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.\" This code is derived from software contributed to Berkeley by

.\" Kenneth Almquist.

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.\"

.\"	@(#)sh.1	8.6 (Berkeley) 5/4/95

.\"

.Dd January 9, 1999

.Os

.Dt SH 1

.Sh NAME

sh \- command interpreter (shell)

.Sh SYNOPSIS

.Nm sh

.Op Fl /+aCefnuvxIimqsVEbc

.Op Fl o Ar longname

.Bk -words

.Op Ar target ...

.Ek

.Sh DESCRIPTION

Sh is the standard command interpreter for the system. The current version

of

.Nm

is in the process of being changed to conform with the

.Tn POSIX

1003.2 and 1003.2a specifications for the shell.  This version has many

features which make it appear similar in some respects to the Korn shell,

but it is not a Korn shell clone (see

.Xr ksh 1 ) .

Only features designated by

.Tn POSIX ,

plus a few Berkeley extensions, are being incorporated into this shell.

We expect

.Tn POSIX

conformance by the time 4.4 BSD is released. This man page is not intended

to be a tutorial or a complete specification of the shell.

.Ss Overview

The shell is a command that reads lines from either a file or the

terminal, interprets them, and generally executes other commands. It is

the program that is running when a user logs into the system (although a

user can select a different shell with the

.Xr chsh 1

command). The shell implements a language that has flow control

constructs, a macro facility that provides a variety of features in

addition to data storage, along with built in history and line editing

capabilities.  It incorporates many features to aid interactive use and

has the advantage that the interpretative language is common to both

interactive and non-interactive use (shell scripts).  That is, commands

can be typed directly to the running shell or can be put into a file and

the file can be executed directly by the shell.

.Ss Invocation

If no args are present and if the standard input of the shell

is connected to a terminal (or if the

.Fl i

flag is set),

and the 

.Fl c

option is not present, the shell is considered an interactive shell.  An

interactive shell generally prompts before each command and handles

programming and command errors differently (as described below). When

first starting, the shell inspects argument 0, and if it begins with a

dash

.Sq - ,

the shell is also considered

a login shell.  This is normally done automatically by the system

when the user first logs in. A login shell first reads commands

from the files

.Pa /etc/profile

and

.Pa .profile

if they exist.

If the environment variable

.Ev ENV

is set on entry to a shell, or is set in the

.Pa .profile

of a login shell, the shell next reads

commands from the file named in

.Ev ENV .

Therefore, a user should place commands that are to be executed only at

login time in the

.Pa .profile

file, and commands that are executed for every shell inside the

.Ev ENV

file.  To set the

.Ev ENV

variable to some file, place the following line in your

.Pa .profile

of your home directory

.Pp

.Dl ENV=$HOME/.shinit; export ENV

.Pp

substituting for

.Dq .shinit

any filename you wish. Since the

.Ev ENV

file is read for every invocation of the shell, including shell scripts

and non-interactive shells, the following paradigm is useful for

restricting commands in the

.Ev ENV

file to interactive invocations. Place commands within the

.Dq case

and

.Dq esac

below (these commands are described later):

.Pp

.Bl -item -compact -offset indent

.It

.Li case $- in *i*)

.Bl -item -compact -offset indent

.It

.Li # commands for interactive use only

.It

.Li ...

.El

.It

.Li esac

.El

.Pp

If command line arguments besides the options have been specified, then

the shell treats the first argument as the name of a file from which to

read commands (a shell script), and the remaining arguments are set as the

positional parameters of the shell ($1, $2, etc).  Otherwise, the shell

reads commands from its standard input.

.Ss Argument List Processing

All of the single letter options have a corresponding name that can be

used as an argument to the

.Fl o

option. The set

.Fl o

name is provided next to the single letter option in

the description below. Specifying a dash

.Dq -

turns the option on, while using a plus

.Dq +

disables the option.

The following options can be set from the command line or

with the

.Xr set 1

builtin (described later).

.Bl -tag -width aaaallexportfoo -offset indent

.It Fl a Em allexport

Export all variables assigned to. (UNIMPLEMENTED for 4.4alpha)

.It Fl c

Read commands from the command line.

No commands will be read from the standard input.

.It Fl C Em noclobber

Don't overwrite existing files with

.Dq > .

