What's all those users in the /etc/passwd file?
The /etc/passwd is supposed to have a line for every user on the system. Amongst the common username and root there is a bunch of other users. Some examples:
timidity:x:114:127:TiMidity++ MIDI sequencer service:/etc/timidity:/bin/false
liquidsoap:x:115:128::/usr/share/liquidsoap:/bin/false
statd:x:116:65534::/var/lib/nfs:/bin/false
gdm:x:117:131:Gnome Display Manager:/var/lib/gdm:/bin/false
mysql:x:118:133:MySQL Server,,,:/nonexistent:/bin/false
- What's the purpose with all these users?
- How can I login as mysql or gdm? What will the password be?
They're what you call "service accounts" and they're used for separation of privileges (so mysql can't read files it doesn't own, for example).
They can't be logged into interactively because of the /bin/false entry. Instead, they are just used for access to the appropriate files.
These accounts are used to run services in the background. Your linux system will have a range of application doing a range of tasks in the background, as you correctly identified SQL is one such service. In order for these services to carry out activities it must have a user attached.
In order to preserve the security of your system these tasks cannot be carried out as root and instead are assigned accounts with do not have shell or login access as identified by /bin/false or /sbin/nologin. This also allows permissions to be assigned only to the files used by each application.
You cannot login as these users for this reason.
Source - linuxquestions.com