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44 .Nd command interpreter (shell)
47 .Op Fl /+abCEefIimnPpTuVvx
48 .Op Fl /+o Ar longname
54 .Op Fl /+abCEefIimnPpTuVvx
55 .Op Fl /+o Ar longname
62 .Op Fl /+abCEefIimnPpTuVvx
63 .Op Fl /+o Ar longname
69 utility is the standard command interpreter for the system.
70 The current version of
74 specification for the shell.
75 It only supports features
78 plus a few Berkeley extensions.
79 This man page is not intended to be a tutorial nor a complete
80 specification of the shell.
82 The shell is a command that reads lines from
83 either a file or the terminal, interprets them, and
84 generally executes other commands.
85 It is the program that is started when a user logs into the system,
86 although a user can select a different shell with the
90 implements a language that has flow control constructs,
91 a macro facility that provides a variety of features in
92 addition to data storage, along with built-in history and line
94 It incorporates many features to
95 aid interactive use and has the advantage that the interpretative
96 language is common to both interactive and non-interactive
98 That is, commands can be typed directly
99 to the running shell or can be put into a file,
100 which can be executed directly by the shell.
103 .\" XXX This next sentence is incredibly confusing.
105 If no arguments are present and if the standard input of the shell
106 is connected to a terminal
110 the shell is considered an interactive shell.
112 generally prompts before each command and handles programming
113 and command errors differently (as described below).
114 When first starting, the shell inspects argument 0, and
115 if it begins with a dash
117 the shell is also considered a login shell.
118 This is normally done automatically by the system
119 when the user first logs in.
120 A login shell first reads commands
125 in a user's home directory,
127 If the environment variable
129 is set on entry to a shell, or is set in the
131 of a login shell, the shell then reads commands from the file named in
133 Therefore, a user should place commands that are to be executed only
136 file, and commands that are executed for every shell inside the
141 variable to some file by placing the following line in the file
143 in the home directory,
146 the filename desired:
148 .Dl "ENV=$HOME/.shinit; export ENV"
150 The first non-option argument specified on the command line
151 will be treated as the
152 name of a file from which to read commands (a shell script), and
153 the remaining arguments are set as the positional parameters
157 Otherwise, the shell reads commands
158 from its standard input.
160 Unlike older versions of
164 script is only sourced on invocation of interactive shells.
166 closes a well-known, and sometimes easily exploitable security
167 hole related to poorly thought out
170 .Ss Argument List Processing
171 All of the single letter options to
173 have a corresponding long name,
174 with the exception of
178 These long names are provided next to the single letter options
179 in the descriptions below.
180 The long name for an option may be specified as an argument to the
184 Once the shell is running,
185 the long name for an option may be specified as an argument to the
190 (described later in the section called
191 .Sx Built-in Commands ) .
192 Introducing an option with a dash
202 will stop option processing and will force the remaining
203 words on the command line to be treated as arguments.
208 options do not have long names.
209 They take arguments and are described after the single letter options.
210 .Bl -tag -width indent
211 .It Fl a Li allexport
212 Flag variables for export when assignments are made to them.
214 Enable asynchronous notification of background job
217 .It Fl C Li noclobber
218 Do not overwrite existing files with
223 command line editor (disables the
225 option if it has been set;
226 set automatically when interactive on terminals).
228 Exit immediately if any untested command fails in non-interactive mode.
229 The exit status of a command is considered to be
230 explicitly tested if the command is part of the list used to control
232 .Ic if , elif , while ,
235 if the command is the left
240 operator; or if the command is a pipeline preceded by the
243 If a shell function is executed and its exit status is explicitly
244 tested, all commands of the function are considered to be tested as
247 Disable pathname expansion.
248 .It Fl I Li ignoreeof
251 from input when in interactive mode.
252 .It Fl i Li interactive
253 Force the shell to behave interactively.
255 Turn on job control (set automatically when interactive).
257 If not interactive, read commands but do not
259 This is useful for checking the
260 syntax of shell scripts.
262 Change the default for the
268 (logical directory layout)
271 (physical directory layout).
272 .It Fl p Li privileged
273 Turn on privileged mode.
274 This mode is enabled on startup
275 if either the effective user or group ID is not equal to the
276 real user or group ID.
277 Turning this mode off sets the
278 effective user and group IDs to the real user and group IDs.
279 When this mode is enabled for interactive shells, the file
280 .Pa /etc/suid_profile
281 is sourced instead of
285 is sourced, and the contents of the
287 variable are ignored.
289 Read commands from standard input (set automatically
290 if no file arguments are present).
292 no effect when set after the shell has already started
293 running (i.e., when set with the
296 .It Fl T Li trapsasync
297 When waiting for a child, execute traps immediately.
298 If this option is not set,
299 traps are executed after the child exits,
302 This nonstandard option is useful for putting guarding shells around
303 children that block signals.
304 The surrounding shell may kill the child
305 or it may just return control to the tty and leave the child alone,
307 .Bd -literal -offset indent
308 sh -T -c "trap 'exit 1' 2 ; some-blocking-program"
311 Write a message to standard error when attempting
312 to expand a variable, a positional parameter or
313 the special parameter
315 that is not set, and if the
316 shell is not interactive, exit immediately.
320 command line editor (disables
324 The shell writes its input to standard error
326 Useful for debugging.
329 (preceded by the value of the
332 to standard error before it is executed.
333 Useful for debugging.
334 .It "\ \ " Em tabcomplete
335 Enables filename completion in the command line editor.
336 Typing a tab character will extend the current input word to match a
338 If more than one filename matches it is only extended to be the common prefix.
339 Typing a second tab character will list all the matching names.
340 Turned on by default in an interactive shell.
345 option causes the commands to be read from the
347 operand instead of from the standard input.
348 Keep in mind that this option only accepts a single string as its
349 argument, hence multi-word strings must be quoted.
353 option takes as its only argument the long name of an option
354 to be enabled or disabled.
355 For example, the following two invocations of
357 both enable the built-in
360 .Bd -literal -offset indent
365 If used without an argument, the
367 option displays the current option settings in a human-readable format.
370 is used without an argument, the current option settings are output
371 in a format suitable for re-input into the shell.
