Package:Bash

File permissions

Common 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 (Oct. repr.)	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

You would notice that rights octal representation is coded with powers of 2. This is a common way to represent bunch two-states settings that can be independently toggled. Indeed, a file does not properly have a list of permissions set, you should see this rather as a a bit string (where a 1 at the position i means ON and a 0 means OFF for the right coded 2ⁱ).

An example is worth 1000 words:

rwx    Octal    Permissions
000      0      None
001      1      Execution only
010      2      Read only
100      4      Write only
111      7      All (ie. Read and Write and Execution)
110      6      All but Execution (ie. Read and Write)

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

File permissions are thus represented with nine bits. The three most significant representing the owner rights and the three least significant representing others rights. For instance, a typical file permission is 640 which means The owner can read an write, the group have a read-only access, and other can't even read it.

Alter permissions meaning

There is actually three more bits that allow you to alter the meaning of other permissions

Subject	Right (Oct. repr.)	Name	Description
File	s/S (4)	Setuid bit	-
	s/S (2)	Setgid bit	-
	t/T (1)	Sticky bit	-
Directory	s/S (4)	Setuid bit	-
	s/S (2)	Setgid bit	-
	t/T (1)	Sticky bit	-

Going further

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

Users, and Groups are named, and numbered. the lower the number the more permissions the account has. For example root user has the number 0, and root group has the number 0. To display this information:

root # cat /etc/passwd
root # cat /etc/group

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>

   Note

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 or delete groups

You can create or delete groups with groupadd.

root # groupadd <group>
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>

Security

Generally you will want to have restrictive yet functional permissions. 777 on everything is a bad idea, especially files containing plain text passwords. 600 is common for files like this, with a high level user. mediawiki's LocalSettings.php has database passwords. A good method to lock this down is to change its permissions to 600, and set the file owner as the webserver's user.

Can I have write permission on a file while not being allowed to read it?

Yes, you can! Example:

root ##i### echo "$USER: You can't read! >:)" > /tmp/test
root ##i### ls -l /tmp/test
-rw-r--r-- 1 root root 6 Oct  2 07:30 /tmp/test
root ##i### chmod o-r+w /tmp/test
root ##i### ls -l /tmp/test
-rw-r---w- 1 root root 6 Oct  2 07:30 /tmp/test
root ##i### cat /tmp/test
root: You can't read! >:)
root ##i### su anyuser
root ##i##$ cat /tmp/test/
cat: /tmp/test: Permission denied
root ##i##$ vi /tmp/test/
---[Permission Denied]---
root ##i##$ echo "$USER: But I can write! :)" >> /tmp/test
root ##i##$ exit
root ##i### cat /tmp/test
root: You can't read! >:)
anyuser: But I can write! :)

I don't know if this has an actual application though. Maybe if you need to allow some users to write (and truncate) logs in the same file but you don't want them to be able read what others wrote...