1DASH(1) BSD General Commands Manual DASH(1)
2
4 dash — command interpreter (shell)
5
7 dash [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
8 [+o option_name] [command_file [argument ...]]
9 dash -c [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
10 [+o option_name] command_string [command_name [argument ...]]
11 dash -s [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
12 [+o option_name] [argument ...]
13
15 dash is the standard command interpreter for the system. The current
16 version of dash is in the process of being changed to conform with the
17 POSIX 1003.2 and 1003.2a specifications for the shell. This version has
18 many features which make it appear similar in some respects to the Korn
19 shell, but it is not a Korn shell clone (see ksh(1)). Only features des‐
20 ignated by POSIX, plus a few Berkeley extensions, are being incorporated
21 into this shell. This man page is not intended to be a tutorial or a
22 complete specification of the shell.
23 Overview
25 The shell is a command that reads lines from either a file or the termi‐
26 nal, interprets them, and generally executes other commands. It is the
27 program that is running when a user logs into the system (although a user
28 can select a different shell with the chsh(1) command). The shell imple‐
29 ments a language that has flow control constructs, a macro facility that
30 provides a variety of features in addition to data storage, along with
31 built in history and line editing capabilities. It incorporates many
32 features to aid interactive use and has the advantage that the interpre‐
33 tative language is common to both interactive and non-interactive use
34 (shell scripts). That is, commands can be typed directly to the running
35 shell or can be put into a file and the file can be executed directly by
36 the shell.
37 Invocation
39 If no args are present and if the standard input of the shell is con‐
40 nected to a terminal (or if the -i flag is set), and the -c option is not
41 present, the shell is considered an interactive shell. An interactive
42 shell generally prompts before each command and handles programming and
43 command errors differently (as described below). When first starting,
44 the shell inspects argument 0, and if it begins with a dash ‘-’, the
45 shell is also considered a login shell. This is normally done automati‐
46 cally by the system when the user first logs in. A login shell first
47 reads commands from the files /etc/profile and .profile if they exist.
48 If the environment variable ENV is set on entry to an interactive shell,
49 or is set in the .profile of a login shell, the shell next reads commands
50 from the file named in ENV. Therefore, a user should place commands that
51 are to be executed only at login time in the .profile file, and commands
52 that are executed for every interactive shell inside the ENV file. To
53 set the ENV variable to some file, place the following line in your
54 .profile of your home directory
55 ENV=$HOME/.shinit; export ENV
57 substituting for “.shinit” any filename you wish.
59 If command line arguments besides the options have been specified, then
61 the shell treats the first argument as the name of a file from which to
62 read commands (a shell script), and the remaining arguments are set as
63 the positional parameters of the shell ($1, $2, etc). Otherwise, the
64 shell reads commands from its standard input.
65 Argument List Processing
67 All of the single letter options that have a corresponding name can be
68 used as an argument to the -o option. The set -o name is provided next
69 to the single letter option in the description below. Specifying a dash
70 “-” turns the option on, while using a plus “+” disables the option. The
71 following options can be set from the command line or with the set
72 builtin (described later).
73 -a allexport Export all variables assigned to.
75 -c Read commands from the command_string operand in‐
77 stead of from the standard input. Special parame‐
78 ter 0 will be set from the command_name operand
79 and the positional parameters ($1, $2, etc.) set
80 from the remaining argument operands.
81 -C noclobber Don't overwrite existing files with “>”.
83 -e errexit If not interactive, exit immediately if any
85 untested command fails. The exit status of a com‐
86 mand is considered to be explicitly tested if the
87 command is used to control an if, elif, while, or
88 until; or if the command is the left hand operand
89 of an “&&” or “||” operator.
90 -f noglob Disable pathname expansion.
92 -n noexec If not interactive, read commands but do not exe‐
94 cute them. This is useful for checking the syntax
95 of shell scripts.
96 -u nounset Write a message to standard error when attempting
98 to expand a variable that is not set, and if the
99 shell is not interactive, exit immediately.
100 -v verbose The shell writes its input to standard error as it
102 is read. Useful for debugging.
103 -x xtrace Write each command to standard error (preceded by
105 a ‘+ ’) before it is executed. Useful for debug‐
106 ging.
107 -I ignoreeof Ignore EOF's from input when interactive.
109 -i interactive Force the shell to behave interactively.
111 -l Make dash act as if it had been invoked as a login
113 shell.
114 -m monitor Turn on job control (set automatically when inter‐
116 active).
117 -s stdin Read commands from standard input (set automati‐
119 cally if no file arguments are present). This op‐
120 tion has no effect when set after the shell has
121 already started running (i.e. with set).
122 -V vi Enable the built-in vi(1) command line editor
124 (disables -E if it has been set).
125 -E emacs Enable the built-in emacs(1) command line editor
127 (disables -V if it has been set).
128 -b notify Enable asynchronous notification of background job
130 completion. (UNIMPLEMENTED for 4.4alpha)
131 Lexical Structure
133 The shell reads input in terms of lines from a file and breaks it up into
134 words at whitespace (blanks and tabs), and at certain sequences of char‐
135 acters that are special to the shell called “operators”. There are two
136 types of operators: control operators and redirection operators (their
137 meaning is discussed later). Following is a list of operators:
138 Control operators:
140 & && ( ) ; ;; | || <newline>
141 Redirection operators:
143 < > >| << >> <& >& <<- <>
144 Quoting
146 Quoting is used to remove the special meaning of certain characters or
147 words to the shell, such as operators, whitespace, or keywords. There
148 are three types of quoting: matched single quotes, matched double quotes,
149 and backslash.
150 Backslash
152 A backslash preserves the literal meaning of the following character,
153 with the exception of ⟨newline⟩. A backslash preceding a ⟨newline⟩ is
154 treated as a line continuation.
155 Single Quotes
157 Enclosing characters in single quotes preserves the literal meaning of
158 all the characters (except single quotes, making it impossible to put
159 single-quotes in a single-quoted string).
160 Double Quotes
162 Enclosing characters within double quotes preserves the literal meaning
163 of all characters except dollarsign ($), backquote (`), and backslash
164 (\). The backslash inside double quotes is historically weird, and
165 serves to quote only the following characters:
166 $ ` " \ <newline>.
167 Otherwise it remains literal.
168 Reserved Words
170 Reserved words are words that have special meaning to the shell and are
171 recognized at the beginning of a line and after a control operator. The
172 following are reserved words:
173 ! elif fi while case
175 else for then { }
176 do done until if esac
177 Their meaning is discussed later.
179 Aliases
181 An alias is a name and corresponding value set using the alias(1) builtin
182 command. Whenever a reserved word may occur (see above), and after
183 checking for reserved words, the shell checks the word to see if it
184 matches an alias. If it does, it replaces it in the input stream with
185 its value. For example, if there is an alias called “lf” with the value
186 “ls -F”, then the input:
187 lf foobar ⟨return⟩
189 would become
191 ls -F foobar ⟨return⟩
193 Aliases provide a convenient way for naive users to create shorthands for
195 commands without having to learn how to create functions with arguments.
