freebsd/bin/sh/sh.1

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.\" Kenneth Almquist.
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.\"	from: @(#)sh.1	8.6 (Berkeley) 5/4/95
.\" $FreeBSD$
.\"
.Dd May 5, 1995
.Dt SH 1
.Os
.Sh NAME
.Nm sh
.Nd command interpreter (shell)
.Sh SYNOPSIS
.Nm
.Op Fl /+abCEefIimnpsTuVvx
.Op Fl /+o Ar longname
.Op Fl c Ar string
.Op Ar arg ...
.Sh DESCRIPTION
The
.Nm
utility is the standard command interpreter for the system.
The current version of
.Nm
is in the process of being changed to
conform with the
.St -p1003.2
specification 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 like
.Xr pdksh 1 .
Only features
designated by POSIX, plus a few Berkeley extensions, are being
incorporated into this shell.
This man page is not intended to be a tutorial nor 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 started 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 builtin 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,
which can be executed directly by the shell.
.Ss Invocation
.\"
.\" XXX This next sentence is incredibly confusing.
.\"
If no arguments are present and if the standard input of the shell
is connected to a terminal
(or if the
.Fl i
option is set),
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
.Pq Li - ,
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 then
.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 then 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.
The user can set the
.Ev ENV
variable to some file by placing the following line in the file
.Pa .profile
in the home directory,
substituting for
.Pa .shinit
the filename desired:
.Pp
.Dl ENV=$HOME/.shinit; export ENV
.Pp
The first non-option argument specified on the command line
will be treated 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.
.Pp
Unlike older versions of
.Nm
the
.Ev ENV
script is only sourced on invocation of interactive shells.  This
closes a well-known, and sometimes easily exploitable security
hole related to poorly thought out
.Ev ENV
scripts.
.Ss Argument List Processing
All of the single letter options to
.Nm
have a corresponding long name,
with the exception of
.Fl c
and
.Fl /+o .
These long names are provided next to the single letter options
in the descriptions below.
The long name for an option may be specified as an argument to the
.Fl /+o
option of
.Xr sh 1 .
Once the shell is running,
the long name for an option may be specified as an argument to the
.Fl /+o
option of the
.Ic set
builtin command
(described later in the section called
.Sx Builtin Commands ) .
Introducing an option with a dash
.Pq Li -
enables the option,
while using a plus
.Pq Li +
disables the option.
A
.Dq Li --
or plain
.Dq Li -
will stop option processing and will force the remaining
words on the command line to be treated as arguments.
The
.Fl /+o
and
.Fl c
options do not have long names.
They take arguments and are described after the single letter options.
.Bl -tag -width indent
.It Fl a Li allexport
Flag variables for export when assignments are made to them.
.It Fl b Li notify
Enable asynchronous notification of background job
completion.
(UNIMPLEMENTED)
.It Fl C Li noclobber
Do not overwrite existing files with
.Dq Li > .
.It Fl E Li emacs
Enable the builtin
.Xr emacs 1
command line editor (disables the
.Fl V
option if it has been set).
.It Fl e Li errexit
Exit immediately if any untested command fails in non-interactive mode.
The exit status of a command is considered to be
explicitly tested if the command is used to control
an if, elif, while, or until; or if the command is the left
hand operand of an
.Dq Li &&
or
.Dq Li ||
operator.
.It Fl f Li noglob
Disable pathname expansion.
.It Fl I Li ignoreeof
Ignore
.Dv EOF Ns ' Ns s
from input when in interactive mode.
.It Fl i Li interactive
Force the shell to behave interactively.
.It Fl m Li monitor
Turn on job control (set automatically when interactive).
.It Fl n Li noexec
If not interactive, read commands but do not
execute them.  This is useful for checking the
syntax of shell scripts.
.It Fl p Li privileged
Turn on privileged mode.  This mode is enabled on startup
if either the effective user or group id is not equal to the
real user or group id.  Turning this mode off sets the
effective user and group ids to the real user and group ids.
When this mode is enabled for interactive shells, the file
.Pa /etc/suid_profile
is sourced instead of
.Pa ~/.profile
after
.Pa /etc/profile
is sourced, and the contents of the
.Ev ENV
variable are ignored.
