I found the list of many BASH variables, https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html
The description says,
These variables are set or used by Bash, but other shells do not normally treat them specially.
And the first variable is,
BASH: The full pathname used to execute the current instance of Bash.
but when I type
BASH in my terminal I get
bash: BASH: command not found
I can confirm from
echo $0 that I am using
Similarly, when I run
BASH_VERSINFO I get the same output, even though the description of the command says,
A readonly array variable (see Arrays) whose members hold version information for this instance of Bash. The values assigned to the array members are as follows:
BASH_VERSINFO The major version number (the release).
BASH_VERSINFO The minor version number (the version).
BASH_VERSINFO The patch level.
BASH_VERSINFO The build version.
BASH_VERSINFO The release status (e.g., beta1).
BASH_VERSINFO The value of MACHTYPE.
Some of the variables like
pwd does work though