<p>Have you a look in /etc/shells?</p> <pre><code>cat /etc/shells </code></pre> <p>Thinking about this further you could also make use of "whatis" command for any shells that are not listed in "/etc/shells". </p> <pre><code>cd /bin; whatis * | grep -i 'shell' </code></pre> <p>This will list any shells in '/bin' that have a description that contains the word "shell". It may also be worth doing this in other directories such as /usr/bin, /sbin. This may produce the odd false positive but should give you something to work with.</p>