196 They can also be used to create lexically obscure code. This use is dis‐
197 couraged.
198 Commands
200 The shell interprets the words it reads according to a language, the
201 specification of which is outside the scope of this man page (refer to
202 the BNF in the POSIX 1003.2 document). Essentially though, a line is
203 read and if the first word of the line (or after a control operator) is
204 not a reserved word, then the shell has recognized a simple command.
205 Otherwise, a complex command or some other special construct may have
206 been recognized.
207 Simple Commands
209 If a simple command has been recognized, the shell performs the following
210 actions:
211 1. Leading words of the form “name=value” are stripped off and
213 assigned to the environment of the simple command. Redirect‐
214 ion operators and their arguments (as described below) are
215 stripped off and saved for processing.
216 2. The remaining words are expanded as described in the section
218 called “Expansions”, and the first remaining word is consid‐
219 ered the command name and the command is located. The remain‐
220 ing words are considered the arguments of the command. If no
221 command name resulted, then the “name=value” variable assign‐
222 ments recognized in item 1 affect the current shell.
223 3. Redirections are performed as described in the next section.
225 Redirections
227 Redirections are used to change where a command reads its input or sends
228 its output. In general, redirections open, close, or duplicate an exist‐
229 ing reference to a file. The overall format used for redirection is:
230 [n] redir-op file
232 where redir-op is one of the redirection operators mentioned previously.
234 Following is a list of the possible redirections. The [n] is an optional
235 number between 0 and 9, as in ‘3’ (not ‘[3]’), that refers to a file de‐
236 scriptor.
237 [n]> file Redirect standard output (or n) to file.
239 [n]>| file Same, but override the -C option.
241 [n]>> file Append standard output (or n) to file.
243 [n]< file Redirect standard input (or n) from file.
245 [n1]<&n2 Copy file descriptor n2 as stdout (or fd n1). fd n2.
247 [n]<&- Close standard input (or n).
249 [n1]>&n2 Copy file descriptor n2 as stdin (or fd n1). fd n2.
251 [n]>&- Close standard output (or n).
253 [n]<> file Open file for reading and writing on standard input (or
255 n).
256 The following redirection is often called a “here-document”.
258 [n]<< delimiter
260 here-doc-text ...
261 delimiter
262 All the text on successive lines up to the delimiter is saved away and
264 made available to the command on standard input, or file descriptor n if
265 it is specified. If the delimiter as specified on the initial line is
266 quoted, then the here-doc-text is treated literally, otherwise the text
267 is subjected to parameter expansion, command substitution, and arithmetic
268 expansion (as described in the section on “Expansions”). If the operator
269 is “<<-” instead of “<<”, then leading tabs in the here-doc-text are
270 stripped.
271 Search and Execution
273 There are three types of commands: shell functions, builtin commands, and
274 normal programs – and the command is searched for (by name) in that or‐
275 der. They each are executed in a different way.
276 When a shell function is executed, all of the shell positional parameters
278 (except $0, which remains unchanged) are set to the arguments of the
279 shell function. The variables which are explicitly placed in the envi‐
280 ronment of the command (by placing assignments to them before the func‐
281 tion name) are made local to the function and are set to the values
282 given. Then the command given in the function definition is executed.
283 The positional parameters are restored to their original values when the
284 command completes. This all occurs within the current shell.
285 Shell builtins are executed internally to the shell, without spawning a
287 new process.
288 Otherwise, if the command name doesn't match a function or builtin, the
290 command is searched for as a normal program in the file system (as de‐
291 scribed in the next section). When a normal program is executed, the
292 shell runs the program, passing the arguments and the environment to the
293 program. If the program is not a normal executable file (i.e., if it
294 does not begin with the "magic number" whose ASCII representation is
295 "#!", so execve(2) returns ENOEXEC then) the shell will interpret the
296 program in a subshell. The child shell will reinitialize itself in this
297 case, so that the effect will be as if a new shell had been invoked to
298 handle the ad-hoc shell script, except that the location of hashed com‐
299 mands located in the parent shell will be remembered by the child.
300 Note that previous versions of this document and the source code itself
302 misleadingly and sporadically refer to a shell script without a magic
303 number as a "shell procedure".
304 Path Search
306 When locating a command, the shell first looks to see if it has a shell
307 function by that name. Then it looks for a builtin command by that name.
308 If a builtin command is not found, one of two things happen:
309 1. Command names containing a slash are simply executed without per‐
311 forming any searches.
312 2. The shell searches each entry in PATH in turn for the command. The
314 value of the PATH variable should be a series of entries separated
315 by colons. Each entry consists of a directory name. The current
316 directory may be indicated implicitly by an empty directory name, or
317 explicitly by a single period.
318 Command Exit Status
320 Each command has an exit status that can influence the behaviour of other
321 shell commands. The paradigm is that a command exits with zero for nor‐
322 mal or success, and non-zero for failure, error, or a false indication.
323 The man page for each command should indicate the various exit codes and
324 what they mean. Additionally, the builtin commands return exit codes, as
325 does an executed shell function.
326 If a command consists entirely of variable assignments then the exit sta‐
328 tus of the command is that of the last command substitution if any, oth‐
329 erwise 0.
330 Complex Commands
332 Complex commands are combinations of simple commands with control opera‐
333 tors or reserved words, together creating a larger complex command. More
334 generally, a command is one of the following:
335 • simple command
337 • pipeline
339 • list or compound-list
341 • compound command
343 • function definition
345 Unless otherwise stated, the exit status of a command is that of the last
347 simple command executed by the command.
348 Pipelines
350 A pipeline is a sequence of one or more commands separated by the control
351 operator |. The standard output of all but the last command is connected
352 to the standard input of the next command. The standard output of the
353 last command is inherited from the shell, as usual.
354 The format for a pipeline is:
356 [!] command1 [| command2 ...]
358 The standard output of command1 is connected to the standard input of
360 command2. The standard input, standard output, or both of a command is
361 considered to be assigned by the pipeline before any redirection speci‐
362 fied by redirection operators that are part of the command.
363 If the pipeline is not in the background (discussed later), the shell
365 waits for all commands to complete.
366 If the reserved word ! does not precede the pipeline, the exit status is
368 the exit status of the last command specified in the pipeline. Other‐
369 wise, the exit status is the logical NOT of the exit status of the last
370 command. That is, if the last command returns zero, the exit status is
371 1; if the last command returns greater than zero, the exit status is
372 zero.
373 Because pipeline assignment of standard input or standard output or both
375 takes place before redirection, it can be modified by redirection. For
376 example:
377 $ command1 2>&1 | command2
379 sends both the standard output and standard error of command1 to the
381 standard input of command2.
