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.

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

man bash

/Shell Variables

Common Variables

PS1

Shell command prompt.

echo $PS1

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

UID

Current user ID.

echo $UID

PPID

ps -fp $PPID

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

echo $BASH_VERSION

5.1.16(1)-release

BASH_SUBSHELL

Subshell nesting level, starting from 0.

echo $BASH_SUBSHELL

SECONDS

Seconds since the shell started.

echo $SECONDS

RANDOM

0 ~ 32767 random number.

echo $RANDOM

Custom Variables

Refer to the later chapter on environment variables