Tips and commands related to Linux file permissions, users and groups.

linux-permissions.md

Linux permissions

In Linux, each and every file is owned by a single user and a single group, and has its own access permissions.

Users

To list all users on a Linux machine:

cat /etc/passwd

Groups

To list all groups on a Linux machine:

cat /etc/group

To check to which groups a user belongs, simply run the command:

groups

Permissions

To check who owns a file, we can use the list command together with the -l option

ls -l <full file name>

The "mode" column

After using the ls -l command, we get a table of files and their owners and permissions. The first column, the "mode" column is made of 10 letters. For example, drw-rw-rw-. These letters mean:

d	rw-	rw-	rw-
File type	User (owner) Permissions	Group Permissions	Other Permissions

File type

In Linux, there are two basic types of files: normal and special. Special files can be identified by files that have a non-hyphen character, such as a letter, in their file type fields, and are handled by the OS differently than normal files.

Character	File Type
-	regular file
d	directory
c	character device
b	block device
l	symbolic link
and others...

Permission codes

Character	Meaning
r	read
w	write
e	execute
-	permission not available

Examples of common modes

mode	explanation
-rw-------	A file that is only accessible by its owner
-rwxr-xr-x	A file that is executable by every user on the system. A "world-executable" file
-rw-rw-rw-	A file that is open to modification by every user on the system. A "world-writable" file
drwxr-xr-x	A directory that every user on the system can read and access
drwxrwx---	A directory that is modifiable (including its contents) by its owner and group
drwxr-x---	A directory that is accessible by its group

Changing file permissions

To change a file permission settings, we use the chmod command. This command can be used by using a combination of numbers or letters as parameters.

The chmod command has a few useful options:

• -R: for recursively applying the same permissions to a directory, and all the subdirectories and files in the directory and its different subdirectories
• -c: to report when a change is made
• -f: to suppress error messages
• -v: (verbose) to output diagnostics for every file or directory processed

Numbers as parameters

We use a sum of three different numbers to set a file permission for the three modes (user, group, others). These special numbers are listed below.

Number	Meaning
4	read permission
2	write permission
1	execute permission

So, for example, if we want to change a file permission to allow the user to do anything (read + write + execute = 4 + 2 + 1 = 7), the group to read and write (4 + 2 = 6), and the others to only execute (1 = 1), we use the following command:

chmod 761 <my file>

Letters as parameters

In this way, we need to use the following syntax to change the file permissions: chmod [references][operator][modes] <file>. Where

• [references]
  • Can be a combination of the following letters, depeding on which modes we want to modify.
    • u: users
    • g: groups
    • o: other
    • a: all
• [operator]
  • +: adds
  • -: removes
  • =: adds the ones specified and removes the ones not specified
• [operator]
  • Can be a combination of the following letters, depeding on which permission we want to modify.
    • r: read
    • w: write
    • x: execute

Examples:

• To remove all permissions from others:
  • chmod o-rwx <file>
• To give read permission to all:
  • chmod a-r <file>

Changing a file's owner and group (chown command)

To change the user owner of a file we can use:

chown <new user owner> <file>

To change the group owner of a file we can use:

chown :<new group owner> <file>

To change both, we can use the syntax:

chown <new user>:<new group> <file>

And to run the change recursively for all folders and files within a folder, we can use the -R option:

chown -R <new user>:<new group> <folder>