372 .Ss Lexical Structure
373 The shell reads input in terms of lines from a file and breaks
374 it up into words at whitespace (blanks and tabs), and at
378 which are special to the shell.
379 There are two types of operators: control operators and
380 redirection operators (their meaning is discussed later).
381 The following is a list of valid operators:
382 .Bl -tag -width indent
383 .It Control operators:
384 .Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
385 .It Li & Ta Li && Ta Li ( Ta Li ) Ta Li \en
386 .It Li ;; Ta Li ; Ta Li | Ta Li ||
388 .It Redirection operators:
389 .Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
390 .It Li < Ta Li > Ta Li << Ta Li >> Ta Li <>
391 .It Li <& Ta Li >& Ta Li <<- Ta Li >|
397 introduces a comment if used at the beginning of a word.
398 The word starting with
400 and the rest of the line are ignored.
404 characters (character code 0) are not allowed in shell input.
406 Quoting is used to remove the special meaning of certain characters
407 or words to the shell, such as operators, whitespace, keywords,
410 There are three types of quoting: matched single quotes,
411 matched double quotes, and backslash.
412 .Bl -tag -width indent
414 Enclosing characters in single quotes preserves the literal
415 meaning of all the characters (except single quotes, making
416 it impossible to put single-quotes in a single-quoted string).
418 Enclosing characters within double quotes preserves the literal
419 meaning of all characters except dollar sign
425 The backslash inside double quotes is historically weird.
426 It remains literal unless it precedes the following characters,
427 which it serves to quote:
428 .Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
429 .It Li $ Ta Li ` Ta Li \&" Ta Li \e\ Ta Li \en
432 A backslash preserves the literal meaning of the following
433 character, with the exception of the newline character
435 A backslash preceding a newline is treated as a line continuation.
438 Keywords or reserved words are words that have special meaning to the
439 shell and are recognized at the beginning of a line and
440 after a control operator.
441 The following are keywords:
442 .Bl -column "doneXX" "elifXX" "elseXX" "untilXX" "whileX" -offset center
443 .It Li \&! Ta { Ta } Ta Ic case Ta Ic do
444 .It Ic done Ta Ic elif Ta Ic else Ta Ic esac Ta Ic fi
445 .It Ic for Ta Ic if Ta Ic then Ta Ic until Ta Ic while
448 An alias is a name and corresponding value set using the
451 Whenever a keyword may occur (see above),
452 and after checking for keywords, the shell
453 checks the word to see if it matches an alias.
454 If it does, it replaces it in the input stream with its value.
455 For example, if there is an alias called
467 Aliases provide a convenient way for naive users to
468 create shorthands for commands without having to learn how
469 to create functions with arguments.
470 Using aliases in scripts is discouraged
471 because the command that defines them must be executed
472 before the code that uses them is parsed.
473 This is fragile and not portable.
475 An alias name may be escaped in a command line, so that it is not
476 replaced by its alias value, by using quoting characters within or
477 adjacent to the alias name.
478 This is most often done by prefixing
479 an alias name with a backslash to execute a function, built-in, or
480 normal program with the same name.
485 The shell interprets the words it reads according to a
486 language, the specification of which is outside the scope
487 of this man page (refer to the BNF in the
490 Essentially though, a line is read and if
491 the first word of the line (or after a control operator)
492 is not a keyword, then the shell has recognized a
494 Otherwise, a complex command or some
495 other special construct may have been recognized.
497 If a simple command has been recognized, the shell performs
498 the following actions:
501 Leading words of the form
503 are stripped off and assigned to the environment of
505 Redirection operators and
506 their arguments (as described below) are stripped
507 off and saved for processing.
509 The remaining words are expanded as described in
511 .Sx Word Expansions ,
512 and the first remaining word is considered the command
513 name and the command is located.
515 words are considered the arguments of the command.
516 If no command name resulted, then the
518 variable assignments recognized in 1) affect the
521 Redirections are performed as described in
525 Redirections are used to change where a command reads its input
527 In general, redirections open, close, or
528 duplicate an existing reference to a file.
530 used for redirection is:
532 .D1 Oo Ar n Oc Ar redir-op file
536 is one of the redirection operators mentioned
538 The following gives some examples of how these
539 operators can be used.
540 Note that stdin and stdout are commonly used abbreviations
541 for standard input and standard output respectively.
542 .Bl -tag -width "1234567890XX" -offset indent
543 .It Oo Ar n Oc Ns Li > Ar file
544 redirect stdout (or file descriptor
548 .It Oo Ar n Oc Ns Li >| Ar file
549 same as above, but override the
552 .It Oo Ar n Oc Ns Li >> Ar file
553 append stdout (or file descriptor
557 .It Oo Ar n Oc Ns Li < Ar file
558 redirect stdin (or file descriptor
562 .It Oo Ar n Oc Ns Li <> Ar file
563 redirect stdin (or file descriptor
567 .It Oo Ar n1 Oc Ns Li <& Ns Ar n2
568 duplicate stdin (or file descriptor
572 .It Oo Ar n Oc Ns Li <&-
573 close stdin (or file descriptor
575 .It Oo Ar n1 Oc Ns Li >& Ns Ar n2
576 duplicate stdout (or file descriptor
580 .It Oo Ar n Oc Ns Li >&-
581 close stdout (or file descriptor
585 The following redirection is often called a
587 .Bd -unfilled -offset indent
588 .Oo Ar n Oc Ns Li << Ar delimiter
594 All the text on successive lines up to the delimiter is
595 saved away and made available to the command on standard
596 input, or file descriptor
601 as specified on the initial line is quoted, then the
603 is treated literally, otherwise the text is subjected to
604 parameter expansion, command substitution, and arithmetic
605 expansion (as described in the section on
606 .Sx Word Expansions ) .
615 .Ss Search and Execution
616 There are three types of commands: shell functions,
617 built-in commands, and normal programs.
618 The command is searched for (by name) in that order.
619 The three types of commands are all executed in a different way.
621 When a shell function is executed, all of the shell positional
624 which remains unchanged) are
625 set to the arguments of the shell function.
626 The variables which are explicitly placed in the environment of
627 the command (by placing assignments to them before the
628 function name) are made local to the function and are set
630 Then the command given in the function definition is executed.
631 The positional parameters are restored to their original values
632 when the command completes.
