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Shell Variables ​

Shell variables are a general term. If you are using a specific shell, such as Bash, you can call it a Bash variable. But it is important to note that shell variables are not environment variables, do not be confused by their capitalization, here are the relevant official documents.

Shell Variables

https://www.gnu.org/software/bash/manual/html_node/Shell-Variables.html

Bash Variables

https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html

You can also view it by using the man command and searching for Shell Variables.

bash
man bash
bash
/Shell Variables

Common Variables ​

PS1 ​

Shell command prompt.

bash
echo $PS1
bash
\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$

UID ​

Current user ID.

bash
echo $UID
bash
1000

PPID ​

bash
ps -fp $PPID
bash
UID          PID    PPID  C STIME TTY          TIME CMD
kuga       46635   46570  0 10:56 ?        00:00:00 sshd: kuga@pts/0

You can see that the parent process of bash is sshd.

BASH_VERSION ​

bash
echo $BASH_VERSION
bash
5.1.16(1)-release

BASH_SUBSHELL ​

Subshell nesting level, starting from 0.

bash
echo $BASH_SUBSHELL
bash
0

SECONDS ​

Seconds since the shell started.

bash
echo $SECONDS
bash
13963

RANDOM ​

0 ~ 32767 random number.

bash
echo $RANDOM
bash
1024

Custom Variables ​

Refer to the later chapter on environment variables