sh(1)
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sh, rsh Command
shell, the command programming language
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SYNTAX
sh [ -acefhiknrstuvx ] [ args ]
rsh [ -acefhiknrstuvx ] [ args ]
DESCRIPTION
Sh is a command programming language that executes commands read
from a terminal or a file. A file of commands must have read and
execute permissions set in order for you to run it-see also umask
under "Special Commands," below. Rsh is a restricted version of
the standard command interpreter sh; it sets up login names and
execution environments whose capabilities are more controlled
than those of the standard shell. See "Invocation," below for
the meaning of arguments to the shell. Sh also provides
editread, an optional interface used for editing command lines
entered from the shell. It also provides a history facility that
saves previously typed commands (see Using the DG/UX System for
more information).
Definitions
A blank is a tab or a space. A name is a sequence of letters,
digits, or underscores beginning with a letter or underscore. A
parameter is a name, a digit, or any of the characters *, @, #,
?, -, $, and !.
Commands
A simple-command is a sequence of non-blank words separated by
blanks. 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 (octal)
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 |
(or, for historical compatibility, 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
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separate process; the shell waits for the last command to
terminate. The exit status of a pipeline is the exit status of
the last command.
A list is a sequence of one or more pipelines separated by ;, &,
&&, or ||, and optionally terminated by ; or &. Of these four
symbols, ; and & have equal precedence, which is lower than that
of && and ||. The symbols && and || also have equal precedence.
A semicolon (;) causes sequential execution of the preceding
pipeline; an ampersand (&) causes asynchronous execution of the
preceding pipeline (i.e., the shell does not wait for that
pipeline to finish). The symbol && (||) executes the list
following it only if the preceding pipeline returns a zero (non-
zero) exit status. An arbitrary number of new-lines may appear
in a list, instead of semicolons, to delimit commands.
A command is either a simple-command or one of the following.
Unless otherwise stated, the value returned by a command is that
of the last simple-command executed in the command.
for name [ in word ... ] do list done
Each time a for command is executed, name is set to the next
word taken from the in word list. If in word ... is
omitted, then the for command executes the do list once for
each positional parameter that is set (see Parameter
Substitution below). 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
pattern that matches word. The form of the patterns is the
same as that used for filename generation (see "Filename
Generation") except that a slash, a leading dot, or a dot
immediately following a slash need not be matched
explicitly.
if list then list [ elif list then list ] ... [ else list ] fi
The list following if is executed and, if it returns a zero
exit status, the list following the first then is executed.
Otherwise, the list following elif is executed and, if its
value is zero, the list following the next then is executed.
Failing that, the else list is executed. If no else list or
then list is executed, then the if command returns a zero
exit status.
while list do list done
A while command repeatedly executes the while list and, if
the exit status of the last command in the list is zero,
executes the do list; otherwise the loop terminates. If no
commands in the do list are executed, then while returns a
zero exit status; use until in place of while to negate the
loop termination test.
(list)
Execute list in a sub-shell.
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{list;}
list is simply executed.
name () {list;}
Define a function referenced by name. The body of the
function is the list of commands between { and }. Execution
of functions is described below (see "Execution").
The following words are recognized only when they are the first
word of a command and when they are not quoted:
if then else elif fi case esac for while until do done { }
Comments
# before a word causes that word and all the following characters
up to a new-line to be ignored.
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 new-
lines are removed.
Parameter Substitution
The character $ introduces substitutable parameters. There are
two types of parameters, positional and keyword. If parameter is
a digit, it is positional. Positional parameters may be assigned
values by set. Keyword parameters (also known as variables) may
be assigned values by writing:
name=value [ name=value ] ...
Pattern-matching is not performed on value. There cannot be a
function and a variable with the same name.
${parameter}
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 * or @, all the
positional parameters, starting with $1, are substituted
(separated by spaces). Parameter $0 is set from argument
zero when the shell is invoked.
${parameter:-word}
If parameter is set and is non-null, substitute its value;
otherwise substitute word.
