Linux Users Groups and Permissions

Linux Permissions

Users and Groups

Linux file and directory permissions are based on users and groups. A Linux user is an account on a Linux operation system that allows an individual or entity to log in and interact with the system. There are two types of user accounts - regular and system users. Regular users are for personal use by an individual and system users are meant to be used for system services.

User management

Add a new user

sudo useradd -m username

-m creates a home directory (e.g. /home/username).
After creating, set a password:

sudo passwd username

Delete a user

sudo userdel -r username

-r removes the home directory and mail spool.

Modify a user

sudo usermod options username

Example: change login name:

sudo usermod -l newname oldname

Example: change home directory:

sudo usermod -d /new/home username

Group management

A Linux group organizes users. A group can contain multiple users and a user can be part of multiple groups. However groups are not hierarchical, meaning you can’t add a group inside another group. Groups are important for managing permissions. Instead of allowing/forbidding access to directories/files for every single user, you can set permissions on a group level that affect multiple users.

Add a group

sudo groupadd groupname

sudo groupdel groupname

Add a user to a group

sudo usermod -aG groupname username

-aG append user to supplementary group(s) (don’t drop existing ones).

Change a user’s primary group

sudo usermod -g groupname username

Every user has one primary group and can belong to multiple supplementary groups.

Check which groups a user is in

groups username

id username

Useful Files (wher this info lives)

/etc/passwd user accounts
/etc/shadow passwords (hashed, only root can read)
etc/group group definitions and memberships

Linux File and Directory Permissions

1. Viewing Permissions with `ls -l`

When you run:

ls -l

You might see output like this:

-rw-r--r--  1 alice students  1234 Sep  7 12:00 notes.txt
drwxr-x---  2 bob   teachers  4096 Sep  7 11:30 projects

Breakdown of columns:

File type & permissions
- -rw-r--r-- file type (- = regular file) and permissions.
- d at the start means directory.
Links number of hard links.
Owner (user) who owns the file (alice).
Group group ownership (students).
Size file size in bytes.
Date/Time last modification.
Name filename.

2. Understanding Permissions (`r`, `w`, `x`)

Permissions are shown in three groups:

-rw-r--r--
 u  g  o

User (owner) first 3 characters (rw-)
Group next 3 characters (r--)
Others last 3 characters (r--)

What `r`, `w`, `x` mean:

For files:
- r can read the file’s contents.
- w can modify or delete the file.
- x can execute the file (if it’s a program or script).
For directories:
- r can list directory contents (ls).
- w can create/delete files in the directory.
- x can enter (cd) into the directory.

3. Setting Permissions

Symbolic Syntax

chmod u+x file.txt

u = user, g = group, o = others, a = all
+ = add, - = remove, = = set exactly

Examples:

chmod g+w file.txt    # give group write access
chmod o-r file.txt    # remove read permission for others
chmod a=r file.txt    # set read-only for everyone

Octal Syntax

Permissions can also be expressed as numbers. You can convert one permission group (r/w/x) to a number by first representing it in binary form. If the permission is set, use letter 1, if it’s not set use letter 0.

r-x = 101
-w- = 010
rwx = 111
--- = 000

If you read the binary number on the right side as octal number (in this case it’s the same as decimal, because the highest number is 7), you can express the permissions for one group in the form of one letter (0-7)

r-x = 101 = 5
-w- = 010 = 2
rwx = 111 = 7
--- = 000 = 0

You can then use three octal letters to set the permissions for user, group and others in one go.

Examples:

644 rw-r--r-- (owner can read/write, group and others read-only)
755 rwxr-xr-x (owner full, group and others can read/execute)
700 rwx------ (only owner can access)

Command:

chmod 755 script.sh

4. Recursive Permissions

Apply changes to all files and subdirectories inside a directory:

chmod -R 755 myfolder

Be careful! This changes everything inside.

5. Changing Group Ownership with `chgrp`

Each file/directory has an owner and a group.

Change the group:

chgrp teachers notes.txt

Recursive (apply to folder and everything inside):

chgrp -R teachers projects/

6. Changing File Ownership with `chown`