SH(1) GENIX Programmer's Manual SH(1)
NAME
sh, for, case, if, while, :, ., break, continue, cd, eval, exec,
exit, export, login, newgrp, read, readonly, set, shift, times,
trap, umask, wait - command language
SYNOPSIS
sh [ -ceiknrstuvx ] [ arg ] ...
DESCRIPTION
Sh is a command programming language that executes commands read
from a terminal or a file. See invocation for the meaning of argu-
ments to the shell.
Commands.
A simple-command is a sequence of non blank words separated by
blanks (a blank is a tab or a space). The first word specifies the
name of the command to be executed. Except as specified below the
remaining words are passed as arguments to the invoked command. The
command name is passed as argument 0 (see exec(2)). The value of a
simple-command is its exit status if it terminates normally or
200+status if it terminates abnormally (see signal(2) for a list of
status values).
A pipeline is a sequence of one or more commands separated by |. The
standard output of each command but the last is connected by a
pipe(2) to the standard input of the next command. Each command is
run as a separate process; the shell waits for the last command to
terminate.
A list is a sequence of one or more pipelines separated by ;, &, &&
or || and optionally terminated by ; or &. ; and & have equal pre-
cedence which is lower than that of && and ||, && and || also have
equal precedence. A semicolon causes sequential execution; an
ampersand causes the preceding pipeline to be executed without wait-
ing for it to finish. The symbol && (||) causes the list following
to be executed only if the preceding pipeline returns a zero (non
zero) value. Newlines may appear in a list, instead of semicolons,
to delimit commands.
A command is either a simple-command or one of the following. The
value returned by a command is that of the last simple-command exe-
cuted in the command.
for name [in word ...] do list done
Each time a for command is executed name is set to the next
word in the for word list If in word ... is omitted then in
"$@" is assumed. Execution ends when there are no more words
in the list.
case word in [pattern [ | pattern ] ... ) list ;;] ... esac
A case command executes the list associated with the first pat-
tern that matches word. The form of the patterns is the same as
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that used for file name generation.
if list then list [elif list then list] ... [else list] fi
The list following if is executed and if it returns zero the
list following then is executed. Otherwise, the list following
elif is executed and if its value is zero the list following
then is executed. Failing that the else list is executed.
while list [do list] done
A while command repeatedly executes the while list and if its
value is zero executes the do list; otherwise the loop ter-
minates. The value returned by a while command is that of the
last executed command in the do list. until may be used in
place of while to negate the loop termination test.
( list )
Execute list in a subshell.
{ list }
list is simply executed.
The following words are only recognized as the first word of a com-
mand and when not quoted.
if then else elif fi case in esac for while until do done { }
Command substitution.
The standard output from a command enclosed in a pair of grave
accents (``) may be used as part or all of a word; trailing newlines
are removed.
Parameter substitution.
The character $ is used to introduce substitutable parameters.
Positional parameters may be assigned values by set. Variables may
be set by writing
name=value [ name=value ] ...
${parameter}
A parameter is a sequence of letters, digits or underscores (a
name), a digit, or any of the characters * @ # ? - $ !. The
value, if any, of the parameter is substituted. The braces are
required only when parameter is followed by a letter, digit, or
underscore that is not to be interpreted as part of its name.
If parameter is a digit then it is a positional parameter. If
parameter is * or @ then all the positional parameters, start-
ing with $1, are substituted separated by spaces. $0 is set
from argument zero when the shell is invoked.
${parameter-word}
If parameter is set then substitute its value; otherwise sub-
stitute word.
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${parameter=word}
If parameter is not set then set it to word; the value of the
parameter is then substituted. Positional parameters may not
be assigned to in this way.
${parameter?word}
If parameter is set then substitute its value; otherwise, print
word and exit from the shell. If word is omitted then a stan-
dard message is printed.
${parameter+word}
If parameter is set then substitute word; otherwise substitute
nothing.
In the above word is not evaluated unless it is to be used as the
substituted string. (So that, for example, echo ${d-`pwd`} will
only execute pwd if d is unset.)
The following parameters are automatically set by the shell.
# The number of positional parameters in decimal.
- Options supplied to the shell on invocation or by set.
? The value returned by the last executed command in
decimal.
$ The process number of this shell.
! The process number of the last background command invoked.
The following parameters are used but not set by the shell.
HOME The default argument (home directory) for the cd command.
PATH The search path for commands (see execution).
MAIL If this variable is set to the name of a mail file then
the shell informs the user of the arrival of mail in the
specified file.
PS1 Primary prompt string, by default `$ '.
PS2 Secondary prompt string, by default `> '.