${parameter:=word}
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If parameter is not set or is null set it to word; the value
of the parameter is substituted. Positional parameters may
not be assigned to in this way.
${parameter:?word}
If parameter is set and is non-null, substitute its value;
otherwise, print word and exit from the shell. If word is
omitted, the message "parameter null or not set" is printed.
${parameter:+word}
If parameter is set and is non-null, substitute word;
otherwise substitute nothing.
In the above, word is not evaluated unless it is to be used as
the substituted string, so that, in the following example, pwd is
executed only if d is not set or is null:
echo ${d:-`pwd`}
If the colon (:) is omitted from the above expressions, the shell
only checks whether parameter is set.
The shell sets these parameters automatically:
# The number of positional parameters in decimal.
- Flags supplied to the shell on invocation or by the set
command.
? The decimal value returned by the last synchronously
executed command.
$ The process number of this shell.
! The process number of the last background command
invoked.
The following parameters are used by the shell:
HOME The default argument (home directory) for the cd
command.
PATH The search path for commands (see "Execution," below).
The user may not change PATH if executing under rsh.
CDPATH
The search path for the cd command.
MAIL If this parameter is set to the name of a mail file and
the MAILPATH parameter is not set, the shell informs
the user of the arrival of mail in the specified file.
MAILCHECK
This parameter specifies how often (in seconds) the
shell will check for mail in the files specified by the
MAILPATH or MAIL parameters. The default value is 600
seconds (10 minutes). If set to 0, the shell will
check before each prompt.
MAILPATH
A colon (:) separated list of filenames. If this
parameter is set, the shell informs the user of the
arrival of mail in any of the specified files. Each
filename can be followed by % and a message that will
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be printed when the modification time changes. The
default message is you have mail.
PS1 Primary prompt string, by default $ .
PS2 Secondary prompt string, by default > .
IFS Internal field separators, normally space, tab, and
new-line.
SHACCT
If this parameter is set to the name of a file writable
by the user, the shell will write an accounting record
in the file for each shell procedure executed.
Accounting routines such as acctcom(1) and acctcms(1M)
can be used to analyze the data collected.
SHELL
When the shell is invoked, it scans the environment
(see "Environment," below) for this name. If it is
found and there is an r in the filename part of its
value, the shell becomes a restricted shell.
The shell gives default values to PATH, PS1, PS2, MAILCHECK and
IFS. HOME and MAIL are set by login(1).
Blank Interpretation
After parameter and command substitution, the results of
substitution are scanned for internal field separator characters
(those found in IFS) and split into distinct arguments where such
characters are found. Explicit null arguments ("" or '') are
retained. Implicit null arguments (those resulting from
parameters that have no values) are removed.
Filename Generation
Following substitution, each command word is scanned for the
characters *, ?, and [. If one of these characters appears the
word is regarded as a pattern. The word is replaced with
alphabetically sorted filenames that match the pattern. If no
filename matches the pattern, the word is left unchanged. The
character . at the start of a filename or immediately following a
/, as well as the character / itself, must be matched explicitly.
* Matches any string, including the null string.
? Matches any single character.
[...]
Matches any one of the enclosed characters. A pair of
characters separated by - matches any character
lexically between the pair, inclusive. If the first
character following the opening [ is a !, any character
not enclosed is matched.
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Quoting
The following characters have a special meaning to the shell and
terminate a word unless quoted:
; & ( ) | ^ < > new-line space tab
You can make a character stand for itself by preceding it with a
\. This is called quoting. The pair \new-line is ignored. All
characters enclosed between a pair of single quote marks (''),
except a single quote, are quoted. Inside double quote marks
(""), parameter and command substitution occurs and \ quotes the
characters \, `, ", 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 new-line is typed and
further input is needed to complete a command, the secondary
prompt (i.e., the value of PS2) is issued.
Input/Output
Before a command is executed, you can redirect its input and
output using a special notation interpreted by the shell. The
following may appear anywhere in a simple-command or may precede
or follow a command. They 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 it is created;
otherwise, it is truncated to zero length.