382 A ; or ⟨newline⟩ terminator causes the preceding AND-OR-list (described
384 next) to be executed sequentially; a & causes asynchronous execution of
385 the preceding AND-OR-list.
386 Note that unlike some other shells, each process in the pipeline is a
388 child of the invoking shell (unless it is a shell builtin, in which case
389 it executes in the current shell – but any effect it has on the environ‐
390 ment is wiped).
391 Background Commands – &
393 If a command is terminated by the control operator ampersand (&), the
394 shell executes the command asynchronously – that is, the shell does not
395 wait for the command to finish before executing the next command.
396 The format for running a command in background is:
398 command1 & [command2 & ...]
400 If the shell is not interactive, the standard input of an asynchronous
402 command is set to /dev/null.
403 Lists – Generally Speaking
405 A list is a sequence of zero or more commands separated by newlines,
406 semicolons, or ampersands, and optionally terminated by one of these
407 three characters. The commands in a list are executed in the order they
408 are written. If command is followed by an ampersand, the shell starts
409 the command and immediately proceeds onto the next command; otherwise it
410 waits for the command to terminate before proceeding to the next one.
411 Short-Circuit List Operators
413 “&&” and “||” are AND-OR list operators. “&&” executes the first com‐
414 mand, and then executes the second command if and only if the exit status
415 of the first command is zero. “||” is similar, but executes the second
416 command if and only if the exit status of the first command is nonzero.
417 “&&” and “||” both have the same priority.
418 Flow-Control Constructs – if, while, for, case
420 The syntax of the if command is
421 if list
423 then list
424 [ elif list
425 then list ] ...
426 [ else list ]
427 fi
428 The syntax of the while command is
430 while list
432 do list
433 done
434 The two lists are executed repeatedly while the exit status of the first
436 list is zero. The until command is similar, but has the word until in
437 place of while, which causes it to repeat until the exit status of the
438 first list is zero.
439 The syntax of the for command is
441 for variable [ in [ word ... ] ]
443 do list
444 done
445 The words following in are expanded, and then the list is executed re‐
447 peatedly with the variable set to each word in turn. Omitting in word
448 ... is equivalent to in "$@".
449 The syntax of the break and continue command is
451 break [ num ]
453 continue [ num ]
454 Break terminates the num innermost for or while loops. Continue contin‐
456 ues with the next iteration of the innermost loop. These are implemented
457 as builtin commands.
458 The syntax of the case command is
460 case word in
462 [(]pattern) list ;;
463 ...
464 esac
465 The pattern can actually be one or more patterns (see Shell Patterns de‐
467 scribed later), separated by “|” characters. The “(” character before
468 the pattern is optional.
469 Grouping Commands Together
471 Commands may be grouped by writing either
472 (list)
474 or
476 { list; }
478 The first of these executes the commands in a subshell. Builtin commands
480 grouped into a (list) will not affect the current shell. The second form
481 does not fork another shell so is slightly more efficient. Grouping com‐
482 mands together this way allows you to redirect their output as though
483 they were one program:
484 { printf " hello " ; printf " world\n" ; } > greeting
486 Note that “}” must follow a control operator (here, “;”) so that it is
488 recognized as a reserved word and not as another command argument.
489 Functions
491 The syntax of a function definition is
492 name () command
494 A function definition is an executable statement; when executed it in‐
496 stalls a function named name and returns an exit status of zero. The
497 command is normally a list enclosed between “{” and “}”.
498 Variables may be declared to be local to a function by using a local com‐
500 mand. This should appear as the first statement of a function, and the
501 syntax is
502 local [variable | -] ...
504 Local is implemented as a builtin command.
506 When a variable is made local, it inherits the initial value and exported
508 and readonly flags from the variable with the same name in the surround‐
509 ing scope, if there is one. Otherwise, the variable is initially unset.
510 The shell uses dynamic scoping, so that if you make the variable x local
511 to function f, which then calls function g, references to the variable x
512 made inside g will refer to the variable x declared inside f, not to the
513 global variable named x.
514 The only special parameter that can be made local is “-”. Making “-” lo‐
516 cal any shell options that are changed via the set command inside the
517 function to be restored to their original values when the function re‐
518 turns.
519 The syntax of the return command is
521 return [exitstatus]
523 It terminates the currently executing function. Return is implemented as
525 a builtin command.
526 Variables and Parameters
528 The shell maintains a set of parameters. A parameter denoted by a name
529 is called a variable. When starting up, the shell turns all the environ‐
530 ment variables into shell variables. New variables can be set using the
531 form
532 name=value
534 Variables set by the user must have a name consisting solely of alphabet‐
536 ics, numerics, and underscores - the first of which must not be numeric.
537 A parameter can also be denoted by a number or a special character as ex‐
538 plained below.
539 Positional Parameters
541 A positional parameter is a parameter denoted by a number (n > 0). The
542 shell sets these initially to the values of its command line arguments
543 that follow the name of the shell script. The set builtin can also be
544 used to set or reset them.
545 Special Parameters
547 A special parameter is a parameter denoted by one of the following spe‐
548 cial characters. The value of the parameter is listed next to its char‐
549 acter.
550 * Expands to the positional parameters, starting from one.
552 When the expansion occurs within a double-quoted string it
553 expands to a single field with the value of each parameter
554 separated by the first character of the IFS variable, or by
555 a ⟨space⟩ if IFS is unset.
556 @ Expands to the positional parameters, starting from one.
558 When the expansion occurs within double-quotes, each posi‐
559 tional parameter expands as a separate argument. If there
560 are no positional parameters, the expansion of @ generates
561 zero arguments, even when @ is double-quoted. What this ba‐
562 sically means, for example, is if $1 is “abc” and $2 is “def
563 ghi”, then "$@" expands to the two arguments:
564 "abc" "def ghi"
566 # Expands to the number of positional parameters.
568 ? Expands to the exit status of the most recent pipeline.
570 - (Hyphen.) Expands to the current option flags (the single-letter op‐
572 tion names concatenated into a string) as specified on invo‐
573 cation, by the set builtin command, or implicitly by the
574 shell.
575 $ Expands to the process ID of the invoked shell. A subshell
577 retains the same value of $ as its parent.
578 ! Expands to the process ID of the most recent background com‐
580 mand executed from the current shell. For a pipeline, the
581 process ID is that of the last command in the pipeline.
582 0 (Zero.) Expands to the name of the shell or shell script.
584 Word Expansions
586 This clause describes the various expansions that are performed on words.
587 Not all expansions are performed on every word, as explained later.
588 Tilde expansions, parameter expansions, command substitutions, arithmetic
590 expansions, and quote removals that occur within a single word expand to
591 a single field. It is only field splitting or pathname expansion that
592 can create multiple fields from a single word. The single exception to
593 this rule is the expansion of the special parameter @ within double-
594 quotes, as was described above.
595 The order of word expansion is:
597 1. Tilde Expansion, Parameter Expansion, Command Substitution, Arith‐
599 metic Expansion (these all occur at the same time).
