1SH(1) General Commands Manual SH(1)
2
6 sh, for, case, if, while, :, ., break, continue, cd, eval, exec, exit,
7 export, login, read, readonly, set, shift, times, trap, umask, wait -
8 command language
9
11 sh [ -ceiknrstuvx ] [ arg ] ...
12
14 Sh is a command programming language that executes commands read from a
15 terminal or a file. See invocation for the meaning of arguments to the
16 shell.
17 Commands.
19 A simple-command is a sequence of non blank words separated by blanks
20 (a blank is a tab or a space). The first word specifies the name of
21 the command to be executed. Except as specified below the remaining
22 words are passed as arguments to the invoked command. The command name
23 is passed as argument 0 (see execve(2)). The value of a simple-command
24 is its exit status if it terminates normally or 200+status if it termi‐
25 nates abnormally (see sigvec(2) for a list of status values).
26 A pipeline is a sequence of one or more commands separated by |. The
28 standard output of each command but the last is connected by a pipe(2)
29 to the standard input of the next command. Each command is run as a
30 separate process; the shell waits for the last command to terminate.
31 A list is a sequence of one or more pipelines separated by ;, &, && or
33 || and optionally terminated by ; or &. ; and & have equal precedence
34 which is lower than that of && and ||, && and || also have equal prece‐
35 dence. A semicolon causes sequential execution; an ampersand causes
36 the preceding pipeline to be executed without waiting for it to finish.
37 The symbol && (||) causes the list following to be executed only if the
38 preceding pipeline returns a zero (non zero) value. Newlines may
39 appear in a list, instead of semicolons, to delimit commands.
40 A command is either a simple-command or one of the following. The
42 value returned by a command is that of the last simple-command executed
43 in the command.
44 for name [in word ...] do list done
46 Each time a for command is executed name is set to the next word
47 in the for word list. If in word ... is omitted, in "$@" is
48 assumed. Execution ends when there are no more words in the
49 list.
50 case word in [pattern [ | pattern ] ... ) list ;;] ... esac
52 A case command executes the list associated with the first pat‐
53 tern that matches word. The form of the patterns is the same as
54 that used for file name generation.
55 if list then list [elif list then list] ... [else list] fi
57 The list following if is executed and if it returns zero the
58 list following then is executed. Otherwise, the list following
59 elif is executed and if its value is zero the list following
60 then is executed. Failing that the else list is executed.
61 while list [do list] done
63 A while command repeatedly executes the while list and if its
64 value is zero executes the do list; otherwise the loop termi‐
65 nates. The value returned by a while command is that of the
66 last executed command in the do list. until may be used in
67 place of while to negate the loop termination test.
68 ( list )
70 Execute list in a subshell.
71 { list }
73 list is simply executed.
74 The following words are only recognized as the first word of a command
76 and when not quoted.
77 if then else elif fi case in esac for while until do done { }
79 Command substitution.
81 The standard output from a command enclosed in a pair of back quotes
82 (``) may be used as part or all of a word; trailing newlines are
83 removed.
84 Parameter substitution.
86 The character $ is used to introduce substitutable parameters. Posi‐
87 tional parameters may be assigned values by set. Variables may be set
88 by writing
89 name=value [ name=value ] ...
91 ${parameter}
93 A parameter is a sequence of letters, digits or underscores (a
94 name), a digit, or any of the characters * @ # ? - $ !. The
95 value, if any, of the parameter is substituted. The braces are
96 required only when parameter is followed by a letter, digit, or
97 underscore that is not to be interpreted as part of its name.
98 If parameter is a digit, it is a positional parameter. If
99 parameter is * or @ then all the positional parameters, starting
100 with $1, are substituted separated by spaces. $0 is set from
101 argument zero when the shell is invoked.
102 ${parameter-word}
104 If parameter is set, substitute its value; otherwise substitute
105 word.
