Not all shells set SHELL, and even in bash, SHELL isn't readonly. If I run for instance 'tcsh' from bash, SHELL will still say "/bin/bash" because tcsh doesn't set SHELL.

But I don't understand why it matters what shell the user is running when calling your script. If the script starts with 

   #!/bin/bash

[download]
then the script will be run by bash, unless explicitly run by another shell. In that case, don't do it.

Note also that you can get a users login shell by just checking the result of getpwnam().

In reply to Re: Environment discovery under Linux by JavaFan
in thread Environment discovery under Linux by The Hindmost

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