bashref.texi

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* Comments::		How to specify comments.
@end menu

When the shell reads input, it proceeds through a
sequence of operations.  If the input indicates the beginning of a
comment, the shell ignores the comment symbol (@samp{#}), and the rest
of that line.
                                
Otherwise, roughly speaking,  the shell reads its input and
divides the input into words and operators, employing the quoting rules
to select which meanings to assign various words and characters.

The shell then parses these tokens into commands and other constructs,
removes the special meaning of certain words or characters, expands
others, redirects input and output as needed, executes the specified
command, waits for the command's exit status, and makes that exit status
available for further inspection or processing.

@node Shell Operation
@subsection Shell Operation

The following is a brief description of the shell's operation when it
reads and executes a command.  Basically, the shell does the
following:

@enumerate
@item
Reads its input from a file (@pxref{Shell Scripts}), from a string
supplied as an argument to the @option{-c} invocation option
(@pxref{Invoking Bash}), or from the user's terminal.

@item
Breaks the input into words and operators, obeying the quoting rules
described in @ref{Quoting}.  These tokens are separated by
@code{metacharacters}.  Alias expansion is performed by this step
(@pxref{Aliases}).

@item
Parses the tokens into simple and compound commands
(@pxref{Shell Commands}).

@item
Performs the various shell expansions (@pxref{Shell Expansions}), breaking
the expanded tokens into lists of filenames (@pxref{Filename Expansion})
and commands and arguments.

@item
Performs any necessary redirections (@pxref{Redirections}) and removes
the redirection operators and their operands from the argument list.

@item
Executes the command (@pxref{Executing Commands}).

@item
Optionally waits for the command to complete and collects its exit
status (@pxref{Exit Status}).

@end enumerate

@node Quoting
@subsection Quoting
@cindex quoting
@menu
* Escape Character::	How to remove the special meaning from a single
			character.
* Single Quotes::	How to inhibit all interpretation of a sequence
			of characters.
* Double Quotes::	How to suppress most of the interpretation of a
			sequence of characters.
* ANSI-C Quoting::	How to expand ANSI-C sequences in quoted strings.
* Locale Translation::	How to translate strings into different languages.
@end menu

Quoting 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.

Each of the shell metacharacters (@pxref{Definitions})
has special meaning to the shell and must be quoted if it is to
represent itself.
When the command history expansion facilities are being used
(@pxref{History Interaction}), the
@var{history expansion} character, usually @samp{!}, must be quoted
to prevent history expansion.  @xref{Bash History Facilities}, for
more details concerning history expansion.

There are three quoting mechanisms: the
@var{escape character}, single quotes, and double quotes.

@node Escape Character
@subsubsection Escape Character
A non-quoted backslash @samp{\} is the Bash escape character.
It preserves the literal value of the next character that follows,
with the exception of @code{newline}.  If a @code{\newline} pair
appears, and the backslash itself is not quoted, the @code{\newline}
is treated as a line continuation (that is, it is removed from
the input stream and effectively ignored).

@node Single Quotes
@subsubsection Single Quotes

Enclosing characters in single quotes (@samp{'}) 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.

@node Double Quotes
@subsubsection Double Quotes

Enclosing characters in double quotes (@samp{"}) preserves the literal value
of all characters within the quotes, with the exception of
@samp{$}, @samp{`}, @samp{\},
and, when history expansion is enabled, @samp{!}.
The characters @samp{$} and @samp{`}
retain their special meaning within double quotes (@pxref{Shell Expansions}).
The backslash retains its special meaning only when followed by one of
the following characters:
@samp{$}, @samp{`}, @samp{"}, @samp{\}, or @code{newline}.
Within double quotes, backslashes that are followed by one of these
characters are removed.  Backslashes preceding characters without a
special meaning are left unmodified.
A double quote may be quoted within double quotes by preceding it with
a backslash.
If enabled, history expansion will be performed unless an @samp{!}
appearing in double quotes is escaped using a backslash.
The backslash preceding the @samp{!} is not removed.

