Or the default shell of any user for that matter. I don’t know if this is recommended or not, but changing from /bin/sh to /bin/bash will enable tab-completion, and since I’m a lazy admin that’s what I’ll do. (Plus, less keystrokes = longer hardware life for my keyboard.)
This command will display the user’s default shell (replace root with desired username):
rivendell:~ root# dscl . -read /Users/root UserShell
Now change the shell (in this case to /bin/bash):
rivendell:~ root# dscl . -change /Users/root UserShell /bin/sh /bin/bash
Make sure it was changed:
rivendell:~ root# dscl . -read /Users/root UserShell UserShell: /bin/bash rivendell:~ root# echo $0 -sh rivendell:~ root# su - rivendell:~ root# echo $0 -bash