Difference between "echo $SHELL" and "which bash"

Your system probably has bash installed in multiple locations, whether as different versions of bash or just symbolic links.

which is not really a useful command for most purposes - it's not really portable or very usable in scripts. In general, type is better. The idea behind which is to do a PATH search for the command you give it as an argument.

$SHELL does not necessarily reflect the currently running shell. Instead, $SHELL is the user's preferred shell, which is typically the one set in /etc/passwd. If you start a different shell after logging in, you can not necessarily expect $SHELL to match the current shell anymore.

As you can see, $SHELL and which are completely unrelated. Neither of these will tell you what shell you are actually running.

A side note: Unfortunately, matching the shell you are currently running to a location in the filesystem is harder than one might think. This is because binaries are loaded into memory in order to run, and in most systems the copy in memory will continue to run fine even after you delete the original from disk (the kernel may keep the disk copy around in "limbo" until it is really no longer needed). I don't think there is any portable way to go about this - you'd have to resort to platform specific methods. For example, on Linux, examining the link /proc/$$/exe should give you a decent idea of what file is running (where $$ is the process ID of your running shell). Unfortunately I am not familiar with Solaris, so I can't help you there.

The other answers are good, but I like to give a demonstration.

% echo $SHELL
/bin/zsh
% which bash
/bin/bash

• The first command tells me which shell will be executed by login when you log in. In my case, /bin/zsh.
• The second command tells me the first occurrence in my $PATH the bash command can be found.

One does not imply the second, nor vice versa.

I'm not going to restate other people's answers but in addition to that I want to point out some things.

• The default shell on Solaris is /bin/bash
• On Solaris /bin is a symlink to ./usr/bin
• The default $PATH on Solaris is /usr/bin:/bin

That's why you're seeing this behavior.