633 This all occurs within the current shell.
635 Shell built-in commands are executed internally to the shell, without
636 spawning a new process.
637 There are two kinds of built-in commands: regular and special.
638 Assignments before special builtins persist after they finish
639 executing and assignment errors, redirection errors and certain
640 operand errors cause a script to be aborted.
641 Special builtins cannot be overridden with a function.
642 Both regular and special builtins can affect the shell in ways
643 normal programs cannot.
645 Otherwise, if the command name does not match a function
646 or built-in command, the command is searched for as a normal
647 program in the file system (as described in the next section).
648 When a normal program is executed, the shell runs the program,
649 passing the arguments and the environment to the program.
650 If the program is not a normal executable file
651 (i.e., if it does not begin with the
661 but appears to be a text file,
662 the shell will run a new instance of
666 Note that previous versions of this document
667 and the source code itself misleadingly and sporadically
668 refer to a shell script without a magic number
670 .Dq "shell procedure" .
672 When locating a command, the shell first looks to see if
673 it has a shell function by that name.
675 built-in command by that name.
676 If a built-in command is not found,
677 one of two things happen:
680 Command names containing a slash are simply executed without
681 performing any searches.
683 The shell searches each entry in the
686 in turn for the command.
689 variable should be a series of
690 entries separated by colons.
691 Each entry consists of a
693 The current directory
694 may be indicated implicitly by an empty directory name,
695 or explicitly by a single period.
697 .Ss Command Exit Status
698 Each command has an exit status that can influence the behavior
699 of other shell commands.
700 The paradigm is that a command exits
701 with zero for normal or success, and non-zero for failure,
702 error, or a false indication.
703 The man page for each command
704 should indicate the various exit codes and what they mean.
705 Additionally, the built-in commands return exit codes, as does
706 an executed shell function.
708 If a command is terminated by a signal, its exit status is 128 plus
710 Signal numbers are defined in the header file
713 Complex commands are combinations of simple commands
714 with control operators or keywords, together creating a larger complex
716 More generally, a command is one of the following:
717 .Bl -item -offset indent
723 list or compound-list
730 Unless otherwise stated, the exit status of a command is
731 that of the last simple command executed by the command.
733 A pipeline is a sequence of one or more commands separated
734 by the control operator
736 The standard output of all but
737 the last command is connected to the standard input
739 The standard output of the last
740 command is inherited from the shell, as usual.
742 The format for a pipeline is:
744 .D1 Oo Li \&! Oc Ar command1 Op Li \&| Ar command2 ...
746 The standard output of
748 is connected to the standard input of
750 The standard input, standard output, or
751 both of a command is considered to be assigned by the
752 pipeline before any redirection specified by redirection
753 operators that are part of the command.
755 Note that unlike some other shells,
757 executes each process in a pipeline with more than one command
758 in a subshell environment and as a child of the
762 If the pipeline is not in the background (discussed later),
763 the shell waits for all commands to complete.
767 does not precede the pipeline, the
768 exit status is the exit status of the last command specified
770 Otherwise, the exit status is the logical
771 NOT of the exit status of the last command.
773 the last command returns zero, the exit status is 1; if
774 the last command returns greater than zero, the exit status
777 Because pipeline assignment of standard input or standard
778 output or both takes place before redirection, it can be
779 modified by redirection.
782 .Dl "command1 2>&1 | command2"
784 sends both the standard output and standard error of
786 to the standard input of
791 or newline terminator causes the preceding
793 (described below in the section called
794 .Sx Short-Circuit List Operators )
795 to be executed sequentially;
798 causes asynchronous execution of the preceding AND-OR-list.
799 .Ss Background Commands (&)
800 If a command is terminated by the control operator ampersand
802 the shell executes the command asynchronously;
803 the shell does not wait for the command to finish
804 before executing the next command.
806 The format for running a command in background is:
808 .D1 Ar command1 Li & Op Ar command2 Li & Ar ...
810 If the shell is not interactive, the standard input of an
811 asynchronous command is set to
813 .Ss Lists (Generally Speaking)
814 A list is a sequence of zero or more commands separated by
815 newlines, semicolons, or ampersands,
816 and optionally terminated by one of these three characters.
818 list are executed in the order they are written.
819 If command is followed by an ampersand, the shell starts the
820 command and immediately proceeds onto the next command;
821 otherwise it waits for the command to terminate before
822 proceeding to the next one.
823 .Ss Short-Circuit List Operators
827 are AND-OR list operators.
829 executes the first command, and then executes the second command
830 if the exit status of the first command is zero.
832 is similar, but executes the second command if the exit
833 status of the first command is nonzero.
837 both have the same priority.
838 .Ss Flow-Control Constructs (if, while, for, case)
842 .Bd -unfilled -offset indent -compact
846 .Ic then Ar list Oc Ar ...
854 .Bd -unfilled -offset indent -compact
860 The two lists are executed repeatedly while the exit status of the
864 command is similar, but has the word
869 repeat until the exit status of the first list is zero.
874 .Bd -unfilled -offset indent -compact
875 .Ic for Ar variable Op Ic in Ar word ...
882 and the following words are omitted,
885 The words are expanded, and then the list is executed
886 repeatedly with the variable set to each word in turn.
891 commands may be replaced with
901 .D1 Ic break Op Ar num
902 .D1 Ic continue Op Ar num
906 command terminates the
915 command continues with the next iteration of the innermost loop.
916 These are implemented as special built-in commands.
921 .Bd -unfilled -offset indent -compact
922 .Ic case Ar word Ic in
923 .Ar pattern Ns Li ) Ar list Li ;;
928 The pattern can actually be one or more patterns
937 command is the exit code of the last command executed in the list or
938 zero if no patterns were matched.
939 .Ss Grouping Commands Together
940 Commands may be grouped by writing either
942 .D1 Li \&( Ns Ar list Ns Li \%)
946 .D1 Li { Ar list Ns Li \&; }
948 The first form executes the commands in a subshell environment.
949 Note that built-in commands thus executed do not affect the current shell.
950 The second form never forks another shell,
951 so it is slightly more efficient.
