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32 .\" from: @(#)sh.1 8.6 (Berkeley) 5/4/95
40 .Nd command interpreter (shell)
43 .Op Fl /+abCEefIimnPpsTuVvx
44 .Op Fl /+o Ar longname
50 utility is the standard command interpreter for the system.
51 The current version of
53 is in the process of being changed to
56 specification for the shell.
57 This version has many features which make
59 similar in some respects to the Korn shell, but it is not a Korn
60 shell clone like pdksh.
62 designated by POSIX, plus a few Berkeley extensions, are being
63 incorporated into this shell.
64 This man page is not intended to be a tutorial nor a complete
65 specification of the shell.
67 The shell is a command that reads lines from
68 either a file or the terminal, interprets them, and
69 generally executes other commands.
70 It is the program that is started when a user logs into the system,
71 although a user can select a different shell with the
75 implements a language that has flow control constructs,
76 a macro facility that provides a variety of features in
77 addition to data storage, along with built-in history and line
79 It incorporates many features to
80 aid interactive use and has the advantage that the interpretative
81 language is common to both interactive and non-interactive
83 That is, commands can be typed directly
84 to the running shell or can be put into a file,
85 which can be executed directly by the shell.
88 .\" XXX This next sentence is incredibly confusing.
90 If no arguments are present and if the standard input of the shell
91 is connected to a terminal
95 the shell is considered an interactive shell.
97 generally prompts before each command and handles programming
98 and command errors differently (as described below).
99 When first starting, the shell inspects argument 0, and
100 if it begins with a dash
102 the shell is also considered a login shell.
103 This is normally done automatically by the system
104 when the user first logs in.
105 A login shell first reads commands
111 If the environment variable
113 is set on entry to a shell, or is set in the
115 of a login shell, the shell then reads commands from the file named in
117 Therefore, a user should place commands that are to be executed only
120 file, and commands that are executed for every shell inside the
125 variable to some file by placing the following line in the file
127 in the home directory,
130 the filename desired:
132 .Dl ENV=$HOME/.shinit; export ENV
134 The first non-option argument specified on the command line
135 will be treated as the
136 name of a file from which to read commands (a shell script), and
137 the remaining arguments are set as the positional parameters
138 of the shell ($1, $2, etc).
139 Otherwise, the shell reads commands
140 from its standard input.
142 Unlike older versions of
146 script is only sourced on invocation of interactive shells.
148 closes a well-known, and sometimes easily exploitable security
149 hole related to poorly thought out
152 .Ss Argument List Processing
153 All of the single letter options to
155 have a corresponding long name,
156 with the exception of
160 These long names are provided next to the single letter options
161 in the descriptions below.
162 The long name for an option may be specified as an argument to the
166 Once the shell is running,
167 the long name for an option may be specified as an argument to the
172 (described later in the section called
173 .Sx Built-in Commands ) .
174 Introducing an option with a dash
184 will stop option processing and will force the remaining
185 words on the command line to be treated as arguments.
190 options do not have long names.
191 They take arguments and are described after the single letter options.
192 .Bl -tag -width indent
193 .It Fl a Li allexport
194 Flag variables for export when assignments are made to them.
196 Enable asynchronous notification of background job
199 .It Fl C Li noclobber
200 Do not overwrite existing files with
205 command line editor (disables the
207 option if it has been set).
209 Exit immediately if any untested command fails in non-interactive mode.
210 The exit status of a command is considered to be
211 explicitly tested if the command is part of the list used to control
213 .Ic if , elif , while ,
216 if the command is the left
221 operator; or if the command is a pipeline preceded by the
224 If a shell function is executed and its exit status is explicitly
225 tested, all commands of the function are considered to be tested as
228 Disable pathname expansion.
229 .It Fl I Li ignoreeof
232 from input when in interactive mode.
233 .It Fl i Li interactive
234 Force the shell to behave interactively.
236 Turn on job control (set automatically when interactive).
238 If not interactive, read commands but do not
240 This is useful for checking the
241 syntax of shell scripts.
243 Change the default for the
249 (logical directory layout)
252 (physical directory layout).
253 .It Fl p Li privileged
254 Turn on privileged mode.
255 This mode is enabled on startup
256 if either the effective user or group id is not equal to the
257 real user or group id.
258 Turning this mode off sets the
259 effective user and group ids to the real user and group ids.
260 When this mode is enabled for interactive shells, the file
261 .Pa /etc/suid_profile
262 is sourced instead of
266 is sourced, and the contents of the
268 variable are ignored.
270 Read commands from standard input (set automatically
271 if no file arguments are present).
273 no effect when set after the shell has already started
274 running (i.e., when set with the
277 .It Fl T Li trapsasync
278 When waiting for a child, execute traps immediately.
279 If this option is not set,
280 traps are executed after the child exits,
283 This nonstandard option is useful for putting guarding shells around
284 children that block signals.
285 The surrounding shell may kill the child
286 or it may just return control to the tty and leave the child alone,
288 .Bd -literal -offset indent
289 sh -T -c "trap 'exit 1' 2 ; some-blocking-program"
293 Write a message to standard error when attempting
294 to expand a variable that is not set, and if the
295 shell is not interactive, exit immediately.
299 command line editor (disables
303 The shell writes its input to standard error
305 Useful for debugging.
308 (preceded by the value of the
311 to standard error before it is executed.
312 Useful for debugging.
317 option causes the commands to be read from the
319 operand instead of from the standard input.
320 Keep in mind that this option only accepts a single string as its
321 argument, hence multi-word strings must be quoted.
325 option takes as its only argument the long name of an option
326 to be enabled or disabled.
327 For example, the following two invocations of
329 both enable the built-in
332 .Bd -literal -offset indent
337 If used without an argument, the
339 option displays the current option settings in a human-readable format.
