Why $SHELL doesn't change when I run new shell?

Question

 $ echo $SHELL
/bin/bash
 $ /bin/ksh93
 $ echo $SHELL
/bin/bash
 $ file /bin/ksh93
/bin/ksh93: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), 
dynamically linked (uses shared libs), for GNU/Linux 2.6.8, stripped

 $ getent passwd test111
test111:x:1008:1008:,,,:/tmp:/bin/ksh93
 $ ssh test@localhost
test@localhost's password:
 $ echo $SHELL
/bin/ksh93
 $ bash
 $ echo $SHELL
/bin/ksh93

I expect the $SHELL to change after running another shell. Why doesn't it?

P.S. However the shell does change, only $SHELL variable remains the same:

 $ dash
 $ echo $SHELL
/bin/bash
 $ T=test ; [[ $T = *est ]] && echo ok
dash: [[: not found

Answer 1

You shouldn't expect this variable to change. It is used to store your default shell path, i.e. the one stored in the password database, not which shell you're currently running.

Answer 2

SHELL is specified by POSIX. Quoting:

SHELL This variable shall represent a pathname of the user's preferred command language interpreter. [...]

Running a different shell does not necessarily indicate a change in the user's preference.

It is good to learn the types of things that POSIX specifies and where to search for them since most *.nix systems aim for POSIX compliance. Environment variables are one of those things.

Answer 3

The SHELL variable stores the parent shell for your session, which sometimes matches with your default login shell. The $SHELL is the parent shell which spawned the current session.

so, if i use bash for my user and do this: chsh -s /bin/zsh user, when I do echo $SHELL, the shell displayed will still be BASH.

see this article.