.It Fl s Li 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. when set with the
.Ic set
command).
.It Fl T Li asynctraps
When waiting for a child, execute traps immediately.
If this option is not set,
traps are executed after the child exits,
as specified in
.St -p1003.2
This nonstandard option is useful for putting guarding shells around
children that block signals.  The surrounding shell may kill the child
or it may just return control to the tty and leave the child alone,
like this:
.Bd -literal -offset indent
sh -T -c "trap 'exit 1' 2 ; some-blocking-program"
.Ed
.Pp
.It Fl u Li 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.
.It Fl V Li vi
Enable the builtin
.Xr vi 1
command line editor (disables
.Fl E
if it has been set).
.It Fl v Li verbose
The shell writes its input to standard error
as it is read.  Useful for debugging.
.It Fl x Li xtrace
Write each command
(preceded by
.Dq Li +\  )
to standard error before it is executed.
Useful for debugging.
.El
.Pp
The
.Fl c
option may be used to pass its string argument to the shell
to be interpreted as input.
Keep in mind that this option only accepts a single string as its
argument, hence multi-word strings must be quoted.
.Pp
The
.Fl /+o
option takes as its only argument the long name of an option
to be enabled or disabled.
For example, the following two invocations of
.Nm
both enable the builtin
.Xr emacs 1
command line editor:
.Bd -literal -offset indent
set -E
set -o emacs
.Ed
.Pp
If used without an argument, the
.Fl o
option displays the current option settings in a human-readable format.
If
.Cm +o
is used without an argument, the current option settings are output
in a format suitable for re-input into the shell.
.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 called
.Dq operators ,
which are special to the shell.
There are two types of operators: control operators and
redirection operators (their meaning is discussed later).
The following is a list of valid operators:
.Bl -tag -width indent
.It Control operators:
.Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
.It Li & Ta Li && Ta Li ( Ta Li ) Ta Li \en
.It Li ;; Ta Li ; Ta Li | Ta Li ||
.El
.It Redirection operators:
.Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
.It Li < Ta Li > Ta Li << Ta Li >> Ta Li <>
.It Li <& Ta Li >& Ta Li <<- Ta Li >|
.El
.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.
.Bl -tag -width indent
.It 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).
.It Double Quotes
Enclosing characters within double quotes preserves the literal
meaning of all characters except dollarsign
.Pq Li $ ,
backquote
.Pq Li ` ,
and backslash
.Pq Li \e .
The backslash inside double quotes is historically weird.
It remains literal unless it precedes the following characters,
which it serves to quote:
.Bl -column "XXX" "XXX" "XXX" "XXX" "XXX" -offset center -compact
.It Li $ Ta Li ` Ta Li \&" Ta Li \e\  Ta Li \en
.El
.It Backslash
A backslash preserves the literal meaning of the following
character, with the exception of the newline character
.Pq Li \en .
A backslash preceding a newline is treated as a line continuation.
.El
.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 "doneXX" "elifXX" "elseXX" "untilXX" "whileX" -offset center
.It Li \&! Ta { Ta } Ta Ic case Ta Ic do
.It Ic done Ta Ic elif Ta Ic else Ta Ic esac Ta Ic fi
.It Ic for Ta Ic if Ta Ic then Ta Ic until Ta Ic while
.El
.Ss Aliases
An alias is a name and corresponding value set using the
.Ic alias
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 Li lf
with the value
.Dq Li ls -F ,
then the input
.Bd -literal -offset indent
lf foobar
.Ed
.Pp
would become
.Bd -literal -offset indent
ls -F foobar
.Ed
.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
.St -p1003.2
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
.It
Leading words of the form
.Dq Li 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
.Sx Word 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 Li name=value
variable assignments recognized in 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] redir-op file
.Pp
The
.Ql redir-op
is one of the redirection operators mentioned
previously.  The following gives some examples of how these
operators can be used.
Note that stdin and stdout are commonly used abbreviations
for standard input and standard output respectively.