342 is used without an argument, the current option settings are output
343 in a format suitable for re-input into the shell.
344 .Ss Lexical Structure
345 The shell reads input in terms of lines from a file and breaks
346 it up into words at whitespace (blanks and tabs), and at
350 which are special to the shell.
351 There are two types of operators: control operators and
352 redirection operators (their meaning is discussed later).
353 The following is a list of valid operators:
354 .Bl -tag -width indent
355 .It Control operators:
356 .Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
357 .It Li & Ta Li && Ta Li ( Ta Li ) Ta Li \en
358 .It Li ;; Ta Li ; Ta Li | Ta Li ||
360 .It Redirection operators:
361 .Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
362 .It Li < Ta Li > Ta Li << Ta Li >> Ta Li <>
363 .It Li <& Ta Li >& Ta Li <<- Ta Li >|
369 introduces a comment if used at the beginning of a word.
370 The word starting with
372 and the rest of the line are ignored.
374 Quoting is used to remove the special meaning of certain characters
375 or words to the shell, such as operators, whitespace, keywords,
378 There are three types of quoting: matched single quotes,
379 matched double quotes, and backslash.
380 .Bl -tag -width indent
382 Enclosing characters in single quotes preserves the literal
383 meaning of all the characters (except single quotes, making
384 it impossible to put single-quotes in a single-quoted string).
386 Enclosing characters within double quotes preserves the literal
387 meaning of all characters except dollarsign
393 The backslash inside double quotes is historically weird.
394 It remains literal unless it precedes the following characters,
395 which it serves to quote:
396 .Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
397 .It Li $ Ta Li ` Ta Li \&" Ta Li \e\ Ta Li \en
400 A backslash preserves the literal meaning of the following
401 character, with the exception of the newline character
403 A backslash preceding a newline is treated as a line continuation.
406 Reserved words are words that have special meaning to the
407 shell and are recognized at the beginning of a line and
408 after a control operator.
409 The following are reserved words:
410 .Bl -column "doneXX" "elifXX" "elseXX" "untilXX" "whileX" -offset center
411 .It Li \&! Ta { Ta } Ta Ic case Ta Ic do
412 .It Ic done Ta Ic elif Ta Ic else Ta Ic esac Ta Ic fi
413 .It Ic for Ta Ic if Ta Ic then Ta Ic until Ta Ic while
416 An alias is a name and corresponding value set using the
419 Whenever a reserved word may occur (see above),
420 and after checking for reserved words, the shell
421 checks the word to see if it matches an alias.
422 If it does, it replaces it in the input stream with its value.
423 For example, if there is an alias called
428 .Bd -literal -offset indent
433 .Bd -literal -offset indent
437 Aliases provide a convenient way for naive users to
438 create shorthands for commands without having to learn how
439 to create functions with arguments.
441 used to create lexically obscure code.
442 This use is discouraged.
444 An alias name may be escaped in a command line, so that it is not
445 replaced by its alias value, by using quoting characters within or
446 adjacent to the alias name.
447 This is most often done by prefixing
448 an alias name with a backslash to execute a function, built-in, or
449 normal program with the same name.
454 The shell interprets the words it reads according to a
455 language, the specification of which is outside the scope
456 of this man page (refer to the BNF in the
459 Essentially though, a line is read and if
460 the first word of the line (or after a control operator)
461 is not a reserved word, then the shell has recognized a
463 Otherwise, a complex command or some
464 other special construct may have been recognized.
466 If a simple command has been recognized, the shell performs
467 the following actions:
470 Leading words of the form
472 are stripped off and assigned to the environment of
474 Redirection operators and
475 their arguments (as described below) are stripped
476 off and saved for processing.
478 The remaining words are expanded as described in
480 .Sx Word Expansions ,
481 and the first remaining word is considered the command
482 name and the command is located.
484 words are considered the arguments of the command.
485 If no command name resulted, then the
487 variable assignments recognized in 1) affect the
490 Redirections are performed as described in
494 Redirections are used to change where a command reads its input
496 In general, redirections open, close, or
497 duplicate an existing reference to a file.
499 used for redirection is:
501 .Dl [n] redir-op file
505 is one of the redirection operators mentioned
507 The following gives some examples of how these
508 operators can be used.
509 Note that stdin and stdout are commonly used abbreviations
510 for standard input and standard output respectively.
511 .Bl -tag -width "1234567890XX" -offset indent
513 redirect stdout (or file descriptor n) to file
515 same as above, but override the
519 append stdout (or file descriptor n) to file
521 redirect stdin (or file descriptor n) from file
523 redirect stdin (or file descriptor n) to and from file
525 duplicate stdin (or file descriptor n1) from file descriptor n2
527 close stdin (or file descriptor n)
529 duplicate stdout (or file descriptor n1) to file descriptor n2
531 close stdout (or file descriptor n)
534 The following redirection is often called a
536 .Bd -literal -offset indent
543 All the text on successive lines up to the delimiter is
544 saved away and made available to the command on standard
545 input, or file descriptor n if it is specified.
547 as specified on the initial line is quoted, then the here-doc-text
548 is treated literally, otherwise the text is subjected to
549 parameter expansion, command substitution, and arithmetic
550 expansion (as described in the section on
551 .Sx Word Expansions ) .
557 in the here-doc-text are stripped.
558 .Ss Search and Execution
559 There are three types of commands: shell functions,
560 built-in commands, and normal programs.
561 The command is searched for (by name) in that order.
562 The three types of commands are all executed in a different way.
564 When a shell function is executed, all of the shell positional
565 parameters (except $0, which remains unchanged) are
566 set to the arguments of the shell function.
567 The variables which are explicitly placed in the environment of
568 the command (by placing assignments to them before the
569 function name) are made local to the function and are set
571 Then the command given in the function definition is executed.
572 The positional parameters are restored to their original values
573 when the command completes.
