bash.1

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is connected via a pipe to a file descriptor in the executing shell,
and that file descriptor is assigned to \fINAME\fP[0].
The standard input of
.I command
is connected via a pipe to a file descriptor in the executing shell,
and that file descriptor is assigned to \fINAME\fP[1].
This pipe is established before any redirections specified by the
command (see
.SM
.B REDIRECTION
below).
The file descriptors can be utilized as arguments to shell commands
and redirections using standard word expansions.
The process ID of the shell spawned to execute the coprocess is
available as the value of the variable \fINAME\fP_PID.
The \fBwait\fP
builtin command may be used to wait for the coprocess to terminate.
.PP
The return status of a coprocess is the exit status of \fIcommand\fP.
.SS Shell Function Definitions
.PP
A shell function is an object that is called like a simple command and
executes a compound command with a new set of positional parameters.
Shell functions are declared as follows:
.TP
\fIname\fP () \fIcompound\-command\fP [\fIredirection\fP]
.PD 0
.TP
\fBfunction\fP \fIname\fP [()] \fIcompound\-command\fP [\fIredirection\fP]
.PD
This defines a function named \fIname\fP.
The reserved word \fBfunction\fP is optional.
If the \fBfunction\fP reserved word is supplied, the parentheses are optional.
The \fIbody\fP of the function is the compound command
.I compound\-command 
(see \fBCompound Commands\fP above).
That command is usually a \fIlist\fP of commands between { and }, but
may be any command listed under \fBCompound Commands\fP above.
\fIcompound\-command\fP is executed whenever \fIname\fP is specified as the
name of a simple command.
Any redirections (see
.SM
.B REDIRECTION
below) specified when a function is defined are performed
when the function is executed.
The exit status of a function definition is zero unless a syntax error
occurs or a readonly function with the same name already exists.
When executed, the exit status of a function is the exit status of the
last command executed in the body.  (See
.SM
.B FUNCTIONS
below.)
.SH COMMENTS
In a non-interactive shell, or an interactive shell in which the
.B interactive_comments
option to the
.B shopt
builtin is enabled (see
.SM
.B "SHELL BUILTIN COMMANDS"
below), a word beginning with
.B #
causes that word and all remaining characters on that line to
be ignored.  An interactive shell without the
.B interactive_comments
option enabled does not allow comments.  The
.B interactive_comments
option is on by default in interactive shells.
.SH QUOTING
\fIQuoting\fP is used to remove the special meaning of certain
characters or words to the shell.  Quoting can be used to 
disable special treatment for special characters, to prevent
reserved words from being recognized as such, and to prevent
parameter expansion.
.PP
Each of the \fImetacharacters\fP listed above under
.SM
.B DEFINITIONS
has special meaning to the shell and must be quoted if it is to
represent itself.
.PP
When the command history expansion facilities are being used
(see
.SM
.B HISTORY EXPANSION
below), the
\fIhistory expansion\fP character, usually \fB!\fP, must be quoted
to prevent history expansion.
.PP
There are three quoting mechanisms: the
.IR "escape character" ,
single quotes, and double quotes.
.PP
A non-quoted backslash (\fB\e\fP) is the
.IR "escape character" .
It preserves the literal value of the next character that follows,
with the exception of <newline>.  If a \fB\e\fP<newline> pair
appears, and the backslash is not itself quoted, the \fB\e\fP<newline>
is treated as a line continuation (that is, it is removed from the
input stream and effectively ignored).
.PP
Enclosing characters in single quotes preserves the literal value
of each character within the quotes.  A single quote may not occur
between single quotes, even when preceded by a backslash.
.PP
Enclosing characters in double quotes preserves the literal value
of all characters within the quotes, with the exception of
.BR $ ,
.BR \` ,
.BR \e ,
and, when history expansion is enabled,
.BR ! .
The characters
.B $
and
.B \`
retain their special meaning within double quotes.  The backslash
retains its special meaning only when followed by one of the following
characters:
.BR $ ,
.BR \` ,
\^\fB"\fP\^,
.BR \e ,
or
.BR <newline> .
A double quote may be quoted within double quotes by preceding it with
a backslash.
