sh - shell, the standard UNIX system command interpreter and command-level language

---

SYNOPSIS

DESCRIPTION

sh, the Bourne shell, is the standard UNIX-system command interpreter. It executes commands read from a terminal or a file.

Definitions

Invocation

OPTIONS

-i

If the -i option 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.

-s

If the -s option 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.

-c string

If the -c option is present commands are read from string.

The remaining options and arguments are described under the set command, under Special Commands, below.

USAGE

Commands

A simple command is a sequence of nonblank 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 execve.2v The value of a simple command is its exit status if it terminates normally, or (octal) 200+status if it terminates abnormally (see sigvec.2 for a list of status values).

A pipeline is a sequence of one or more commands separated by `|' (or, for historical compatibility, by `^'). The standard output of each command but the last is connected by a pipe (see pipe.2v to the standard input of the next command. Each command is run as a separate process; the shell normally waits for the last command to terminate before prompting for or accepting the next input line. The exit status of a pipeline is the exit status of its last command.

A list is a sequence of one or more simple commands or 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 (;) sequentially executes the preceding pipeline; an ampersand (&) asynchronously executes the preceding pipeline (the shell does not wait for that pipeline to finish). The symbols && and || are used to indicate conditional execution of the list that follows. With && , list is executed only if the preceding pipeline (or command) returns a zero exit status. With ||, list is executed only if the preceding pipeline (or command) returns a nonzero exit status. An arbitrary number of NEWLINE characters may appear in a list, instead of semicolons, to delimit commands.

A command is either a simple command or one of the following constructions. Unless otherwise stated, the value returned by a command is that of the last simple command executed in the construction.

for name [ in word ... ] do list done

Each time a for command is executed, name is set to the next word taken from the in word list. If in word ... is omitted, then the for command executes the do list once for each positional parameter that is set (see Parameter Substitution below). Execution ends when there are no more words in the list.

case word in [pattern[ | pattern] ... ) list ;; ] ... esac

A case command executes the list associated with the first pattern that matches word. The form of the patterns is the same as that used for filename generation (see Filename Generation) except that a slash, a leading dot, or a dot immediately following a slash need not be matched explicitly.

if list then list [ elif list then list ] ... [ else list ] fi

The list following if is executed and, if it returns a zero exit status, the list following the first then is executed. Otherwise, the list following elif is executed and, if its value is zero, the list following the next then is executed. Failing that, the else list is executed. If no else list or then list is executed, then the if command returns a zero exit status.

while list do list done

A while command repeatedly executes the while list and, if the exit status of the last command in the list is zero, executes the do list; otherwise the loop terminates. If no commands in the do list are executed, then 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 subshell.

{list;}

list is simply executed.

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

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 NEWLINE characters 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 NEWLINE character (\NEWLINE), both the backslash and the NEWLINE are removed (see Quoting, later). 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

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 SPACE characters). Parameter $0 is set from argument zero when the shell is invoked.

If the colon (:) is omitted from the following expressions, the shell only checks whether parameter is set or not.

${parameter:-word}

If parameter is set and is nonnull, 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 may not be assigned to in this way.

${parameter:?word}

If parameter is set and is nonnull, 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 nonnull, 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`}

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.

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 primary prompt.

MAILPATH

A colon (:) separated list of filenames. If this parameter is set, the shell informs the user of the arrival of mail in any of the specified files. Each filename can be followed by % and a message that will 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.

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 to the invoked command. Note: 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 EOF); 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 EOF. If, however, `-' is appended to:

• leading TAB characters are stripped from word before the shell input is read (but after parameter and command substitution is done on word),
  
• leading TAB characters are stripped from the shell input as it is read and before each line is compared with word, and
• shell input is read up to the first line that literally matches the resulting word, or to an EOF.

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:

• parameter and command substitution occurs,
• (escaped) \NEWLINE is ignored, and
• `\' 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 (namely, file 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.

