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File permissions

Revision as of 11:47, September 27, 2014 by Pytony (talk | contribs) (Added a general overview of user rights in Linux systems.)

File permissions

With Linux, the most common way to handle user rights provides three distinct rights on files. The meaning of these rights for directories (which are files in Linux) is slightly different.

Subject Right Description Typical granted commands
File r (4) Read cat f, less f, grep f, file f
w (2) Write sed -i f, shred f, truncate f, vi f
x (1) Execution /absolute/path/to/f, relative/path/to/f
Directory r (4) List contents ls d
w (2) Create/Remove files touch d/a_file, mkdir d/a_dir, rm d/a_file, rmdir d/a_dir, chmod d/a_file, chown d/a_dir
x (1) Browse hierarchy cd d, pushd d

File permissions are split into three categories of users:

The owner of the file (u as user)
Typically the creator of the file
The group of the file (g as group)
Typically the main group of the owner
The others (o as others)
Anybody else

As you would have notice, this does not provide a fine-grained way to manage permissions, but this is quite light, simple, and sufficient for most usages. However, if you think you need a really fine-grained level, you should consider looking at SELinux.

Manage user and groups

Add user

You can add user with useradd.

root # useradd -g users -G wheel,portage,audio,video,usb,cdrom,tty -m <username>

Delete user

You can delete user with userdel.

root # userdel <username>

If you want to remove user files as well (home directory and mail spool, use the -r option:

root # userdel -r <username>

List groups

You can list groups with group.

user $ groups
user $ groups <username>

Add or remove user from group

You can add or remove user from group with gpasswd.

root # gpasswd -a <user> <group>
root # gpasswd -d <user> <group>

Create new group

You can create new group with groupadd.

root # groupadd <group>

Delete group

You can also delete group with groupadd.

root # groupdel <group>

Manage rights on files

Change file permissions

You can change file permissions with chmod.

user $ chmod <u><g><o> <file>

Where <u>, <g> and <o> are respectively the octal representation of the rights you want to set for the owner, the group and others.

7 = 4+2+1 (read/write/execute)
6 = 4+2 (read/write)
5 = 4+1 (read/execute)
4 = 4 (read)
3 = 2+1 (write/execute)
2 = 2 (write)
1 = 1 (execute)

Change owner and group of file

You can change owner and group of a file with chown.

root # chown <user>:<group> <file>

You can change owner of a directory and children recursively with:

root # chown -R <user>:<group> <folder>