>>word Use file word as standard output. If the file
exists output is appended to it (by first seeking
to the end-of-file); otherwise, the file is
created.
<<[-]word The shell input is read up to a line that is the
same as word, or to an end-of-file. The resulting
document becomes the standard input. If any
character of word is quoted, no interpretation is
placed upon the characters of the document;
otherwise, parameter and command substitution
occurs, (unescaped) \new-line is ignored, and \
must be used to quote the characters \, $, `, and
the first character of word. If - is appended to
<<, all leading tabs are stripped from word and
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from the document.
<&digit Use the file associated with file descriptor digit
as standard input. Similarly for the standard
output using >&digit.
<&- The standard input is closed. Similarly for the
standard output using >&-.
Note that when the shell creates a file, the base mode is 666,
rather than 777. The mode is then filtered through the current
umask. See umask under "Special Commands," below, and umask(1).
If any of the above is preceded by a digit, the file descriptor
associated with the file is that specified by the digit (instead
of the default 0 or 1). For example:
... 2>&1
associates file descriptor 2 with the file currently associated
with file descriptor 1.
The order in which redirections are specified is significant.
The shell evaluates redirections from left to right. For
example:
... 1>xxx 2>&1
first associates file descriptor 1 with file xxx. It then
associates file descriptor 2 with xxx. If the order of
redirections were reversed, file descriptor 2 would be associated
with the terminal (assuming file descriptor 1 had been) and file
descriptor 1 would be associated with file xxx.
If a command is followed by & the default standard input for the
command is the empty file /dev/null. Otherwise, the environment
for the execution of a command contains the file descriptors of
the invoking shell as modified by input/output specifications.
Redirection of output is not allowed in the restricted shell.
Environment
The environment (see environ(5)) is a list of name-value pairs
that is passed to an executed program in the same way as a normal
argument list. 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. If the user modifies the value of any of
these parameters or creates new parameters, none of these affects
the environment unless the export command is used to bind the
shell's parameter to the environment (see also set -a). A
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parameter may be removed from the environment with the unset
command.
The environment seen by any executed command is thus composed of
any unmodified name-value pairs originally inherited by the
shell, minus any pairs removed by unset, plus any modifications
or additions, all of which must be noted in export commands.
You can augment the environment for any simple-command by
prefixing it with one or more assignments to parameters. Thus:
TERM=605x cmd
and
(export TERM; TERM=605x; cmd)
are equivalent (as far as the execution of cmd is concerned).
If the -k flag is set, all keyword arguments are placed in the
environment, even if they occur after the command name. The
following first 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, with the exception
of signal 11. See also the trap command below.
Execution
Each time a command is executed, the above substitutions are
made. If the command name matches one of the special commands
listed below, it is executed in the shell process. If the
command name does not match a special command, but matches the
name of a defined function, the function is executed in the shell
process (note how this differs from the execution of shell
procedures). The positional parameters $1, $2, .... are set to
the arguments of the function. If the command name matches
neither a special command nor the name of a defined function, a
new process is created and the system tries to execute the
command via exec(2).
The shell parameter PATH defines the search path for the
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directory containing the command. Alternative directory names
are separated by a colon (:). The default path is :/bin:/usr/bin
(specifying the current directory, /bin, and /usr/bin, in that
order). Note that the current directory is specified by a null
pathname, which can appear immediately after the equal sign or
between the colon delimiters anywhere else in the path list.
If the command name contains a / the search path is not used;
such commands will not be executed by the restricted shell.
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 sub-shell is spawned to read it. A parenthesized
command is also executed in a sub-shell.
The shell remembers a command's location in the search path (to
help avoid unnecessary execs later). If the command was found in
a relative directory, its location must be re-determined whenever
the current directory changes. The shell forgets all remembered
locations whenever the PATH variable is changed or the hash -r
command is executed (see below).
Special Commands
Input/output redirection is now permitted for these commands.
File descriptor 1 is the default output location.
: No effect; the command does nothing. A zero exit code is
returned.
. file
Read and execute commands from file and return. The search
path specified by PATH finds the directory containing file.