574 This all occurs within the current shell.
576 Shell built-in commands are executed internally to the shell, without
577 spawning a new process.
579 Otherwise, if the command name does not match a function
580 or built-in command, the command is searched for as a normal
581 program in the file system (as described in the next section).
582 When a normal program is executed, the shell runs the program,
583 passing the arguments and the environment to the program.
584 If the program is not a normal executable file
585 (i.e., if it does not begin with the
595 the shell will interpret the program in a subshell.
596 The child shell will reinitialize itself in this case,
597 so that the effect will be
598 as if a new shell had been invoked to handle the ad-hoc shell script,
599 except that the location of hashed commands located in
600 the parent shell will be remembered by the child.
602 Note that previous versions of this document
603 and the source code itself misleadingly and sporadically
604 refer to a shell script without a magic number
606 .Qq shell procedure .
608 When locating a command, the shell first looks to see if
609 it has a shell function by that name.
611 built-in command by that name.
612 If a built-in command is not found,
613 one of two things happen:
616 Command names containing a slash are simply executed without
617 performing any searches.
619 The shell searches each entry in
621 in turn for the command.
624 variable should be a series of
625 entries separated by colons.
626 Each entry consists of a
628 The current directory
629 may be indicated implicitly by an empty directory name,
630 or explicitly by a single period.
632 .Ss Command Exit Status
633 Each command has an exit status that can influence the behavior
634 of other shell commands.
635 The paradigm is that a command exits
636 with zero for normal or success, and non-zero for failure,
637 error, or a false indication.
638 The man page for each command
639 should indicate the various exit codes and what they mean.
640 Additionally, the built-in commands return exit codes, as does
641 an executed shell function.
643 If a command is terminated by a signal, its exit status is 128 plus
645 Signal numbers are defined in the header file
648 Complex commands are combinations of simple commands
649 with control operators or reserved words, together creating a larger complex
651 More generally, a command is one of the following:
652 .Bl -item -offset indent
658 list or compound-list
665 Unless otherwise stated, the exit status of a command is
666 that of the last simple command executed by the command.
668 A pipeline is a sequence of one or more commands separated
669 by the control operator |.
670 The standard output of all but
671 the last command is connected to the standard input
673 The standard output of the last
674 command is inherited from the shell, as usual.
676 The format for a pipeline is:
678 .Dl [!] command1 [ | command2 ...]
680 The standard output of command1 is connected to the standard
682 The standard input, standard output, or
683 both of a command is considered to be assigned by the
684 pipeline before any redirection specified by redirection
685 operators that are part of the command.
687 If the pipeline is not in the background (discussed later),
688 the shell waits for all commands to complete.
692 does not precede the pipeline, the
693 exit status is the exit status of the last command specified
695 Otherwise, the exit status is the logical
696 NOT of the exit status of the last command.
698 the last command returns zero, the exit status is 1; if
699 the last command returns greater than zero, the exit status
702 Because pipeline assignment of standard input or standard
703 output or both takes place before redirection, it can be
704 modified by redirection.
707 .Dl $ command1 2>&1 | command2
709 sends both the standard output and standard error of
711 to the standard input of
716 or newline terminator causes the preceding
718 (described below in the section called
719 .Sx Short-Circuit List Operators )
720 to be executed sequentially;
723 causes asynchronous execution of the preceding AND-OR-list.
725 Note that unlike some other shells,
727 executes each process in the pipeline as a child of the
730 Shell built-in commands are the exception to this rule.
731 They are executed in the current shell, although they do not affect its
732 environment when used in pipelines.
733 .Ss Background Commands (&)
734 If a command is terminated by the control operator ampersand
736 the shell executes the command asynchronously;
737 the shell does not wait for the command to finish
738 before executing the next command.
740 The format for running a command in background is:
741 .Bd -literal -offset indent
742 command1 & [command2 & ...]
745 If the shell is not interactive, the standard input of an
746 asynchronous command is set to /dev/null.
747 .Ss Lists (Generally Speaking)
748 A list is a sequence of zero or more commands separated by
749 newlines, semicolons, or ampersands,
750 and optionally terminated by one of these three characters.
752 list are executed in the order they are written.
753 If command is followed by an ampersand, the shell starts the
754 command and immediately proceeds onto the next command;
755 otherwise it waits for the command to terminate before
756 proceeding to the next one.
757 .Ss Short-Circuit List Operators
761 are AND-OR list operators.
763 executes the first command, and then executes the second command
764 if the exit status of the first command is zero.
766 is similar, but executes the second command if the exit
767 status of the first command is nonzero.
771 both have the same priority.
772 .Ss Flow-Control Constructs (if, while, for, case)
777 .\" XXX Use .Dl to work around broken handling of .Ic inside .Bd and .Ed .
781 .Dl [ Ic elif Ar list
782 .Dl Ic then Ar list ] ...
783 .Dl [ Ic else Ar list ]
793 The two lists are executed repeatedly while the exit status of the
797 command is similar, but has the word
802 repeat until the exit status of the first list is zero.
807 .Dl Ic for Ar variable Op Ic in Ar word ...
813 and the following words are omitted,
816 The words are expanded, and then the list is executed
817 repeatedly with the variable set to each word in turn.
822 commands may be replaced with
832 .Dl Ic break Op Ar num
833 .Dl Ic continue Op Ar num
837 command terminates the
846 command continues with the next iteration of the innermost loop.
847 These are implemented as built-in commands.
852 .Dl Ic case Ar word Ic in
857 The pattern can actually be one or more patterns
864 .Ss Grouping Commands Together
865 Commands may be grouped by writing either
866 .Bd -literal -offset indent
871 .Bd -literal -offset indent
875 The first form executes the commands in a subshell.
876 Note that built-in commands thus executed do not affect the current shell.
877 The second form does not fork another shell,
878 so it is slightly more efficient.
