Accounts & Groups

2008 Oct 18

Accounts

To login to a linux or unix system a user must have an account. Every process runs with the permissions set up with an acoount. Accounts are created and managed using the /etc/passwd file, which contains one line per account on the system. Each of these lines is a a series of colon separted fields, e.g.:

root:&%(*&^(%*&ty6tf/j765:0:0:root:/root:/bin/csh
bin:*:1:1:bin:/bin:
daemon:*2:2:daemon:/sbin/
ftp:*:8:11:FTP User:/home/ftp:
somebody:*:99:99:Sobebody:/:
nerd:86^%87865877\67:317:313:Hacker:/home/

• Login Name : This is the name given at the login prompt
• Encrypted password : The is the password given at the login prompt but after being encrypted - Since no password encrypts to "*" this value prevents anyone from logging in with the login name on this line
• UID : User ID number
• GID : Group ID number
• Information : Free form information not used by system, e.g. full name
• Home Directory : directory user is placed in after loggin in, usually where the user keeps files
• Login Shell : The name of the shell program that will be excecuted after login, such as sh (Bourne shell), bash (Bourne-again shell), csh C shell, ksh, (Korn shell), etc. or could be any program to exceute when the user logs in.

Any user can read the passwd file, it is world readable but not write it. Because of this some systems do not have the encrypted password in the file but put them in a shadow password file called /etc/shadow or /etc/secure. Only users with superuser (root) privileges can access the shadow password file.

Groups

To simplify managing the permission, users can be assigned to groups and these groups can have permissions set. All groups are in the file /etc/group which has one line for each group defined on the system, e.g.:

daemon:x:5:root,bin
sales:x:31:bob,anne,clyde

• Group Name
• Encrypted password : never used, set to "x" or "*"
• Group Members : a comma separated list of login names

There is a special user called root, also called the super-user, who has the maximum (or all) privileges on the system - the ability to read, write, alter any setting, execute any file or program. The UID of root is 0, the actual name does not have to be root but its UID must be 0. Multiple users can have UID 0.