bsh(1)bsh(1)NAME
bsh, jsh - shell, the standard/job control command programming language
SYNOPSISbsh [ -acefhiknprstuvx ] [ args ]
jsh [ -acefhiknprstuvx ] [ args ]
DESCRIPTION
Note: This is the Bourne shell description. All references to sh and
shell pertain to bsh and all references below to /usr/lib/rsh, the
restricted shell, no longer apply to the Bourne shell. (See sh(1)).
bsh is a command programming language that executes commands read from a
terminal or a file.
jsh is an interface to the shell that provides all the functionality of
sh and enables Job Control (see Job Control below).
See Invocation below for the meaning of arguments to the shell.
See CAVEATS below for interaction with Korn shell builtins.
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 |. 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. 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 (that is, the shell does
not wait for that pipeline to finish). The symbol && (||) causes the
list following it to be executed only if the preceding pipeline returns a
zero (nonzero) exit status. An arbitrary number of newlines can appear
in a list, instead of semicolons, to delimit commands.
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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,
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, 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, the while command returns a zero exit
status; until can be used in place of while to negate the loop
termination test.
(list) Execute list in a subshell.
{list;} list is executed in the current (that is, parent) shell. The {
must be followed by a space.
name () {list;}
Define a function that is referenced by name. The body of the
function is the list of commands between { and }. The list can
appear on the same line as the {. If it does, the { and list
must be separated by a space. The } cannot be on the same line
as list; it must be on a newline. Execution of functions is
described below (see Execution). The { and } are unnecessary
if the body of the function is a command as defined above,
under Commands.
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The following words are only recognized as the first word of a command
and when not quoted:
if then else elif fi case esac for while until do done { }
Comments
A word beginning with # causes that word and all the following characters
up to a newline to be ignored.
Command Substitution
The shell reads commands from the string between two grave accents (``)
and the standard output from these commands can be used as all or part of
a word. Trailing newlines from the standard output are removed.
No interpretation is done on the string before the string is read, except
to remove backslashes (\) used to escape other characters. Backslashes
can be used to escape a grave accent (`) or another backslash (\) and are
removed before the command string is read. Escaping grave accents allows
nested command substitution. If the command substitution lies within a
pair of double quotes (" ... `...` ... "), a backslash used to escape a
double quote (\") is removed; otherwise, it is left intact.
If a backslash is used to escape a newline character (\newline), both the
backslash and the newline are removed (see the later section on
"Quoting"). In addition, backslashes used to escape dollar signs (\$)
are removed. Since no interpretation is done on the command string
before it is read, inserting a backslash to escape a dollar sign has no
effect. Backslashes that precede characters other than \, `, ", newline,
and $ are left intact when the command string is read.
Parameter Substitution
The character $ is used to introduce substitutable parameters. There are
two types of parameters, positional and keyword. If parameter is a
digit, it is a positional parameter. Positional parameters can be
assigned values by set. Keyword parameters (also known as variables) can
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 $0 is set from argument zero when
the shell is invoked.
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${parameter:-word} If parameter is set and is non-null, substitute its
value; otherwise substitute word.
${parameter:=word} If parameter is not set or is null set it to word;
the value of the parameter is substituted.
Positional parameters cannot 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 or not.
The following parameters are automatically set by the shell:
* Expands to the positional parameters, beginning with 1.
@ Expands to the positional parameters beginning with 1, except when
expanded within double quotes, in which case each positional
parameter expands as a separate field.
# 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. $ reports the process ID of the
parent shell in all shell constructs, including pipelines, and in
parenthesized subshells.
! 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, set
to the user's login directory by login(1) from the password
file (see passwd(4)).
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PATH The search path for commands (see Execution below). The user
cannot 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
checks for the arrival of 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 checks 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 to 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 newline.
SHACCT If this parameter is set to the name of a file writable by the
user, the shell writes an accounting record in the file for
each shell procedure executed.
SHELL When the shell is invoked, it scans the environment (see
Environment below) for this name. If it is found and 'rsh' is
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. The original
whitespace characters (space, tab, and newline) are always considered
internal field separators.