The special parameters @samp{*} and @samp{@@} have special meaning
when in double quotes (@pxref{Shell Parameter Expansion}).

@node ANSI-C Quoting
@subsubsection ANSI-C Quoting
@cindex quoting, ANSI

Words of the form @code{$'@var{string}'} are treated specially.  The
word expands to @var{string}, with backslash-escaped characters replaced
as specified by the ANSI C standard.  Backslash escape sequences, if
present, are decoded as follows:

@table @code
@item \a
alert (bell)
@item \b
backspace
@item \e
@itemx \E
an escape character (not ANSI C)
@item \f
form feed
@item \n
newline
@item \r
carriage return
@item \t
horizontal tab
@item \v
vertical tab
@item \\
backslash
@item \'
single quote
@item \"
double quote
@item \@var{nnn}
the eight-bit character whose value is the octal value @var{nnn}
(one to three digits)
@item \x@var{HH}
the eight-bit character whose value is the hexadecimal value @var{HH}
(one or two hex digits)
@item \u@var{HHHH}
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
@var{HHHH} (one to four hex digits)
@item \U@var{HHHHHHHH}
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
@var{HHHHHHHH} (one to eight hex digits)
@item \c@var{x}
a control-@var{x} character
@end table

@noindent
The expanded result is single-quoted, as if the dollar sign had not
been present.

@node Locale Translation
@subsubsection Locale-Specific Translation
@cindex localization
@cindex internationalization
@cindex native languages
@cindex translation, native languages

A double-quoted string preceded by a dollar sign (@samp{$}) will cause
the string to be translated according to the current locale.
If the current locale is @code{C} or @code{POSIX}, the dollar sign
is ignored.
If the string is translated and replaced, the replacement is
double-quoted.

@vindex LC_MESSAGES
@vindex TEXTDOMAIN
@vindex TEXTDOMAINDIR
Some systems use the message catalog selected by the @env{LC_MESSAGES}
shell variable.  Others create the name of the message catalog from the
value of the @env{TEXTDOMAIN} shell variable, possibly adding a
suffix of @samp{.mo}.  If you use the @env{TEXTDOMAIN} variable, you
may need to set the @env{TEXTDOMAINDIR} variable to the location of
the message catalog files.  Still others use both variables in this
fashion:
@env{TEXTDOMAINDIR}/@env{LC_MESSAGES}/LC_MESSAGES/@env{TEXTDOMAIN}.mo.

@node Comments
@subsection Comments
@cindex comments, shell

In a non-interactive shell, or an interactive shell in which the
@code{interactive_comments} option to the @code{shopt}
builtin is enabled (@pxref{The Shopt Builtin}),
a word beginning with @samp{#}
causes that word and all remaining characters on that line to
be ignored.  An interactive shell without the @code{interactive_comments}
option enabled does not allow comments.  The @code{interactive_comments}
option is on by default in interactive shells.
@xref{Interactive Shells}, for a description of what makes
a shell interactive.

@node Shell Commands
@section Shell Commands
@cindex commands, shell

A simple shell command such as @code{echo a b c} consists of the command
itself followed by arguments, separated by spaces.

More complex shell commands are composed of simple commands arranged together
in a variety of ways: in a pipeline in which the output of one command
becomes the input of a second, in a loop or conditional construct, or in
some other grouping.

@menu
* Simple Commands::		The most common type of command.
* Pipelines::			Connecting the input and output of several
				commands.
* Lists::			How to execute commands sequentially.
* Compound Commands::		Shell commands for control flow.
* Coprocesses::			Two-way communication between commands.
* GNU Parallel::		Running commands in parallel.
@end menu

@node Simple Commands
@subsection Simple Commands
@cindex commands, simple

A simple command is the kind of command encountered most often.
It's just a sequence of words separated by @code{blank}s, terminated
by one of the shell's control operators (@pxref{Definitions}).  The
first word generally specifies a command to be executed, with the
rest of the words being that command's arguments.