952 Grouping commands together this way allows the user to
953 redirect their output as though they were one program:
954 .Bd -literal -offset indent
955 { echo -n "hello"; echo " world"; } > greeting
958 The syntax of a function definition is
960 .D1 Ar name Li \&( \&) Ar command
962 A function definition is an executable statement; when
963 executed it installs a function named
975 Variables may be declared to be local to a function by
979 This should appear as the first statement of a function,
982 .D1 Ic local Oo Ar variable ... Oc Op Fl
986 command is implemented as a built-in command.
988 When a variable is made local, it inherits the initial
989 value and exported and readonly flags from the variable
990 with the same name in the surrounding scope, if there is
992 Otherwise, the variable is initially unset.
994 uses dynamic scoping, so that if the variable
996 is made local to function
998 which then calls function
1000 references to the variable
1004 will refer to the variable
1008 not to the global variable named
1011 The only special parameter that can be made local is
1015 local causes any shell options that are
1018 command inside the function to be
1019 restored to their original values when the function
1026 .D1 Ic return Op Ar exitstatus
1028 It terminates the current executional scope, returning from the previous
1029 nested function, sourced script, or shell instance, in that order.
1032 command is implemented as a special built-in command.
1033 .Ss Variables and Parameters
1034 The shell maintains a set of parameters.
1036 denoted by a name is called a variable.
1038 the shell turns all the environment variables into shell
1040 New variables can be set using the form
1042 .D1 Ar name Ns = Ns Ar value
1044 Variables set by the user must have a name consisting solely
1045 of alphabetics, numerics, and underscores.
1046 The first letter of a variable name must not be numeric.
1047 A parameter can also be denoted by a number
1048 or a special character as explained below.
1049 .Ss Positional Parameters
1050 A positional parameter is a parameter denoted by a number greater than zero.
1051 The shell sets these initially to the values of its command line
1052 arguments that follow the name of the shell script.
1055 built-in command can also be used to set or reset them.
1056 .Ss Special Parameters
1057 Special parameters are parameters denoted by a single special character
1059 They are shown in the following list, exactly as they would appear in input
1060 typed by the user or in the source of a shell script.
1063 Expands to the positional parameters, starting from one.
1065 the expansion occurs within a double-quoted string
1066 it expands to a single field with the value of each parameter
1067 separated by the first character of the
1074 Expands to the positional parameters, starting from one.
1076 the expansion occurs within double-quotes, each positional
1077 parameter expands as a separate argument.
1078 If there are no positional parameters, the
1081 generates zero arguments, even when
1084 What this basically means, for example, is
1097 .Bd -literal -offset indent
1101 Expands to the number of positional parameters.
1103 Expands to the exit status of the most recent pipeline.
1105 (hyphen) Expands to the current option flags (the single-letter
1106 option names concatenated into a string) as specified on
1109 built-in command, or implicitly
1112 Expands to the process ID of the invoked shell.
1114 retains the same value of
1118 Expands to the process ID of the most recent background
1119 command executed from the current shell.
1121 pipeline, the process ID is that of the last command in the
1123 If this parameter is referenced, the shell will remember
1124 the process ID and its exit status until the
1126 built-in command reports completion of the process.
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 This is normally set to
1168 .Sx White Space Splitting
1169 section for more details.
1171 The current line number in the script or function.
1173 The name of a mail file, that will be checked for the arrival of new
1180 separated list of file names, for the shell to check for incoming
1182 This variable overrides the
1185 There is a maximum of 10 mailboxes that can be monitored at once.
1187 The default search path for executables.
1190 section for details.
1192 The parent process ID of the invoked shell.
1193 This is set at startup
1194 unless this variable is in the environment.
1195 A later change of parent process ID is not reflected.
1196 A subshell retains the same value of
1199 The primary prompt string, which defaults to
1201 unless you are the superuser, in which case it defaults to
1204 The secondary prompt string, which defaults to
1207 The prefix for the trace output (if
1214 This clause describes the various expansions that are
1216 Not all expansions are performed on
1217 every word, as explained later.
1219 Tilde expansions, parameter expansions, command substitutions,
1220 arithmetic expansions, and quote removals that occur within
1221 a single word expand to a single field.
1223 splitting or pathname expansion that can create multiple
1224 fields from a single word.
1225 The single exception to this rule is
1226 the expansion of the special parameter
1228 within double-quotes,
1229 as was described above.
1231 The order of word expansion is:
1234 Tilde Expansion, Parameter Expansion, Command Substitution,
1235 Arithmetic Expansion (these all occur at the same time).
1237 Field Splitting is performed on fields generated by step (1)
1242 Pathname Expansion (unless the
1244 option is in effect).
1251 character is used to introduce parameter expansion, command
1252 substitution, or arithmetic expansion.
1253 .Ss Tilde Expansion (substituting a user's home directory)
1254 A word beginning with an unquoted tilde character
1257 subjected to tilde expansion.
1258 All the characters up to a slash
1260 or the end of the word are treated as a username
1261 and are replaced with the user's home directory.
1263 username is missing (as in
1265 the tilde is replaced with the value of the
1267 variable (the current user's home directory).
1268 .Ss Parameter Expansion
1269 The format for parameter expansion is as follows:
1271 .D1 Li ${ Ns Ar expression Ns Li }
1275 consists of all characters until the matching
1279 escaped by a backslash or within a single-quoted or double-quoted
1280 string, and characters in
1281 embedded arithmetic expansions, command substitutions, and variable
1282 expansions, are not examined in determining the matching
1284 If the variants with
1290 occur within a double-quoted string,
1291 as an extension there may be unquoted parts
1292 (via double-quotes inside the expansion);
1294 within such parts are also not examined in determining the matching
1297 The simplest form for parameter expansion is:
1299 .D1 Li ${ Ns Ar parameter Ns Li }
1301 The value, if any, of
1305 The parameter name or symbol can be enclosed in braces, which are
1306 optional except for positional parameters with more than one digit or
1307 when parameter is followed by a character that could be interpreted as
1309 If a parameter expansion occurs inside double-quotes:
1312 Pathname expansion is not performed on the results of the
1315 Field splitting is not performed on the results of the
1316 expansion, with the exception of the special parameter
1320 In addition, a parameter expansion can be modified by using one of the
1322 .Bl -tag -width indent
1323 .It Li ${ Ns Ar parameter Ns Li :- Ns Ar word Ns Li }
1327 is unset or null, the expansion of
1329 is substituted; otherwise, the value of
1332 .It Li ${ Ns Ar parameter Ns Li := Ns Ar word Ns Li }
1333 Assign Default Values.
1336 is unset or null, the expansion of
1346 does not prevent field splitting or pathname expansion.