Input/Output
A command's input and output can be redirected using a special notation
interpreted by the shell. The following can appear anywhere in a
simple-command or can precede or follow a command and are not passed on
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as arguments to the invoked command. Note that parameter and 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 After parameter and command substitution is done on word, the
shell input is read up to the first line that literally matches
the resulting word, or to an end-of-file. If, however, - is
appended to <<:
1. Leading tabs are stripped from word before the shell input
is read (but after parameter and command substitution is
done on word).
2. Leading tabs are stripped from the shell input as it is
read and before each line is compared with word.
3. Shell input is read up to the first line that literally
matches the resulting word, or to an end-of-file.
If any character of word is quoted (see Quoting, later), no
additional processing is done to the shell input. If no
characters of word are quoted:
1. Parameter and command substitution occurs.
2. (Escaped) \newline is ignored.
3. \ must be used to quote the characters \, $, and `.
The resulting document becomes the standard input.
<&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 >&-.
If any of the above is preceded by a digit, the file descriptor that will
be associated with the file is that specified by the digit (instead of
the default 0 or 1). For example:
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... 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 left-to-right. For example:
... 1>xxx 2>&1
first associates file descriptor 1 with file xxx. It associates file
descriptor 2 with the file associated with file descriptor 1 (that is,
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.
Using the terminology introduced on the first page, under Commands, if a
command is composed of several simple commands, redirection is evaluated
for the entire command before it is evaluated for each simple command.
That is, the shell evaluates redirection for the entire list, then each
pipeline within the list, then each command within each pipeline, then
each list within each command.
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.
Filename Generation
Before a command is executed, 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 is found that 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.
Quoting
The following characters have a special meaning to the shell and cause
termination of a word unless quoted:
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; & ( ) | ^ < > newline space tab
A character can be quoted that is, made to stand for itself) by preceding
it with a backslash (\) or inserting it between a pair of quote marks (''
or ""). During processing, the shell can quote certain characters to
prevent them from taking on a special meaning. Backslashes used to quote
a single character are removed from the word before the command is
executed. The pair \newline is removed from a word before command and
parameter substitution.
All characters enclosed between a pair of single quote marks (''), except
a single quote, are quoted by the shell. Backslash has no special
meaning inside a pair of single quotes. A single quote can be quoted
inside a pair of double quote marks (for example, "'").
Inside a pair of double quote marks (""), parameter and command
substitution occurs and the shell quotes the results to avoid blank
interpretation and filename generation. If $* is within a pair of double
quotes, the positional parameters are substituted and quoted, separated
by quoted spaces ("$1 $2 ..."); however, if $@ is within a pair of double
quotes, the positional parameters are substituted and quoted, separated
by unquoted spaces ("$1" "$2" ...). \ quotes the characters \, `, ", and
$. The pair \newline is removed before parameter and command
substitution. If a backslash precedes characters other than \, `, ", $,
and newline, then the backslash itself is quoted by the shell.
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, the secondary prompt that is, the value
of PS2) is issued.
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 parameter can
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.
The environment for any simple-command can be augmented by prefixing it
with one or more assignments to parameters. Thus these two commands are
equivalent (as far as the execution of cmd is concerned if cmd is not a
Special Command):
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TERM=450 cmd
(export TERM; TERM=450; cmd)
If cmd is a Special Command, then
TERM=45 cmd
modifies the TERM variable in the current shell.
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
When a command is run in the background (cmd &) under sh, it can receive
INTERRUPT and QUIT signals but ignores them by default. (A background
process can override this default behavior via trap or signal. For
details, see the description of trap, below, or signal(2).) When a
command is run in the background under jsh, however, it does not receive
INTERRUPT or QUIT signals.
Otherwise signals have the values inherited by the shell from its parent,
with the exception of signal 11 (SIGSEGV). See also the trap command
below.
Execution
Each time a command is executed, the command substitution, parameter
substitution, blank interpretation, input/output redirection, and
filename generation listed above are carried out. If the command name
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). If the command name does not match the name of a defined
function, but matches one of the Special Commands listed below, it is
executed in the shell process. The positional parameters $1, $2, and so
on 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 an attempt is made to execute the command via
exec(2).