If enabled, history expansion will be performed unless an
.B !
appearing in double quotes is escaped using a backslash.
The backslash preceding the
.B !
is not removed.
.PP
The special parameters
.B *
and
.B @
have special meaning when in double
quotes (see
.SM
.B PARAMETERS
below).
.PP
Words of the form \fB$\fP\(aq\fIstring\fP\(aq are treated specially.  The
word expands to \fIstring\fP, with backslash-escaped characters replaced
as specified by the ANSI C standard.  Backslash escape sequences, if
present, are decoded as follows:
.RS
.PD 0
.TP
.B \ea
alert (bell)
.TP
.B \eb
backspace
.TP
.B \ee
.TP
.B \eE
an escape character
.TP   
.B \ef
form feed
.TP  
.B \en
new line
.TP     
.B \er
carriage return
.TP
.B \et
horizontal tab
.TP   
.B \ev
vertical tab
.TP
.B \e\e
backslash
.TP
.B \e\(aq
single quote
.TP
.B \e\(dq
double quote
.TP   
.B \e\fInnn\fP
the eight-bit character whose value is the octal value \fInnn\fP
(one to three digits)
.TP
.B \ex\fIHH\fP
the eight-bit character whose value is the hexadecimal value \fIHH\fP
(one or two hex digits)
.TP
.B \eu\fIHHHH\fP
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
\fIHHHH\fP (one to four hex digits)
.TP
.B \eU\fIHHHHHHHH\fP
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
\fIHHHHHHHH\fP (one to eight hex digits)
.TP
.B \ec\fIx\fP
a control-\fIx\fP character
.PD
.RE
.LP
The expanded result is single-quoted, as if the dollar sign had
not been present.
.PP
A double-quoted string preceded by a dollar sign (\fB$\fP\(dq\fIstring\fP\(dq)
will cause the string to be translated according to the current locale.
If the current locale is \fBC\fP or \fBPOSIX\fP, the dollar sign
is ignored.
If the string is translated and replaced, the replacement is
double-quoted.
.SH PARAMETERS
A
.I parameter
is an entity that stores values.
It can be a
.IR name ,
a number, or one of the special characters listed below under
.BR "Special Parameters" .
A
.I variable
is a parameter denoted by a
.IR name .
A variable has a \fIvalue\fP and zero or more \fIattributes\fP.
Attributes are assigned using the
.B declare
builtin command (see
.B declare
below in
.SM
.BR "SHELL BUILTIN COMMANDS" ).
.PP
A parameter is set if it has been assigned a value.  The null string is
a valid value.  Once a variable is set, it may be unset only by using
the
.B unset
builtin command (see
.SM
.B SHELL BUILTIN COMMANDS
below).
.PP
A
.I variable
may be assigned to by a statement of the form
.RS
.PP
\fIname\fP=[\fIvalue\fP]
.RE
.PP
If
.I value
is not given, the variable is assigned the null string.  All
.I values
undergo tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote
removal (see
.SM
.B EXPANSION
below).  If the variable has its
.B integer
attribute set, then
.I value
is evaluated as an arithmetic expression even if the $((...)) expansion is
not used (see
.B "Arithmetic Expansion"
below).
Word splitting is not performed, with the exception
of \fB"$@"\fP as explained below under
.BR "Special Parameters" .
Pathname expansion is not performed.
Assignment statements may also appear as arguments to the
.BR alias ,
.BR declare ,
.BR typeset ,
.BR export ,
.BR readonly ,
and
.B local
builtin commands.
.PP
In the context where an assignment statement is assigning a value
to a shell variable or array index, the += operator can be used to
append to or add to the variable's previous value.
When += is applied to a variable for which the \fIinteger\fP attribute has been
set, \fIvalue\fP is evaluated as an arithmetic expression and added to the
variable's current value, which is also evaluated.
When += is applied to an array variable using compound assignment (see
.B Arrays
below), the
variable's value is not unset (as it is when using =), and new values are
appended to the array beginning at one greater than the array's maximum index
(for indexed arrays) or added as additional key\-value pairs in an
associative array.
When applied to a string-valued variable, \fIvalue\fP is expanded and
appended to the variable's value.