600 2. Field Splitting is performed on fields generated by step (1) unless
602 the IFS variable is null.
603 3. Pathname Expansion (unless set -f is in effect).
605 4. Quote Removal.
607 The $ character is used to introduce parameter expansion, command substi‐
609 tution, or arithmetic evaluation.
610 Tilde Expansion (substituting a user's home directory)
612 A word beginning with an unquoted tilde character (~) is subjected to
613 tilde expansion. All the characters up to a slash (/) or the end of the
614 word are treated as a username and are replaced with the user's home di‐
615 rectory. If the username is missing (as in ~/foobar), the tilde is re‐
616 placed with the value of the HOME variable (the current user's home di‐
617 rectory).
618 Parameter Expansion
620 The format for parameter expansion is as follows:
621 ${expression}
623 where expression consists of all characters until the matching “}”. Any
625 “}” escaped by a backslash or within a quoted string, and characters in
626 embedded arithmetic expansions, command substitutions, and variable ex‐
627 pansions, are not examined in determining the matching “}”.
628 The simplest form for parameter expansion is:
630 ${parameter}
632 The value, if any, of parameter is substituted.
634 The parameter name or symbol can be enclosed in braces, which are op‐
636 tional except for positional parameters with more than one digit or when
637 parameter is followed by a character that could be interpreted as part of
638 the name. If a parameter expansion occurs inside double-quotes:
639 1. Pathname expansion is not performed on the results of the expansion.
641 2. Field splitting is not performed on the results of the expansion,
643 with the exception of @.
644 In addition, a parameter expansion can be modified by using one of the
646 following formats.
647 ${parameter:-word} Use Default Values. If parameter is unset or null,
649 the expansion of word is substituted; otherwise,
650 the value of parameter is substituted.
651 ${parameter:=word} Assign Default Values. If parameter is unset or
653 null, the expansion of word is assigned to parame‐
654 ter. In all cases, the final value of parameter is
655 substituted. Only variables, not positional param‐
656 eters or special parameters, can be assigned in
657 this way.
658 ${parameter:?[word]} Indicate Error if Null or Unset. If parameter is
660 unset or null, the expansion of word (or a message
661 indicating it is unset if word is omitted) is writ‐
662 ten to standard error and the shell exits with a
663 nonzero exit status. Otherwise, the value of pa‐
664 rameter is substituted. An interactive shell need
665 not exit.
666 ${parameter:+word} Use Alternative Value. If parameter is unset or
668 null, null is substituted; otherwise, the expansion
669 of word is substituted.
670 In the parameter expansions shown previously, use of the colon in the
672 format results in a test for a parameter that is unset or null; omission
673 of the colon results in a test for a parameter that is only unset.
674 ${#parameter} String Length. The length in characters of the
676 value of parameter.
677 The following four varieties of parameter expansion provide for substring
679 processing. In each case, pattern matching notation (see Shell
680 Patterns), rather than regular expression notation, is used to evaluate
681 the patterns. If parameter is * or @, the result of the expansion is un‐
682 specified. Enclosing the full parameter expansion string in double-
683 quotes does not cause the following four varieties of pattern characters
684 to be quoted, whereas quoting characters within the braces has this ef‐
685 fect.
686 ${parameter%word} Remove Smallest Suffix Pattern. The word is ex‐
688 panded to produce a pattern. The parameter expan‐
689 sion then results in parameter, with the smallest
690 portion of the suffix matched by the pattern
691 deleted.
692 ${parameter%%word} Remove Largest Suffix Pattern. The word is ex‐
694 panded to produce a pattern. The parameter expan‐
695 sion then results in parameter, with the largest
696 portion of the suffix matched by the pattern
697 deleted.
698 ${parameter#word} Remove Smallest Prefix Pattern. The word is ex‐
700 panded to produce a pattern. The parameter expan‐
701 sion then results in parameter, with the smallest
702 portion of the prefix matched by the pattern
703 deleted.
704 ${parameter##word} Remove Largest Prefix Pattern. The word is ex‐
706 panded to produce a pattern. The parameter expan‐
707 sion then results in parameter, with the largest
708 portion of the prefix matched by the pattern
709 deleted.
710 Command Substitution
712 Command substitution allows the output of a command to be substituted in
713 place of the command name itself. Command substitution occurs when the
714 command is enclosed as follows:
715 $(command)
717 or (“backquoted” version):
719 `command`
721 The shell expands the command substitution by executing command in a sub‐
723 shell environment and replacing the command substitution with the stan‐
724 dard output of the command, removing sequences of one or more ⟨newline⟩s
725 at the end of the substitution. (Embedded ⟨newline⟩s before the end of
726 the output are not removed; however, during field splitting, they may be
727 translated into ⟨space⟩s, depending on the value of IFS and quoting that
728 is in effect.)
729 Arithmetic Expansion
731 Arithmetic expansion provides a mechanism for evaluating an arithmetic
732 expression and substituting its value. The format for arithmetic expan‐
733 sion is as follows:
734 $((expression))
736 The expression is treated as if it were in double-quotes, except that a
738 double-quote inside the expression is not treated specially. The shell
739 expands all tokens in the expression for parameter expansion, command
740 substitution, and quote removal.
741 Next, the shell treats this as an arithmetic expression and substitutes
743 the value of the expression.
744 White Space Splitting (Field Splitting)
746 After parameter expansion, command substitution, and arithmetic expansion
747 the shell scans the results of expansions and substitutions that did not
748 occur in double-quotes for field splitting and multiple fields can re‐
749 sult.
750 The shell treats each character of the IFS as a delimiter and uses the
752 delimiters to split the results of parameter expansion and command sub‐
753 stitution into fields.
754 Pathname Expansion (File Name Generation)
756 Unless the -f flag is set, file name generation is performed after word
757 splitting is complete. Each word is viewed as a series of patterns, sep‐
758 arated by slashes. The process of expansion replaces the word with the
759 names of all existing files whose names can be formed by replacing each
760 pattern with a string that matches the specified pattern. There are two
761 restrictions on this: first, a pattern cannot match a string containing a
762 slash, and second, a pattern cannot match a string starting with a period
763 unless the first character of the pattern is a period. The next section
764 describes the patterns used for both Pathname Expansion and the case com‐
765 mand.
766 Shell Patterns
768 A pattern consists of normal characters, which match themselves, and
769 meta-characters. The meta-characters are “!”, “*”, “?”, and “[”. These
770 characters lose their special meanings if they are quoted. When command
771 or variable substitution is performed and the dollar sign or back quotes
772 are not double quoted, the value of the variable or the output of the
773 command is scanned for these characters and they are turned into meta-
774 characters.