879 Grouping commands together this way allows the user to
880 redirect their output as though they were one program:
881 .Bd -literal -offset indent
882 { echo -n "hello"; echo " world"; } > greeting
885 The syntax of a function definition is
886 .Bd -literal -offset indent
890 A function definition is an executable statement; when
891 executed it installs a function named name and returns an
893 The command is normally a list
899 Variables may be declared to be local to a function by
903 This should appear as the first statement of a function,
905 .Bd -ragged -offset indent
913 command is implemented as a built-in command.
915 When a variable is made local, it inherits the initial
916 value and exported and readonly flags from the variable
917 with the same name in the surrounding scope, if there is
919 Otherwise, the variable is initially unset.
921 uses dynamic scoping, so that if the variable
923 is made local to function
925 which then calls function
927 references to the variable
931 will refer to the variable
935 not to the global variable named
938 The only special parameter that can be made local is
942 local causes any shell options that are
943 changed via the set command inside the function to be
944 restored to their original values when the function
950 .Bd -ragged -offset indent
955 It terminates the current executional scope, returning from the previous
956 nested function, sourced script, or shell instance, in that order.
959 command is implemented as a built-in command.
960 .Ss Variables and Parameters
961 The shell maintains a set of parameters.
963 denoted by a name is called a variable.
965 the shell turns all the environment variables into shell
967 New variables can be set using the form
968 .Bd -literal -offset indent
972 Variables set by the user must have a name consisting solely
973 of alphabetics, numerics, and underscores.
974 The first letter of a variable name must not be numeric.
975 A parameter can also be denoted by a number
976 or a special character as explained below.
977 .Ss Positional Parameters
978 A positional parameter is a parameter denoted by a number greater than zero.
979 The shell sets these initially to the values of its command line
980 arguments that follow the name of the shell script.
983 built-in command can also be used to set or reset them.
984 .Ss Special Parameters
985 A special parameter is a parameter denoted by a special one-character
987 The special parameters recognized by the
991 are shown in the following list, exactly as they would appear in input
992 typed by the user or in the source of a shell script.
995 Expands to the positional parameters, starting from one.
997 the expansion occurs within a double-quoted string
998 it expands to a single field with the value of each parameter
999 separated by the first character of the
1008 Expands to the positional parameters, starting from one.
1010 the expansion occurs within double-quotes, each positional
1011 parameter expands as a separate argument.
1012 If there are no positional parameters, the
1015 generates zero arguments, even when
1018 What this basically means, for example, is
1027 .Bd -literal -offset indent
1031 Expands to the number of positional parameters.
1033 Expands to the exit status of the most recent pipeline.
1035 (hyphen) Expands to the current option flags (the single-letter
1036 option names concatenated into a string) as specified on
1037 invocation, by the set built-in command, or implicitly
1040 Expands to the process ID of the invoked shell.
1042 retains the same value of $ as its parent.
1044 Expands to the process ID of the most recent background
1045 command executed from the current shell.
1047 pipeline, the process ID is that of the last command in the
1050 (zero) Expands to the name of the shell or shell script.
1053 This clause describes the various expansions that are
1055 Not all expansions are performed on
1056 every word, as explained later.
1058 Tilde expansions, parameter expansions, command substitutions,
1059 arithmetic expansions, and quote removals that occur within
1060 a single word expand to a single field.
1062 splitting or pathname expansion that can create multiple
1063 fields from a single word.
1064 The single exception to this rule is
1065 the expansion of the special parameter
1067 within double-quotes,
1068 as was described above.
1070 The order of word expansion is:
1073 Tilde Expansion, Parameter Expansion, Command Substitution,
1074 Arithmetic Expansion (these all occur at the same time).
1076 Field Splitting is performed on fields generated by step (1)
1081 Pathname Expansion (unless the
1083 option is in effect).
1090 character is used to introduce parameter expansion, command
1091 substitution, or arithmetic evaluation.
1092 .Ss Tilde Expansion (substituting a user's home directory)
1093 A word beginning with an unquoted tilde character
1096 subjected to tilde expansion.
1097 All the characters up to a slash
1099 or the end of the word are treated as a username
1100 and are replaced with the user's home directory.
1102 username is missing (as in ~/foobar), the tilde is replaced
1103 with the value of the HOME variable (the current user's
1105 .Ss Parameter Expansion
1106 The format for parameter expansion is as follows:
1107 .Bd -literal -offset indent
1111 where expression consists of all characters until the matching
1115 escaped by a backslash or within a quoted string, and characters in
1116 embedded arithmetic expansions, command substitutions, and variable
1117 expansions, are not examined in determining the matching
1120 The simplest form for parameter expansion is:
1121 .Bd -literal -offset indent
1125 The value, if any, of parameter is substituted.
1127 The parameter name or symbol can be enclosed in braces, which are
1128 optional except for positional parameters with more than one digit or
1129 when parameter is followed by a character that could be interpreted as
1131 If a parameter expansion occurs inside double-quotes:
1134 Pathname expansion is not performed on the results of the
1137 Field splitting is not performed on the results of the
1138 expansion, with the exception of the special parameter
1142 In addition, a parameter expansion can be modified by using one of the
1144 .Bl -tag -width indent
1145 .It Li ${parameter:-word}
1147 If parameter is unset or
1148 null, the expansion of word is
1149 substituted; otherwise, the value of
1150 parameter is substituted.
1151 .It Li ${parameter:=word}
1152 Assign Default Values.
1153 If parameter is unset
1154 or null, the expansion of word is
1155 assigned to parameter.
