shell Builtin variables（shell内建变量）

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$BASH

The path to the Bash binary itself

bash$ echo $BASH

/bin/bash

$BASH_ENV

An environmental variable pointing to a Bash startup file to be read when a script is invoked

$BASH_SUBSHELL

A variable indicating the subshell level. This is a new addition to Bash, version 3.

See Example 21-1 for usage.

$BASHPID

Process ID of the current

$BASH_VERSINFO


A 6-element array containing version information about the installed release of Bash. This is similar

to $BASH_VERSION, below, but a bit more detailed.












$BASH_VERSION

The version of Bash installed on the system

$CDPATH

A colon-separated list of search paths available to the cd command, similar in function to the $PATH

variable for binaries. The $CDPATH variable may be set in the local ~/.bashrc file.

$DIRSTACK

The top value in the directory stack [41] (affected by pushd and popd)

This builtin variable corresponds to the dirs command, however dirs shows the entire contents of the

directory stack.

$EDITOR

The default editor invoked by a script, usually vi or emacs .

$EUID

"effective" user ID number

Identification number of whatever identity the current user has assumed, perhaps by means of su.

The $EUID is not necessarily the same as the $UID.

$FUNCNAME

Name of the current function

















$GLOBIGNORE

A list of filename patterns to be excluded from matching in globbing.

$GROUPS

Groups current user belongs to

This is a listing (array) of the group id numbers for current user, as recorded in /etc/passwd and

/etc/group.



$HOME

Home directory of the user, usually /home/username (see Example 10-7)

$HOSTNAME

The hostname command assigns the system host name at bootup in an init script. However, the

gethostname() function sets the Bash internal variable $HOSTNAME. See also Example 10-7.

$HOSTTYPE

host type

Like $MACHTYPE, identifies the system hardware.

$IFS

internal field separator

This variable determines how Bash recognizes fields, or word boundaries, when it interprets character

strings.

$IFS defaults to whitespace (space, tab, and newline), but may be changed, for example, to parse a

comma-separated data file. Note that $* uses the first character held in $IFS.



$IGNOREEOF

Ignore EOF: how many end-of-files (control-D) the shell will ignore before logging out.

$LC_COLLATE

Often set in the .bashrc or /etc/profile files, this variable controls collation order in filename

expansion and pattern matching. If mishandled, LC_COLLATE can cause unexpected results in

filename globbing.

As of version 2.05 of Bash, filename globbing no longer distinguishes between

lowercase and uppercase letters in a character range between brackets. For example, ls

[A-M]* would match both File1.txt and file1.txt . To revert to the customary

behavior of bracket matching, set LC_COLLATE to C by an export LC_COLLATE=C in /etc/profile and/or ~/.bashrc .

$LC_CTYPE

This internal variable controls character interpretation in globbing and pattern matching.

$LINENO

This variable is the line number of the shell script in which this variable appears. It has significance

only within the script in which it appears, and is chiefly useful for debugging purposes.







$MACHTYPE

machine type

Identifies the system hardware.

$OLDPWD

Old working directory ("OLD-Print-Working-Directory", previous directory you were in).

$OSTYPE

operating system type

$PATH

Path to binaries, usually /usr/bin/, /usr/X11R6/bin/, /usr/local/bin, etc.

PATH=${PATH}:/opt/bin appends the /opt/bin directory to the current path. In a script, it

may be expedient to temporarily add a directory to the path in this way. When the script exits, this

restores the original $PATH (a child process, such as a script, may not change the environment of the

parent process, the shell).

$PIPESTATUS

Array variable holding exit status(es) of last executed foreground pipe.















The members of the $PIPESTATUS array hold the exit status of each respective command executed

in a pipe. $PIPESTATUS[0] holds the exit status of the first command in the pipe,

$PIPESTATUS[1] the exit status of the second command, and so on

$PPID

The $PPID of a process is the process ID (pid) of its parent process. [42]

Compare this with the pidof command.

$PROMPT_COMMAND

A variable holding a command to be executed just before the primary prompt, $PS1 is to be

displayed.

$PS1

This is the main prompt, seen at the command-line.

$PS2

The secondary prompt, seen when additional input is expected. It displays as ">".

$PS3

The tertiary prompt, displayed in a select loop (see Example 11-29).

$PS4

The quartenary prompt, shown at the beginning of each line of output when invoking a script with the

-x option. It displays as "+".

$PWD

Working directory (directory you are in at the time)

$REPLY

The default value when a variable is not supplied to read. Also applicable to select menus, but only

supplies the item number of the variable chosen, not the value of the variable itself.

$SECONDS

The number of seconds the script has been running.

$SHELLOPTS

The list of enabled shell options, a readonly variable.

$SHLVL

Shell level, how deeply Bash is nested. [43] If, at the command-line, $SHLVL is 1, then in a script it

will increment to 2.

$TMOUT

If the $TMOUT environmental variable is set to a non-zero value time, then the shell prompt will

time out after $time seconds. This will cause a logout.

$UID

User ID number

Current user's user identification number, as recorded in /etc/passwd

This is the current user's real id, even if she has temporarily assumed another identity through su.

$UID is a readonly variable, not subject to change from the command line or within a script, and is

the counterpart to the id builtin.

$0, $1, $2, etc.

Positional parameters, passed from command line to script, passed to a function, or set to a variable

(see Example 4-5 and Example 15-16)

$#

Number of command-line arguments [44] or positional parameters (see Example 36-2)

$*

All of the positional parameters, seen as a single word

"$*" must be quoted.

$@

Same as $*, but each parameter is a quoted string, that is, the parameters are passed on intact, without

interpretation or expansion. This means, among other things, that each parameter in the argument list

is seen as a separate word.

The $@ and $* parameters differ only when between double quotes.



$-

Flags passed to script (using set).

$!

PID (process ID) of last job run in background

$_

Special variable set to final argument of previous command executed

$?

Exit status of a command, function, or the script itself (see Example 24-7)

$$

Process ID ( PID) of the script itself