.Bl -tag -width "1234567890XX" -offset indent
.It Li [n]> file
redirect stdout (or file descriptor n) to file
.It Li [n]>| file
same as above, but override the
.Fl C
option
.It Li [n]>> file
append stdout (or file descriptor n) to file
.It Li [n]< file
redirect stdin (or file descriptor n) from file
.It Li [n]<> file
redirect stdin (or file descriptor n) to and from file
.It Li [n1]<&n2
duplicate stdin (or file descriptor n1) from file descriptor n2
.It Li [n]<&-
close stdin (or file descriptor n)
.It Li [n1]>&n2
duplicate stdout (or file descriptor n1) to file descriptor n2
.It Li [n]>&-
close stdout (or file descriptor n)
.El
.Pp
The following redirection is often called a
.Dq here-document .
.Bd -literal -offset indent
[n]<< delimiter
	here-doc-text
	...
delimiter
.Ed
.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
.Sx Word Expansions ) .
If the operator is
.Dq Li <<-
instead of
.Dq Li << ,
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.
The command is searched for (by name) in that order.
The three types of commands are all 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 builtin commands are executed internally to the shell, without
spawning a new process.
.Pp
Otherwise, if the command name does not match a function
or builtin command, 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
.Qq magic number
whose
.Tn ASCII
representation is
.Qq #! ,
resulting in an
.Er ENOEXEC
return value from
.Xr execve 2 )
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
.Qq shell procedure .
.Ss Path Search
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.
.Pp
If a command is terminated by a signal, its exit status is 128 plus
the signal number.  Signal numbers are defined in the header file
.Aq Pa sys/signal.h .
.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 -item -offset indent
.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
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
.Ql command1
to the standard input of
.Ql command2 .
.Pp
A
.Dq Li \&;
or newline terminator causes the preceding
AND-OR-list
(described below in the section called
.Sx Short-Circuit List Operators )
to be executed sequentially;
an
.Dq Li &
causes asynchronous execution of the preceding AND-OR-list.
.Pp
Note that unlike some other shells,
.Nm
executes each process in the pipeline as a child of the
.Nm
process.
Shell builtin commands are the exception to this rule.
They are executed in the current shell, although they do not affect its
environment when used in pipelines.
.Ss Background Commands (&)
If a command is terminated by the control operator ampersand
.Pq Li & ,
the shell executes the command asynchronously;
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:
.Bd -literal -offset indent
command1 & [command2 & ...]
.Ed
.Pp
If the shell is not interactive, the standard input of an
asynchronous command is set to /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 Li &&
and
.Dq Li ||
are AND-OR list operators.
.Dq Li &&
executes the first command, and then executes the second command
if the exit status of the first command is zero.
.Dq Li ||
is similar, but executes the second command if the exit
status of the first command is nonzero.
.Dq Li &&
and
.Dq Li ||
both have the same priority.
.Ss Flow-Control Constructs (if, while, for, case)
The syntax of the
.Ic if
command is:
.\"
.\" XXX Use .Dl to work around broken handling of .Ic inside .Bd and .Ed .
.\"
.Dl Ic if Ar list
.Dl Ic then Ar list
.Dl [ Ic elif Ar list
.Dl Ic then Ar list ] ...
.Dl [ Ic else Ar list ]
.Dl Ic fi
.Pp
The syntax of the
.Ic while
command is:
.Dl Ic while Ar list
.Dl Ic do Ar list
.Dl Ic done
.Pp
The two lists are executed repeatedly while the exit status of the
first list is zero.
The
.Ic until
command is similar, but has the word
.Ic until
in place of
.Ic while ,
which causes it to
repeat until the exit status of the first list is zero.
.Pp
The syntax of the
.Ic for
command is:
.Dl Ic for Ar variable Ic in Ar word ...
.Dl Ic do Ar list
.Dl Ic done
.Pp
The words are expanded, and then the list is executed
repeatedly with the variable set to each word in turn.
The
.Ic do
and
.Ic done
commands may be replaced with
.Dq Li {
and
.Dq Li } .
.Pp
The syntax of the
.Ic break
and
.Ic continue
commands is:
.Dl Ic break Op Ar num
.Dl Ic continue Op Ar num
.Pp
The
.Ic break
command terminates the
.Ar num
innermost
.Ic for
or
.Ic while
loops.
The
.Ic continue
command continues with the next iteration of the innermost loop.
These are implemented as builtin commands.
.Pp
The syntax of the
.Ic case
command is
.Dl Ic case Ar word Ic in
.Dl pattern) list ;;
.Dl ...