1157 final value of parameter is
1159 Only variables, not positional
1160 parameters or special parameters, can be
1161 assigned in this way.
1162 .It Li ${parameter:?[word]}
1163 Indicate Error if Null or Unset.
1165 parameter is unset or null, the expansion of
1166 word (or a message indicating it is unset if
1167 word is omitted) is written to standard
1168 error and the shell exits with a nonzero
1170 Otherwise, the value of
1171 parameter is substituted.
1173 interactive shell need not exit.
1174 .It Li ${parameter:+word}
1175 Use Alternate Value.
1176 If parameter is unset
1177 or null, null is substituted;
1178 otherwise, the expansion of word is
1182 In the parameter expansions shown previously, use of the colon in the
1183 format results in a test for a parameter that is unset or null; omission
1184 of the colon results in a test for a parameter that is only unset.
1185 .Bl -tag -width indent
1186 .It Li ${#parameter}
1188 The length in characters of
1189 the value of parameter.
1192 The following four varieties of parameter expansion provide for substring
1194 In each case, pattern matching notation
1196 .Sx Shell Patterns ) ,
1197 rather than regular expression notation,
1198 is used to evaluate the patterns.
1199 If parameter is one of the special parameters
1203 the result of the expansion is unspecified.
1204 Enclosing the full parameter expansion string in double-quotes does not
1205 cause the following four varieties of pattern characters to be quoted,
1206 whereas quoting characters within the braces has this effect.
1207 .Bl -tag -width indent
1208 .It Li ${parameter%word}
1209 Remove Smallest Suffix Pattern.
1211 is expanded to produce a pattern.
1213 parameter expansion then results in
1214 parameter, with the smallest portion of the
1215 suffix matched by the pattern deleted.
1216 .It Li ${parameter%%word}
1217 Remove Largest Suffix Pattern.
1219 is expanded to produce a pattern.
1221 parameter expansion then results in
1222 parameter, with the largest portion of the
1223 suffix matched by the pattern deleted.
1224 .It Li ${parameter#word}
1225 Remove Smallest Prefix Pattern.
1227 is expanded to produce a pattern.
1229 parameter expansion then results in
1230 parameter, with the smallest portion of the
1231 prefix matched by the pattern deleted.
1232 .It Li ${parameter##word}
1233 Remove Largest Prefix Pattern.
1235 is expanded to produce a pattern.
1237 parameter expansion then results in
1238 parameter, with the largest portion of the
1239 prefix matched by the pattern deleted.
1241 .Ss Command Substitution
1242 Command substitution allows the output of a command to be substituted in
1243 place of the command name itself.
1244 Command substitution occurs when
1245 the command is enclosed as follows:
1246 .Bd -literal -offset indent
1250 or the backquoted version:
1251 .Bd -literal -offset indent
1255 The shell expands the command substitution by executing command in a
1256 subshell environment and replacing the command substitution
1257 with the standard output of the command,
1258 removing sequences of one or more newlines at the end of the substitution.
1259 Embedded newlines before the end of the output are not removed;
1260 however, during field splitting, they may be translated into spaces
1261 depending on the value of
1263 and the quoting that is in effect.
1264 .Ss Arithmetic Expansion
1265 Arithmetic expansion provides a mechanism for evaluating an arithmetic
1266 expression and substituting its value.
1267 The format for arithmetic expansion is as follows:
1268 .Bd -literal -offset indent
1272 The expression is treated as if it were in double-quotes, except
1273 that a double-quote inside the expression is not treated specially.
1275 shell expands all tokens in the expression for parameter expansion,
1276 command substitution, and quote removal.
1278 Next, the shell treats this as an arithmetic expression and
1279 substitutes the value of the expression.
1280 .Ss White Space Splitting (Field Splitting)
1281 After parameter expansion, command substitution, and
1282 arithmetic expansion the shell scans the results of
1283 expansions and substitutions that did not occur in double-quotes for
1284 field splitting and multiple fields can result.
1286 The shell treats each character of the
1288 as a delimiter and uses
1289 the delimiters to split the results of parameter expansion and command
1290 substitution into fields.
1291 .Ss Pathname Expansion (File Name Generation)
1295 file name generation is performed
1296 after word splitting is complete.
1298 viewed as a series of patterns, separated by slashes.
1300 process of expansion replaces the word with the names of
1301 all existing files whose names can be formed by replacing
1302 each pattern with a string that matches the specified pattern.
1303 There are two restrictions on this: first, a pattern cannot match
1304 a string containing a slash, and second,
1305 a pattern cannot match a string starting with a period
1306 unless the first character of the pattern is a period.
1307 The next section describes the patterns used for both
1308 Pathname Expansion and the
1312 A pattern consists of normal characters, which match themselves,
1313 and meta-characters.
1314 The meta-characters are
1320 These characters lose their special meanings if they are quoted.
1321 When command or variable substitution is performed and the dollar sign
1322 or back quotes are not double-quoted, the value of the
1323 variable or the output of the command is scanned for these
1324 characters and they are turned into meta-characters.
1328 matches any string of characters.
1331 matches any single character.
1334 introduces a character class.
1335 The end of the character class is indicated by a
1343 rather than introducing a character class.
1344 A character class matches any of the characters between the square brackets.
1345 A range of characters may be specified using a minus sign.
1346 The character class may be complemented by making an exclamation point
1348 the first character of the character class.
1352 in a character class, make it the first character listed
1358 make it the first or last character listed.
1359 .Ss Built-in Commands
1360 This section lists the commands which
1361 are built-in because they need to perform some operation
1362 that cannot be performed by a separate process.
1364 these, built-in versions of essential utilities
1365 are provided for efficiency.
1366 .Bl -tag -width indent
1368 A null command that returns a 0 (true) exit value.
1370 The commands in the specified file are read and executed by the shell.
1373 command may be used to return to the
1380 characters, it is used as is.
