The user system in Unix allows different people to log in to the same machine without interfering with one another. At the lowest level, users are nothing more than numbers (user IDs), but login names help to avert confusion and boredom.
The plain-text file
/etc/passwd maps login names to user IDs, and it looks something like this:
root:x:0:0:Superuser:/root:/bin/sh daemon:*:1:1:daemon:/usr/sbin:/bin/sh bin:*:2:2:bin:/bin:/bin/sh sys:*:3:3:sys:/dev:/bin/sh nobody:*:65534:65534:nobody:/home:/bin/false juser:x:3119:1000:J. Random User:/home/juser:/bin/bash beazley:x:143:1000:David Beazley:/home/beazley:/bin/bash
The format is straightforward. Each line represents one user, with seven fields separated by colons:
The user (login) ...