BSH(1) SysV BSH(1)
NAME
bsh - the Bourne shell command language
SYNOPSIS
bsh [ -acefhiknstuvx ] [ -Dname=value ... ] [ args ]
DESCRIPTION
Note: /bin/bsh is the Bourne shell (/bin/sh) as it was supplied at
SR10.3. It is provided only for backwards compatibility purposes and is
unsupported at this release. We recommend that you use /bin/ksh (the
Korn shell) instead.
bsh is a command programming language that executes commands read from a
terminal or a file. 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.
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.
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 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 worrd ; the value of
the parameter is substituted. Positional parameters may not be
assigned to in this way.
${parameter:?word}
If parameter is set and is non-null, substitute its value;
otherwise, print word and exit from the shell. If word is omitted,
the message "parameter null or not set" is printed.
${parameter:+word}
If parameter is set and is non-null, substitute word; otherwise
substitute nothing.
In the above, word is not evaluated unless it is to be used as the
substituted string, so that, in the following example, pwd is executed
only if d is not set or is null:
echo ${d:-`pwd`}
If the colon (:) is omitted from the above expressions, the shell only
checks whether parameter is set 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).
CDPATH
The search path for the cd command.
SHENV
If this parameter is set, the Shell performs parameter
substitution on the value to generate the pathname of the
startup script containing commands that the Shell executes
every time a new shell is invoked. No error results if the
file specified by the SHENV parameter doesn't exist or can't be
read.
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.
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.
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.
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 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 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.
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) 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 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. 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. 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 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.
echo [ arg ... ]
Echo arguments. See echo(1) for usage and description.
eval [ arg ... ]
The arguments are read as input to the shell and the resulting
command(s) executed.
exec [ arg ... ]
The command specified by the arguments is executed in place of this
shell without creating a new process. Input/output arguments may
appear and, if no other arguments are given, cause the shell
input/output to be modified.
exit [ n ]
Causes a shell to exit with the exit status specified by n. If n is
omitted the exit status is that of the last command executed (an
end-of-file will also cause the shell to exit.)
export [ name ... ]
The given names are marked for automatic export to the environment
of subsequently-executed commands. If no arguments are given,
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 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 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 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/bsh. 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.
-Dname=value
Use the -D option to specify a parameter name, that will be set
to value, then passed into the shell's environment. This SysV
option is useful for tailoring the environment of a shell
invoked from a program that is not another shell (such as the
Display Manager). If you set the SHENV parameter using this
option, the startup script it specifies will be run. Any
number of -D options can be specified.
The remaining flags and arguments are described under the set command
above.
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
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 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.
FILES
/etc/profile
$HOME/.profile
/tmp/sh*
/dev/null
SEE ALSO
cd(1), echo(1), env(1), getopts(1), inlib(1), intro(1), ksh(1), login(1),
newgrp(1), pwd(1), rootnode(1), test(1), umask(1), wait(1).
dup(2), exec(2), fork(2), pipe(2), profile(4), signal(2), ulimit(2) in
the SysV Programmer's Reference.