775 An asterisk (“*”) matches any string of characters. A question mark
777 matches any single character. A left bracket (“[”) introduces a charac‐
778 ter class. The end of the character class is indicated by a (“]”); if
779 the “]” is missing then the “[” matches a “[” rather than introducing a
780 character class. A character class matches any of the characters between
781 the square brackets. A range of characters may be specified using a mi‐
782 nus sign. The character class may be complemented by making an exclama‐
783 tion point the first character of the character class.
784 To include a “]” in a character class, make it the first character listed
786 (after the “!”, if any). To include a minus sign, make it the first or
787 last character listed.
788 Builtins
790 This section lists the builtin commands which are builtin because they
791 need to perform some operation that can't be performed by a separate
792 process. In addition to these, there are several other commands that may
793 be builtin for efficiency (e.g. printf(1), echo(1), test(1), etc).
794 :
796 true A null command that returns a 0 (true) exit value.
798 . file
800 The commands in the specified file are read and executed by the
801 shell.
802 alias [name[=string ...]]
804 If name=string is specified, the shell defines the alias name with
805 value string. If just name is specified, the value of the alias
806 name is printed. With no arguments, the alias builtin prints the
807 names and values of all defined aliases (see unalias).
808 bg [job] ...
810 Continue the specified jobs (or the current job if no jobs are
811 given) in the background.
812 command [-p] [-v] [-V] command [arg ...]
814 Execute the specified command but ignore shell functions when
815 searching for it. (This is useful when you have a shell function
816 with the same name as a builtin command.)
817 -p search for command using a PATH that guarantees to find all
819 the standard utilities.
820 -V Do not execute the command but search for the command and
822 print the resolution of the command search. This is the
823 same as the type builtin.
824 -v Do not execute the command but search for the command and
826 print the absolute pathname of utilities, the name for
827 builtins or the expansion of aliases.
828 cd -
830 cd [-LP] [directory]
832 Switch to the specified directory (default HOME). If an entry for
833 CDPATH appears in the environment of the cd command or the shell
834 variable CDPATH is set and the directory name does not begin with
835 a slash, then the directories listed in CDPATH will be searched
836 for the specified directory. The format of CDPATH is the same as
837 that of PATH. If a single dash is specified as the argument, it
838 will be replaced by the value of OLDPWD. The cd command will
839 print out the name of the directory that it actually switched to
840 if this is different from the name that the user gave. These may
841 be different either because the CDPATH mechanism was used or be‐
842 cause the argument is a single dash. The -P option causes the
843 physical directory structure to be used, that is, all symbolic
844 links are resolved to their respective values. The -L option
845 turns off the effect of any preceding -P options.
846 echo [-n] args...
848 Print the arguments on the standard output, separated by spaces.
849 Unless the -n option is present, a newline is output following the
850 arguments.
851 If any of the following sequences of characters is encountered
853 during output, the sequence is not output. Instead, the specified
854 action is performed:
855 \b A backspace character is output.
857 \c Subsequent output is suppressed. This is normally used at
859 the end of the last argument to suppress the trailing new‐
860 line that echo would otherwise output.
861 \f Output a form feed.
863 \n Output a newline character.
865 \r Output a carriage return.
867 \t Output a (horizontal) tab character.
869 \v Output a vertical tab.
871 \0digits
873 Output the character whose value is given by zero to three
874 octal digits. If there are zero digits, a nul character
875 is output.
876 \\ Output a backslash.
878 All other backslash sequences elicit undefined behaviour.
880 eval string ...
882 Concatenate all the arguments with spaces. Then re-parse and exe‐
883 cute the command.
884 exec [command arg ...]
886 Unless command is omitted, the shell process is replaced with the
887 specified program (which must be a real program, not a shell
888 builtin or function). Any redirections on the exec command are
889 marked as permanent, so that they are not undone when the exec
890 command finishes.
891 exit [exitstatus]
893 Terminate the shell process. If exitstatus is given it is used as
894 the exit status of the shell; otherwise the exit status of the
895 preceding command is used.
896 export name ...
898 export -p
900 The specified names are exported so that they will appear in the
901 environment of subsequent commands. The only way to un-export a
902 variable is to unset it. The shell allows the value of a variable
903 to be set at the same time it is exported by writing
904 export name=value
906 With no arguments the export command lists the names of all ex‐
908 ported variables. With the -p option specified the output will be
909 formatted suitably for non-interactive use.
910 fc [-e editor] [first [last]]
912 fc -l [-nr] [first [last]]
914 fc -s [old=new] [first]
916 The fc builtin lists, or edits and re-executes, commands previ‐
917 ously entered to an interactive shell.
918 -e editor
920 Use the editor named by editor to edit the commands. The
921 editor string is a command name, subject to search via the
922 PATH variable. The value in the FCEDIT variable is used as
923 a default when -e is not specified. If FCEDIT is null or
924 unset, the value of the EDITOR variable is used. If EDITOR
925 is null or unset, ed(1) is used as the editor.
926 -l (ell)
928 List the commands rather than invoking an editor on them.
929 The commands are written in the sequence indicated by the
930 first and last operands, as affected by -r, with each com‐
931 mand preceded by the command number.
932 -n Suppress command numbers when listing with -l.
934 -r Reverse the order of the commands listed (with -l) or
936 edited (with neither -l nor -s).
937 -s Re-execute the command without invoking an editor.
939 first
941 last Select the commands to list or edit. The number of previ‐
943 ous commands that can be accessed are determined by the
944 value of the HISTSIZE variable. The value of first or last
945 or both are one of the following:
946 [+]number
948 A positive number representing a command number;
949 command numbers can be displayed with the -l option.
950 -number
952 A negative decimal number representing the command
953 that was executed number of commands previously.
954 For example, -1 is the immediately previous command.
955 string
957 A string indicating the most recently entered command that
958 begins with that string. If the old=new operand is not
959 also specified with -s, the string form of the first oper‐
960 and cannot contain an embedded equal sign.
961 The following environment variables affect the execution of fc:
963 FCEDIT Name of the editor to use.
965 HISTSIZE The number of previous commands that are accessible.
967 fg [job]
969 Move the specified job or the current job to the foreground.
970 getopts optstring var
972 The POSIX getopts command, not to be confused with the Bell Labs
973 -derived getopt(1).
974 The first argument should be a series of letters, each of which
976 may be optionally followed by a colon to indicate that the option
977 requires an argument. The variable specified is set to the parsed
978 option.
979 The getopts command deprecates the older getopt(1) utility due to
981 its handling of arguments containing whitespace.
982 The getopts builtin may be used to obtain options and their argu‐
984 ments from a list of parameters. When invoked, getopts places the
985 value of the next option from the option string in the list in the
986 shell variable specified by var and its index in the shell vari‐
987 able OPTIND. When the shell is invoked, OPTIND is initialized to
988 1. For each option that requires an argument, the getopts builtin
989 will place it in the shell variable OPTARG. If an option is not
990 allowed for in the optstring, then OPTARG will be unset.
