Definition:
It defines the three modes of a file or directory: read, write, and execute. When you want to control who can perform these actions on a file, you decide and assign specific permissions to each user.
Syntax:
chmod [options] permissions filenameSet Permissions with chmod (Step-by-Step for Files and Folders):
Step 1: Decide who you want to permit:
u→ user (the file owner)g→ groupo→ others (everyone else)a→ all (user + group + others)
Step 2:Decide what permission you want to give by Symbolic or Numeric Method
List of permissions:
r→ read (ability to read the file)w→ write (ability to edit or delete the file)x→ execute (ability to run the file/program)
Step 3: Symbolic Method
Use letters (u, g, o) with +, -, =:
u→ user/ownerg→ groupo→ others+→ adds permission-→ removes permission=→ sets exact permission
Example:
chmod u+x file.txt
chmod g-w file.txt
chmod o=r file.txtExplanation:
u+x→ Owner can now execute the file.g-w→ Group cannot write to the file anymore.o=r→ it lets others read the file only, removing all other permissions.
Note: This method is useful when you want to change specific permissions without affecting others.
Step 3: Numeric Method
Assign numbers to permissions (r=4, w=2, x=1) and combine them to set access.
Examples:
chmod 755 file.txt # owner=rwx, group=r-x, others=r-x
chmod 644 file.txt # owner=rw-, group=r--, others=r--Explanation:
- 7 = 4+2+1 → rwx
- 5 = 4+1 → r-x
- 5 = 4+1 → r-x
When the number 7 is given to the owner, they can read, write, and execute the file, while 5 is given to the group and others, which means they can only read and execute it.
Step 4: Apply the chmod command on the file or folder:
chmod [options] permissions filenameStep 5: Check the permissions to verify:
ls -l file.txtOutput will show something like:
-rwxr-xr-x 1 user group 0 Apr 05 12:00 file.txt