You must have read and execute permission for the file.
break [ n ]
Exit from the enclosing for or while loop, if any. If n is
specified, break n levels.
continue [ n ]
Resume the next iteration of the enclosing for or while
loop. If n is specified, resume at the nth enclosing loop.
cd [ arg ]
Change the current directory to arg. The shell parameter
HOME is the default arg. The shell parameter CDPATH defines
the search path for the directory containing arg.
Alternative directory names are separated by a colon (:).
The default path is <null> (specifying the current
directory). Note that the current directory is specified by
a null pathname, which can appear immediately after the
equal sign or between the colon delimiters anywhere else in
the path list. If arg begins with a / the search path is
not used. Otherwise, each directory in the path is searched
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for arg. The cd command may not be executed by rsh.
echo [ arg ... ]
Echo arguments. See echo(1) for usage and description.
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
arguments may appear and, if no other arguments are given,
cause the shell input/output to be modified.
exit [ n ]
Causes a shell to exit with the exit status specified by n.
If n is omitted the exit status is that of the last command
executed. An end-of-file will also cause the shell to exit.
export [ name ... ]
The given names are marked for automatic export to the
environment of subsequently-executed commands. If no
arguments are given, a list of all names that are exported
in this shell is printed. Function names may not be
exported.
hash [ -r ] [ name ... ]
This option is available in DG/UX only. The shell finds and
remembers the location in the search path of each command
specified by name. The -r option makes the shell forget all
remembered locations. If no arguments are given,
information about remembered commands is presented. Hits is
the number of times a command has been invoked by the shell
process. Cost is a measure of the work required to locate a
command in the search path. Some situations require that
the stored location of a command be recalculated. These
commands are indicated by an asterisk (*) adjacent to the
hits information. Cost is incremented when the
recalculation is done.
newgrp [ arg ... ]
Equivalent to exec newgrp arg .... See newgrp(1).
pwd Print the current working directory. See pwd(1).
read [ name ... ]
One line is read from the standard input and the first word
is assigned to the first name, the second word to the second
name, etc., with leftover words assigned to the last name.
The return code is 0 unless an end-of-file is encountered.
readonly [ name ... ]
The given names are marked readonly and the values of these
names may not be changed by subsequent assignment. If no
arguments are given, a list of all readonly names is
printed.
return [ n ]
Causes a function to exit with the return value specified by
n. If n is omitted, the return status is that of the last
command executed.
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set [ --aefhkntuvx [ arg ... ] ]
-a Mark variables that are modified or created for export.
-e Exit immediately if a command exits with a non-zero
exit status.
-f Disable filename generation.
-h Find and remember function commands as functions are
defined (function commands are normally located when
the function is executed).
-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.
-- Do not change any of the flags; useful in setting $1 to
-.
Using + rather than - turns these flags off. These flags
can also be used upon invocation of the shell. The current
setting of flags may be found in $-. The remaining
arguments are positional parameters and are assigned, in
order, to $1, $2, .... If no arguments are given, the
values of all names are printed.
shift [ n ]
The positional parameters from $n+1 ... are renamed $1 ....
If n is not given, it is assumed to be 1.
test
Evaluate conditional expressions. See test(1) for usage and
description.
times
Print the accumulated user and system times for processes
run from the shell.
trap [ arg ] [ n ] ...
Read and execute the command arg 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. Any attempt to set a
trap on a signal that was ignored on entry to the current
shell is ineffective. An attempt to trap on signal 11
(memory fault) or signal 18 (termination of child process)
produces an error. If arg is absent all trap(s) n are reset
to their original values. If arg is the null string this
signal is ignored by the shell and by the commands it
invokes. If n is 0 the command arg is executed on exit from
the shell. The trap command with no arguments prints a list
of commands associated with each signal number.
type [ name ... ]
For each name, indicate how it would be interpreted if used
as a command name.
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ulimit [ -fp ] [ n ]
Imposes a size limit of n
-f Imposes a size limit of n blocks on files written by
child processes (files of any size may be read). With
no argument, the current limit is printed.