IFS Internal field separators, normally space, tab, and new-
line.
Blank interpretation.
After parameter and command substitution, any results of substitu-
tion are scanned for internal field separator characters (those
found in $IFS) and split into distinct arguments where such charac-
ters are found. Explicit null arguments ("" or '') are retained.
Implicit null arguments (those resulting from parameters that have
no values) are removed.
File name generation.
Following substitution, each command word is scanned for the charac-
ters *, ? and [. If one of these characters appears then the word is
regarded as a pattern. The word is replaced with alphabetically
sorted file names that match the pattern. If no file name is found
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that matches the pattern then the word is left unchanged. The char-
acter . at the start of a file name or immediately following a /,
and the character /, must be matched explicitly.
* Matches any string, including the null string.
? Matches any single character.
[...]
Matches any one of the characters enclosed. A pair of charac-
ters separated by - matches any character lexically between the
pair.
Quoting.
The following characters have a special meaning to the shell and
cause termination of a word unless quoted.
; & ( ) | < > newline space tab
A character may be quoted by preceding it with a \. \newline is
ignored. All characters enclosed between a pair of quote marks
(''), except a single quote, are quoted. Inside double quotes ("")
parameter and command substitution occurs and \ quotes the charac-
ters \ ` " and $.
"$*" is equivalent to "$1 $2 ..." whereas
"$@" is equivalent to "$1" "$2" ... .
Prompting.
When used interactively, the shell prompts with the value of PS1
before reading a command. If at any time a newline is typed and
further input is needed to complete a command then the secondary
prompt ($PS2) is issued.
Input output.
Before a command is executed its input and output may be redirected
using a special notation interpreted by the shell. The following
may appear anywhere in a simple-command or may precede or follow a
command and are not passed on to the invoked command. Substitution
occurs before word or digit is used.
<word
Use file word as standard input (file descriptor 0).
>word
Use file word as standard output (file descriptor 1). If the
file does not exist then it is created; otherwise it is trun-
cated to zero length.
>>word
Use file word as standard output. If the file exists then out-
put is appended (by seeking to the end); otherwise the file is
created.
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<<word
The shell input is read up to a line the same as word, or end
of file. The resulting document becomes the standard input.
If any character of word is quoted then no interpretation is
placed upon the characters of the document; otherwise, parame-
ter and command substitution occurs, \newline is ignored, and \
is used to quote the characters \ $ ` and the first character
of word.
<&digit
The standard input is duplicated from file descriptor digit;
see dup(2). Similarly for the standard output using >.
<&- The standard input is closed. Similarly for the standard out-
put using >.
If one of the above is preceded by a digit then the file descriptor
created is that specified by the digit (instead of the default 0 or
1). For example,
... 2>&1
creates file descriptor 2 to be a duplicate of file descriptor 1.
If a command is followed by & then the default standard input for
the command is the empty file (/dev/null). Otherwise, the environ-
ment for the execution of a command contains the file descriptors of
the invoking shell as modified by input output specifications.
Environment.
The environment is a list of name-value pairs that is passed to an
executed program in the same way as a normal argument list; see
exec(2) and environ(5). The shell interacts with the environment in
several ways. On invocation, the shell scans the environment and
creates a parameter for each name found, giving it the corresponding
value. Executed commands inherit the same environment. If the user
modifies the values of these parameters or creates new ones, none of
these affects the environment unless the export command is used to
bind the shell's parameter to the environment. The environment seen
by any executed command is thus composed of any unmodified name-
value pairs originally inherited by the shell, plus any modifica-
tions or additions, all of which must be noted in export commands.
The environment for any simple-command may be augmented by prefixing
it with one or more assignments to parameters. Thus these two lines
are equivalent
TERM=450 cmd args
(export TERM; TERM=450; cmd args)
If the -k flag is set, all keyword arguments are placed in the
environment, even if the occur after the command name. The
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following prints `a=b c' and `c':
echo a=b c
set -k
echo a=b c
Signals.
The INTERRUPT and QUIT signals for an invoked command are ignored if
the command is followed by &; otherwise signals have the values
inherited by the shell from its parent. (But see also trap.)
Execution.
Each time a command is executed the above substitutions are carried
out. Except for the `special commands' listed below a new process
is created and an attempt is made to execute the command via an
exec(2).
The shell parameter $PATH defines the search path for the directory
containing the command. Each alternative directory name is
separated by a colon (:). The default path is :/bin:/usr/bin. If
the command name contains a / then the search path is not used.
Otherwise, each directory in the path is searched for an executable
file. If the file has execute permission but is not an a.out file,
it is assumed to be a file containing shell commands. A subshell
(i.e., a separate process) is spawned to read it. A parenthesized
command is also executed in a subshell.