1381 Otherwise, the shell searches the
1384 If it is not found in the
1386 it is sought in the current working directory.
1388 A built-in equivalent of
1390 .It Ic alias Oo Ar name Ns Oo = Ns Ar string Oc ... Oc
1392 .Ar name Ns = Ns Ar string
1393 is specified, the shell defines the alias
1399 is specified, the value of the alias
1402 With no arguments, the
1404 built-in command prints the names and values of all defined aliases
1407 Alias values are written with appropriate quoting so that they are
1408 suitable for re-input to the shell.
1412 .It Ic bg Op Ar job ...
1413 Continue the specified jobs
1414 (or the current job if no jobs are given)
1416 .It Ic builtin Ar cmd Op Ar arg ...
1417 Execute the specified built-in command,
1419 This is useful when the user wishes to override a shell function
1420 with the same name as a built-in command.
1421 .It Ic bind Oo Fl aeklrsv Oc Oo Ar key Oo Ar command Oc Oc
1422 List or alter key bindings for the line editor.
1423 This command is documented in
1425 .It Ic cd Oo Fl L | P Oc Op Ar directory
1426 Switch to the specified
1428 or to the directory specified in the
1430 environment variable if no
1439 then the directories listed in the
1442 searched for the specified
1446 is unset, the current directory is searched.
1449 is the same as that of
1451 In an interactive shell,
1454 command will print out the name of the directory
1455 that it actually switched to
1456 if this is different from the name that the user gave.
1457 These may be different either because the
1459 mechanism was used or because a symbolic link was crossed.
1463 option is specified,
1465 is handled physically and symbolic links are resolved before
1467 components are processed.
1470 option is specified,
1472 is handled logically.
1473 This is the default.
1478 .It Ic command Oo Fl p Oc Op Ar utility Op Ar argument ...
1479 .It Ic command Oo Fl v | V Oc Op Ar utility
1480 The first form of invocation executes the specified
1482 as a simple command (see the
1488 option is specified, the command search is performed using a
1491 that is guaranteed to find all of the standard utilities.
1495 option is specified,
1497 is not executed but a description of its interpretation by the shell is
1499 For ordinary commands the output is the path name; for shell built-in
1500 commands, shell functions and keywords only the name is written.
1501 Aliases are printed as
1502 .Dq Ic alias Ar name Ns = Ns Ar value .
1506 option is identical to
1508 except for the output.
1510 .Dq Ar utility Ic is Ar description
1522 .It Ic echo Oo Fl e | n Oc Op Ar string ...
1523 Print a space-separated list of the arguments to the standard output
1524 and append a newline character.
1525 .Bl -tag -width indent
1527 Suppress the output of the trailing newline.
1529 Process C-style backslash escape sequences.
1531 understands the following character escapes:
1532 .Bl -tag -width indent
1534 Alert (ring the terminal bell)
1538 Suppress the trailing newline (this has the side-effect of truncating the
1539 line if it is not the last character)
1541 The ESC character (ASCII 0x1b)
1555 (Zero) The character whose octal value is nnn
1560 is not enclosed in quotes then the backslash itself must be escaped
1561 with a backslash to protect it from the shell.
1563 .Bd -literal -offset indent
1572 $ echo -e a\e\e\e\eb
1581 options may be specified.
1582 .It Ic eval Ar string ...
1583 Concatenate all the arguments with spaces.
1584 Then re-parse and execute the command.
1585 .It Ic exec Op Ar command Op arg ...
1589 the shell process is replaced with the specified program
1590 (which must be a real program, not a shell built-in command or function).
1591 Any redirections on the
1593 command are marked as permanent,
1594 so that they are not undone when the
1597 .It Ic exit Op Ar exitstatus
1598 Terminate the shell process.
1602 it is used as the exit status of the shell;
1603 otherwise the exit status of the preceding command is used.
1604 .It Ic export Ar name ...
1605 .It Ic export Op Fl p
1606 The specified names are exported so that they will
1607 appear in the environment of subsequent commands.
1608 The only way to un-export a variable is to
1611 The shell allows the value of a variable to be set
1612 at the same time as it is exported by writing
1613 .Bd -literal -offset indent
1617 With no arguments the export command lists the names
1618 of all exported variables.
1621 option is specified, the exported variables are printed as
1622 .Dq Ic export Ar name Ns = Ns Ar value
1623 lines, suitable for re-input to the shell.
1625 A null command that returns a non-zero (false) exit value.
1626 .It Ic fc Oo Fl e Ar editor Oc Op Ar first Op Ar last
1627 .It Ic fc Fl l Oo Fl nr Oc Op Ar first Op Ar last
1628 .It Ic fc Fl s Oo Ar old Ns = Ns Ar new Oc Op Ar first
1631 built-in command lists, or edits and re-executes,
1632 commands previously entered to an interactive shell.
1633 .Bl -tag -width indent
1635 Use the editor named by
1637 to edit the commands.
1638 The editor string is a command name,
1639 subject to search via the
1644 variable is used as a default when
1649 is null or unset, the value of the
1656 is used as the editor.
1658 List the commands rather than invoking
1660 The commands are written in the
1661 sequence indicated by the first and last operands, as
1664 with each command preceded by the command number.
1666 Suppress command numbers when listing with
1669 Reverse the order of the commands listed
1678 Re-execute the command without invoking an editor.
1681 Select the commands to list or edit.
1682 The number of previous commands that can be accessed
1683 are determined by the value of the
1690 or both are one of the following:
1691 .Bl -tag -width indent
1693 A positive number representing a command number;
1694 command numbers can be displayed with the
1698 A negative decimal number representing the
1699 command that was executed
1702 commands previously.