106 ${parameter=word}
108 If parameter is not set, set it to word; the value of the param‐
109 eter is then substituted. Positional parameters may not be
110 assigned to in this way.
111 ${parameter?word}
113 If parameter is set, substitute its value; otherwise, print word
114 and exit from the shell. If word is omitted, a standard message
115 is printed.
116 ${parameter+word}
118 If parameter is set, substitute word; otherwise substitute noth‐
119 ing.
120 In the above word is not evaluated unless it is to be used as the sub‐
122 stituted string. (So that, for example, echo ${d-´pwd´} will only exe‐
123 cute pwd if d is unset.)
124 The following parameters are automatically set by the shell.
126 # The number of positional parameters in decimal.
128 - Options supplied to the shell on invocation or by set.
129 ? The value returned by the last executed command in deci‐
130 mal.
131 $ The process number of this shell.
132 ! The process number of the last background command
133 invoked.
134 The following parameters are used but not set by the shell.
136 HOME The default argument (home directory) for the cd command.
138 PATH The search path for commands (see execution).
139 MAIL If this variable is set to the name of a mail file, the
140 shell informs the user of the arrival of mail in the
141 specified file.
142 PS1 Primary prompt string, by default '$ '.
143 PS2 Secondary prompt string, by default '> '.
144 IFS Internal field separators, normally space, tab, and new‐
145 line. IFS is ignored if sh is running as root or if the
146 effective user id differs from the real user id.
147 Blank interpretation.
149 After parameter and command substitution, any results of substitution
150 are scanned for internal field separator characters (those found in
151 $IFS) and split into distinct arguments where such characters are
152 found. Explicit null arguments ("" or ´´) are retained. Implicit null
153 arguments (those resulting from parameters that have no values) are
154 removed.
155 File name generation.
157 Following substitution, each command word is scanned for the characters
158 *, ? and [. If one of these characters appears, the word is regarded
159 as a pattern. The word is replaced with alphabetically sorted file
160 names that match the pattern. If no file name is found that matches
161 the pattern, the word is left unchanged. The character . at the start
162 of a file name or immediately following a /, and the character /, must
163 be matched explicitly.
164 * Matches any string, including the null string.
166 ? Matches any single character.
167 [...] Matches any one of the characters enclosed. A pair of charac‐
168 ters separated by - matches any character lexically between the
169 pair.
170 Quoting.
172 The following characters have a special meaning to the shell and cause
173 termination of a word unless quoted.
174 ; & ( ) | < > newline space tab
176 A character may be quoted by preceding it with a \. \newline is
178 ignored. All characters enclosed between a pair of quote marks (´´),
179 except a single quote, are quoted. Inside double quotes ("") parameter
180 and command substitution occurs and \ quotes the characters \ ´ " and
181 $.
182 "$*" is equivalent to "$1 $2 ..." whereas
184 "$@" is equivalent to "$1" "$2" ... .
185 Prompting.
187 When used interactively, the shell prompts with the value of PS1 before
188 reading a command. If at any time a newline is typed and further input
189 is needed to complete a command, the secondary prompt ($PS2) is issued.
190 Input output.
192 Before a command is executed its input and output may be redirected
193 using a special notation interpreted by the shell. The following may
194 appear anywhere in a simple-command or may precede or follow a command
195 and are not passed on to the invoked command. Substitution occurs
196 before word or digit is used.
197 <word Use file word as standard input (file descriptor 0).
199 >word Use file word as standard output (file descriptor 1). If the
201 file does not exist, it is created; otherwise it is truncated to
202 zero length.
203 >>word Use file word as standard output. If the file exists, output is
205 appended (by seeking to the end); otherwise the file is created.
206 <<word The shell input is read up to a line the same as word, or end of
208 file. The resulting document becomes the standard input. If
209 any character of word is quoted, no interpretation is placed
210 upon the characters of the document; otherwise, parameter and
211 command substitution occurs, \newline is ignored, and \ is used
212 to quote the characters \ $ ´ and the first character of word.