(UNIMPLEMENTED for 4.4alpha)

.It Fl e Em errexit

If not interactive, exit immediately if any untested command fails.

The exit status of a command is considered to be

explicitly tested if the command is used to control

an

.Ic if ,

.Ic elif ,

.Ic while ,

or

.Ic until ;

or if the command is the left hand operand of an

.Dq &&

or

.Dq ||

operator.

.It Fl f Em noglob

Disable pathname expansion.

.It Fl n Em noexec

If not interactive, read commands but do not execute them.  This is useful

for checking the syntax of shell scripts.

.It Fl u Em nounset

Write a message to standard error when attempting to expand a variable

that is not set, and if the shell is not interactive, exit immediately.

(UNIMPLEMENTED for 4.4alpha)

.It Fl v Em verbose

The shell writes its input to standard error as it is read.  Useful for

debugging.

.It Fl x Em xtrace

Write each command to standard error (preceded

by a

.Sq +\  )

before it is executed.  Useful for debugging.

.It Fl q Em quietprofile

If the

.Fl v

or

.Fl x

options have been set, do not apply them when reading

initialization files, these being

.Pa /etc/profile ,

.Pa .profile ,

and the file specified by the

.Ev ENV

environment variable.

.It Fl I Em ignoreeof

Ignore EOF's from input when interactive.

.It Fl i Em interactive

Force the shell to behave interactively.

.It Fl m Em monitor

Turn on job control (set automatically when interactive).

.It Fl s Em stdin

Read commands from standard input (set automatically if no file arguments

are present).  This option has no effect when set after the shell has

already started running (i.e. with

.Xr set 1 ) .

.It Fl V Em vi

Enable the built-in

.Xr vi 1

command line editor (disables

.Fl E

if it has been set).

.It Fl E Em emacs

Enable the built-in

.Xr emacs 1

command line editor (disables

.Fl V

if it has been set).

.It Fl b Em notify

Enable asynchronous notification of background job completion.

(UNIMPLEMENTED for 4.4alpha)

.El

.Ss Lexical Structure

The shell reads input in terms of lines from a file and breaks it up into

words at whitespace (blanks and tabs), and at certain sequences of

characters that are special to the shell called

.Dq operators .

There are two types of operators: control operators and redirection

operators (their meaning is discussed later). Following is a list of operators:

.Bl -ohang -offset indent

.It "Control operators:"

.Dl &  &&  (  )  ;  ;; | || <newline>

.It "Redirection operator:"

.Dl <  >  >|  <<  >>  <&  >&  <<-  <>

.El

.Ss Quoting

Quoting is used to remove the special meaning of certain characters or

words to the shell, such as operators, whitespace, or keywords.  There are

three types of quoting: matched single quotes, matched double quotes, and

backslash.

.Ss Backslash

A backslash preserves the literal meaning of the following

character, with the exception of

.Aq newline .

A backslash preceding a

.Aq newline

is treated as a line continuation.

.Ss Single Quotes

Enclosing characters in single quotes preserves the literal meaning of all

the characters (except single quotes, making it impossible to put

single-quotes in a single-quoted string).

.Ss Double Quotes

Enclosing characters within double quotes preserves the literal

meaning of all characters except dollarsign

.Pq $ ,

backquote

.Pq ` ,

and backslash

.Pq \e .

The backslash inside double quotes is historically weird, and serves to

quote only the following characters:

.Dl $  `  \*q  \e  <newline> .

Otherwise it remains literal.

.Ss Reserved Words

Reserved words are words that have special meaning to the

shell and are recognized at the beginning of a line and

after a control operator.  The following are reserved words:

.Bl -column while while while while while -offset indent

.It ! Ta elif Ta fi Ta while Ta case

.It else Ta for Ta then Ta { Ta }

.It do Ta done Ta until Ta if Ta esac

.El

.Pp

Their meaning is discussed later.

.Ss Aliases

An alias is a name and corresponding value set using the

.Xr alias 1

builtin command.  Whenever a reserved word may occur (see above),

and after checking for reserved words, the shell

checks the word to see if it matches an alias. If it does,

it replaces it in the input stream with its value.  For example,

if there is an alias called

.Dq lf

with the value

.Dq "ls -F" ,

then the input:

.Pp

.Dl lf foobar <return>

.Pp

would become

.Pp

.Dl ls -F foobar <return>

.Pp

Aliases provide a convenient way for naive users to create shorthands for

commands without having to learn how to create functions with arguments.

They can also be used to create lexically obscure code.  This use is

discouraged.

.Ss Commands

The shell interprets the words it reads according to a language, the

specification of which is outside the scope of this man page (refer to the

BNF in the

.Tn POSIX

1003.2 document).  Essentially though, a line is read and if the first

word of the line (or after a control operator) is not a reserved word,

then the shell has recognized a simple command.  Otherwise, a complex

command or some other special construct may have been recognized.

.Ss Simple Commands

If a simple command has been recognized, the shell performs

the following actions:

.Bl -enum -offset indent

.It

Leading words of the form

.Dq name=value

are stripped off and assigned to the environment of the simple command.

Redirection operators and their arguments (as described below) are

stripped off and saved for processing.

.It

The remaining words are expanded as described in

the section called

.Dq Expansions ,

and the first remaining word is considered the command name and the

command is located.  The remaining words are considered the arguments of

the command. If no command name resulted, then the

.Dq name=value

variable assignments recognized in item 1 affect the current shell.

.It

Redirections are performed as described in the next section.

.El

.Ss Redirections

Redirections are used to change where a command reads its input or sends

its output.  In general, redirections open, close, or duplicate an

existing reference to a file.  The overall format used for redirection is:

.Pp

.Dl [n] Va redir-op Ar file

.Pp

where

.Va redir-op

is one of the redirection operators mentioned previously.  Following is a

list of the possible redirections. The

.Bq n

is an optional number, as in

.Sq 3

(not

.Sq Bq 3 ,

that refers to a file descriptor.

.Bl -tag -width aaabsfiles -offset indent

.It [n] Ns > file

Redirect standard output (or n) to file.

.It [n] Ns >| file

Same, but override the

.Fl C

option.

.It [n] Ns >> file

Append standard output (or n) to file.

.It [n] Ns < file

Redirect standard input (or n) from file.

.It [n1] Ns <& Ns n2

Duplicate standard input (or n1) from file descriptor n2.

.It [n] Ns <&-

Close standard input (or n).

.It [n1] Ns >& Ns n2

Duplicate standard output (or n1) from n2.

.It [n] Ns >&-

Close standard output (or n).

.It [n] Ns <> file

Open file for reading and writing on standard input (or n).

.El

.Pp

The following redirection is often called a

.Dq here-document .

.Bl -item -offset indent

.It

.Li [n]<< delimiter

.Dl here-doc-text...

.Li delimiter

.El

.Pp

All the text on successive lines up to the delimiter is saved away and

made available to the command on standard input, or file descriptor n if

it is specified.  If the delimiter as specified on the initial line is

quoted, then the here-doc-text is treated literally, otherwise the text is

subjected to parameter expansion, command substitution, and arithmetic

expansion (as described in the section on

.Dq Expansions ) .

If the operator is

.Dq <<-

instead of

.Dq << ,

then leading tabs in the here-doc-text are stripped.

.Ss Search and Execution

There are three types of commands: shell functions, builtin commands, and

normal programs -- and the command is searched for (by name) in that

order.  They each are executed in a different way.

.Pp

When a shell function is executed, all of the shell positional parameters

(except $0, which remains unchanged) are set to the arguments of the shell

function. The variables which are explicitly placed in the environment of

the command (by placing assignments to them before the function name) are

made local to the function and are set to the values given. Then the

command given in the function definition is executed.  The positional

parameters are restored to their original values when the command

completes. This all occurs within the current shell.

.Pp

Shell builtins are executed internally to the shell, without spawning a

new process.

.Pp

Otherwise, if the command name doesn't match a function or builtin, the

command is searched for as a normal program in the filesystem (as

described in the next section). When a normal program is executed, the

shell runs the program, passing the arguments and the environment to the

program. If the program is not a normal executable file (i.e., if it does

not begin with the "magic number" whose

.Tn ASCII

representation is "#!", so

.Xr execve 2

returns

.Er ENOEXEC

then) the shell will interpret the program in a subshell.  The child shell

will reinitialize itself in this case, so that the effect will be as if a

new shell had been invoked to handle the ad-hoc shell script, except that

the location of hashed commands located in the parent shell will be

remembered by the child.