-p Changes the pipe size to n (UNIX/RT only).
If no option is given, -f is assumed.
umask [ nnn ]
The user file-creation mask is set to nnn (see umask(2)).
If nnn is omitted, the current value of the mask is printed.
Note that the shell and any programs running under the
shell, like ed(1), create files with a maximum permission of
666, even if you set the mask to 000. The mask value is
subtracted from 777 to arrive at the final mode, however. A
mask of 012 yields a mode of 665, for example. You must use
chmod to add the execution permission. This is especially
important if you are creating a shell program, since it must
have read and execute permissions in order to run.
unset [ name ... ]
For each name, remove the corresponding variable or
function. The variables PATH, PS1, PS2, MAILCHECK and IFS
cannot be unset.
wait [ n ]
Wait for the specified process and report its termination
status. If n is not given, all currently active child
processes are waited for and the return code is zero.
Invocation
If the shell is invoked through exec(2) and the first character
of argument zero is -, commands are initially read from
/etc/profile and from $HOME/.profile, if such files exist.
Thereafter, commands are read as described below, which is also
the case when the shell is invoked as /bin/sh. The flags below
are interpreted by the shell on invocation only. Unless the -c
or -s flag is specified, the first argument is assumed to be the
name of a file containing commands, and the remaining arguments
are passed as positional parameters to that command file:
-c string Commands are read from string.
-s If no arguments remain, commands are read from the
standard input. Any remaining arguments specify the
positional parameters. Shell output (except for
Special Commands) is written to file descriptor 2.
-i If the shell input and output are attached to a
terminal, this shell is interactive. In this case
TERMINATE is ignored (so that kill 0 does not kill
an interactive shell) and INTERRUPT is caught and
ignored (so that wait is interruptible). In all
cases, QUIT is ignored by the shell.
-r The shell is a restricted shell.
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The remaining flags and arguments are described under the set
command above.
Rsh Only
Rsh sets up login names and execution environments that are more
controlled than those of the standard shell. Rsh is identical to
sh, except that the following are disallowed:
changing directory (see cd(1))
setting the value of $PATH and $SHELL
specifying command names containing /
redirecting output (> and >>)
The restrictions above are enforced after .profile is
interpreted.
When a command to be executed is a shell procedure, rsh invokes
sh to execute it. Thus, you can give procedures to the end-user
shell that have access to the full power of the standard shell,
while imposing a limited menu of commands; this scheme assumes
that the end-user does not have write and execute permissions in
the same directory.
The net effect of these rules is to give the writer of the
.profile complete control over user actions, by performing
guaranteed setup actions and leaving the user in an appropriate
directory (probably not the login directory).
The system administrator often sets up a directory of commands
(i.e., /usr/rbin) that can be safely invoked by rsh. Some
systems also provide a restricted editor red.
EXIT STATUS
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 execution of the shell file is abandoned
except under special conditions:
The "echo" and "pwd" built-in commands have counterparts in
the /bin directory. If you use /bin/echo or /bin/pwd in a
shell script rather than the built-in echo or bin, then
execution of the script will continue after an error.
Otherwise, the shell returns the exit status of the last command
executed (see also the exit command above).
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FILES
/etc/profile
$HOME/.profile
/tmp/sh*
/dev/null
SEE ALSO
acctcom(1), cd(1), echo(1), env(1), login(1), newgrp(1), pwd(1),
test(1), umask(1).
acctcms(1M) in the System Manager's Reference for the DG/UX
System
dup(2), exec(2), fork(2), pipe(2), signal(2), ulimit(2),
umask(2), wait(2), a.out(4), profile(4), environ(5) in the
Programmer's Reference for the DG/UX System
WARNINGS
If a command is executed, and a command with the same name is
installed in a directory in the search path before the directory
where the original command was found, the shell continues to exec
the original command. Use the hash command to correct this
situation.
If you move the current directory or one above it, pwd may not
give the correct response. Use the cd command with a full
pathname to correct this situation.
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