HOWTO: Detect bash from shell script

There are a bunch of environment variables that you can look at but many of them will not detect if a different shell is spawned from bash. Consider the following:

bash$ echo "SHELL: $SHELL, shell: $shell, ARGV[0]: $0, PS1: $PS1, prompt: $prompt"
SHELL: /bin/bash, shell: , ARGV[0]: -bash, PS1: bash$ , prompt: 

bash$ csh
[lorien:~] daveshawley% echo "SHELL: $SHELL, shell: $shell, \$0: $0, PS1: $PS1, prompt: $prompt"
SHELL: /bin/bash, shell: /bin/tcsh, ARGV[0]: csh, PS1: bash$ , prompt: [%m:%c3] %n%#

[lorien:~] daveshawley% bash -r
bash$ echo "SHELL: $SHELL, shell: $shell, ARGV[0]: $0, PS1: $PS1, prompt: $prompt"
SHELL: /bin/bash, shell: , ARGV[0]: sh, PS1: bash$ , prompt:

bash$ zsh
% echo "SHELL: $SHELL, shell: $shell, ARGV[0]: $0, PS1: $PS1, prompt: $prompt"
SHELL: /bin/bash, shell: , ARGV[0]: zsh, PS1: % , prompt: % 

% ksh
$ echo "SHELL: $SHELL, shell: $shell, ARGV[0]: $0, PS1: $PS1, prompt: $prompt"
SHELL: /bin/bash, shell: , ARGV[0]: ksh, PS1: bash$ , prompt: 

There are a number of variables specific to the various shells except that they have a habit of being inherited by sub-shells which is where the environment thing really breaks. The only thing that almost works is ps -o command -p $$. This technically gives you the command name that the shell is running as. In most cases this will work... since applications are started with some variant of the exec system call and it allows for the name of the command and the executable to differ, it is possible for this to fail as well. Consider:

bash$ exec -a "-csh" bash
bash$ echo "$0, $SHELL, $BASH"
-csh, /bin/bash, /bin/bash
bash$ ps -o command -p $$
COMMAND
-csh
bash$

Another trick is to use lsof -p $$ | awk '(NR==2) {print $1}'. This is probably as close as you can get if you are lucky enough to have lsof handy.

This works also

[ -z "$BASH_VERSION" ] && return