.Dl Ic esac
.Pp
The pattern can actually be one or more patterns
(see
.Sx Shell Patterns
described later),
separated by
.Dq Li \&|
characters.
.Ss Grouping Commands Together
Commands may be grouped by writing either
.Bd -literal -offset indent
(list)
.Ed
.Pp
or
.Bd -literal -offset indent
{ list; }
.Ed
.Pp
The first form executes the commands in a subshell.
Note that builtin commands thus executed do not affect the current shell.
The second form does not fork another shell,
so it is slightly more efficient.
Grouping commands together this way allows the user to
redirect their output as though they were one program:
.Bd -literal -offset indent
{ echo -n "hello"; echo " world"; } > greeting
.Ed
.Ss Functions
The syntax of a function definition is
.Bd -literal -offset indent
name ( ) command
.Ed
.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 Li {
and
.Dq Li } .
.Pp
Variables may be declared to be local to a function by
using the
.Ic local
command.
This should appear as the first statement of a function,
and the syntax is:
.Bd -ragged -offset indent
.Ic local
.Op Ar variable ...
.Op Ar -
.Ed
.Pp
The
.Ic local
command 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 the variable
.Em x
is made local to function
.Em f ,
which then calls function
.Em g ,
references to the variable
.Em x
made inside
.Em g
will refer to the variable
.Em x
declared inside
.Em f ,
not to the global variable named
.Em x .
.Pp
The only special parameter than can be made local is
.Dq Li - .
Making
.Dq Li -
local causes 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
.Ic return
command is
.Bd -ragged -offset indent
.Ic return
.Op Ar exitstatus
.Ed
.Pp
It terminates the currently executing function.
The
.Ic return
command 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
.Bd -literal -offset indent
name=value
.Ed
.Pp
Variables set by the user must have a name consisting solely
of alphabetics, numerics, and underscores.
The first letter of a variable name 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 greater than zero.
The shell sets these initially to the values of its command line
arguments that follow the name of the shell script.  The
.Ic set
builtin command 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 -hang
.It Li *
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
.Aq space
if
.Ev IFS
is unset.
.It Li @
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
.Li @
generates zero arguments, even when
.Li @
is double-quoted.  What this basically means, for example, is
if $1 is
.Dq abc
and $2 is
.Dq def ghi ,
then
.Qq Li $@
expands to
the two arguments:
.Bd -literal -offset indent
"abc"   "def ghi"
.Ed
.It Li #
Expands to the number of positional parameters.
.It Li \&?
Expands to the exit status of the most recent pipeline.
.It Li -
(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 Li $
Expands to the process ID of the invoked shell.  A subshell
retains the same value of $ as its parent.
.It Li \&!
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 Li 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
.Li @
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 the
.Fl f
option is in effect).
.It
Quote Removal.
.El
.Pp
The
.Dq Li $
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
.Pq Li ~
is
subjected to tilde expansion.
All the characters up to a slash
.Pq Li /
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 ~/foobar), the tilde is replaced
with the value of the HOME variable (the current user's
home directory).
.Ss Parameter Expansion
The format for parameter expansion is as follows:
.Bd -literal -offset indent
${expression}
.Ed
.Pp
where expression consists of all characters until the matching
.Dq Li } .
Any
.Dq Li }
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 Li } .
.Pp
The simplest form for parameter expansion is:
.Bd -literal -offset indent
${parameter}
.Ed
.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 the special parameter
.Li @ .
.El
.Pp
In addition, a parameter expansion can be modified by using one of the
following formats.
.Bl -tag -width indent
.It Li ${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 Li ${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 Li ${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 Li ${parameter:+word}
Use Alternate Value.  If parameter is unset
or null, null is substituted;
otherwise, the expansion of word is
substituted.
.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.
.It Li ${#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
.Sx Shell Patterns ) ,
rather than regular expression notation,
is used to evaluate the patterns.
If parameter is one of the special parameters
.Li *
or
.Li @ ,
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 indent
.It Li ${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 Li ${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 Li ${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 Li ${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:
.Bd -literal -offset indent
$(command)
.Ed
.Pp
or the backquoted version:
.Bd -literal -offset indent
`command`
.Ed
.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 newlines at the end of the substitution.