991 optstring is a string of recognized option letters (see
993 getopt(3)). If a letter is followed by a colon, the option is ex‐
994 pected to have an argument which may or may not be separated from
995 it by white space. If an option character is not found where ex‐
996 pected, getopts will set the variable var to a “?”; getopts will
997 then unset OPTARG and write output to standard error. By specify‐
998 ing a colon as the first character of optstring all errors will be
999 ignored.
1000 After the last option getopts will return a non-zero value and set
1002 var to “?”.
1003 The following code fragment shows how one might process the argu‐
1005 ments for a command that can take the options [a] and [b], and the
1006 option [c], which requires an argument.
1007 while getopts abc: f
1009 do
1010 case $f in
1011 a | b) flag=$f;;
1012 c) carg=$OPTARG;;
1013 \?) echo $USAGE; exit 1;;
1014 esac
1015 done
1016 shift `expr $OPTIND - 1`
1017 This code will accept any of the following as equivalent:
1019 cmd -acarg file file
1021 cmd -a -c arg file file
1022 cmd -carg -a file file
1023 cmd -a -carg -- file file
1024 hash -rv command ...
1026 The shell maintains a hash table which remembers the locations of
1027 commands. With no arguments whatsoever, the hash command prints
1028 out the contents of this table. Entries which have not been
1029 looked at since the last cd command are marked with an asterisk;
1030 it is possible for these entries to be invalid.
1031 With arguments, the hash command removes the specified commands
1033 from the hash table (unless they are functions) and then locates
1034 them. With the -v option, hash prints the locations of the com‐
1035 mands as it finds them. The -r option causes the hash command to
1036 delete all the entries in the hash table except for functions.
1037 pwd [-LP]
1039 builtin command remembers what the current directory is rather
1040 than recomputing it each time. This makes it faster. However, if
1041 the current directory is renamed, the builtin version of pwd will
1042 continue to print the old name for the directory. The -P option
1043 causes the physical value of the current working directory to be
1044 shown, that is, all symbolic links are resolved to their respec‐
1045 tive values. The -L option turns off the effect of any preceding
1046 -P options.
1047 read [-p prompt] [-r] variable [...]
1049 The prompt is printed if the -p option is specified and the stan‐
1050 dard input is a terminal. Then a line is read from the standard
1051 input. The trailing newline is deleted from the line and the line
1052 is split as described in the section on word splitting above, and
1053 the pieces are assigned to the variables in order. At least one
1054 variable must be specified. If there are more pieces than vari‐
1055 ables, the remaining pieces (along with the characters in IFS that
1056 separated them) are assigned to the last variable. If there are
1057 more variables than pieces, the remaining variables are assigned
1058 the null string. The read builtin will indicate success unless
1059 EOF is encountered on input, in which case failure is returned.
1060 By default, unless the -r option is specified, the backslash “\”
1062 acts as an escape character, causing the following character to be
1063 treated literally. If a backslash is followed by a newline, the
1064 backslash and the newline will be deleted.
1065 readonly name ...
1067 readonly -p
1069 The specified names are marked as read only, so that they cannot
1070 be subsequently modified or unset. The shell allows the value of
1071 a variable to be set at the same time it is marked read only by
1072 writing
1073 readonly name=value
1075 With no arguments the readonly command lists the names of all read
1077 only variables. With the -p option specified the output will be
1078 formatted suitably for non-interactive use.
1079 printf format [arguments ...]
1081 printf formats and prints its arguments, after the first, under
1082 control of the format. The format is a character string which
1083 contains three types of objects: plain characters, which are sim‐
1084 ply copied to standard output, character escape sequences which
1085 are converted and copied to the standard output, and format speci‐
1086 fications, each of which causes printing of the next successive
1087 argument.
1088 The arguments after the first are treated as strings if the corre‐
1090 sponding format is either b, c or s; otherwise it is evaluated as
1091 a C constant, with the following extensions:
1092 • A leading plus or minus sign is allowed.
1094 • If the leading character is a single or double quote,
1095 the value is the ASCII code of the next character.
1096 The format string is reused as often as necessary to satisfy the
1098 arguments. Any extra format specifications are evaluated with
1099 zero or the null string.
1100 Character escape sequences are in backslash notation as defined in
1102 ANSI X3.159-1989 (“ANSI C89”). The characters and their meanings
1103 are as follows:
1104 \a Write a <bell> character.
1106 \b Write a <backspace> character.
1108 \f Write a <form-feed> character.
1110 \n Write a <new-line> character.
1112 \r Write a <carriage return> character.
1114 \t Write a <tab> character.
1116 \v Write a <vertical tab> character.
1118 \\ Write a backslash character.
1120 \num Write an 8-bit character whose ASCII value is the
1122 1-, 2-, or 3-digit octal number num.
1123 Each format specification is introduced by the percent character
1125 (``%''). The remainder of the format specification includes, in
1126 the following order:
1127 Zero or more of the following flags:
1129 # A `#' character specifying that the value should
1131 be printed in an ``alternative form''. For b, c,
1132 d, and s formats, this option has no effect. For
1133 the o format the precision of the number is in‐
1134 creased to force the first character of the output
1135 string to a zero. For the x (X) format, a non-
1136 zero result has the string 0x (0X) prepended to
1137 it. For e, E, f, g, and G formats, the result
1138 will always contain a decimal point, even if no
1139 digits follow the point (normally, a decimal point
1140 only appears in the results of those formats if a
1141 digit follows the decimal point). For g and G
1142 formats, trailing zeros are not removed from the
1143 result as they would otherwise be.
1144 - A minus sign `-' which specifies left adjustment
1146 of the output in the indicated field;
1147 + A `+' character specifying that there should al‐
1149 ways be a sign placed before the number when using
1150 signed formats.
1151 ‘ ’ A space specifying that a blank should be left be‐
1153 fore a positive number for a signed format. A `+'
1154 overrides a space if both are used;
1155 0 A zero `0' character indicating that zero-padding
1157 should be used rather than blank-padding. A `-'
1158 overrides a `0' if both are used;
1159 Field Width:
1161 An optional digit string specifying a field width; if the
1162 output string has fewer characters than the field width it
1163 will be blank-padded on the left (or right, if the left-
1164 adjustment indicator has been given) to make up the field
1165 width (note that a leading zero is a flag, but an embedded
1166 zero is part of a field width);
1167 Precision:
1169 An optional period, ‘.’, followed by an optional digit
1170 string giving a precision which specifies the number of
1171 digits to appear after the decimal point, for e and f for‐
1172 mats, or the maximum number of bytes to be printed from a
1173 string (b and s formats); if the digit string is missing,
1174 the precision is treated as zero;
1175 Format:
1177 A character which indicates the type of format to use (one
1178 of diouxXfwEgGbcs).
1179 A field width or precision may be ‘*’ instead of a digit string.
1181 In this case an argument supplies the field width or precision.
1182 The format characters and their meanings are:
1184 diouXx The argument is printed as a signed decimal (d or i),
1186 unsigned octal, unsigned decimal, or unsigned hexadec‐
1187 imal (X or x), respectively.