1347 Only variables, not positional
1348 parameters or special parameters, can be
1349 assigned in this way.
1350 .It Li ${ Ns Ar parameter Ns Li :? Ns Oo Ar word Oc Ns Li }
1351 Indicate Error if Null or Unset.
1354 is unset or null, the expansion of
1356 (or a message indicating it is unset if
1358 is omitted) is written to standard
1359 error and the shell exits with a nonzero
1361 Otherwise, the value of
1365 interactive shell need not exit.
1366 .It Li ${ Ns Ar parameter Ns Li :+ Ns Ar word Ns Li }
1367 Use Alternate Value.
1370 is unset or null, null is substituted;
1371 otherwise, the expansion of
1376 In the parameter expansions shown previously, use of the colon in the
1377 format results in a test for a parameter that is unset or null; omission
1378 of the colon results in a test for a parameter that is only unset.
1382 inherits the type of quoting
1383 (unquoted, double-quoted or here-document)
1384 from the surroundings,
1385 with the exception that a backslash that quotes a closing brace is removed
1386 during quote removal.
1387 .Bl -tag -width indent
1388 .It Li ${# Ns Ar parameter Ns Li }
1390 The length in characters of
1395 The following four varieties of parameter expansion provide for substring
1397 In each case, pattern matching notation
1399 .Sx Shell Patterns ) ,
1400 rather than regular expression notation,
1401 is used to evaluate the patterns.
1402 If parameter is one of the special parameters
1406 the result of the expansion is unspecified.
1407 Enclosing the full parameter expansion string in double-quotes does not
1408 cause the following four varieties of pattern characters to be quoted,
1409 whereas quoting characters within the braces has this effect.
1410 .Bl -tag -width indent
1411 .It Li ${ Ns Ar parameter Ns Li % Ns Ar word Ns Li }
1412 Remove Smallest Suffix Pattern.
1415 is expanded to produce a pattern.
1417 parameter expansion then results in
1419 with the smallest portion of the
1420 suffix matched by the pattern deleted.
1421 .It Li ${ Ns Ar parameter Ns Li %% Ns Ar word Ns Li }
1422 Remove Largest Suffix Pattern.
1425 is expanded to produce a pattern.
1427 parameter expansion then results in
1429 with the largest portion of the
1430 suffix matched by the pattern deleted.
1431 .It Li ${ Ns Ar parameter Ns Li # Ns Ar word Ns Li }
1432 Remove Smallest Prefix Pattern.
1435 is expanded to produce a pattern.
1437 parameter expansion then results in
1439 with the smallest portion of the
1440 prefix matched by the pattern deleted.
1441 .It Li ${ Ns Ar parameter Ns Li ## Ns Ar word Ns Li }
1442 Remove Largest Prefix Pattern.
1445 is expanded to produce a pattern.
1447 parameter expansion then results in
1449 with the largest portion of the
1450 prefix matched by the pattern deleted.
1452 .Ss Command Substitution
1453 Command substitution allows the output of a command to be substituted in
1454 place of the command name itself.
1455 Command substitution occurs when
1456 the command is enclosed as follows:
1458 .D1 Li $( Ns Ar command Ns Li )\&
1460 or the backquoted version:
1462 .D1 Li ` Ns Ar command Ns Li `
1464 The shell expands the command substitution by executing command
1465 and replacing the command substitution
1466 with the standard output of the command,
1467 removing sequences of one or more newlines at the end of the substitution.
1468 Embedded newlines before the end of the output are not removed;
1469 however, during field splitting, they may be translated into spaces
1470 depending on the value of
1472 and the quoting that is in effect.
1473 The command is executed in a subshell environment,
1474 except that the built-in commands
1480 return information about the main shell environment
1481 if they are the only command in a command substitution
1482 and the substitutions in the command cannot cause side effects
1483 (such as from assigning values to variables or referencing
1485 .Ss Arithmetic Expansion
1486 Arithmetic expansion provides a mechanism for evaluating an arithmetic
1487 expression and substituting its value.
1488 The format for arithmetic expansion is as follows:
1490 .D1 Li $(( Ns Ar expression Ns Li ))
1494 is treated as if it were in double-quotes, except
1495 that a double-quote inside the expression is not treated specially.
1497 shell expands all tokens in the
1499 for parameter expansion,
1500 command substitution,
1501 arithmetic expansion
1504 The allowed expressions are a subset of C expressions,
1506 .Bl -tag -width "Variables" -offset indent
1508 All values are of type
1511 Decimal, octal (starting with
1513 and hexadecimal (starting with
1517 Shell variables can be read and written
1518 and contain integer constants.
1521 .It Binary operators
1522 .Li "* / % + - << >> < <= > >= == != & ^ | && ||"
1523 .It Assignment operators
1524 .Li "= += -= *= /= %= <<= >>= &= ^= |="
1525 .It Conditional operator
1529 The result of the expression is substituted in decimal.
1530 .Ss White Space Splitting (Field Splitting)
1531 After parameter expansion, command substitution, and
1532 arithmetic expansion the shell scans the results of
1533 expansions and substitutions that did not occur in double-quotes for
1534 field splitting and multiple fields can result.
1536 The shell treats each character of the
1538 variable as a delimiter and uses
1539 the delimiters to split the results of parameter expansion and command
1540 substitution into fields.
1541 .Ss Pathname Expansion (File Name Generation)
1545 file name generation is performed
1546 after word splitting is complete.
1548 viewed as a series of patterns, separated by slashes.
1550 process of expansion replaces the word with the names of
1551 all existing files whose names can be formed by replacing
1552 each pattern with a string that matches the specified pattern.
1553 There are two restrictions on this: first, a pattern cannot match
1554 a string containing a slash, and second,
1555 a pattern cannot match a string starting with a period
1556 unless the first character of the pattern is a period.
1557 The next section describes the patterns used for both
1558 Pathname Expansion and the
1562 A pattern consists of normal characters, which match themselves,
1563 and meta-characters.