Embedded newlines before the end of the output are not removed;
however, during field splitting, they may be translated into spaces
depending on the value of
.Ev IFS
and the quoting that is in effect.
.Ss Arithmetic Expansion
Arithmetic expansion provides a mechanism for evaluating an arithmetic
expression and substituting its value.
The format for arithmetic expansion is as follows:
.Bd -literal -offset indent
$((expression))
.Ed
.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 uses
the delimiters to split the results of parameter expansion and command
substitution into fields.
.Ss Pathname Expansion (File Name Generation)
Unless the
.Fl f
option 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
.Ic case
command.
.Ss Shell Patterns
A pattern consists of normal characters, which match themselves,
and meta-characters.
The meta-characters are
.Dq Li \&! ,
.Dq Li * ,
.Dq Li \&? ,
and
.Dq Li [ .
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 Li *
matches any string of characters.
A question mark
.Pq Li \&?
matches any single character.
A left bracket
.Pq Li [
introduces a character class.
The end of the character class is indicated by a
.Dq Li \&] ;
if the
.Dq Li \&]
is missing then the
.Dq Li [
matches a
.Dq Li [
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
.Pq Li !\&
the first character of the character class.
.Pp
To include a
.Dq Li \&]
in a character class, make it the first character listed
(after the
.Dq Li \&! ,
if any).
To include a
.Dq Li - ,
make it the first or last character listed.
.Ss Builtin Commands
This section lists the commands which
are builtin because they need to perform some operation
that cannot be performed by a separate process.  In addition to
these, a builtin version of the
.Xr test 1
command is provided for efficiency.
.Bl -tag -width indent
.It Ic \&:
A null command that returns a 0 (true) exit value.
.It Ic \&. Ar file
The commands in the specified file are read and executed by the shell.
If
.Ar file
contains any
.Dq /
characters, it is used as is.  Otherwise, the shell searches the
.Ev PATH
for the file.  If it is not found in the
.Ev PATH ,
it is sought in the current working directory.
.It Ic alias Op Ar name ...
.It Ic alias Op Ar name Ns = Ns Ar string ...
If
.Ar name Ns = Ns Ar 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 command prints the names and values of all defined aliases
(see
.Ic unalias ) .
Alias values are written with appropriate quoting so that they are
suitable for reinput to the shell.
.It Ic bg Op Ar job ...
Continue the specified jobs
(or the current job if no jobs are given)
in the background.
.It Ic command Ar cmd Op Ar arg ...
Execute the specified builtin command,
.Ar cmd .
This is useful when the user wishes to override a shell function
with the same name as a builtin command.
.It Ic cd Oo Fl LP Oc Op Ar directory
Switch to the specified
.Ar directory ,
or to the directory specified in the
.Ev HOME
environment variable if no
.Ar directory
is specified.
If
.Ar directory
is not found as a subdirectory of the current directory
(and does not begin with
.Pa / , ./ ,
or
.Pa ../ ) ,
then the directories listed in the
.Ev CDPATH
variable will be
searched for the specified
.Ar directory .
The format of
.Ar 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.
.Pp
If the
.Fl P
option is specified,
.Pa ..
is handled physically and symbolic links are resolved before
.Pa ..
components are processed.
If the
.Fl L
option is specified,
.Pa ..
is handled logically.
This is the default.
.It Ic chdir
A synonym for the
.Ic cd
builtin command.
.It Ic echo Oo Fl en Oc Ar string
Print
.Ar string
to the standard output with a newline appended.
.Bl -tag -width indent
.It Fl n
Suppress the output of the trailing newline.
.It Fl e
Process C-style backslash escape sequences.
.Ic echo
understands the following character escapes:
.Bl -tag -width indent
.It \ea
Alert (ring the terminal bell)
.It \eb
Backspace
.It \ec
Suppress the trailing newline (this has the side-effect of truncating the
line if it is not the last character)
.It \ee
The ESC character (ASCII 0x1b)
.It \ef
Formfeed
.It \en
Newline
.It \er
Carriage return
.It \et
Horizontal tab
.It \ev
Vertical tab
.It \e\e
Literal backslash
.It \e0nnn
(Zero) The character whose octal value is nnn
.El
.Pp
If
.Ar string
is not enclosed in quotes then the backslash itself must be escaped
with a backslash to protect it from the shell. For example
.Bd -literal -offset indent
$ echo -e "a\evb"
a
 b
$ echo -e a\e\evb
a
 b
$ echo -e "a\e\eb"
a\eb
$ echo -e a\e\e\e\eb
a\eb
.Ed
.El
.It Ic eval Ar string ...