The return status (@pxref{Exit Status}) of a simple command is
its exit status as provided
by the @sc{posix} 1003.1 @code{waitpid} function, or 128+@var{n} if
the command was terminated by signal @var{n}.

@node Pipelines
@subsection Pipelines
@cindex pipeline
@cindex commands, pipelines

A @code{pipeline} is a sequence of simple commands separated by one of
the control operators @samp{|} or @samp{|&}.

@rwindex time
@rwindex !
@cindex command timing
The format for a pipeline is
@example
[@code{time} [@code{-p}]] [@code{!}] @var{command1} [ [@code{|} or @code{|&}] @var{command2} @dots{}]
@end example

@noindent
The output of each command in the pipeline is connected via a pipe
to the input of the next command.
That is, each command reads the previous command's output.  This
connection is performed before any redirections specified by the
command.

If @samp{|&} is used, the standard error of @var{command1} is connected to
@var{command2}'s standard input through the pipe; it is shorthand for
@code{2>&1 |}.  This implicit redirection of the standard error is
performed after any redirections specified by the command.

The reserved word @code{time} causes timing statistics
to be printed for the pipeline once it finishes.
The statistics currently consist of elapsed (wall-clock) time and
user and system time consumed by the command's execution.
The @option{-p} option changes the output format to that specified
by @sc{posix}.
When the shell is in @sc{posix} mode (@pxref{Bash POSIX Mode}),
it does not recognize @code{time} as a reserved word if the next
token begins with a @samp{-}.
The @env{TIMEFORMAT} variable may be set to a format string that
specifies how the timing information should be displayed.
@xref{Bash Variables}, for a description of the available formats.
The use of @code{time} as a reserved word permits the timing of
shell builtins, shell functions, and pipelines.  An external
@code{time} command cannot time these easily.

When the shell is in @sc{posix} mode (@pxref{Bash POSIX Mode}), @code{time}
may be followed by a newline.  In this case, the shell displays the
total user and system time consumed by the shell and its children.
The @env{TIMEFORMAT} variable may be used to specify the format of
the time information.

If the pipeline is not executed asynchronously (@pxref{Lists}), the
shell waits for all commands in the pipeline to complete.

Each command in a pipeline is executed in its own subshell
(@pxref{Command Execution Environment}).  The exit
status of a pipeline is the exit status of the last command in the
pipeline, unless the @code{pipefail} option is enabled
(@pxref{The Set Builtin}).
If @code{pipefail} is enabled, the pipeline's return status is the
value of the last (rightmost) command to exit with a non-zero status,
or zero if all commands exit successfully.
If the reserved word @samp{!} precedes the pipeline, the
exit status is the logical negation of the exit status as described
above.
The shell waits for all commands in the pipeline to terminate before
returning a value.

@node Lists
@subsection Lists of Commands
@cindex commands, lists

A @code{list} is a sequence of one or more pipelines separated by one
of the operators @samp{;}, @samp{&}, @samp{&&}, or @samp{||},
and optionally terminated by one of @samp{;}, @samp{&}, or a
@code{newline}.

Of these list operators, @samp{&&} and @samp{||}
have equal precedence, followed by @samp{;} and @samp{&},
which have equal precedence.

A sequence of one or more newlines may appear in a @code{list}
to delimit commands, equivalent to a semicolon.

If a command is terminated by the control operator @samp{&},
the shell executes the command asynchronously in a subshell.
This is known as executing the command in the @var{background}.
The shell does not wait for the command to finish, and the return
status is 0 (true).
When job control is not active (@pxref{Job Control}),
the standard input for asynchronous commands, in the absence of any
explicit redirections, is redirected from @code{/dev/null}.