Finding out the default shell of a user within a shell script

The environment variable, SHELL would always expands to the default login shell of the invoking user (gets the value from /etc/passwd).

For any other given user, you need to do some processing with /etc/passwd, here is a simple awk snippet:

awk -F: -v user="foobar" '$1 == user {print $NF}' /etc/passwd

Replace foobar with the actual username.

If you have ldap (or something similar in place), use getent to get the database instead of directly parsing /etc/passwd:

getent passwd | awk -F: -v user="foobar" '$1 == user {print $NF}'

or cleanest approach, let getent do the parsing (thanks to @Kusalananda):

getent passwd foobar | awk -F: '{print $NF}'

Since getent isn't a standard command on MacOS, you may wish to use a lower level getpwuid call that will consult the naming services the machine is configured for. This may require calling out to perl or python, which are pretty common across most Unix-like platforms

eg this will return the shell for the current user:

perl -e '@x=getpwuid($<); print $x[8]'

You can get the user's default shell from the environment like this:

echo $SHELL

But not like this:

echo $0

The latter will just tell you what shell your script is currently using. So you want the first option above.