1703 For example, -1 is the immediately previous command.
1705 A string indicating the most recently entered command
1706 that begins with that string.
1709 operand is not also specified with
1711 the string form of the first operand cannot contain an embedded equal sign.
1715 The following environment variables affect the execution of
1717 .Bl -tag -width ".Ev HISTSIZE"
1719 Name of the editor to use for history editing.
1721 The number of previous commands that are accessible.
1726 or the current job to the foreground.
1727 .It Ic getopts Ar optstring Ar var
1733 command deprecates the older
1736 The first argument should be a series of letters, each possibly
1737 followed by a colon which indicates that the option takes an argument.
1738 The specified variable is set to the parsed option.
1740 the next argument is placed into the shell variable
1742 If an option takes an argument, it is placed into the shell variable
1744 If an invalid option is encountered,
1748 It returns a false value (1) when it encounters the end of the options.
1749 .It Ic hash Oo Fl rv Oc Op Ar command ...
1750 The shell maintains a hash table which remembers the locations of commands.
1751 With no arguments whatsoever, the
1753 command prints out the contents of this table.
1754 Entries which have not been looked at since the last
1756 command are marked with an asterisk;
1757 it is possible for these entries to be invalid.
1761 command removes each specified
1763 from the hash table (unless they are functions) and then locates it.
1768 prints the locations of the commands as it finds them.
1773 command to delete all the entries in the hash table except for functions.
1774 .It Ic jobid Op Ar job
1775 Print the process id's of the processes in the specified
1779 argument is omitted, use the current job.
1780 .It Ic jobs Oo Fl lps Oc Op Ar job ...
1781 Print information about the specified jobs, or all jobs if no
1784 The information printed includes job ID, status and command name.
1788 option is specified, the PID of each job is also printed.
1791 option is specified, only the process IDs for the process group leaders
1792 are printed, one per line.
1795 option is specified, only the PIDs of the job commands are printed, one per
1797 .It Ic local Oo Ar variable ... Oc Op Fl
1801 .It Ic pwd Op Fl L | P
1802 Print the path of the current directory.
1803 The built-in command may
1804 differ from the program of the same name because the
1805 built-in command remembers what the current directory
1806 is rather than recomputing it each time.
1809 However, if the current directory is
1811 the built-in version of
1813 will continue to print the old name for the directory.
1817 option is specified, symbolic links are resolved.
1820 option is specified, the shell's notion of the current directory
1821 is printed (symbolic links are not resolved).
1822 This is the default.
1823 .It Ic read Oo Fl p Ar prompt Oc Oo Fl t Ar timeout Oc Oo Fl er Oc Ar variable ...
1829 and the standard input is a terminal.
1831 read from the standard input.
1832 The trailing newline
1833 is deleted from the line and the line is split as
1834 described in the section on
1835 .Sx White Space Splitting (Field Splitting)
1837 the pieces are assigned to the variables in order.
1838 If there are more pieces than variables, the remaining
1839 pieces (along with the characters in
1841 that separated them)
1842 are assigned to the last variable.
1843 If there are more variables than pieces, the remaining
1844 variables are assigned the null string.
1846 Backslashes are treated specially, unless the
1850 If a backslash is followed by
1851 a newline, the backslash and the newline will be
1853 If a backslash is followed by any other
1854 character, the backslash will be deleted and the following
1855 character will be treated as though it were not in
1861 option is specified and the
1863 elapses before any input is supplied,
1866 command will return an exit status of 1 without assigning any values.
1869 value may optionally be followed by one of
1874 to explicitly specify seconds, minutes or hours.
1875 If none is supplied,
1881 option exists only for backward compatibility with older scripts.
1882 .It Ic readonly Oo Fl p Oc Op Ar name ...
1885 is marked as read only,
1886 so that it cannot be subsequently modified or unset.
1887 The shell allows the value of a variable to be set
1888 at the same time as it is marked read only
1889 by using the following form:
1890 .Bd -literal -offset indent
1894 With no arguments the
1896 command lists the names of all read only variables.
1899 option is specified, the read-only variables are printed as
1900 .Dq Ic readonly Ar name Ns = Ns Ar value
1901 lines, suitable for re-input to the shell.
1902 .It Ic return Op Ar exitstatus
1906 .It Ic set Oo Fl /+abCEefIimnpTuVvx Oc Oo Fl /+o Ar longname Oc Oo
1907 .Fl c Ar string Oc Op Fl - Ar arg ...
1910 command performs three different functions:
1913 With no arguments, it lists the values of all shell variables.
1915 If options are given,
1916 either in short form or using the long
1917 .Dq Fl /+o Ar longname
1919 it sets or clears the specified options as described in the section called
1920 .Sx Argument List Processing .
1924 option is specified,
1926 will replace the shell's positional parameters with the subsequent
1928 If no arguments follow the
1931 all the positional parameters will be cleared,
1932 which is equivalent to executing the command
1936 flag may be omitted when specifying arguments to be used
1937 as positional replacement parameters.
1938 This is not recommended,
1939 because the first argument may begin with a dash
1945 command will interpret as a request to enable or disable options.
1947 .It Ic setvar Ar variable Ar value
1948 Assigns the specified
1953 is intended to be used in functions that
1954 assign values to variables whose names are passed as parameters.
1955 In general it is better to write
1956 .Bd -literal -offset indent
1961 .It Ic shift Op Ar n
1962 Shift the positional parameters
1967 A shift sets the value of $1 to the value of $2,
1968 the value of $2 to the value of $3, and so on,
1969 decreasing the value of $# by one.
1970 If there are zero positional parameters, shifting does not do anything.
1972 A built-in equivalent of
1975 Print the amount of time spent executing the shell and its children.
1976 The first output line shows the user and system times for the shell
1977 itself, the second one contains the user and system times for the
1979 .It Ic trap Oo Ar action Oc Ar signal ...
