Ever wondered how can we see the other 'unset' environment variables belongs to Bash shell or some command or tool ?
The technique that I follow is to look in 2 places: One in the Binaries of the program which the application uses and other in the MAN page.
To view all the printable strings in a Bash shell: # strings /bin/bash | grep -P '[A-Z]+'
To print all the ENV variables related to 'history' command:
# man bash | sed 's/[[:cntrl:]].//g' | egrep -x ' +HIST[[:alpha:]]*'
A URL that contains all the Bash variables explaining each: