Changing File Permissions

The chmod command changes permissions of a file or directory. Only the file owner or root user can change permissions.

Why is the command named chmod instead of chperm?

Permissions used to be referred to as modes of access, so the command chmod really means change the modes of access.

There are two methods to change file or directory permissions:

  1. Symbolic Method

    • Good for changing one set of permissions at a time.
    • Uses symbols like u (user), g (group), o (other) and +, -, =.
    • Example: chmod u+x file.sh → add execute permission for owner.

  2. Octal (Numeric) Method

    • Uses octal values for permissions (r=4, w=2, x=1).
    • Requires specifying all three sets (user, group, other) at once.

    • Requires understanding octal values for each permission.

    • Example: chmod 754 file.sh

      • 7 (rwx) → user,5 (r-x) → group, 4 (r--) → others
chmod 755 filename  # User (rwx), Group (r-x), Others (r-x)
chmod 644 filename  # User (rw-), Group (r--), Others (r--)
chmod 700 filename  # User (rwx), No access for others
  • Use symbolic for small, targeted changes.
  • Use octal for full permission setup.


Here is the permission value table

Number Permission Meaning
7 rwx Read, Write, Execute
6 rw- Read, Write
5 r-x Read, Execute
4 r-- Read only
3 -wx Write, Execute
2 -w- Write only
1 --x Execute only
0 --- No permissions 
chmod [<SET><ACTION><PERMISSIONS>]... FILE

  • 1. Permission Sets : < SET >

    • User(u),Group(g),Others(o),All(a)

  • 2. Action Symbols : < ACTION>

    • Modify permissions using symbols: Add (+), remove (-), or set (=) permissions.

  • 3. Permissions < PERMISSIONS>

    • Read(r),Write(w),Execute(x)

Example : 

chmod u+x filename  # Add execute for user
chmod g-w filename  # Remove write for group
chmod o=r filename  # Set read-only for others
chmod u=rwx,g=rx,o= filename  # Set full access for user, read/execute for group, and no access for others
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