1188 f The argument is printed in the style [-]ddd.ddd where
1190 the number of d's after the decimal point is equal to
1191 the precision specification for the argument. If the
1192 precision is missing, 6 digits are given; if the pre‐
1193 cision is explicitly 0, no digits and no decimal point
1194 are printed.
1195 eE The argument is printed in the style [-]d.ddde±dd
1197 where there is one digit before the decimal point and
1198 the number after is equal to the precision specifica‐
1199 tion for the argument; when the precision is missing,
1200 6 digits are produced. An upper-case E is used for an
1201 `E' format.
1202 gG The argument is printed in style f or in style e (E)
1204 whichever gives full precision in minimum space.
1205 b Characters from the string argument are printed with
1207 backslash-escape sequences expanded.
1208 The following additional backslash-escape sequences
1209 are supported:
1210 \c Causes dash to ignore any remaining characters
1212 in the string operand containing it, any re‐
1213 maining string operands, and any additional
1214 characters in the format operand.
1215 \0num Write an 8-bit character whose ASCII value is
1217 the 1-, 2-, or 3-digit octal number num.
1218 c The first character of argument is printed.
1220 s Characters from the string argument are printed until
1222 the end is reached or until the number of bytes indi‐
1223 cated by the precision specification is reached; if
1224 the precision is omitted, all characters in the string
1225 are printed.
1226 % Print a `%'; no argument is used.
1228 In no case does a non-existent or small field width cause trunca‐
1230 tion of a field; padding takes place only if the specified field
1231 width exceeds the actual width.
1232 set [{ -options | +options | -- }] arg ...
1234 The set command performs three different functions.
1235 With no arguments, it lists the values of all shell variables.
1237 If options are given, it sets the specified option flags, or
1239 clears them as described in the section called Argument List
1240 Processing. As a special case, if the option is -o or +o and no
1241 argument is supplied, the shell prints the settings of all its op‐
1242 tions. If the option is -o, the settings are printed in a human-
1243 readable format; if the option is +o, the settings are printed in
1244 a format suitable for reinput to the shell to affect the same op‐
1245 tion settings.
1246 The third use of the set command is to set the values of the
1248 shell's positional parameters to the specified args. To change
1249 the positional parameters without changing any options, use “--”
1250 as the first argument to set. If no args are present, the set
1251 command will clear all the positional parameters (equivalent to
1252 executing “shift $#”.)
1253 shift [n]
1255 Shift the positional parameters n times. A shift sets the value
1256 of $1 to the value of $2, the value of $2 to the value of $3, and
1257 so on, decreasing the value of $# by one. If n is greater than
1258 the number of positional parameters, shift will issue an error
1259 message, and exit with return status 2.
1260 test expression
1262 [ expression ]
1264 The test utility evaluates the expression and, if it evaluates to
1265 true, returns a zero (true) exit status; otherwise it returns 1
1266 (false). If there is no expression, test also returns 1 (false).
1267 All operators and flags are separate arguments to the test util‐
1269 ity.
1270 The following primaries are used to construct expression:
1272 -b file True if file exists and is a block special file.
1274 -c file True if file exists and is a character special file.
1276 -d file True if file exists and is a directory.
1278 -e file True if file exists (regardless of type).
1280 -f file True if file exists and is a regular file.
1282 -g file True if file exists and its set group ID flag is
1284 set.
1285 -h file True if file exists and is a symbolic link.
1287 -k file True if file exists and its sticky bit is set.
1289 -n string True if the length of string is nonzero.
1291 -p file True if file is a named pipe (FIFO).
1293 -r file True if file exists and is readable.
1295 -s file True if file exists and has a size greater than
1297 zero.
1298 -t file_descriptor
1300 True if the file whose file descriptor number is
1301 file_descriptor is open and is associated with a
1302 terminal.
1303 -u file True if file exists and its set user ID flag is set.
1305 -w file True if file exists and is writable. True indicates
1307 only that the write flag is on. The file is not
1308 writable on a read-only file system even if this
1309 test indicates true.
1310 -x file True if file exists and is executable. True indi‐
1312 cates only that the execute flag is on. If file is
1313 a directory, true indicates that file can be
1314 searched.
1315 -z string True if the length of string is zero.
1317 -L file True if file exists and is a symbolic link. This
1319 operator is retained for compatibility with previous
1320 versions of this program. Do not rely on its exis‐
1321 tence; use -h instead.
1322 -O file True if file exists and its owner matches the effec‐
1324 tive user id of this process.
1325 -G file True if file exists and its group matches the effec‐
1327 tive group id of this process.
1328 -S file True if file exists and is a socket.
1330 file1 -nt file2
1332 True if file1 and file2 exist and file1 is newer
1333 than file2.
1334 file1 -ot file2
1336 True if file1 and file2 exist and file1 is older
1337 than file2.
1338 file1 -ef file2
1340 True if file1 and file2 exist and refer to the same
1341 file.
1342 string True if string is not the null string.
1344 s1 = s2 True if the strings s1 and s2 are identical.
1346 s1 != s2 True if the strings s1 and s2 are not identical.
1348 s1 < s2 True if string s1 comes before s2 based on the ASCII
1350 value of their characters.
1351 s1 > s2 True if string s1 comes after s2 based on the ASCII
1353 value of their characters.
1354 n1 -eq n2 True if the integers n1 and n2 are algebraically
1356 equal.
1357 n1 -ne n2 True if the integers n1 and n2 are not algebraically
1359 equal.
1360 n1 -gt n2 True if the integer n1 is algebraically greater than
1362 the integer n2.
1363 n1 -ge n2 True if the integer n1 is algebraically greater than
1365 or equal to the integer n2.
1366 n1 -lt n2 True if the integer n1 is algebraically less than
1368 the integer n2.
1369 n1 -le n2 True if the integer n1 is algebraically less than or
1371 equal to the integer n2.
1372 These primaries can be combined with the following operators:
1374 ! expression True if expression is false.
1376 expression1 -a expression2
1378 True if both expression1 and expression2 are true.
1379 expression1 -o expression2
1381 True if either expression1 or expression2 are true.
1382 (expression) True if expression is true.
1384 The -a operator has higher precedence than the -o operator.
1386 times Print the accumulated user and system times for the shell and for
1388 processes run from the shell. The return status is 0.
1389 trap [action signal ...]
1391 Cause the shell to parse and execute action when any of the speci‐
1392 fied signals are received. The signals are specified by signal
1393 number or as the name of the signal. If signal is 0 or EXIT, the
1394 action is executed when the shell exits. action may be empty
1395 (''), which causes the specified signals to be ignored. With
1396 action omitted or set to `-' the specified signals are set to
1397 their default action. When the shell forks off a subshell, it re‐
1398 sets trapped (but not ignored) signals to the default action. The
1399 trap command has no effect on signals that were ignored on entry
1400 to the shell. trap without any arguments cause it to write a list
1401 of signals and their associated action to the standard output in a
1402 format that is suitable as an input to the shell that achieves the
1403 same trapping results.