.Pp

Note that previous versions of this document and the source code itself

misleadingly and sporadically refer to a shell script without a magic

number as a "shell procedure".

.Ss Path Search

.Pp

When locating a command, the shell first looks to see if it has a shell

function by that name.  Then it looks for a builtin command by that name.

If a builtin command is not found, one of two things happen:

.Bl -enum

.It

Command names containing a slash are simply executed without performing

any searches.

.It

The shell searches each entry in

.Ev PATH

in turn for the command. The value of the

.Ev PATH

variable should be a series of entries separated by colons.  Each entry

consists of a directory name. The current directory may be indicated

implicitly by an empty directory name, or explicitly by a single period.

.El

.Ss Command Exit Status

Each command has an exit status that can influence the behavior

of other shell commands.  The paradigm is that a command exits

with zero for normal or success, and non-zero for failure,

error, or a false indication.  The man page for each command

should indicate the various exit codes and what they mean.

Additionally, the builtin commands return exit codes, as does

an executed shell function.

.Ss Complex Commands

Complex commands are combinations of simple commands with control

operators or reserved words, together creating a larger complex command.

More generally, a command is one of the following:

.Bl -bullet

.It

simple command

.It

pipeline

.It

list or compound-list

.It

compound command

.It

function definition

.El

.Pp

Unless otherwise stated, the exit status of a command is that of the last

simple command executed by the command.

.Ss Pipelines

.Pp

A pipeline is a sequence of one or more commands separated

by the control operator |.  The standard output of all but

the last command is connected to the standard input

of the next command.  The standard output of the last

command is inherited from the shell, as usual.

.Pp

The format for a pipeline is:

.Pp

.Dl [!] command1 [ | command2 ...]

.Pp

The standard output of command1 is connected to the standard input of

command2. The standard input, standard output, or both of a command is

considered to be assigned by the pipeline before any redirection specified

by redirection operators that are part of the command.

.Pp

If the pipeline is not in the background (discussed later), the shell

waits for all commands to complete.

.Pp

If the reserved word ! does not precede the pipeline, the exit status is

the exit status of the last command specified in the pipeline.

Otherwise, the exit status is the logical NOT of the exit status of the

last command.  That is, if the last command returns zero, the exit status

is 1; if the last command returns greater than zero, the exit status is

zero.

.Pp

Because pipeline assignment of standard input or standard output or both

takes place before redirection, it can be modified by redirection.  For

example:

.Pp

.Dl $ command1 2>&1 | command2

.Pp

sends both the standard output and standard error of command1

to the standard input of command2.

.Pp

A ; or <newline> terminator causes the preceding AND-OR-list (described

next) to be executed sequentially; a & causes asynchronous execution of

the preceding AND-OR-list.

.Pp

Note that unlike some other shells, each process in the pipeline is a

child of the invoking shell (unless it is a shell builtin, in which case

it executes in the current shell -- but any effect it has on the

environment is wiped).

.Ss Background Commands -- &

If a command is terminated by the control operator ampersand (&), the

shell executes the command asynchronously -- that is, the shell does not

wait for the command to finish before executing the next command.

.Pp

The format for running a command in background is:

.Pp

.Dl command1 & [command2 & ...]

.Pp

If the shell is not interactive, the standard input of an asynchronous

command is set to

.Pa /dev/null .

.Ss Lists -- Generally Speaking

A list is a sequence of zero or more commands separated by newlines,

semicolons, or ampersands, and optionally terminated by one of these three

characters. The commands in a list are executed in the order they are

written. If command is followed by an ampersand, the shell starts the

command and immediately proceed onto the next command; otherwise it waits

for the command to terminate before proceeding to the next one.

.Ss Short-Circuit List Operators

.Dq &&

and

.Dq ||

are AND-OR list operators.

.Dq &&

executes the first command, and then executes the second command iff the

exit status of the first command is zero.

.Dq ||

is similar, but executes the second command iff the exit status of the first

command is nonzero.

.Dq &&

and

.Dq ||

both have the same priority.

