SH(1) (Essential Utilities) SH(1)
NAME
sh, rsh - shell, the standard/restricted command programming
language
SYNOPSIS
sh [ -acefhiknrstuvx ] [ args ]
rsh [ -acefhiknrstuvx ] [ args ]
DESCRIPTION
sh is a command programming language that executes commands
read from a terminal or a file. rsh is a restricted version
of the standard command interpreter sh; it is used to set 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.
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.
Page 1 May 1989
SH(1) (Essential Utilities) SH(1)
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 && (||) causes the list following it to be executed
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 file-name
generation (see ``File Name 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.
Page 2 May 1989
SH(1) (Essential Utilities) SH(1)
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 the while command returns a zero exit
status; until may be used in place of while to negate
the loop termination test.
(list)
Execute list in a sub-shell.
{list;}
list is executed in the current (that is, parent)
shell.
name () {list;}
Define a function which is 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 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 new-line to be ignored.
Command Substitution
The shell reads commands from the string between two grave
accents (``) and the standard output from these commands may
be used as all or part of a word. Trailing new-lines 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 may 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
Page 3 May 1989
SH(1) (Essential Utilities) SH(1)
lies within a pair of double quotes (" ...` ...` ... "), a
backslash used to escape a double quote (\") will be
removed; otherwise, it will be left intact.
If a backslash is used to escape a new-line character
(\new-line), both the backslash and the new-line 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 \, `, ", new-line, 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 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}
If parameter is not set or is null set it to word; the
Page 4 May 1989
SH(1) (Essential Utilities) SH(1)
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 or not.
The following parameters are automatically set by the shell:
# 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
Page 5 May 1989
SH(1) (Essential Utilities) SH(1)
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 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 will check
before each prompt.
MAILPATH
A colon (:) separated list of file names. If
this parameter is set, the shell informs the user
of the arrival of mail in any of the specified
files. Each file name can be followed by % and a
message that will 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.
SHELL
When the shell is invoked, it scans the
environment (see ``Environment'' below) for this
name. If it is found and 'rsh' is the file name
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
Page 6 May 1989
SH(1) (Essential Utilities) SH(1)
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.
Input/Output
A command's 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 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, and
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
Page 7 May 1989
SH(1) (Essential Utilities) SH(1)
done to the shell input. If no characters of
word are quoted:
1) parameter and command substitution occurs,
2) (escaped) \new-line is ignored, and
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 which will be 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 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 (i.e., 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 will be evaluated for the entire
Page 8 May 1989
SH(1) (Essential Utilities) SH(1)
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.
File Name 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 file names that match
the pattern. If no file name is found that matches the
pattern, the word is left unchanged. The character . at the
start of a file name 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:
; & ( ) | ^ < > new-line space tab
A character may be quoted (i.e., made to stand for itself)
Page 9 May 1989
SH(1) (Essential Utilities) SH(1)
by preceding it with a backslash (\) or inserting it between
a pair of quote marks ('' or ""). During processing, the
shell may 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 \new-line 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 may 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 file name 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 \new-line is
removed before parameter and command substitution. If a
backslash precedes characters other than \, `, ", $, and
new-line, 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 new-line is
typed and further input is needed to complete a command, the
secondary prompt (i.e., 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
Page 10 May 1989
SH(1) (Essential Utilities) SH(1)
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 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.
The environment for any simple-command may be augmented by
prefixing it with one or more assignments to parameters.
Thus:
TERM=450 cmd and
(export TERM; TERM=450; 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 (but see also the trap command
below).
Execution
Each time a command is executed, the above substitutions are
carried out. If the command name matches one of the Special
Commands listed below, it is executed in the shell process.
Page 11 May 1989
SH(1) (Essential Utilities) SH(1)
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 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
:/bin:/usr/bin (specifying the current directory, /bin, and
/usr/bin, in that order). Note that the current directory
is specified by a null path name, which 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 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 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.
: No effect; the command does nothing. A zero exit code
Page 12 May 1989
SH(1) (Essential Utilities) SH(1)
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 path name,
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 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
Page 13 May 1989
SH(1) (Essential Utilities) SH(1)
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, 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 will be done are indicated by
an asterisk (*) adjacent to the hits information. Cost
will be incremented when the recalculation is done.
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
Page 14 May 1989
SH(1) (Essential Utilities) SH(1)
to the first name, the second word to the second name,
etc., with leftover words assigned to the last name.
Lines can be continued using \new-line. Characters
other than new-line 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 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 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.
set [ --aefhkntuvx [ arg ... ] ]
-a Mark variables which are modified or created for
export.
-e Exit immediately if a command exits with a non-
zero exit status.
-f Disable file name 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
Page 15 May 1989
SH(1) (Essential Utilities) SH(1)
the shell. The current set 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 ] ...
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 attempt to trap on signal 11 (memory fault) 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.
ulimit [ n ]
Impose a size limit of n blocks on files written by the
shell and its child processes (files of any size may be
read). If n is omitted, the current limit is printed.
You may lower your own ulimit, but only a super-user
(see su(1M)) can raise a ulimit.
umask [ nnn ]
The user file-creation mask is set to nnn (see
Page 16 May 1989
SH(1) (Essential Utilities) SH(1)
umask(1)). If nnn is omitted, the current value of the
mask is printed.
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 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 will be 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.
Page 17 May 1989
SH(1) (Essential Utilities) SH(1)
-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.
rsh Only
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 rsh are identical to those
of sh, except that the following are disallowed:
changing directory (see cd(1)),
setting the value of $PATH,
specifying path or command names containing /,
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 file name part of the last entry in
the /etc/passwd file (see passwd(4)); (2) the environment
variable SHELL exists and rsh is the file name part of its
value; (3) the shell is invoked and rsh is the file name
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, 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 (probably not the login
directory).
Page 18 May 1989
SH(1) (Essential Utilities) SH(1)
The system administrator often sets up a directory of
commands (i.e., /usr/rbin) that can be safely invoked by a
restricted shell. 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. Otherwise, the shell returns the exit status of
the last command executed (see also the exit command above).
FILES
/etc/profile
$HOME/.profile
/tmp/sh*
/dev/null
SEE ALSO
cd(1), echo(1), env(1), getopts(1), intro(1), login(1),
newgrp(1), pwd(1), test(1), umask(1), wait(1).
dup(2), exec(2), fork(2), pipe(2), profile(4), signal(2),
ulimit(2) in the Programmer's Reference Manual.
CAVEATS
Words used for filenames in input/output redirection are not
interpreted for filename generation (see ``File Name
Generation,'' above). For example, cat file1 >a* will
create a file named 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
Page 19 May 1989
SH(1) (Essential Utilities) SH(1)
the system can keep track of.)
BUGS
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
will continue 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 path name to correct this situation.
Not all the processes of a 3- or more-stage pipeline are
children of the shell, and thus cannot be waited for.
For wait n, if n is not an active process id, all your
shell's currently active background processes are waited for
and the return code will be zero.
Page 20 May 1989