Users and rights

Introduction

Files and directories on the Linux system belong to an owner and a group. You can set read, write and execute permissions on a file or directory for owner, group and others. Users can belong to one or more groups. The command chmod is used to set the permissions, the command chown to change the owner and the command chgrp to change the group.

• https://wiki.archlinux.org/index.php/Users_and_Groups
• https://help.ubuntu.com/lts/serverguide/user-management
• http://www.cyberciti.biz/tips/unix-or-linux-commands-for-changing-user-rights.html
• http://www.cyberciti.biz/faq/how-to-use-chmod-and-chown-command/
• http://www.thinkplexx.com/learn/article/unix/command/chmod-permissions-flags-explained-600-0600-700-777-100-etc

Show ownership and permissions

ls -l /path/to/files/*

Example output:

Directory	User	Group	Other	Number of links	Owner	Group	Size	Modified date/time	Object name
-	rwx	rw-	r--	1	wilbert	users	464843	Apr 6 16:09	file1.txt
-	rwx	rwx	r--	1	wilbert	users	1398792	Apr 6 16:09	file2.sh
d	rwx	rw-	r--	2	wilbert	users	4096	Apr 17 23:16	directory
-	rwx	rw-	r--	2	wilbert	users	93	Apr 17 23:16	link_to_file1.txt

Add, remove and modify users and groups

Disable (lock) the root account:

sudo passwd -l root

Enable the root account by specifying a password for it:

sudo passwd

Add a user account and home folder, delete user:

sudo adduser username

Delete user:

sudo deluser username

Lock (l) or unlock (u) a user account:

sudo passwd -l username
sudo passwd -u username

Add or delete a personalized group:

sudo addgroup groupname
sudo delgroup groupname

Add a user to a group:

sudo adduser username groupname

Change permissions on files and directories

flags

User/group:

• u user/owner
• g group
• o other

Permissions:

• r read
• w write
• x execute

Options:

• -R full recurive
• + add permission
• - remove permission

Examples:

sudo chmod -R u+rwx /path/to/files
sudo chmod -R go-x /path/to/files

Octal

There are four OCTAL (0..7) digits, which control permissions. Mostly only three are used, for more info on the first digit:

• https://en.wikipedia.org/wiki/Setuid

Permissions:

1 = execute (x)
2 = write (w)
4 = read (r)

The octal number is the sum of those free permissions, i.e.

3 (1+2) = can execute and write
6 (2+4) = can write and read

Permissins are set for owner, group and others, depending on the position of the digit:

0	owner	group	others
0	x=1 w=2 r=4	x=1 w=2 r=4	x=1 w=2 r=4

Examples:

chmod 600 file – owner can read and write
chmod 700 file – owner can read, write and execute
chmod 666 file – all can read and write
chmod 777 file – all can read, write and execute

Change ownership of files and directories

Note: option -R applies the change recursively to matching files and directories

Owner:

sudo chown user /path/to/file_or_dir
sudo chown -R user /path/to/dir

Group:

sudo chgrp -R group /path/to/file_or_dir
sudo chgrp -R group /path/to/dir