List all possible owners and groups

Question

Using either Java or bash how can I list all possible owners and groups.

For instance if I was to do the following command in a shell with an obviously wrong owner:

chown ljafdj ~/Desktop/test.txt

My output would be

chown: invalid user: `ljafdj'

The same would be for chgrp

So how can I get a list of valid users using bash or Java?

It's for an Android application and I need to display the possibilities to my user so they can change if needed.

Answer 1

Groups in Android are hardcoded and "are used to isolate processes and grant permissions." However, if you want to get the online username, you can use AccountManager.getAccounts().

Answer 2

Using the shell ("bash"):

For users:

getent passwd

For groups:

getent group

Just parse for the first field (before the colon) to get the actual names.