The shell parameter PATH defines the search path for the directory
containing the command. Alternative directory names are separated by a
colon (:). The default path is:
:/usr/sbin:/usr/bsd:/bin:/usr/bin:/usr/bin/X11
specifying the current directory, /usr/sbin, /usr/bsd, /bin, /usr/bin,
and /usr/bin/X11, in that order. Note that the current directory is
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specified by a null pathname. It can appear immediately after the equal
sign, between two colon delimiters anywhere in the path list, or at the
end of the path list. If the command name contains a / the search path
is not used; such commands are not 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 subshell is spawned to
read it. A parenthesized command is also executed in a subshell.
The location in the search path where a command was found is remembered
by the shell (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. When Job Control is
enabled, additional Special Commands are added to the shell's environment
(see Job Control).
: 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 is used to find the
directory containing 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 n-th
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. It 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 for arg. The cd command cannot be
executed by rsh.
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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 can 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 also causes
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, variable names that have
been marked for export during the current shell's
execution are listed. (Variable names exported from
a parent shell are listed only if they have been
exported again during the current shell's execution.)
Function names are not exported.
getopts Use in shell scripts to support command syntax
standards (see intro(1)); it parses positional
parameters and checks for legal options. See
getopts(1) for usage and description.
hash [ -r ] [ name ... ]
For each name, the location in the search path of the
command specified by name is determined and
remembered by the shell. The -r option causes the
shell to 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. If a command is found in a
"relative" directory in the search path, after
changing to that directory, the stored location of
that command is recalculated. Commands for which
this is done are indicated by an asterisk (*)
adjacent to the hits information. cost is
incremented when the recalculation is done.
limit [ -h ] [ resource [maximum-use ] ]
Limits the consumption by the current process and
each process it creates to not individually exceed
maximum-use on the specified resource. If no
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maximum-use is given, then the current limit is
printed; if no resource is given, then all
limitations are given. If the -h flag is given, the
hard limits are used instead of the current limits.
The hard limits impose a ceiling on the values of the
current limits. Only the superuser can raise the
hard limits, but a user can lower or raise the
current limits within the legal range.
Resources controllable currently include cputime, the
maximum number of cpu-seconds to be used by each
process, filesize, the largest single file that can
be created, datasize, the maximum growth of the data
region via sbrk(2) beyond the end of the program
text, stacksize, the maximum size of the
automatically-extended stack region, coredumpsize,
the size of the largest core dump created, memoryuse,
the maximum amount of physical memory a process can
have allocated to it at a given time, descriptors,
the maximum number of open files, and vmemory, the
maximum total virtual size of the process, including
text, data, heap, shared memory, mapped files, stack,
and so on.
The maximum-use can be given as a (floating point or
integer) number followed by a scale factor. For all
limits other than cputime the default scale is k or
kilobytes (1024 bytes); a scale factor of m or
megabytes can also be used. For cputime the default
scaling is seconds, while m for minutes or h for
hours, or a time of the form mm:ss giving minutes and
seconds can be used.
For both resource names and scale factors,
unambiguous prefixes of the names suffice.
newgrp [ arg ... ] Equivalent to exec newgrp arg .... See newgrp(1) for
usage and description.
pwd Print the current working directory. See pwd(1) for
usage and description.
read [ name ... ] One line is read from the standard input and, using
the internal field separator, IFS (normally space or
tab), to delimit word boundaries, the first word is
assigned to the first name, the second word to the
second name, and so on, with leftover words assigned
to the last name. Lines can be continued using
\newline. Characters other than newline can be
quoted by preceding them with a backslash. These
backslashes are removed before words are assigned to
names, and no interpretation is done on the character
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that follows the backslash. The return code is 0
unless an end-of-file is encountered.
readonly [ name ... ]
The given names are marked readonly and the values of
the these names cannot 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.
set [ --aefhkntuvx [ arg ... ] ]
-a Mark variables that are modified or created for
export.
-e Exit immediately if a command exits with a
nonzero exit status.