1404 Examples:
1406 trap
1408 List trapped signals and their corresponding action
1410 trap '' INT QUIT tstp 30
1412 Ignore signals INT QUIT TSTP USR1
1414 trap date INT
1416 Print date upon receiving signal INT
1418 type [name ...]
1420 Interpret each name as a command and print the resolution of the
1421 command search. Possible resolutions are: shell keyword, alias,
1422 shell builtin, command, tracked alias and not found. For aliases
1423 the alias expansion is printed; for commands and tracked aliases
1424 the complete pathname of the command is printed.
1425 ulimit [-H | -S] [-a | -tfdscmlpnv [value]]
1427 Inquire about or set the hard or soft limits on processes or set
1428 new limits. The choice between hard limit (which no process is
1429 allowed to violate, and which may not be raised once it has been
1430 lowered) and soft limit (which causes processes to be signaled but
1431 not necessarily killed, and which may be raised) is made with
1432 these flags:
1433 -H set or inquire about hard limits
1435 -S set or inquire about soft limits. If neither -H nor
1437 -S is specified, the soft limit is displayed or both
1438 limits are set. If both are specified, the last one
1439 wins.
1440 The limit to be interrogated or set, then, is chosen by specifying
1442 any one of these flags:
1443 -a show all the current limits
1445 -t show or set the limit on CPU time (in seconds)
1447 -f show or set the limit on the largest file that can be
1449 created (in 512-byte blocks)
1450 -d show or set the limit on the data segment size of a
1452 process (in kilobytes)
1453 -s show or set the limit on the stack size of a process
1455 (in kilobytes)
1456 -c show or set the limit on the largest core dump size
1458 that can be produced (in 512-byte blocks)
1459 -m show or set the limit on the total physical memory
1461 that can be in use by a process (in kilobytes)
1462 -l show or set the limit on how much memory a process can
1464 lock with mlock(2) (in kilobytes)
1465 -p show or set the limit on the number of processes this
1467 user can have at one time
1468 -n show or set the limit on the number files a process
1470 can have open at once
1471 -v show or set the limit on the total virtual memory that
1473 can be in use by a process (in kilobytes)
1474 -r show or set the limit on the real-time scheduling pri‐
1476 ority of a process
1477 If none of these is specified, it is the limit on file size that
1479 is shown or set. If value is specified, the limit is set to that
1480 number; otherwise the current limit is displayed.
1481 Limits of an arbitrary process can be displayed or set using the
1483 sysctl(8) utility.
1484 umask [mask]
1486 Set the value of umask (see umask(2)) to the specified octal
1487 value. If the argument is omitted, the umask value is printed.
1488 unalias [-a] [name]
1490 If name is specified, the shell removes that alias. If -a is
1491 specified, all aliases are removed.
1492 unset [-fv] name ...
1494 The specified variables and functions are unset and unexported.
1495 If -f or -v is specified, the corresponding function or variable
1496 is unset, respectively. If a given name corresponds to both a
1497 variable and a function, and no options are given, only the vari‐
1498 able is unset.
1499 wait [job]
1501 Wait for the specified job to complete and return the exit status
1502 of the last process in the job. If the argument is omitted, wait
1503 for all jobs to complete and return an exit status of zero.
1504 Command Line Editing
1506 When dash is being used interactively from a terminal, the current com‐
1507 mand and the command history (see fc in Builtins) can be edited using vi-
1508 mode command-line editing. This mode uses commands, described below,
1509 similar to a subset of those described in the vi man page. The command
1510 ‘set -o vi’ enables vi-mode editing and places sh into vi insert mode.
1511 With vi-mode enabled, sh can be switched between insert mode and command
1512 mode. It is similar to vi: typing ⟨ESC⟩ enters vi command mode. Hitting
1513 ⟨return⟩ while in command mode will pass the line to the shell.
1514
1516 Errors that are detected by the shell, such as a syntax error, will cause
1517 the shell to exit with a non-zero exit status. If the shell is not an
1518 interactive shell, the execution of the shell file will be aborted. Oth‐
1519 erwise the shell will return the exit status of the last command exe‐
1520 cuted, or if the exit builtin is used with a numeric argument, it will
1521 return the argument.
1522
1524 HOME Set automatically by login(1) from the user's login directory
1525 in the password file (passwd(4)). This environment variable
1526 also functions as the default argument for the cd builtin.
1527 PATH The default search path for executables. See the above sec‐
1529 tion Path Search.
1530 CDPATH The search path used with the cd builtin.
1532 MAIL The name of a mail file, that will be checked for the arrival
1534 of new mail. Overridden by MAILPATH.
1535 MAILCHECK The frequency in seconds that the shell checks for the arrival
1537 of mail in the files specified by the MAILPATH or the MAIL
1538 file. If set to 0, the check will occur at each prompt.
1539 MAILPATH A colon “:” separated list of file names, for the shell to
1541 check for incoming mail. This environment setting overrides
1542 the MAIL setting. There is a maximum of 10 mailboxes that can
1543 be monitored at once.
1544 PS1 The primary prompt string, which defaults to “$ ”, unless you
1546 are the superuser, in which case it defaults to “# ”.
1547 PS2 The secondary prompt string, which defaults to “> ”.
1549 PS4 Output before each line when execution trace (set -x) is en‐
1551 abled, defaults to “+ ”.
1552 IFS Input Field Separators. This is normally set to ⟨space⟩,
1554 ⟨tab⟩, and ⟨newline⟩. See the White Space Splitting section
1555 for more details.
1556 TERM The default terminal setting for the shell. This is inherited
1558 by children of the shell, and is used in the history editing
1559 modes.
1560 HISTSIZE The number of lines in the history buffer for the shell.
1562 PWD The logical value of the current working directory. This is
1564 set by the cd command.
1565 OLDPWD The previous logical value of the current working directory.
1567 This is set by the cd command.
1568 PPID The process ID of the parent process of the shell.
1570
1572 $HOME/.profile
1573 /etc/profile
1575
1577 csh(1), echo(1), getopt(1), ksh(1), login(1), printf(1), test(1),
1578 getopt(3), passwd(5), environ(7), sysctl(8)
1579
1581 dash is a POSIX-compliant implementation of /bin/sh that aims to be as
1582 small as possible. dash is a direct descendant of the NetBSD version of
1583 ash (the Almquist SHell), ported to Linux in early 1997. It was renamed
1584 to dash in 2002.
1585
1587 Setuid shell scripts should be avoided at all costs, as they are a sig‐
1588 nificant security risk.
1589 PS1, PS2, and PS4 should be subject to parameter expansion before being
1591 displayed.
1592BSD January 19, 2003 BSD