Concatenate all the arguments with spaces.
Then re-parse and execute the command.
.It Ic exec Op Ar command Op arg ...
Unless
.Ar command
is omitted,
the shell process is replaced with the specified program
(which must be a real program, not a shell builtin command 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 Ic 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 Ic export Ar name ...
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
.Ic unset
it.
The shell allows the value of a variable to be set
at the same time as it is exported by writing
.Bd -literal -offset indent
export name=value
.Ed
.Pp
With no arguments the export command lists the names
of all exported variables.
.It Ic fc Oo Fl e Ar editor Oc Op Ar first Op Ar last
.It Ic fc Fl l Oo Fl nr Oc Op Ar first Op Ar last
.It Ic fc Fl s Oo Ar old Ns = Ns Ar new Oc Op Ar first
The
.Ic fc
builtin command lists, or edits and re-executes,
commands previously entered to an interactive shell.
.Bl -tag -width indent
.It Fl e Ar editor
Use the editor named by
.Ar 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
.Fl l .
.It Fl r
Reverse the order of the commands listed
(with
.Fl l )
or edited
(with neither
.Fl l
nor
.Fl s ) .
.It Fl s
Re-execute the command without invoking an editor.
.It Ar first
.It Ar last
Select the commands to list or edit.
The number of previous commands that can be accessed
are determined by the value of the
.Ev HISTSIZE
variable.
The value of
.Ar first
or
.Ar last
or both are one of the following:
.Bl -tag -width indent
.It Ar [+]num
A positive number representing a command number;
command numbers can be displayed with the
.Fl l
option.
.It Ar -num
A negative decimal number representing the
command that was executed
.Ar num
of
commands previously.
For example, -1 is the immediately previous command.
.It Ar string
A string indicating the most recently entered command
that begins with that string.
If the
.Ar old=new
operand is not also specified with
.Fl s ,
the string form of the first operand cannot contain an embedded equal sign.
.El
.El
.Pp
The following environment variables affect the execution of
.Ic fc :
.Bl -tag -width indent
.It Ev FCEDIT
Name of the editor to use.
.It Ev HISTSIZE
The number of previous commands that are accessible.
.El
.It Ic fg Op Ar job
Move the specified
.Ar job
or the current job to the foreground.
.It Ic getopts Ar optstring Ar var
The POSIX
.Ic getopts
command.
The
.Ic getopts
command deprecates the older
.Xr getopt 1
command.
The first argument should be a series of letters, each possibly
followed by a colon which indicates that the option takes an argument.
The specified variable is set to the parsed option.  The index of
the next argument is placed into the shell variable
.Ev OPTIND .
If an option takes an argument, it is placed into the shell variable
.Ev OPTARG .
If an invalid option is encountered,
.Ev var
is set to
.Dq Li \&? .
It returns a false value (1) when it encounters the end of the options.
.It Ic hash Oo Fl rv Oc Op Ar command ...
The shell maintains a hash table which remembers the locations of commands.
With no arguments whatsoever, the
.Ic hash
command prints out the contents of this table.
Entries which have not been looked at since the last
.Ic cd
command are marked with an asterisk;
it is possible for these entries to be invalid.
.Pp
With arguments, the
.Ic hash
command removes each specified
.Ar command
from the hash table (unless they are functions) and then locates it.
With the
.Fl v
option,
.Ic hash
prints the locations of the commands as it finds them.
The
.Fl r
option causes the
.Ic hash
command to delete all the entries in the hash table except for functions.
.It Ic jobid Op Ar job
Print the process id's of the processes in the specified
.Ar job .
If the
.Ar job
argument is omitted, use the current job.
.It Xo
.Ic jobs
.Op Fl ls
.Op Ar job ...
.Xc
Print information about the specified jobs, or all jobs if no
.Ar job
argument is given.