Special commands.
The following commands are executed in the shell process and except
where specified no input output redirection is permitted for such
commands.
: No effect; the command does nothing.
. file
Read and execute commands from file and return. The search
path $PATH is used to find the directory containing file.
break [n]
Exit from the enclosing for or while loop, if any. If n is
specified then break n levels.
continue [n]
Resume the next iteration of the enclosing for or while loop.
If n is specified then resume at the n-th enclosing loop.
cd [arg]
Change the current directory to arg. The shell parameter $HOME
is the default arg.
eval [arg ...]
The arguments are read as input to the shell and the resulting
command(s) executed.
exec [arg ...]
The command specified by the arguments is executed in place of
this shell without creating a new process. Input output argu-
ments may appear and if no other arguments are given cause the
shell input output to be modified.
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exit [n]
Causes a non interactive shell to exit with the exit status
specified by n. If n is omitted then the exit status is that of
the last command executed. (An end of file will also exit from
the shell.)
export [name ...]
The given names are marked for automatic export to the environ-
ment of subsequently-executed commands. If no arguments are
given then a list of exportable names is printed.
login [arg ...]
Equivalent to `exec login arg ...'.
newgrp [arg ...]
Equivalent to `exec newgrp arg ...'.
read name ...
One line is read from the standard input; successive words of
the input are assigned to the variables name in order, with
leftover words to the last variable. The return code is 0
unless the end-of-file is encountered.
readonly [name ...]
The given names are marked readonly and the values of the these
names may not be changed by subsequent assignment. If no argu-
ments are given then a list of all readonly names is printed.
set [-eknptuvx [arg ...]]
-e If non interactive then exit immediately if a command fails.
-k All keyword arguments are placed in the environment for a
command, not just those that precede the command name.
-n Read commands but do not execute them.
-t Exit after reading and executing one command.
-u Treat unset variables as an error when substituting.
-v Print shell input lines as they are read.
-x Print commands and their arguments as they are executed.
- Turn off the -x and -v options.
These flags can also be used upon invocation of the shell. The
current set of flags may be found in $-.
Remaining arguments are positional parameters and are assigned,
in order, to $1, $2, etc. If no arguments are given then the
values of all names are printed.
shift
The positional parameters from $2... are renamed $1...
times
Print the accumulated user and system times for processes run
from the shell.
trap [arg] [n] ...
Arg is a command to be read and executed when the shell
receives signal(s) n. (Note that arg is scanned once when the
trap is set and once when the trap is taken.) Trap commands are
executed in order of signal number. If arg is absent then all
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trap(s) n are reset to their original values. If arg is the
null string then this signal is ignored by the shell and by
invoked commands. If n is 0 then the command arg is executed
on exit from the shell, otherwise upon receipt of signal n as
numbered in signal(2). Trap with no arguments prints a list of
commands associated with each signal number.
umask [ nnn ]
The user file creation mask is set to the octal value nnn (see
umask(2)). If nnn is omitted, the current value of the mask is
printed.
wait [n]
Wait for the specified process and report its termination
status. If n is not given then all currently active child
processes are waited for. The return code from this command is
that of the process waited for.
Invocation.
If the first character of argument zero is -, commands are read from
$HOME/.profile, if such a file exists. Commands are then read as
described below. The following flags are interpreted by the shell
when it is invoked.
-c string If the -c flag is present then commands are read from
string.
-s If the -s flag is present or if no arguments remain then
commands are read from the standard input. Shell output
is written to file descriptor 2.
-i If the -i flag is present or if the shell input and out-
put are attached to a terminal (as told by gtty) then
this shell is interactive. In this case the terminate
signal SIGTERM (see signal(2)) is ignored (so that `kill
0' does not kill an interactive shell) and the interrupt
signal SIGINT is caught and ignored (so that wait is
interruptable). In all cases SIGQUIT is ignored by the
shell.
The remaining flags and arguments are described under the set com-
mand.
FILES
$HOME/.profile
/tmp/sh*
/dev/null
SEE ALSO
csh(1), test(1), exec(2),
DIAGNOSTICS
Errors detected by the shell, such as syntax errors cause the shell
to return a non zero exit status. If the shell is being used non
interactively then execution of the shell file is abandoned.
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Otherwise, the shell returns the exit status of the last command
executed (see also exit).
BUGS
IF << is used to provide standard input to an asynchronous process
invoked by &, the shell gets mixed up about naming the input docu-
ment. A garbage file /tmp/sh* is created, and the shell complains
about not being able to find the file by another name.
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