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

; & ( ) | ^ < > NEWLINES* SPACES* TABS*

A character may be quoted (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 \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 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 \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

Environment

The environment for any simple command may be augmented by prefixing it with one or more assignments to parameters. Thus:

TERM=450cmd

and

(export TERM; TERM=450;cmd)

are equivalent (as far as the execution of cmd is concerned).

If the -k option 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

Execution

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/ucb:/bin:/usr/bin (specifying /usr/ucb, /bin, and /usr/bin, in addition to the current directory). Directories are searched in order. The current directory is specified by a null path name, which can appear immediately after the equal sign (PATH=:...), between the colon delimiters (...::...) anywhere else 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 binary executable (see a.out.5 for details) or an executable script (with a first line beginning with #!) it is assumed to be a file containing shell commands, and 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.

:

No effect; the command does nothing. A zero exit code is returned.

. filename

Read and execute commands from filename and return. The search path specified by PATH is used to find the directory containing filename.

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 argument. The shell parameter HOME is the default argument. The shell parameter CDPATH defines the search path for the directory containing argument. Alternative directory names are separated by a colon (:). The default path is NULL (specifying the current directory). Note: 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 argument begins with a / the search path is not used. Otherwise, each directory in the path is searched for argument.

echo [ argument ... ]

Echo arguments. See echo.1v for usage and description.

eval [ argument ... ]

The arguments are read as input to the shell and the resulting command(s) executed.

exec [ argument ... ]

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, modify the shell's input/output.

exit [ n ]

Exit a shell with the exit status specified by n. If n is omitted the exit status is that of the last command executed (an EOF 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 parse positional parameters and check 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.

login [ argument ... ]

Equivalent to `exec login argument....' See login.1 for usage and description.

newgrp [ argument ... ]

Equivalent to `exec newgrp argument....' 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 a SPACE or TAB character), 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 \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 that follows the backslash. The return code is 0 unless an EOF 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 ]

Exit a function with the return value specified by n. If n is omitted, the return status is that of the last command executed.

set [ -aefhkntuvx- [ argument ... ] ]

-a

Mark variables which 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 options; useful in setting $1 to `-'.

Using `+' rather than `-' turns off these options. These options can also be used upon invocation of the shell. The current set of options may be found in `$-'. The remaining arguments are positional parameters and are assigned, in order, to $1, $2, and so on. If no arguments are given, the values of all names are printed.

shift [ n ]

The positional parameters are shifted to the left, from position n+1 to position 1, and so on. Previous values for $1 through $n are discarded. If n is not given, it is assumed to be 1.

test

Evaluate conditional expressions. See test.1v 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: 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. 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.

umask [ ooo ]

The user file-creation mode mask is set to ooo (see csh.1 The three octal digits refer to read/write/execute permissions for owner, group, and others, respectively. The value of each specified digit is subtracted from the corresponding ``digit'' specified by the system for the creation of a file. For example, umask 022 removes group and others write permission (files normally created with mode 777 become mode 755; files created with mode 666 become mode 644). The current value of the mask is printed if ooo is omitted.

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 background process whose process ID is n and report its termination status. If n is omitted, all the shell's currently active background processes are waited for and the return code will be zero.

EXIT STATUS

Errors detected by the shell, such as syntax errors, 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).

ENVIRONMENT

The environment variables LC_CTYPE, LANG, and LC_default control the character classification throughout all command line parsing. These variables are checked in the following order: LC_CTYPE, LANG, and LC_default. When a valid value is found, remaining environment variables for character classification are ignored. For example, a new setting for LANG does not override the current valid character classification rules of LC_CTYPE. When none of the values is valid, the shell character classification defaults to the POSIX.1 ``C'' locale.

FILES

/etc/profile

$HOME/.profile

/tmp/sh*

/dev/null

/usr/lib/rsh

SEE ALSO

[a manual with the abbreviation DMBG]

WARNINGS

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 does not 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 the shell's currently active background processes are waited for and the return code will be zero.

Created by unroff & hp-tools. © somebody (See intro for details). All Rights Reserved. Last modified 11/5/97