The information printed includes job ID, status and command name.
.Pp
If the
.Fl l
option is specified, the PID of each job is also printed.
If the
.Fl s
option is specified, only the PID's of the jobs are printed, one per line.
.It Ic pwd Op Fl LP
Print the path of the current directory.  The builtin command may
differ from the program of the same name because the
builtin command remembers what the current directory
is rather than recomputing it each time.  This makes
it faster.  However, if the current directory is
renamed,
the builtin version of
.Xr pwd 1
will continue to print the old name for the directory.
.Pp
If the
.Fl P
option is specified, symbolic links are resolved.
If the
.Fl L
option is specified, the shell's notion of the current directory
is printed (symbolic links are not resolved).
This is the default.
.It Ic read Oo Fl p Ar prompt Oc Oo Fl t Ar timeout Oc Oo Fl er Oc Ar variable ...
The
.Ar prompt
is printed if the
.Fl p
option is specified
and the standard input is a terminal.  Then a line is
read from the standard input.  The trailing newline
is deleted from the line and the line is split as
described in the section on
.Sx White Space Splitting (Field Splitting)
above, and
the pieces are assigned to the variables in order.
If there are more pieces than variables, the remaining
pieces (along with the characters in
.Ev IFS
that separated them)
are assigned to the last variable.
If there are more variables than pieces, the remaining
variables are assigned the null string.
.Pp
Backslashes are treated specially, unless the
.Fl r
option is
specified.  If a backslash is followed by
a newline, the backslash and the newline will be
deleted.  If a backslash is followed by any other
character, the backslash will be deleted and the following
character will be treated as though it were not in
.Ev IFS ,
even if it is.
.Pp
If the
.Fl t
option is specified and the
.Ar timeout
elapses before any input is supplied,
the
.Ic read
command will return without assigning any values.
The
.Ar timeout
value may optionally be followed by one of
.Dq s ,
.Dq m
or
.Dq h
to explicitly specify seconds, minutes or hours.
If none is supplied,
.Dq s
is assumed.
.Pp
The
.Fl e
option exists only for backward compatibility with older scripts.
.It Ic readonly Ar name ...
Each specified
.Ar name
is marked as read only,
so that it cannot be subsequently modified or unset.
The shell allows the value of a variable to be set
at the same time as it is marked read only
by using the following form:
.Bd -literal -offset indent
readonly name=value
.Ed
.Pp
With no arguments the
.Ic readonly
command lists the names of all read only variables.
.It Ic set Oo Fl /+abCEefIimnpTuVvx Oc Oo Fl /+o Ar longname Oc Oo
.Fl c Ar string Oc Op Fl - Ar arg ...
The
.Ic set
command performs three different functions:
.Bl -item
.It
With no arguments, it lists the values of all shell variables.
.It
If options are given,
either in short form or using the long
.Dq Fl /+o Ar longname
form,
it sets or clears the specified options as described in the section called
.Sx Argument List Processing .
.It
If the
.Dq Fl -
option is specified,
.Ic set
will replace the shell's positional parameters with the subsequent
arguments.
If no arguments follow the
.Dq Fl -
option,
all the positional parameters will be cleared,
which is equivalent to executing the command
.Dq Li shift $# .
The
.Dq Fl -
flag may be ommitted when specifying arguments to be used
as positional replacement parameters.
This is not recommended,
because the first argument may begin with a dash
.Pq Li -
or a plus
.Pq Li + ,
which the
.Ic set
command will interpret as a request to enable or disable options.
.El
.It Ic setvar Ar variable Ar value
Assigns the specified
.Ar value
to the specified
.Ar variable .
.Ic Setvar
is intended to be used in functions that
assign values to variables whose names are passed as parameters.
In general it is better to write
.Bd -literal -offset indent
variable=value
.Ed
rather than using
.Ic setvar .
.It Ic shift Op Ar n
Shift the positional parameters
.Ar n
times, or once if
.Ar n
is not specified.
A shift sets the value of $1 to the value of $2,
the value of $2 to the value of $3, and so on,
decreasing the value of $# by one.
If there are zero positional parameters, shifting does not do anything.
.It Ic trap Oo Ar action Oc Ar signal ...
