Difference between $SHELL and $0
Q. What is the difference between $SHELL and $0?
A. SHELL is just an environment variable that gives the abolute path to the user's default login shell.
$0 gives the absolute path of the current shell.
For example, if you launch a new shell from the current shell, $SHELL will not change but $0 will.
% echo "\$0 is $0; \$SHELL is $SHELL" $0 is /usr/bin/zsh; $SHELL is /usr/bin/zsh % dash $ echo "\$0 is $0; \$SHELL is $SHELL" $0 is dash; $SHELL is /usr/bin/zsh $ bash $ echo "\$0 is $0; \$SHELL is $SHELL" $0 is bash; $SHELL is /usr/bin/zsh $ sh $ echo "\$0 is $0; \$SHELL is $SHELL" $0 is sh; $SHELL is /usr/bin/zsh
A running shell does not control $SHELL. It is set by the 'login' process and is exported to the child processes. The 'login' process uses /etc/passwd file to set the $SHELL variable.
See also: 'man 1 login'
Tested this with
% dpkg -l zsh dash bash sh | cut -c 1-72 dpkg-query: no packages found matching sh Desired=Unknown/Install/Remove/Purge/Hold | Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Tri |/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad) ||/ Name Version Architecture Description +++-==============-============-============-=========================== ii bash 4.3-11+b1 amd64 GNU Bourne Again SHell ii dash 0.5.7-4+b1 amd64 POSIX-compliant shell ii zsh 5.0.7-5 amd64 shell with lots of features