Bash Internal Variables
=====
$BASH
The path to bash binary itself
$ echo $BASH
/bin/bash
$BASH_SUBSHELL
Indicating subshell level
$ (echo $BASH_SUBSHELL; (echo $BASH_SUBSHELL))
1
2
$BASHPID
current Process ID. Similar as $$, but in subshell, it will shown as real Process ID, not parent Process ID as $$
$ echo $$; echo $BASHPID ; ( cd /usr; pstree -p | grep $$; echo "$BASHPID" )
21443
21443
|-sshd(8595)---sshd(21440)---sshd(21442)---bash(21443)---bash(27203)-+-grep(27205)
27203
$BASH_VERSINFO[n]
A 6-element array containing version info for running Bash. Similar as $BASH_VERSION, which is a string for current info.
$ for i in "${BASH_VERSINFO[@]}"; do echo $i; done
4 # Major version
2 # Minor version
46 # Patch level
2 # Build version
release # Release status
x86_64-redhat-linux-gnu # Arch
$CDPATH
A colon-separated list of search paths available to the cd command
$ cd bash-4.2.46
-bash: cd: bash-4.2.46: No such file or directory
$ CDPATH=/usr/share/doc
$ cd bash-4.2.46
/usr/share/doc/bash-4.2.46
$ pwd
/usr/share/doc/bash-4.2.46
$FUNCNAME
Current function name.
$ function matrix () {
> echo "$FUNCNAME is running"
> }
$ matrix
matrix is running
$GLOBIGNORE
A list of filename patterns to be excluded from matching in globbing.
$ ls te*
test.sh
$ GLOBIGNORE=te*
$ ls te*
ls: cannot access te*: No such file or directory
$HOME
Home Directory of current user.
$HOSTNAME
Current hostname.
$HOSTTYPE
Arch
$ echo $HOSTTYPE
x86_64
$IFS
internal field separator
Note: When working with $*, the first character held in $IFS will be used.
$ bash -c 'set w x y z; IFS=":-;"; echo "$*"'
w:x:y:z
$ printf "%q" "$IFS"
$' \t\n'
$ echo "$IFS" | cat -vte
^I$
$
$ var1="a+b+c"; IFS='+'; echo $var1
a b c
Note: $IFS treats whitespace differently than other characters. No matter how many continous whitespaces, they will be taken as a single splitter.
$ var=' a b c '; IFS=' '; echo $var | tr ' ' '#'
a#b#c
$ var='::a::b:c:::'; IFS=':'; echo $var | tr ' ' '#'
##a##b#c##
$IGNOREEOF
Ignore EOF: how many end-of-files (control-D) the shell will ignore before logging out.
$LINENO
line number of shell script
$ cat /tmp/test.sh | awk '{print NR, $0}'
1 #!/bin/bash
2
3 echo test
4 echo $LINENO
$ /tmp/test.sh
test
4
$OLDPWD
Last working directory
$ pwd
/tmp
$ cd ~
$ echo $OLDPWD
/tmp
$PIPESTATUS
Array variable holding exit status(es) of last executed foreground pipe.
$ ls -al | wc -l
110
$ echo ${PIPESTATUS[@]}
0 0
$PIPESTATUS is a “volatile” variable. It needs to be captured immediately. It will always show last command exit status(es).
$ ls -al | wc -l
110
$ test
$ echo ${PIPESTATUS[@]}
1
$ test
$ echo $?
1
$PROMPT_COMMAND
holding a command to be executed just before the primary prompt, $PS1 is to be displayed.
$ export PROMPT_COMMAND='echo "Current date and time: $(date)"'
Current date and time: Sat Sep 16 18:07:54 UTC 2023
$
Current date and time: Sat Sep 16 18:08:21 UTC 2023
$PS1 / $PS2 / $PS3 / $PS4
$PS1 main prompt, seen at the command-line.
$PS2 secondary prompt, seen when additional input is expected. It displays as “>”.
$PS3 tertiary prompt, displayed in a select loop
$PS4 quartenary prompt, shown at the beginning of each line of output when invoking a script with the -x
$ PS3='Please choose: '; select answer in "Yes" "No"; do echo "Let's check PS3 Value, and your answer is $REPLY"; break; done
1) Yes
2) No
Please choose: 1
Let's check PS3 Value, and your answer is 1
$ cat /tmp/test.sh
#!/bin/bash
# PS4='___'
echo " test"
$ echo $PS4; bash -x /tmp/test.sh
+
+ echo ' test'
test
$ cat /tmp/test.sh
#!/bin/bash
PS4='___'
echo " test"
$ bash -x /tmp/test.sh
+ PS4=___
___echo ' test'
test
$REPLY
default value when a variable is not supplied to read. In a login shell, it was how many seconds has been log in.
$ echo $SECONDS
715
$SHELLOPTS
list of enabled shell options, a readonly variable.
$ echo $SHELLOPTS; set -o verbose; echo $SHELLOPTS
braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor
braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor:verbose
$SHLVL
shell level, how deeply Bash is nested.
$ echo $SHLVL; bash -c 'echo $SHLVL'
1
2