Cause the shell to parse and execute
.Ar action
when any specified
.Ar signal
is received.
The signals are specified by signal number.
The
.Ar action
may be null or omitted;
the former causes the specified signal to be ignored
and the latter causes the default action to be taken.
When the shell forks off a subshell,
it resets trapped (but not ignored) signals to the default action.
The
.Ic trap
command has no effect on signals that were ignored on entry to the shell.
.It Ic type Op Ar name ...
Interpret each
.Ar name
as a command and print the resolution of the command search.
Possible resolutions are:
shell keyword, alias, shell builtin command, command, tracked alias
and not found.
For aliases the alias expansion is printed;
for commands and tracked aliases
the complete pathname of the command is printed.
.It Ic ulimit Oo Fl HSabcdflmnust Oc Op Ar limit
Set or display resource limits (see
.Xr getrlimit 2 ) .
If
.Ar limit
is specified, the named resource will be set;
otherwise the current resource value will be displayed.
.Pp
If
.Fl H
is specified, the hard limits will be set or displayed.
While everybody is allowed to reduce a hard limit,
only the superuser can increase it.
The
.Fl S
option
specifies the soft limits instead.  When displaying limits,
only one of
.Fl S
or
.Fl H
can be given.
The default is to display the soft limits,
and to set both the hard and the soft limits.
.Pp
Option
.Fl a
causes the
.Ic ulimit
command to display all resources.
The parameter
.Ar limit
is not acceptable in this mode.
.Pp
The remaining options specify which resource value is to be
displayed or modified.
They are mutually exclusive.
.Bl -tag -width indent
.It Fl b Ar sbsize
The maximum size of socket buffer usage, in bytes.
.It Fl c Ar coredumpsize
The maximal size of core dump files, in 512-byte blocks.
.It Fl d Ar datasize
The maximal size of the data segment of a process, in kilobytes.
.It Fl f Ar filesize
The maximal size of a file, in 512-byte blocks.
.It Fl l Ar lockedmem
The maximal size of memory that can be locked by a process, in
kilobytes.
.It Fl m Ar memoryuse
The maximal resident set size of a process, in kilobytes.
.It Fl n Ar nofiles
The maximal number of descriptors that could be opened by a process.
.It Fl s Ar stacksize
The maximal size of the stack segment, in kilobytes.
.It Fl t Ar time
The maximal amount of CPU time to be used by each process, in seconds.
.It Fl u Ar userproc
The maximal number of simultaneous processes for this user ID.
.El
.It Ic umask Op Ar mask
Set the file creation mask (see
.Xr umask 2 )
to the octal value specified by
.Ar mask .
If the argument is omitted, the current mask value is printed.
.It Ic unalias Oo Fl a Oc Op Ar name
If
.Ar name
is specified, the shell removes that alias.
If
.Fl a
is specified, all aliases are removed.
.It Ic unset Ar name ...
The specified variables and functions are unset and unexported.
If a given
.Ar name
corresponds to both a variable and a function,
both the variable and the function are unset.
.It Ic wait Op Ar job
Wait for the specified
.Ar job
to complete and return the exit status of the last process in the
.Ar job .
If the argument is omitted, wait for all jobs to complete
and return an exit status of zero.
.El
.Ss Commandline Editing
When
.Nm
is being used interactively from a terminal, the current command
and the command history
(see
.Ic fc
in
.Sx Builtin Commands )
can be edited using vi-mode command line editing.
This mode uses commands similar
to a subset of those described in the vi man page.
The command
.Dq Li set -o vi
(or
.Dq Li set -V )
enables vi-mode editing and places
.Nm
into vi insert mode.  With vi-mode enabled,
.Nm
can be switched between insert mode and command mode by typing
.Aq ESC .
Hitting
.Aq return
while in command mode will pass the line to the shell.
.Pp
Similarly, the
.Dq Li set -o emacs
(or
.Dq Li set -E )
command can be used to enable a subset of
emacs-style command line editing features.
.Sh SEE ALSO
.Xr builtin 1 ,
.Xr echo 1 ,
.Xr expr 1 ,
.Xr pwd 1 ,
.Xr test 1
.Sh HISTORY
A
.Nm
command appeared in
.At v1 .