1564 The meta-characters are
1570 These characters lose their special meanings if they are quoted.
1571 When command or variable substitution is performed and the dollar sign
1572 or back quotes are not double-quoted, the value of the
1573 variable or the output of the command is scanned for these
1574 characters and they are turned into meta-characters.
1578 matches any string of characters.
1581 matches any single character.
1584 introduces a character class.
1585 The end of the character class is indicated by a
1593 rather than introducing a character class.
1594 A character class matches any of the characters between the square brackets.
1595 A range of characters may be specified using a minus sign.
1596 The character class may be complemented by making an exclamation point
1600 the first character of the character class.
1604 in a character class, make it the first character listed
1612 make it the first or last character listed.
1613 .Ss Built-in Commands
1614 This section lists the built-in commands.
1615 .Bl -tag -width indent
1617 A null command that returns a 0 (true) exit value.
1619 The commands in the specified file are read and executed by the shell.
1622 command may be used to return to the
1629 characters, it is used as is.
1630 Otherwise, the shell searches the
1633 If it is not found in the
1635 it is sought in the current working directory.
1637 A built-in equivalent of
1639 .It Ic alias Oo Ar name Ns Oo = Ns Ar string Oc Ar ... Oc
1641 .Ar name Ns = Ns Ar string
1642 is specified, the shell defines the alias
1648 is specified, the value of the alias
1651 With no arguments, the
1653 built-in command prints the names and values of all defined aliases
1656 Alias values are written with appropriate quoting so that they are
1657 suitable for re-input to the shell.
1661 .It Ic bg Op Ar job ...
1662 Continue the specified jobs
1663 (or the current job if no jobs are given)
1665 .It Ic bind Oo Fl aeklrsv Oc Oo Ar key Oo Ar command Oc Oc
1666 List or alter key bindings for the line editor.
1667 This command is documented in
1669 .It Ic break Op Ar num
1671 .Sx Flow-Control Constructs
1673 .It Ic builtin Ar cmd Op Ar arg ...
1674 Execute the specified built-in command,
1676 This is useful when the user wishes to override a shell function
1677 with the same name as a built-in command.
1678 .It Ic cd Oo Fl L | P Oc Op Ar directory
1679 Switch to the specified
1681 or to the directory specified in the
1683 environment variable if no
1692 then the directories listed in the
1695 searched for the specified
1699 is unset, the current directory is searched.
1702 is the same as that of
1704 In an interactive shell,
1707 command will print out the name of the directory
1708 that it actually switched to
1709 if this is different from the name that the user gave.
1710 These may be different either because the
1712 mechanism was used or because a symbolic link was crossed.
1716 option is specified,
1718 is handled physically and symbolic links are resolved before
1720 components are processed.
1723 option is specified,
1725 is handled logically.
1726 This is the default.
1731 .It Ic command Oo Fl p Oc Op Ar utility Op Ar argument ...
1732 .It Ic command Oo Fl p Oc Fl v Ar utility
1733 .It Ic command Oo Fl p Oc Fl V Ar utility
1734 The first form of invocation executes the specified
1736 ignoring shell functions in the search.
1739 is a special builtin,
1740 it is executed as if it were a regular builtin.
1744 option is specified, the command search is performed using a
1747 that is guaranteed to find all of the standard utilities.
1751 option is specified,
1753 is not executed but a description of its interpretation by the shell is
1755 For ordinary commands the output is the path name; for shell built-in
1756 commands, shell functions and keywords only the name is written.
1757 Aliases are printed as
1758 .Dq Ic alias Ar name Ns = Ns Ar value .
1762 option is identical to
1764 except for the output.
1766 .Dq Ar utility Ic is Ar description
1772 a special shell builtin,
1779 .It Ic continue Op Ar num
1781 .Sx Flow-Control Constructs
1783 .It Ic echo Oo Fl e | n Oc Op Ar string ...
1784 Print a space-separated list of the arguments to the standard output
1785 and append a newline character.
1786 .Bl -tag -width indent
1788 Suppress the output of the trailing newline.
1790 Process C-style backslash escape sequences.
1793 command understands the following character escapes:
1794 .Bl -tag -width indent
1796 Alert (ring the terminal bell)
1800 Suppress the trailing newline (this has the side-effect of truncating the
1801 line if it is not the last character)
1819 (Zero) The character whose octal value is
1825 is not enclosed in quotes then the backslash itself must be escaped
1826 with a backslash to protect it from the shell.
1828 .Bd -literal -offset indent
1837 $ echo -e a\e\e\e\eb
1846 options may be specified.
1847 .It Ic eval Ar string ...
1848 Concatenate all the arguments with spaces.
1849 Then re-parse and execute the command.
1850 .It Ic exec Op Ar command Op Ar arg ...
1854 the shell process is replaced with the specified program
1855 (which must be a real program, not a shell built-in command or function).
1856 Any redirections on the
1858 command are marked as permanent,
1859 so that they are not undone when the
1862 .It Ic exit Op Ar exitstatus
1863 Terminate the shell process.
1867 it is used as the exit status of the shell.
1868 Otherwise, if the shell is executing an
1870 trap, the exit status of the last command before the trap is used;
1871 if the shell is executing a trap for a signal,
1872 the shell exits by resending the signal to itself.
1873 Otherwise, the exit status of the preceding command is used.
1874 The exit status should be an integer between 0 and 255.
1875 .It Ic export Ar name ...
1876 .It Ic export Op Fl p
1877 The specified names are exported so that they will
1878 appear in the environment of subsequent commands.
1879 The only way to un-export a variable is to
1882 The shell allows the value of a variable to be set
1883 at the same time as it is exported by writing
1885 .D1 Ic export Ar name Ns = Ns Ar value
1887 With no arguments the
1889 command lists the names
1890 of all exported variables.
1893 option is specified, the exported variables are printed as
1894 .Dq Ic export Ar name Ns = Ns Ar value
1895 lines, suitable for re-input to the shell.
1897 A null command that returns a non-zero (false) exit value.
1898 .It Ic fc Oo Fl e Ar editor Oc Op Ar first Op Ar last
1899 .It Ic fc Fl l Oo Fl nr Oc Op Ar first Op Ar last
1900 .It Ic fc Fl s Oo Ar old Ns = Ns Ar new Oc Op Ar first
1903 built-in command lists, or edits and re-executes,
1904 commands previously entered to an interactive shell.
