shell Builtin variables（shell内建变量）

末蓝、 2021-11-10 07:52 279阅读 0赞

内容来自 ：abs-guide

***$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\[n\]***

*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.*

> > *for n in 0 1 2 3 4 5*
> 
> > *do*
> 
> > *echo "BASH\_VERSINFO\[$n\] = $\{BASH\_VERSINFO\[$n\]\}"*
> 
> > *done*
> 
> > *\# BASH\_VERSINFO\[0\] = 3 \# Major version no.*
> 
> > *\# BASH\_VERSINFO\[1\] = 00 \# Minor version no.*
> 
> > *\# BASH\_VERSINFO\[2\] = 14 \# Patch level.*
> 
> > *\# BASH\_VERSINFO\[3\] = 1 \# Build version.*
> 
> > *\# BASH\_VERSINFO\[4\] = release \# Release status.*
> 
> > *\# BASH\_VERSINFO\[5\] = i386-redhat-linux-gnu \# Architecture*

***$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*

> > > *xyz23 ()*
> 
> > > *\{* 
> 
> > > *echo "$FUNCNAME now executing." \# xyz23 now executing.*
> 
> > > *\}*
> 
> > > *xyz23*
> 
> > > *echo "FUNCNAME = $FUNCNAME" \# FUNCNAME =*
> 
> > > *\# Null value outside a 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.*

> *root\# **echo $GROUPS***
> 
> *0*
> 
> *root\# **echo $\{GROUPS\[1\]\}***
> 
> *1*
> 
> *root\# **echo $\{GROUPS\[5\]\}***
> 
> *6*

***$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.*

> *bash$ **echo "$IFS"***
> 
> *(With $IFS set to default, a blank line displays.)*
> 
> *bash$ **echo "$IFS" | cat -vte***
> 
> *^I$*
> 
> *$*
> 
> *(Show whitespace: here a single space, ^I \[horizontal tab\],*
> 
> *and newline, and display "$" at end-of-line.)*
> 
> *bash$ **bash -c 'set w x y z; IFS=":-;"; echo "$\*"'***
> 
> *w:x:y:z*
> 
> *(Read commands from string and assign any arguments to pos params.)*

***$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.*

> > *\# \*\*\* BEGIN DEBUG BLOCK \*\*\**
> 
> > *last\_cmd\_arg=$\_ \# Save it.*
> 
> > *echo "At line number $LINENO, variable \\"v1\\" = $v1"*
> 
> > *echo "Last command argument processed = $last\_cmd\_arg"*
> 
> > *\# \*\*\* END DEBUG BLOCK \*\*\**

***$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.*

> > *bash$*
> 
> > *echo $PIPESTATUS*
> 
> > *0*
> 
> > *bash$ls -al | bogus\_command*
> 
> > *bash: bogus\_command: command not found*
> 
> > *bash$*
> 
> > *echo $\{PIPESTATUS\[1\]\}*
> 
> > *127*
> 
> > *bash$ls -al | bogus\_command*
> 
> > *bash: bogus\_command: command not found*
> 
> > *bash$*
> 
> > *echo $?*
> 
> > *127*

*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.*

> *\#!/bin/bash*
> 
> *\# If $IFS set, but empty,*
> 
> *\#+ then "$\*" and "$@" do not echo positional params as expected.*
> 
> *mecho () \# Echo positional parameters.*
> 
> *\{* 
> 
> *echo "$1,$2,$3";*
> 
> *\}*
> 
> *IFS="" \# Set, but empty.*
> 
> *set a b c \# Positional parameters.*
> 
> *mecho "$\*" \# abc,,*
> 
> *\# ^^*
> 
> *mecho $\* \# a,b,c*
> 
> *mecho $@ \# a,b,c*
> 
> *mecho "$@" \# a,b,c*

***$-***

*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*