.Ss Flow-Control Constructs -- if, while, for, case

The syntax of the if command is

.Bd -literal -offset indent

if list

then list

[ elif list

then    list ] ...

[ else list ]

fi

.Ed

.Pp

The syntax of the while command is

.Bd -literal -offset indent

while list

do   list

done

.Ed

.Pp

The two lists are executed repeatedly while the exit status of the

first list is zero.  The until command is similar, but has the word

until in place of while, which causes it to

repeat until the exit status of the first list is zero.

.Pp

The syntax of the for command is

.Bd -literal -offset indent

for variable in word...

do   list

done

.Ed

.Pp

The words are expanded, and then the list is executed repeatedly with the

variable set to each word in turn.  do and done may be replaced with

.Dq {

and

.Dq } .

.Pp

The syntax of the break and continue command is

.Bd -literal -offset indent

break [ num ]

continue [ num ]

.Ed

.Pp

Break terminates the num innermost for or while loops.

Continue continues with the next iteration of the innermost loop.

These are implemented as builtin commands.

.Pp

The syntax of the case command is

.Bd -literal -offset indent

case word in

pattern) list ;;

\&...

esac

.Ed

.Pp

The pattern can actually be one or more patterns (see Shell

Patterns described later), separated by

.Dq \*(Ba

characters.

.Ss Grouping Commands Together

Commands may be grouped by writing either

.Pp

.Dl (list)

.Pp

or

.Pp

.Dl { list; }

.Pp

The first of these executes the commands in a subshell. Builtin commands

grouped into a (list) will not affect the current shell. The second form

does not fork another shell so is slightly more efficient. Grouping

commands together this way allows you to redirect their output as though

they were one program:

.Pp

.Bd -literal -offset indent

{ echo \*q hello \\c\*q ; echo \*q world" } > greeting

.Ed

.Pp

.Ss Functions

The syntax of a function definition is

.Pp

.Dl name ( ) command

.Pp

A function definition is an executable statement; when executed it

installs a function named name and returns an exit status of zero.  The

command is normally a list enclosed between

.Dq {

and

.Dq } .

.Pp

Variables may be declared to be local to a function by using a local

command.  This should appear as the first statement of a function, and the

syntax is

.Pp

.Dl local [ variable | - ] ...

.Pp

Local is implemented as a builtin command.

.Pp

When a variable is made local, it inherits the initial value and exported

and readonly flags from the variable with the same name in the surrounding

scope, if there is one.  Otherwise, the variable is initially unset.  The

shell uses dynamic scoping, so that if you make the variable x local to

function f, which then calls function g, references to the variable x made

inside g will refer to the variable x declared inside f, not to the global

variable named x.

.Pp

The only special parameter than can be made local is

.Dq - .

Making

.Dq -

local any shell options that are changed via the set command inside the

function to be restored to their original values when the function

returns.

.Pp

The syntax of the return command is

.Pp

.Dl return [ exitstatus ]

.Pp

It terminates the currently executing function.  Return is

implemented as a builtin command.

.Ss Variables and Parameters

The shell maintains a set of parameters.  A parameter denoted by a name is

called a variable. When starting up, the shell turns all the environment

variables into shell variables.  New variables can be set using the form

.Pp

.Dl name=value

.Pp

Variables set by the user must have a name consisting solely of

alphabetics, numerics, and underscores - the first of which must not be

numeric.  A parameter can also be denoted by a number or a special

character as explained below.

.Ss Positional Parameters

A positional parameter is a parameter denoted by a number (n > 0). The

shell sets these initially to the values of its command line arguments

that follow the name of the shell script. The

.Xr set 1

builtin can also be used to set or reset them.

.Ss Special Parameters

A special parameter is a parameter denoted by one of the following special

characters.  The value of the parameter is listed next to its character.

.Bl -tag -width thinhyphena

.It *

Expands to the positional parameters, starting from one.  When the

expansion occurs within a double-quoted string it expands to a single

field with the value of each parameter separated by the first character of

the

.Ev IFS

variable, or by a <space> if

.Ev IFS

is unset.

.It @

Expands to the positional parameters, starting from one.  When

the expansion occurs within double-quotes, each positional

parameter expands as a separate argument.