213 <&digit
215 The standard input is duplicated from file descriptor digit; see
216 dup(2). Similarly for the standard output using >.
217 <&- The standard input is closed. Similarly for the standard output
219 using >.
220 If one of the above is preceded by a digit, the file descriptor created
222 is that specified by the digit (instead of the default 0 or 1). For
223 example,
224 ... 2>&1
226 creates file descriptor 2 to be a duplicate of file descriptor 1.
228 If a command is followed by & then the default standard input for the
230 command is the empty file (/dev/null). Otherwise, the environment for
231 the execution of a command contains the file descriptors of the invok‐
232 ing shell as modified by input output specifications.
233 Environment.
235 The environment is a list of name-value pairs that is passed to an exe‐
236 cuted program in the same way as a normal argument list; see execve(2)
237 and environ(7). The shell interacts with the environment in several
238 ways. On invocation, the shell scans the environment and creates a
239 parameter for each name found, giving it the corresponding value. Exe‐
240 cuted commands inherit the same environment. If the user modifies the
241 values of these parameters or creates new ones, none of these affects
242 the environment unless the export command is used to bind the shell's
243 parameter to the environment. The environment seen by any executed
244 command is thus composed of any unmodified name-value pairs originally
245 inherited by the shell, plus any modifications or additions, all of
246 which must be noted in export commands.
247 The environment for any simple-command may be augmented by prefixing it
249 with one or more assignments to parameters. Thus these two lines are
250 equivalent
251 TERM=450 cmd args
253 (export TERM; TERM=450; cmd args)
254 If the -k flag is set, all keyword arguments are placed in the environ‐
256 ment, even if the occur after the command name. The following prints
257 'a=b c' and 'c':
258 echo a=b c
259 set -k
260 echo a=b c
261 Signals.
263 The INTERRUPT and QUIT signals for an invoked command are ignored if
264 the command is followed by &; otherwise signals have the values inher‐
265 ited by the shell from its parent. (But see also trap.)
266 Execution.
268 Each time a command is executed the above substitutions are carried
269 out. Except for the 'special commands' listed below a new process is
270 created and an attempt is made to execute the command via an execve(2).
271 The shell parameter $PATH defines the search path for the directory
273 containing the command. Each alternative directory name is separated
274 by a colon (:). The default path is :/bin:/usr/bin. If the command
275 name contains a /, the search path is not used. Otherwise, each direc‐
276 tory in the path is searched for an executable file. If the file has
277 execute permission but is not an a.out file, it is assumed to be a file
278 containing shell commands. A subshell (i.e., a separate process) is
279 spawned to read it. A parenthesized command is also executed in a sub‐
280 shell.
281 Special commands.
283 The following commands are executed in the shell process and except
284 where specified no input output redirection is permitted for such com‐
285 mands.
286 # For non-interactive shells, everything following the # is
288 treated as a comment, i.e. the rest of the line is ignored. For
289 interactive shells, the # has no special effect.
290 : No effect; the command does nothing.
292 . file Read and execute commands from file and return. The search path
293 $PATH is used to find the directory containing file.
294 break [n]
295 Exit from the enclosing for or while loop, if any. If n is
296 specified, break n levels.
297 continue [n]
298 Resume the next iteration of the enclosing for or while loop.
299 If n is specified, resume at the n-th enclosing loop.
300 cd [arg]
301 Change the current directory to arg. The shell parameter $HOME
302 is the default arg.
303 eval [arg ...]
304 The arguments are read as input to the shell and the resulting
305 command(s) executed.
306 exec [arg ...]
307 The command specified by the arguments is executed in place of
308 this shell without creating a new process. Input output argu‐
309 ments may appear and if no other arguments are given cause the
310 shell input output to be modified.
311 exit [n]
312 Causes a non interactive shell to exit with the exit status
313 specified by n. If n is omitted, the exit status is that of the
314 last command executed. (An end of file will also exit from the
315 shell.)
