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original sh5.1 massaged, nroffed and htmlized.
sh5(1) sh5(1)
NAME
sh5, rsh5 - shell, the standard/restricted command programming language
SYNTAX
sh5 [ -acefhiknrstuvx ] [ args ]
rsh5 [ -acefhiknrstuvx ] [ args ]
DESCRIPTION
The sh5 program is a command line interpreter and programming language
that executes commands read from a terminal or a file. The rsh5 pro-
gram is a restricted version of the standard command interpreter sh5.
It is used to set up login names and execution environments whose capa-
bilities are more controlled than those of the standard shell. See
Invocation below for the meaning of arguments to the shell. This ver-
sion of the shell is from SYSTEM V Rel 2. For further information
about the standard Version 7 shell interpreter, see sh(1).
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. For
further information, see exec(2). The value of a simple command is its
exit status if it terminates normally, or (octal) 200+status if it ter-
minates abnormally. For a list of status values, see signal(2).
A pipeline is a sequence of one or more commands separated by | (or,
for historical compatibility, by ^). The standard output of each com-
mand 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 (non-zero) exit status. An arbitrary number of
new-lines may appear in a list, instead of semicolons, to delimit com-
mands.
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
status, the list following the first then is executed. Other-
wise, 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 the while command returns a zero
exit status. The until command may be used in place of while to
negate the loop termination test.
(list)
Execute list in a sub-shell.
{list;}
Simply executes list from current 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. For further information, see Exe-
cution.
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 charac-
ters up to a new-line to be ignored.
Command Substitution
The standard output from a command enclosed in a pair of grave accents
( `command` ) may be used as part or all of a word. Trailing new-lines
are removed.
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
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. For further information,
see Execution below. The user may not change PATH if
executing under rsh5.
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 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.
SHELL When the shell is invoked, it scans the environment for
this name. For further information, see Environment
below. If it is found and there is an 'r' in 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 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.
File Name 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 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 (that is, made to stand for itself) by
preceding it with a \. 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 (that is,
the value of PS2) is issued.
Input/Output
Before a command is executed, its input and output may be redirected
using a special notation interpreted by the shell. The following may
appear anywhere in a simple-command or may precede or follow a command
and are not passed on to the invoked command. Substitution occurs
before word or digit is used:
<word Use file word as standard input (file descriptor 0).
>word Use file word as standard output (file descriptor 1). If
the file does not exist 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 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 >&-.
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:
This 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:
The 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.
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 is a list of name-value pairs that is passed to an
executed program in the same way as a normal argument list. For
further information, see environ(5). The shell interacts with the
environment in several ways. On invocation, the shell scans the
environment and creates a parameter for each name found, giving it the
corresponding value. 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 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
(export TERM; TERM=450; cmd)
These 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. For further information, 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. 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 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 location in the search path where a command was found is remembered
by the shell (to help avoid unnecessary exec 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 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
rsh5.
echo [ arg... ]
Each given name is 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... ]
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. There are certain situations which require that the
stored location of a command be 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 .... For usage and description,
see newgrp(1).
pwd Print the current working directory. For usage and description,
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 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
-.
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 [ -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. For further
information, see umask(2). 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 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/sh5. 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.
-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.
rsh5 Only
The rsh5 shell 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 sh5, 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.
When a command to be executed is found to be a shell procedure, rsh5
invokes sh5 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 has
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 (that
is, /usr/rbin ) that can be safely invoked by rsh5. 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).
CAVEATS
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.
FILES
/etc/profile
$HOME/.profile
/tmp/sh*
/dev/null
SEE ALSO
cd(1), echo(1), login(1), newgrp(1), printenv(1), pwd(1), sh(1),
test(1)
dup(2), exec(2), fork(2), pipe(2), signal(2), ulimit(2), umask(2),
wait(2)
a.out(5), profile(5), environ(5)