If there are no positional parameters, the

expansion of @ generates zero arguments, even when @ is

double-quoted.  What this basically means, for example, is

if $1 is

.Dq abc

and $2 is

.Dq def ghi ,

then

.Qq $@

expands to

the two arguments:

.Pp

.Sm off

.Dl \*q abc \*q \  \*q def\ ghi \*q

.Sm on

.It #

Expands to the number of positional parameters.

.It ?

Expands to the exit status of the most recent pipeline.

.It - (Hyphen.)

Expands to the current option flags (the single-letter

option names concatenated into a string) as specified on

invocation, by the set builtin command, or implicitly

by the shell.

.It $

Expands to the process ID of the invoked shell.  A subshell

retains the same value of $ as its parent.

.It !

Expands to the process ID of the most recent background

command executed from the current shell.  For a

pipeline, the process ID is that of the last command in the

pipeline.

.It 0 (Zero.)

Expands to the name of the shell or shell script.

.El

.Ss Word Expansions

This clause describes the various expansions that are performed on words.

Not all expansions are performed on every word, as explained later.

.Pp

Tilde expansions, parameter expansions, command substitutions, arithmetic

expansions, and quote removals that occur within a single word expand to a

single field.  It is only field splitting or pathname expansion that can

create multiple fields from a single word. The single exception to this

rule is the expansion of the special parameter @ within double-quotes, as

was described above.

.Pp

The order of word expansion is:

.Bl -enum

.It

Tilde Expansion, Parameter Expansion, Command Substitution,

Arithmetic Expansion (these all occur at the same time).

.It

Field Splitting is performed on fields

generated by step (1) unless the

.Ev IFS

variable is null.

.It

Pathname Expansion (unless set

.Fl f

is in effect).

.It

Quote Removal.

.El

.Pp

The $ character is used to introduce parameter expansion, command

substitution, or arithmetic evaluation.

.Ss Tilde Expansion (substituting a user's home directory)

A word beginning with an unquoted tilde character (~) is

subjected to tilde expansion.  All the characters up to

a slash (/) or the end of the word are treated as a username

and are replaced with the user's home directory.  If the

username is missing (as in

.Pa ~/foobar ) ,

the tilde is replaced with the value of the

.Va HOME

variable (the current user's home directory).

.Ss Parameter Expansion

The format for parameter expansion is as follows:

.Pp

.Dl ${expression}

.Pp

where expression consists of all characters until the matching

.Dq } .

Any

.Dq }

escaped by a backslash or within a quoted string, and characters in

embedded arithmetic expansions, command substitutions, and variable

expansions, are not examined in determining the matching

.Dq } .

.Pp

The simplest form for parameter expansion is:

.Pp

.Dl ${parameter}

.Pp

The value, if any, of parameter is substituted.

.Pp

The parameter name or symbol can be enclosed in braces, which are

optional except for positional parameters with more than one digit or

when parameter is followed by a character that could be interpreted as

part of the name.

If a parameter expansion occurs inside

double-quotes:

.Bl -enum

.It

Pathname expansion is not performed on the results of the

expansion.

.It

Field splitting is not performed on the results of the

expansion, with the exception of @.

.El

.Pp

In addition, a parameter expansion can be modified by using one of the

following formats.

.Bl -tag -width aaparameterwordaaaaa

.It ${parameter:-word}

Use Default Values.  If parameter is unset or null, the expansion of word

is substituted; otherwise, the value of parameter is substituted.

.It ${parameter:=word}

Assign Default Values.  If parameter is unset or null, the expansion of

word is assigned to parameter.  In all cases, the final value of parameter

is substituted.  Only variables, not positional parameters or special

parameters, can be assigned in this way.

.It ${parameter:?[word]}

Indicate Error if Null or Unset.  If parameter is unset or null, the

expansion of word (or a message indicating it is unset if word is omitted)

is written to standard error and the shell exits with a nonzero exit

status.  Otherwise, the value of parameter is substituted.  An interactive

shell need not exit.

.It ${parameter:+word}

Use Alternative Value.  If parameter is unset or null, null is

substituted; otherwise, the expansion of word is substituted.

.El

.Pp

In the parameter expansions shown previously, use of the colon in the

format results in a test for a parameter that is unset or null; omission

of the colon results in a test for a parameter that is only unset.