316 export [name ...]
317 The given names are marked for automatic export to the environ‐
318 ment of subsequently-executed commands. If no arguments are
319 given, a list of exportable names is printed.
320 login [arg ...]
321 Equivalent to 'exec login arg ...'.
322 read name ...
323 One line is read from the standard input; successive words of
324 the input are assigned to the variables name in order, with
325 leftover words to the last variable. The return code is 0
326 unless the end-of-file is encountered.
327 readonly [name ...]
328 The given names are marked readonly and the values of the these
329 names may not be changed by subsequent assignment. If no argu‐
330 ments are given, a list of all readonly names is printed.
331 set [-eknptuvx [arg ...]]
332 -e If non interactive, exit immediately if a command fails.
333 -k All keyword arguments are placed in the environment for a
334 command, not just those that precede the command name.
335 -n Read commands but do not execute them.
336 -t Exit after reading and executing one command.
337 -u Treat unset variables as an error when substituting.
338 -v Print shell input lines as they are read.
339 -x Print commands and their arguments as they are executed.
340 - Turn off the -x and -v options.
341 These flags can also be used upon invocation of the shell. The
343 current set of flags may be found in $-.
344 Remaining arguments are positional parameters and are assigned,
346 in order, to $1, $2, etc. If no arguments are given, the values
347 of all names are printed.
348 shift The positional parameters from $2... are renamed $1...
350 times Print the accumulated user and system times for processes run
352 from the shell.
353 trap [arg] [n] ...
355 Arg is a command to be read and executed when the shell receives
356 signal(s) n. (Note that arg is scanned once when the trap is
357 set and once when the trap is taken.) Trap commands are exe‐
358 cuted in order of signal number. If arg is absent, all trap(s)
359 n are reset to their original values. If arg is the null
360 string, this signal is ignored by the shell and by invoked com‐
361 mands. If n is 0, the command arg is executed on exit from the
362 shell, otherwise upon receipt of signal n as numbered in
363 sigvec(2). Trap with no arguments prints a list of commands
364 associated with each signal number.
365 umask [ nnn ]
367 The user file creation mask is set to the octal value nnn (see
368 umask(2)). If nnn is omitted, the current value of the mask is
369 printed.
370 wait [n]
372 Wait for the specified process and report its termination sta‐
373 tus. If n is not given, all currently active child processes
374 are waited for. The return code from this command is that of
375 the process waited for.
376 Invocation.
378 If the first character of argument zero is -, commands are read from
379 $HOME/.profile, if such a file exists. Commands are then read as
380 described below. The following flags are interpreted by the shell when
381 it is invoked.
382 -c string If the -c flag is present, commands are read from string.
383 -s If the -s flag is present or if no arguments remain then
384 commands are read from the standard input. Shell output is
385 written to file descriptor 2.
386 -i If the -i flag is present or if the shell input and output
387 are attached to a terminal (as told by gtty) then this shell
388 is interactive. In this case the terminate signal SIGTERM
389 (see sigvec(2)) is ignored (so that 'kill 0' does not kill
390 an interactive shell) and the interrupt signal SIGINT is
391 caught and ignored (so that wait is interruptible). In all
392 cases SIGQUIT is ignored by the shell.
393 The remaining flags and arguments are described under the set command.
395
397 $HOME/.profile
398 /tmp/sh*
399 /dev/null
400
402 csh(1), test(1), execve(2), environ(7)
403
405 Errors detected by the shell, such as syntax errors cause the shell to
406 return a non zero exit status. If the shell is being used non interac‐
407 tively then execution of the shell file is abandoned. Otherwise, the
408 shell returns the exit status of the last command executed (see also
409 exit).
410
412 If << is used to provide standard input to an asynchronous process
413 invoked by &, the shell gets mixed up about naming the input document.
414 A garbage file /tmp/sh* is created, and the shell complains about not
415 being able to find the file by another name.
4167th Edition May 5, 1986 SH(1)