1905 .Bl -tag -width indent
1907 Use the editor named by
1909 to edit the commands.
1912 string is a command name,
1913 subject to search via the
1918 variable is used as a default when
1923 is null or unset, the value of the
1930 is used as the editor.
1932 List the commands rather than invoking
1934 The commands are written in the
1935 sequence indicated by the
1939 operands, as affected by
1941 with each command preceded by the command number.
1943 Suppress command numbers when listing with
1946 Reverse the order of the commands listed
1955 Re-execute the command without invoking an editor.
1958 Select the commands to list or edit.
1959 The number of previous commands that can be accessed
1960 are determined by the value of the
1967 or both are one of the following:
1968 .Bl -tag -width indent
1969 .It Oo Cm + Oc Ns Ar num
1970 A positive number representing a command number;
1971 command numbers can be displayed with the
1975 A negative decimal number representing the
1976 command that was executed
1979 commands previously.
1980 For example, \-1 is the immediately previous command.
1982 A string indicating the most recently entered command
1983 that begins with that string.
1985 .Ar old Ns = Ns Ar new
1986 operand is not also specified with
1988 the string form of the first operand cannot contain an embedded equal sign.
1992 The following variables affect the execution of
1994 .Bl -tag -width ".Va HISTSIZE"
1996 Name of the editor to use for history editing.
1998 The number of previous commands that are accessible.
2003 or the current job to the foreground.
2004 .It Ic getopts Ar optstring var
2011 command deprecates the older
2014 The first argument should be a series of letters, each possibly
2015 followed by a colon which indicates that the option takes an argument.
2016 The specified variable is set to the parsed option.
2018 the next argument is placed into the shell variable
2020 If an option takes an argument, it is placed into the shell variable
2022 If an invalid option is encountered,
2026 It returns a false value (1) when it encounters the end of the options.
2027 .It Ic hash Oo Fl rv Oc Op Ar command ...
2028 The shell maintains a hash table which remembers the locations of commands.
2029 With no arguments whatsoever, the
2031 command prints out the contents of this table.
2032 Entries which have not been looked at since the last
2034 command are marked with an asterisk;
2035 it is possible for these entries to be invalid.
2039 command removes each specified
2041 from the hash table (unless they are functions) and then locates it.
2046 prints the locations of the commands as it finds them.
2051 command to delete all the entries in the hash table except for functions.
2052 .It Ic jobid Op Ar job
2053 Print the process IDs of the processes in the specified
2057 argument is omitted, use the current job.
2058 .It Ic jobs Oo Fl lps Oc Op Ar job ...
2059 Print information about the specified jobs, or all jobs if no
2062 The information printed includes job ID, status and command name.
2066 option is specified, the PID of each job is also printed.
2069 option is specified, only the process IDs for the process group leaders
2070 are printed, one per line.
2073 option is specified, only the PIDs of the job commands are printed, one per
2076 A built-in equivalent of
2078 that additionally supports sending signals to jobs.
2079 .It Ic local Oo Ar variable ... Oc Op Fl
2084 A built-in equivalent of
2086 .It Ic pwd Op Fl L | P
2087 Print the path of the current directory.
2088 The built-in command may
2089 differ from the program of the same name because the
2090 built-in command remembers what the current directory
2091 is rather than recomputing it each time.
2094 However, if the current directory is
2096 the built-in version of
2098 will continue to print the old name for the directory.
2102 option is specified, symbolic links are resolved.
2105 option is specified, the shell's notion of the current directory
2106 is printed (symbolic links are not resolved).
2107 This is the default.
2108 .It Ic read Oo Fl p Ar prompt Oc Oo
2109 .Fl t Ar timeout Oc Oo Fl er Oc Ar variable ...
2115 and the standard input is a terminal.
2117 read from the standard input.
2118 The trailing newline
2119 is deleted from the line and the line is split as
2120 described in the section on
2121 .Sx White Space Splitting (Field Splitting)
2123 the pieces are assigned to the variables in order.
2124 If there are more pieces than variables, the remaining
2125 pieces (along with the characters in
2127 that separated them)
2128 are assigned to the last variable.
2129 If there are more variables than pieces, the remaining
2130 variables are assigned the null string.
2132 Backslashes are treated specially, unless the
2136 If a backslash is followed by
2137 a newline, the backslash and the newline will be
2139 If a backslash is followed by any other
2140 character, the backslash will be deleted and the following
2141 character will be treated as though it were not in
2147 option is specified and the
2149 elapses before a complete line of input is supplied,
2152 command will return an exit status of 1 without assigning any values.
2155 value may optionally be followed by one of
2160 to explicitly specify seconds, minutes or hours.
2161 If none is supplied,
2167 option exists only for backward compatibility with older scripts.
2168 .It Ic readonly Oo Fl p Oc Op Ar name ...
2171 is marked as read only,
2172 so that it cannot be subsequently modified or unset.
2173 The shell allows the value of a variable to be set
2174 at the same time as it is marked read only
2175 by using the following form:
2177 .D1 Ic readonly Ar name Ns = Ns Ar value
2179 With no arguments the
2181 command lists the names of all read only variables.
2184 option is specified, the read-only variables are printed as
2185 .Dq Ic readonly Ar name Ns = Ns Ar value
2186 lines, suitable for re-input to the shell.
2187 .It Ic return Op Ar exitstatus
2191 .It Ic set Oo Fl /+abCEefIimnpTuVvx Oc Oo Fl /+o Ar longname Oc Oo
2192 .Fl c Ar string Oc Op Fl - Ar arg ...
2195 command performs three different functions:
2198 With no arguments, it lists the values of all shell variables.
2200 If options are given,
2201 either in short form or using the long
2202 .Dq Fl /+o Ar longname
2204 it sets or clears the specified options as described in the section called
2205 .Sx Argument List Processing .
2209 option is specified,
2211 will replace the shell's positional parameters with the subsequent
2213 If no arguments follow the
2216 all the positional parameters will be cleared,
2217 which is equivalent to executing the command
2221 flag may be omitted when specifying arguments to be used
2222 as positional replacement parameters.