.Bl -tag -width aaparameterwordaaaaa

.It ${#parameter}

String Length.  The length in characters of

the value of parameter.

.El

.Pp

The following four varieties of parameter expansion provide for substring

processing.  In each case, pattern matching notation (see Shell Patterns),

rather than regular expression notation, is used to evaluate the patterns.

If parameter is * or @, the result of the expansion is unspecified.

Enclosing the full parameter expansion string in double-quotes does not

cause the following four varieties of pattern characters to be quoted,

whereas quoting characters within the braces has this effect.

.Bl -tag -width aaparameterwordaaaaa

.It ${parameter%word}

Remove Smallest Suffix Pattern.  The word is expanded to produce a

pattern.  The parameter expansion then results in parameter, with the

smallest portion of the suffix matched by the pattern deleted.

.It ${parameter%%word}

Remove Largest Suffix Pattern.  The word is expanded to produce a pattern.

The parameter expansion then results in parameter, with the largest

portion of the suffix matched by the pattern deleted.

.It ${parameter#word}

Remove Smallest Prefix Pattern.  The word is expanded to produce a

pattern.  The parameter expansion then results in parameter, with the

smallest portion of the prefix matched by the pattern deleted.

.It ${parameter##word}

Remove Largest Prefix Pattern.  The word is expanded to produce a pattern.

The parameter expansion then results in parameter, with the largest

portion of the prefix matched by the pattern deleted.

.El

.Ss Command Substitution

Command substitution allows the output of a command to be substituted in

place of the command name itself.  Command substitution occurs when

the command is enclosed as follows:

.Pp

.Dl $(command)

.Pp

or

.Po

.Dq backquoted

version

.Pc :

.Pp

.Dl `command`

.Pp

The shell expands the command substitution by executing command in a

subshell environment and replacing the command substitution with the

standard output of the command, removing sequences of one or more

<newline>s at the end of the substitution.  (Embedded <newline>s before

the end of the output are not removed; however, during field splitting,

they may be translated into <space>s, depending on the value of

.Ev IFS

and quoting that is in effect.)

.Ss Arithmetic Expansion

.Pp

Arithmetic expansion provides a mechanism for evaluating an arithmetic

expression and substituting its value. The format for arithmetic

expansion is as follows:

.Pp

.Dl $((expression))

.Pp

The expression is treated as if it were in double-quotes, except

that a double-quote inside the expression is not treated specially.  The

shell expands all tokens in the expression for parameter expansion,

command substitution, and quote removal.

.Pp

Next, the shell treats this as an arithmetic expression and

substitutes the value of the expression.

.Ss White Space Splitting (Field Splitting)

After parameter expansion, command substitution, and

arithmetic expansion the shell scans the results of

expansions and substitutions that did not occur in double-quotes for

field splitting and multiple fields can result.

.Pp

The shell treats each character of the

.Ev IFS

as a delimiter and use the delimiters to split the results of parameter

expansion and command substitution into fields.

.Ss Pathname Expansion (File Name Generation)

Unless the

.Fl f

flag is set, file name generation is performed after word splitting is

complete.  Each word is viewed as a series of patterns, separated by

slashes.  The process of expansion replaces the word with the names of all

existing files whose names can be formed by replacing each pattern with a

string that matches the specified pattern. There are two restrictions on

this: first, a pattern cannot match a string containing a slash, and

second, a pattern cannot match a string starting with a period unless the

first character of the pattern is a period. The next section describes the

patterns used for both Pathname Expansion and the

.Xr case 1

command.

.Ss Shell Patterns

A pattern consists of normal characters, which match themselves,

and meta-characters.   The meta-characters are

.Dq ! ,

.Dq * ,

.Dq ? ,

and

.Dq [ .

These characters lose their special meanings if they are quoted.  When

command or variable substitution is performed and the dollar sign or back

quotes are not double quoted, the value of the variable or the output of

the command is scanned for these characters and they are turned into

meta-characters.