-f Disable filename generation.
-h Locate 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 - causes these flags to be turned
off. These flags can also be used upon invocation of
the shell. The current set of flags can 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
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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 ] ...
The command arg is 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. Any attempt to set a trap on a
signal that was ignored on entry to the current shell
is ineffective. An error results when an attempt is
made to trap signal 11 (SIGSEGV--segmentation fault).
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.
ulimit [ n ] Impose a size limit of n blocks on files written by
the shell and its child processes (files of any size
can be read). If n is omitted, the current limit is
printed. You can lower your own ulimit, but only a
superuser (see su(1M)) can raise a ulimit.
umask [ nnn ] The user file creation mask is set to nnn (see
umask(1)). If nnn is omitted, the current value of
the mask is printed.
unlimit [ -h ] [ resource ]
Removes the limitation on resource. If no resource
is specified, then all resource limitations are
removed. If -h is given, the corresponding hard
limits are removed. Only the superuser can do this.
unset [ name ... ] For each name, remove the corresponding variable or
function. The variables PATH, PS1, PS2, MAILCHECK
and IFS cannot be unset.
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wait [ n ] Wait for your background process whose process id is
n and report its termination status. If n is
omitted, all your shell's currently active background
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; Note that 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
If the -c flag is present, commands are read from string.
-s If the -s flag is present or 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 -i flag is present or 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.
-p If the -p flag is present, the shell skips the processing of the
system profile (/etc/profile) and the user profile (.profile) when
it starts.
-r If the -r flag is present the shell is a restricted shell.
The remaining flags and arguments are described under the set command
above.
Job Control (jsh)
When the shell is invoked as jsh, Job Control is enabled in addition to
all of the functionality described previously for sh. Typically Job
Control is enabled for the interactive shell only. Noninteractive shells
typically do not benefit from the added functionality of Job Control.
With Job Control enabled every command or pipeline the user enters at the
terminal is called a job. All jobs exist in one of the following states:
foreground, background, or stopped. These terms are defined as follows:
1) a job in the foreground has read and write access to the controlling
terminal; 2) a job in the background is denied read access and has
conditional write access to the controlling terminal (see stty(1)); 3) a
stopped job is a job that has been placed in a suspended state, usually
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as a result of a SIGTSTP signal (see signal(2)). Jobs in the foreground
can be stopped by INTERRUPT or QUIT signals from the keyboard; background
jobs cannot be stopped by these signals.
Every job the shell starts is assigned a positive integer, called a job
number, which is tracked by the shell and is used, later, as an
identifier to indicate a specific job. Additionally the shell keeps
track of the current and previous jobs. The current job is the most
recent job to be started or restarted. The previous job is the first
noncurrent job.
The acceptable syntax for a Job Identifier is of the form:
%jobid
where jobid can be specified in any of the following formats:
% or + For the current job.
- For the previous job.
?string Specify the job for which the command line uniquely contains
string.
n For job number n, where n is a job number.
pref Where pref is a unique prefix of the command name (for example,
if the command ls -l foo were running in the background, it
could be referred to as %ls); pref cannot contain blanks unless
it is quoted.
When Job Control is enabled, the following commands are added to the
user's environment to manipulate jobs:
bg [%jobid ...] Resumes the execution of a stopped job in the
background. If %jobid is omitted the current job is
assumed.
fg [%jobid ...] Resumes the execution of a stopped job in the
foreground, also moves an executing background job
into the foreground. If %jobid is omitted the
current job is assumed.
jobs [-p|-l] [%jobid ...]
jobs -x command [arguments]
Reports all jobs that are stopped or executing in the
background. If %jobid is omitted, all jobs that are
stopped or running in the background are reported.
The following options modify/enhance the output of
jobs:
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bsh(1)bsh(1)-l Report the process group ID and working
directory of the jobs.
-p Report only the process group ID of the jobs.
