• The chmod command (short for "change mode") allows you to modify file and directory permissions (for security purposes).
• The chown command (short for "change owner") allows you to re-allocate the owner of a file or directory (for security purposes).

Permissions

Each file in Linux has three types of permissions:

• Read (r) - Allows a user to view the contents of a file.
• Write (w) - Grants the ability to modify or delete a file.
• Execute (x) - Permits a file to be run as a program.

If none of these permissions are set, it is represented with -.

Users

Permissions are assigned to three categories of users:

• Owner - The user who owns the file.
• Group - A set of users who share file access.
• Others - All other users.

Creating/modifying users and groups

• You can use the useradd and groupadd commands to create users and groups on Linux.
• You can assign one (or many users) to one (or many groups) with usermod -aG group1,group2 user1 user2. The -a flag is important as it will append groups to the users. Using -G without -a will replace the users' current groups with the new ones.

Changing ownership

You can change ownership of a file/directory using chown:

• To change the owner (but leave the group) you can simply run: chown newowner file.txt
• To change the group (but leave the owner) you can run: chown :newgroup file.txt
• To change the owner AND the group simultaneously, you can run: chown newowner:newgroup file.txt
• To change the permissions of all files within a directory you can use the -R to recursively modify permissions: chown -R newowner:newgroup directory/.
• These commands can be run on many files with wildcards chown newowner *.txt or multiple paths chown newowner file1.txt file2.txt

Understanding permissions

You can view permissions when running ls in "list" mode: ls -l

The first column gives you a hint for the type of the object. It can have the following values:

• - regular file
• d directory
• l symlink - symbolic reference to another file or directory, similar to a shortcut
• c character device - file representing a device that handles data as a stream of bytes (e.g. serial ports, sounds cards, etc.)
• b block device - file representing a device that handles data in blocks (e.g. hard disks, USB cameras, etc.)
• p named pipe - a FIFO queue (pipe) that is represented as a file on the linux filesystem
• s socket - similar to pipes but supporting network communication or inter-process communication over the network
• D door - special file used in Solaris-based systems (not common in Linux) for inter-process communication

Then the permissions follow:

• r means read permission.
• w means write permission.
• x means executable permission.
• - means the permission is not set.

The 9 following r/w/x/- characters form the below pattern:

• The first set of three rwx characters represents the permissions for the owner.
• The second set of three rwx characters represents the permissions for the group.
• The third set of three rwx characters represents the permissions for others.

Changing permissions

Using chmod you can modify permissions either symbolically or numerically:

• Symbolic mode - chmod u+x file.txt adds execute permission to the file's owner (u for user/owner).
• Numeric mode - Permissions are represented by octal numbers. You can set permissions with values from 0 to 7.

Explaining Symbolic Mode

You can add and remove permissions with the + and - symbols respectively.

Explaining Numeric Mode

In the past, I've been really confused at the seemingly random numbers used in chmod commands. If I haven't used chmod recently, I also forget which values represent read/write/execute. Below is a handy summary!

r|w|x -|-|- 4|2|1

You can combine permissions by adding the required values above. This results in the following permissions for each value from 0-7:

value|permissions|explanation|binary -|-|-|- 0|No permissions|None of 1, 2 or 4 have been provided|000 1|Execute|Just the value 1|001 2|Write|Just the value 2|010 3|Write and Execute|Sum of 1 and 2, hence both permissions 1 and 2 are included|011 4|Read|Just the value 4|100 5|Read and Execute|Sum of 1 and 4, hence both permissions 1 and 4 are included|101 6|Read and Write|Sum of 2 and 4, hence both permissions 2 and 4 are included|110 7|Read, Write and Execute|Sum of 1, 2 and 4, hence all permissions are included|111

If I'm sharing code with others, I'll typically use the verbose symbolic mode for easier readability, but for speed and ease it can be handy to use numeric mode.