Q. What is the difference between \$SHELL and \$0?

A.

• \$SHELL is just an environment variable that gives the absolute 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

• Change the default shell
• Which shell am I using?