-x Replace any jobid found in command or arguments
with the corresponding process group ID, and
then execute command passing it arguments.
kill [-signal] %jobid
Builtin version of kill to provide the functionality
of the kill command for processes identified with a
jobid.
stop %jobid . . . Stops the execution of a background job(s).
suspend Stops the execution of the current shell (but not if
it is the login shell).
wait [%jobid ...] wait builtin accepts a job identifier. If %jobid is
omitted, wait behaves as described above under
Special Commands.
Restricted Shell (/usr/lib/rsh) Only
/usr/lib/rsh is used to set up login names and execution environments
whose capabilities are more controlled than those of the standard shell.
The actions of /usr/lib/rsh are identical to those of sh, except that the
following are disallowed:
o changing directory (see cd(1))
o setting the value of $PATH
o specifying path or command names containing /
o redirecting output (> and >>)
The restrictions above are enforced after .profile is interpreted.
A restricted shell can be invoked in one of the following ways: (1) rsh
is the filename part of the last entry in the /etc/passwd file (see
passwd(4)); (2) the environment variable SHELL exists and rsh is the
filename part of its value; (3) the shell is invoked and rsh is the
filename part of argument 0; (4) the shell is invoke with the -r option.
When a command to be executed is found to be a shell procedure,
/usr/lib/rsh invokes sh to execute it. Thus, it is possible to provide
to the end-user shell procedures 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 that the writer of the .profile (see
profile(4)) has complete control over user actions by performing
guaranteed setup actions and leaving the user in an appropriate directory
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(probably not the login directory).
The system administrator often sets up a directory of commands (that is,
/usr/rbin) that can be safely invoked by a restricted shell. IRIX
provides a restricted editor, red(1).
EXIT STATUS
Errors detected by the shell, such as syntax errors, cause the shell to
return a nonzero exit status. If the shell is being used
noninteractively execution of the shell file is abandoned. Otherwise,
the shell returns the exit status of the last command executed (see also
the exit command above).
jsh Only
If the shell is invoked as jsh and an attempt is made to exit the shell
while there are stopped jobs, the shell issues one warning:
UX:jsh:WARNING:there are stopped jobs
This is the only message. If another exit attempt is made and there are
still stopped jobs, they are sent a SIGHUP signal from the kernel and the
shell is exited.
FILES
/etc/profile
$HOME/.profile
/tmp/sh*
/dev/null
SEE ALSOcd(1), echo(1), env(1), getopts(1), intro(1), login(1), newgrp(1),
pwd(1), systune(1m), test(1), umask(1), wait(1), xargs(1), dup(2),
exec(2), fork(2), getrlimit(2), pipe(2), signal(2), ulimit(2),
profile(4).
CAVEATS
Positional parameters have a range of 0 to 9. Attempting to use the
positional parameter $10 gives the contents of $1 followed by a 0, which
is probably not the desired result.
Words used for filenames in input/output redirection are not interpreted
for filename generation (see Filename Generation, above). For example,
cat file1 >a* creates a file with the name a*.
Because commands in pipelines are run as separate processes, variables
set in a pipeline have no effect on the parent shell.
If you get the error message "cannot fork, too many processes", try using
the wait(1) command to clean up your background processes. If this
doesn't help, the system process table is probably full or you have too
many active foreground processes. (There is a limit to the number of
process ids associated with your login, and to the number the system can
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keep track of.)
For compatibility with the POSIX builtin executables linked to
/sbin/builtin_exec, the Korn shell parameter expansion
${parameter##pattern} has been implemented only for the specific pattern
'*/' to emulate basename. This allows the Bourne shell builtins to work
correctly when called with fullpaths e.g. /sbin/jobs.
NOTES
Sometimes, particularly when using wildcards, the shell will fail to
execute a command, and complain with the message
Arg list or environment too large
This can often be avoided by using multiple commands, the xargs(1)
command, or by increasing the ncargs kernel parameter with the
systune(1m) command.
BUGS
Only the last process in a pipeline can be waited for.
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.
Prior to IRIX Release 5.0, the rsh command invoked the restricted shell.
This restricted shell command is /usr/lib/rsh and it can be executed by
using the full pathname. Beginning with IRIX Release 5.0, the rsh
command is the remote shell. See rsh_bsd(1C).
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