2223 This is not recommended,
2224 because the first argument may begin with a dash
2230 command will interpret as a request to enable or disable options.
2232 .It Ic setvar Ar variable value
2233 Assigns the specified
2239 command is intended to be used in functions that
2240 assign values to variables whose names are passed as parameters.
2241 In general it is better to write
2242 .Dq Ar variable Ns = Ns Ar value
2245 .It Ic shift Op Ar n
2246 Shift the positional parameters
2251 A shift sets the value of
2260 decreasing the value of
2263 If there are zero positional parameters, shifting does not do anything.
2265 A built-in equivalent of
2268 Print the amount of time spent executing the shell and its children.
2269 The first output line shows the user and system times for the shell
2270 itself, the second one contains the user and system times for the
2272 .It Ic trap Oo Ar action Oc Ar signal ...
2274 Cause the shell to parse and execute
2279 The signals are specified by name or number.
2280 In addition, the pseudo-signal
2282 may be used to specify an
2284 that is performed when the shell terminates.
2287 may be an empty string or a dash
2289 the former causes the specified signal to be ignored
2290 and the latter causes the default action to be taken.
2293 is another way to request the default action, for compatibility reasons this
2294 usage is not recommended though.
2295 In a subshell environment,
2296 the shell resets trapped (but not ignored) signals to the default action.
2299 command has no effect on signals that were ignored on entry to the shell.
2305 command to display a list of valid signal names.
2307 A null command that returns a 0 (true) exit value.
2308 .It Ic type Op Ar name ...
2311 as a command and print the resolution of the command search.
2312 Possible resolutions are:
2313 shell keyword, alias, special shell builtin, shell builtin, command,
2316 For aliases the alias expansion is printed;
2317 for commands and tracked aliases
2318 the complete pathname of the command is printed.
2319 .It Ic ulimit Oo Fl HSabcdflmnstuv Oc Op Ar limit
2320 Set or display resource limits (see
2324 is specified, the named resource will be set;
2325 otherwise the current resource value will be displayed.
2329 is specified, the hard limits will be set or displayed.
2330 While everybody is allowed to reduce a hard limit,
2331 only the superuser can increase it.
2335 specifies the soft limits instead.
2336 When displaying limits,
2342 The default is to display the soft limits,
2343 and to set both the hard and the soft limits.
2349 command to display all resources.
2352 is not acceptable in this mode.
2354 The remaining options specify which resource value is to be
2355 displayed or modified.
2356 They are mutually exclusive.
2357 .Bl -tag -width indent
2359 The maximum size of socket buffer usage, in bytes.
2360 .It Fl c Ar coredumpsize
2361 The maximal size of core dump files, in 512-byte blocks.
2362 .It Fl d Ar datasize
2363 The maximal size of the data segment of a process, in kilobytes.
2364 .It Fl f Ar filesize
2365 The maximal size of a file, in 512-byte blocks.
2366 .It Fl l Ar lockedmem
2367 The maximal size of memory that can be locked by a process, in
2369 .It Fl m Ar memoryuse
2370 The maximal resident set size of a process, in kilobytes.
2372 The maximal number of descriptors that could be opened by a process.
2373 .It Fl s Ar stacksize
2374 The maximal size of the stack segment, in kilobytes.
2376 The maximal amount of CPU time to be used by each process, in seconds.
2377 .It Fl u Ar userproc
2378 The maximal number of simultaneous processes for this user ID.
2379 .It Fl v Ar virtualmem
2380 The maximal virtual size of a process, in kilobytes.
2382 .It Ic umask Oo Fl S Oc Op Ar mask
2383 Set the file creation mask (see
2385 to the octal or symbolic (see
2389 If the argument is omitted, the current mask value is printed.
2392 option is specified, the output is symbolic, otherwise the output is octal.
2393 .It Ic unalias Oo Fl a Oc Op Ar name ...
2394 The specified alias names are removed.
2397 is specified, all aliases are removed.
2398 .It Ic unset Oo Fl fv Oc Ar name ...
2399 The specified variables or functions are unset and unexported.
2402 option is specified or no options are given, the
2404 arguments are treated as variable names.
2407 option is specified, the
2409 arguments are treated as function names.
2410 .It Ic wait Op Ar job
2411 Wait for the specified
2413 to complete and return the exit status of the last process in the
2415 If the argument is omitted, wait for all jobs to complete
2416 and return an exit status of zero.
2418 .Ss Commandline Editing
2421 is being used interactively from a terminal, the current command
2422 and the command history
2426 .Sx Built-in Commands )
2429 command line editing.
2430 This mode uses commands similar
2431 to a subset of those described in the
2449 can be switched between insert mode and command mode by typing
2453 while in command mode will pass the line to the shell.
2456 .Dq Li "set -o emacs"
2459 command can be used to enable a subset of
2461 command line editing features.
2463 The following environment variables affect the execution of
2465 .Bl -tag -width ".Ev LANGXXXXXX"
2467 Initialization file for interactive shells.
2468 .It Ev LANG , Ev LC_*
2470 These are inherited by children of the shell,
2471 and is used in a limited manner by the shell itself.
2473 An absolute pathname for the current directory,
2474 possibly containing symbolic links.
2475 This is used and updated by the shell.
2477 The default terminal setting for the shell.
2478 This is inherited by children of the shell, and is used in the history
2482 Additionally, all environment variables are turned into shell variables
2484 which may affect the shell as described under
2485 .Sx Special Variables .
2487 Errors that are detected by the shell, such as a syntax error, will
2488 cause the shell to exit with a non-zero exit status.
2489 If the shell is not an interactive shell, the execution of the shell
2490 file will be aborted.
2491 Otherwise the shell will return the exit status of the last command
2494 builtin is used with a numeric argument, it
2495 will return the argument.
2501 .Xr emacs 1 Pq Pa pkgsrc/editors/emacs ,
2515 command, the Thompson shell, appeared in
2517 It was superseded in
2519 by the Bourne shell, which inherited the name
2524 was rewritten in 1989 under the
2526 license after the Bourne shell from
2531 was originally written by
2532 .An Kenneth Almquist .
2536 utility does not recognize multibyte characters.
2538 The characters generated by filename completion should probably be quoted
2539 to ensure that the filename is still valid after the input line has been
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