The shell that Linux used in the first place was Bourne, and then Linux released Bash, which is an enhanced version of Bourne, and most Linux systems now use Bash by default.

Bourne Shell Variables

Home and path are the two most commonly used bourne shell variables

HOME

Current user’s home directory.

1

echo $HOME
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prints current user's home directory

PATH

Paths of binary files, which are runnable programs.

1

echo $PATH
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prints the executable files' directories

Bash Variables

The number of environment variables in bash is so large that only a few commonly used ones are listed here:

BASH

Print the directory where bash is stored.

1

echo $BASH
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prints current bash shell's directory

BASH_VERSION

Prints bash’s version

1

echo $BASH_VERSION
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prints current bash shell's version

BASHPID

Current Bash’s process ID.

1

echo $BASHPID
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prints current bash shell's process ID

GROUPS

Current user’s group IDs:

1

echo $GROUPS
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prints current user's group ID

HISTFILE

The directory of history file:

1

echo $HISTFILE
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prints the history file's location

HISTFILESIZE

The allowed lines contained in the history file:

1

echo $HISTFILESIZE
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prints the history file's allowed lines

HISTSIZE

The allowed lines contained in the history list:

1

echo $HISTSIZE
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prints the history's allowed lines

HOSTNAME

Current host’s name:

1

echo $HOSTNAME
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prints the current host name

HOSTTYPE

Current machine type:

1

echo $HOSTTYPE
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prints the current host type

LINENO

The line number that the current shell or script is running: 

1

echo $LINENO
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prints the current shell's current line number

MACHTYPE

The CPU architecture, company name, and type of the system which Bash is running: 

1

echo $MACHTYPE
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prints the current machine's type

OSTYPE

The operating system that Bash is running on:

1

echo $OSTYPE
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prints the current operating system's type

PPID

The shell’s parent’s process ID:

1

echo $$    # get current shell's process ID
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prints the current shell's process ID

Switch to a new shell:

1

bash

Check parent shell’s process ID:

1

echo $PPID
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prints the current shell's parent shell ID

RANDOM

Generate a random integer number between 0 and 32767

1

echo $RANDOM
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prints a random number between 0 and 32767

SECONDS

The number of seconds since shell started:

1

echo $SECONDS
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prints the seconds since shell started

SHELL

The full path of the shell we are using

1

echo $SHELL
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prints the bash shell's location

SHLVL

The deep level of the current shell, every time a new instance of bash is started, the variable incremented by one: 

1

echo $SHLVL
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prints the current bash shell's deep level

Switch to a new bash, this starts a new bash instance:

1

bash

Check current bash’s deep level:

1

echo $SHLVL
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prints the current bash shell's deep level

UID

The user ID of the current user:

1

echo $UID
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prints the current user's ID

References 5.1 Bourne Shell Variables, 5.2 Bash Variables

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