 The number of previous commands that are accessible.
2358 The starting directory of
2361 Input Field Separators.
2362 This is normally set to
2368 .Sx White Space Splitting
2369 section for more details.
2371 The name of a mail file, that will be checked for the arrival of new
2378 separated list of file names, for the shell to check for incoming
2380 This environment setting overrides the
2383 There is a maximum of 10 mailboxes that can be monitored at once.
2385 The default search path for executables.
2388 section for details.
2390 The primary prompt string, which defaults to
2392 unless you are the superuser, in which case it defaults to
2395 The secondary prompt string, which defaults to
2398 The prefix for the trace output (if
2404 The default terminal setting for the shell.
2405 This is inherited by children of the shell, and is used in the history
2409 Errors that are detected by the shell, such as a syntax error, will
2410 cause the shell to exit with a non-zero exit status.
2411 If the shell is not an interactive shell, the execution of the shell
2412 file will be aborted.
2413 Otherwise the shell will return the exit status of the last command
2416 builtin is used with a numeric argument, it
2417 will return the argument.
2434 command, the Thompson shell, appeared in
2436 It was superseded in
2438 by the Bourne shell, which inherited the name
2443 was rewritten in 1989 under the
2445 license after the Bourne shell from
2450 was originally written by
2451 .An Kenneth Almquist .
2455 utility does not recognize multibyte characters.
projects / FreeBSD / releng / 8.2.git / blob