1981 Cause the shell to parse and execute
1986 The signals are specified by name or number.
1987 In addition, the pseudo-signal
1989 may be used to specify an action that is performed when the shell terminates.
1992 may be an empty string or a dash
1994 the former causes the specified signal to be ignored
1995 and the latter causes the default action to be taken.
1998 is another way to request the default action, for compatibility reasons this
1999 usage is not recommended though.
2000 When the shell forks off a subshell,
2001 it resets trapped (but not ignored) signals to the default action.
2004 command has no effect on signals that were ignored on entry to the shell.
2010 command to display a list of valid signal names.
2012 A null command that returns a 0 (true) exit value.
2013 .It Ic type Op Ar name ...
2016 as a command and print the resolution of the command search.
2017 Possible resolutions are:
2018 shell keyword, alias, shell built-in command, command, tracked alias
2020 For aliases the alias expansion is printed;
2021 for commands and tracked aliases
2022 the complete pathname of the command is printed.
2023 .It Ic ulimit Oo Fl HSabcdflmnstuv Oc Op Ar limit
2024 Set or display resource limits (see
2028 is specified, the named resource will be set;
2029 otherwise the current resource value will be displayed.
2033 is specified, the hard limits will be set or displayed.
2034 While everybody is allowed to reduce a hard limit,
2035 only the superuser can increase it.
2039 specifies the soft limits instead.
2040 When displaying limits,
2046 The default is to display the soft limits,
2047 and to set both the hard and the soft limits.
2053 command to display all resources.
2056 is not acceptable in this mode.
2058 The remaining options specify which resource value is to be
2059 displayed or modified.
2060 They are mutually exclusive.
2061 .Bl -tag -width indent
2063 The maximum size of socket buffer usage, in bytes.
2064 .It Fl c Ar coredumpsize
2065 The maximal size of core dump files, in 512-byte blocks.
2066 .It Fl d Ar datasize
2067 The maximal size of the data segment of a process, in kilobytes.
2068 .It Fl f Ar filesize
2069 The maximal size of a file, in 512-byte blocks.
2070 .It Fl l Ar lockedmem
2071 The maximal size of memory that can be locked by a process, in
2073 .It Fl m Ar memoryuse
2074 The maximal resident set size of a process, in kilobytes.
2076 The maximal number of descriptors that could be opened by a process.
2077 .It Fl s Ar stacksize
2078 The maximal size of the stack segment, in kilobytes.
2080 The maximal amount of CPU time to be used by each process, in seconds.
2081 .It Fl u Ar userproc
2082 The maximal number of simultaneous processes for this user ID.
2083 .It Fl v Ar virtualmem
2084 The maximal virtual size of a process, in kilobytes.
2086 .It Ic umask Oo Fl S Oc Op Ar mask
2087 Set the file creation mask (see
2089 to the octal or symbolic (see
2093 If the argument is omitted, the current mask value is printed.
2096 option is specified, the output is symbolic, otherwise the output is octal.
2097 .It Ic unalias Oo Fl a Oc Op Ar name ...
2098 The specified alias names are removed.
2101 is specified, all aliases are removed.
2102 .It Ic unset Oo Fl fv Oc Ar name ...
2103 The specified variables or functions are unset and unexported.
2106 option is specified or no options are given, the
2108 arguments are treated as variable names.
2111 option is specified, the
2113 arguments are treated as function names.
2114 .It Ic wait Op Ar job
2115 Wait for the specified
2117 to complete and return the exit status of the last process in the
2119 If the argument is omitted, wait for all jobs to complete
2120 and return an exit status of zero.
2122 .Ss Commandline Editing
2125 is being used interactively from a terminal, the current command
2126 and the command history
2130 .Sx Built-in Commands )
2131 can be edited using vi-mode command line editing.
2132 This mode uses commands similar
2133 to a subset of those described in the vi man page.
2138 enables vi-mode editing and places
2140 into vi insert mode.
2141 With vi-mode enabled,
2143 can be switched between insert mode and command mode by typing
2147 while in command mode will pass the line to the shell.
2153 command can be used to enable a subset of
2154 emacs-style command line editing features.
2156 The following environment variables affect the execution of
2158 .Bl -tag -width ".Ev HISTSIZE"
2160 The search path used with the
2164 The fallback editor used with the
2167 If not set, the default editor is
2170 The default editor used with the
2174 The number of previous commands that are accessible.
2176 The starting directory of
2179 Input Field Separators.
2180 This is normally set to
2186 .Sx White Space Splitting
2187 section for more details.
2189 The name of a mail file, that will be checked for the arrival of new
2196 separated list of file names, for the shell to check for incoming
2198 This environment setting overrides the
2201 There is a maximum of 10 mailboxes that can be monitored at once.
2203 The default search path for executables.
2206 section for details.
2208 The primary prompt string, which defaults to
2210 unless you are the superuser, in which case it defaults to
2213 The secondary prompt string, which defaults to
2216 The prefix for the trace output (if
2222 The default terminal setting for the shell.
2223 This is inherited by children of the shell, and is used in the history
2227 Errors that are detected by the shell, such as a syntax error, will
2228 cause the shell to exit with a non-zero exit status.
2229 If the shell is not an interactive shell, the execution of the shell
2230 file will be aborted.
2231 Otherwise the shell will return the exit status of the last command
2232 executed, or if the exit builtin is used with a numeric argument, it
2233 will return the argument.
2252 command, the Thompson shell, appeared in
2254 It was superseded in
2256 by the Bourne shell, which inherited the name
2261 was rewritten in 1989 under the
2263 license after the Bourne shell from
2268 was originally written by
2269 .An Kenneth Almquist .
2273 utility does not recognize multibyte characters.
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