.Pp

An asterisk

.Pq Dq *

matches any string of characters.  A question mark matches any single

character. A left bracket

.Pq Dq \&[

introduces a character class.  The end of

the character class is indicated by a

.Pq Dq \&] ;

if the

.Dq \&]

is missing then the

.Dq \&[

matches a

.Dq \&[

rather than introducing a character class.  A character class matches any

of the characters between the square brackets.  A range of characters may

be specified using a minus sign. The character class may be complemented

by making an exclamation point the first character of the character class.

.Pp

To include a

.Dq \&]

in a character class, make it the first character listed (after the

.Dq \&! ,

if any).  To include a minus sign, make it the first or last character listed

.Ss Builtins

.Pp

This section lists the builtin commands which are builtin because they

need to perform some operation that can't be performed by a separate

process. In addition to these, there are several other commands that may

be builtin for efficiency (e.g.

.Xr printf 1 ,

.Xr echo 1 ,

.Xr test 1 ,

etc).

.Bl -tag -width 5n

.It :

A null command that returns a 0 (true) exit value.

.It \&. file

The commands in the specified file are read and executed by the shell.

.It alias Op Ar name Ns Op Ar "=string ..."

If

.Ar name=string

is specified, the shell defines the alias

.Ar name

with value

.Ar string .

If just

.Ar name

is specified, the value of the alias

.Ar name

is printed. With no arguments, the

.Ic alias

builtin prints the

names and values of all defined aliases (see

.Ic unalias ) .

.It bg [ Ar job ] ...

Continue the specified jobs (or the current job if no

jobs are given) in the background.

.It command Ar command Ar arg...

Execute the specified builtin command.  (This is useful when you

have a shell function with the same name as a builtin command.)

.It cd Op Ar directory

Switch to the specified directory (default

.Ev $HOME ) .

If an entry for

.Ev CDPATH

appears in the environment of the

.Ic cd

command or the shell variable

.Ev CDPATH

is set and the directory name does not begin with a slash, then the

directories listed in

.Ev CDPATH

will be searched for the specified directory.  The format of

.Ev CDPATH

is the same as that of

.Ev PATH .

In an interactive shell, the

.Ic cd

command will print out the name of the

directory that it actually switched to if this is different from the name

that the user gave.  These may be different either because the

.Ev CDPATH

mechanism was used or because a symbolic link was crossed.

.It eval Ar string...

Concatenate all the arguments with spaces.  Then re-parse and execute

the command.

.It exec Op Ar command arg...

Unless command is omitted, the shell process is replaced with the

specified program (which must be a real program, not a shell builtin or

function).  Any redirections on the

.Ic exec

command are marked as permanent, so that they are not undone when the

.Ic exec

command finishes.

.It exit Op Ar exitstatus

Terminate the shell process.  If

.Ar exitstatus

is given it is used as the exit status of the shell; otherwise the

exit status of the preceding command is used.

.It export Ar name...

.It export Fl p

The specified names are exported so that they will appear in the

environment of subsequent commands. The only way to un-export a variable

is to unset it. The shell allows the value of a variable to be set at the

same time it is exported by writing

.Pp

.Dl export name=value

.Pp

With no arguments the export command lists the names of all exported variables.

With the

.Fl p

option specified the output will be formatted suitably for non-interactive use.

.It Xo fc Op Fl e Ar editor

.Op Ar first Op Ar last

.Xc

.It Xo fc Fl l

.Op Fl nr

.Op Ar first Op Ar last

.Xc

.It Xo fc Fl s Op Ar old=new

.Op Ar first

.Xc

The

.Ic fc

builtin lists, or edits and re-executes, commands previously entered

to an interactive shell.

.Bl -tag -width 5n

.It Fl e No editor

Use the editor named by editor to edit the commands.  The

editor string is a command name, subject to search via the

.Ev PATH

variable.  The value in the

.Ev FCEDIT

variable is used as a default when

.Fl e

is not specified.  If

.Ev FCEDIT

is null or unset, the value of the

.Ev EDITOR

variable is used.  If

.Ev EDITOR

is null or unset,

.Xr ed 1

is used as the editor.

.It Fl l No (ell)

List the commands rather than invoking an editor on them.  The commands

are written in the sequence indicated by the first and last operands, as

affected by

.Fl r ,

with each command preceded by the command number.

.It Fl n

Suppress command numbers when listing with -l.

.It Fl r

Reverse the order of the commands listed (with

.Fl l )

or edited (with neither