.SS Positional Parameters
.PP
A
.I positional parameter
is a parameter denoted by one or more
digits, other than the single digit 0.  Positional parameters are
assigned from the shell's arguments when it is invoked,
and may be reassigned using the
.B set
builtin command.  Positional parameters may not be assigned to
with assignment statements.  The positional parameters are
temporarily replaced when a shell function is executed (see
.SM
.B FUNCTIONS
below).
.PP
When a positional parameter consisting of more than a single
digit is expanded, it must be enclosed in braces (see
.SM
.B EXPANSION
below).
.SS Special Parameters
.PP
The shell treats several parameters specially.  These parameters may
only be referenced; assignment to them is not allowed.
.PD 0
.TP
.B *
Expands to the positional parameters, starting from one.  When the
expansion occurs within double quotes, it expands to a single word
with the value of each parameter separated by the first character
of the 
.SM
.B IFS
special variable.  That is, "\fB$*\fP" is equivalent
to "\fB$1\fP\fIc\fP\fB$2\fP\fIc\fP\fB...\fP", where
.I c
is the first character of the value of the
.SM
.B IFS
variable.  If
.SM
.B IFS
is unset, the parameters are separated by spaces.
If
.SM
.B IFS
is null, the parameters are joined without intervening separators.
.TP
.B @
Expands to the positional parameters, starting from one.  When the
expansion occurs within double quotes, each parameter expands to a
separate word.  That is, "\fB$@\fP" is equivalent to
"\fB$1\fP" "\fB$2\fP" ...
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part of the original
word, and the expansion of the last parameter is joined with the last
part of the original word.
When there are no positional parameters, "\fB$@\fP" and 
.B $@
expand to nothing (i.e., they are removed).
.TP
.B #
Expands to the number of positional parameters in decimal.
.TP
.B ?
Expands to the exit status of the most recently executed foreground
pipeline.
.TP
.B \-
Expands to the current option flags as specified upon invocation, 
by the
.B set
builtin command, or those set by the shell itself
(such as the
.B \-i
option).
.TP
.B $
Expands to the process ID of the shell.  In a () subshell, it
expands to the process ID of the current shell, not the
subshell.
.TP
.B !
Expands to the process ID of the most recently executed background
(asynchronous) command.
.TP
.B 0
Expands to the name of the shell or shell script.  This is set at
shell initialization.  If
.B bash
is invoked with a file of commands,
.B $0
is set to the name of that file.  If
.B bash
is started with the
.B \-c
option, then
.B $0
is set to the first argument after the string to be
executed, if one is present.  Otherwise, it is set
to the file name used to invoke
.BR bash ,
as given by argument zero.
.TP
.B _
At shell startup, set to the absolute pathname used to invoke the
shell or shell script being executed as passed in the environment
or argument list.
Subsequently, expands to the last argument to the previous command,
after expansion.
Also set to the full pathname used to invoke each command executed
and placed in the environment exported to that command.
When checking mail, this parameter holds the name of the mail file
currently being checked.
.PD
.SS Shell Variables
.PP
The following variables are set by the shell:
.PP
.PD 0
.TP
.B BASH
Expands to the full file name used to invoke this instance of
.BR bash .
.TP
.B BASHOPTS
A colon-separated list of enabled shell options.  Each word in
the list is a valid argument for the
.B \-s
option to the
.B shopt
builtin command (see
.SM
.B "SHELL BUILTIN COMMANDS"
below).  The options appearing in
.SM
.B BASHOPTS
are those reported as
.I on
by \fBshopt\fP.
If this variable is in the environment when
.B bash
starts up, each shell option in the list will be enabled before
reading any startup files.
This variable is read-only.
.TP
.B BASHPID
Expands to the process ID of the current \fBbash\fP process.
This differs from \fB$$\fP under certain circumstances, such as subshells
that do not require \fBbash\fP to be re-initialized.
.TP
.B BASH_ALIASES
An associative array variable whose members correspond to the internal
list of aliases as maintained by the \fBalias\fP builtin.
Elements added to this array appear in the alias list; unsetting array
elements cause aliases to be removed from the alias list.
.TP
.B BASH_ARGC
An array variable whose values are the number of parameters in each
frame of the current \fBbash\fP execution call stack.