How to su to another user in mac OS X?

Question

I came from Linux world , and not familiar with OS X admin. Now I want to su to another user , but it just not working ... Where goes wrong here ?

smallufo@miniserver:~ $ whoami
smallufo
smallufo@miniserver:~ $ su - postgres
Password:
smallufo@miniserver:~ $ whoami
smallufo
smallufo@miniserver:~ $

I input correct password , but why I cannot su to postgres ?

Sorry if it is a dumb question , but I've been stuck with it the whole night...

Thanks a lot !

Answer 1

su should work normally in OS X. @Scott Warren's idea about the postgres user's shell is a possible explanation, but OS X doesn't use /etc/passwd except during bootup. Normal users (and groups etc) are stored in .plist files under /var/db/dslocal/nodes/Default/. The easy way to look at (and modify) them is with the dscl command:

dscl . -read /Users/postgres  # prints all user attributes
dscl . -read /Users/postgres UserShell # prints just the default shell

If necessary, you can set the shell the same way:

sudo dscl . -create /Users/postgres UserShell /bin/bash

Answer 2

I don't have Postgres installed but check your /etc/passwd file. The last entry for that user account will be the shell that user uses. In my passwd file the accounts that shouldn't login (_lp,_postfix,etc.) have a shell of /usr/bin/false but accounts that can login are usually /bin/sh.