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

Show, add, remove and modify users and groups

List groups and its users

getent group groupname1

Format:

groupname1:x:group_id:username1,username2,etc

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 username1

Delete user:

sudo deluser username1

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

sudo passwd -l username1
sudo passwd -u username1

Add or delete a group:

sudo addgroup groupname1
sudo delgroup groupname1

Add an existing user to a group:

sudo adduser username1 groupname1

Remove user from a group:

sudo deluser username1 groupname1

Change permissions on files and directories

Flag method

User/group:

• u user/owner
• g group
• o other

Permissions:

• r read
• w write
• x execute

Options:

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

Examples:

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

Octal method

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.

1+2+4 = 7 : can execute, write and read
  2+4 = 6 : can write and read

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

who:	owner			group			others
may:	x	w	r	x	w	r	x	w	r
add:	1	2	4	1	2	4	1	2	4

Examples:

chmod 600 file – owner r,w
chmod 700 file – owner r,w,x
chmod 777 file – all can r,w,x

Change ownership of files and directories

Note: option -R (recursive) applies the change all files and (sub)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