chmod changes file permissions - who can read, write, and execute a file. Understanding permissions fixes "Permission denied" errors and secures your files.

The Permission System

Every file has three permission types:

Permission Letter Number Meaning
Read r 4 View contents
Write w 2 Modify contents
Execute x 1 Run as program

And three user categories:

Category Letter Who
Owner u The file's owner
Group g Users in the file's group
Others o Everyone else

View Permissions

ls -l filename

Output:

-rwxr-xr-- 1 john staff 1024 Dec 15 file.sh

Breaking down -rwxr-xr--:

Characters Meaning
- File type (- = file, d = directory)
rwx Owner: read, write, execute
r-x Group: read, execute
r-- Others: read only

Numeric Mode

Each permission has a number:

Permission Value
Read (r) 4
Write (w) 2
Execute (x) 1

Add them up for each category:

Number Meaning
7 rwx (4+2+1)
6 rw- (4+2)
5 r-x (4+1)
4 r-- (4)
0 --- (none)

Common Permission Numbers

Mode Meaning Use case
755 Owner: all. Others: read/execute Executable scripts
644 Owner: read/write. Others: read Normal files
700 Owner only Private files
600 Owner read/write only Secrets, SSH keys
777 Everyone: everything Almost never needed

Change Permissions (Numeric)

chmod 755 script.sh
chmod 644 document.txt
chmod 600 ~/.ssh/id_rsa

Symbolic Mode

Use letters instead of numbers:

chmod u+x file        # Add execute for owner
chmod g-w file        # Remove write for group
chmod o=r file        # Set others to read only
chmod a+r file        # Add read for all (a = all)

Operators:

  • + adds permission
  • - removes permission
  • = sets exact permission

Make a Script Executable

The most common use:

chmod +x script.sh

Now you can run it:

./script.sh

Recursive Changes (-R)

Apply to directory and contents:

chmod -R 755 folder/

Be careful - this changes everything inside.

Common Scenarios

Fix "Permission denied" on a script:

chmod +x script.sh

Secure SSH keys (required by SSH):

chmod 600 ~/.ssh/id_rsa
chmod 644 ~/.ssh/id_rsa.pub
chmod 700 ~/.ssh

Make a file read-only:

chmod 444 important.txt

Reset to default permissions:

chmod 644 file.txt    # Regular file
chmod 755 folder/     # Directory

Directory Permissions

Directories need execute permission to be entered:

Permission For directories
r List contents
w Create/delete files inside
x Enter the directory 
chmod 755 folder/    # Normal directory
chmod 700 private/   # Private directory

View Numeric Permissions

stat -f "%Lp" filename

Shows just the number like 755 or 644.

Common Mistakes

Using 777:

chmod 777 file

Gives everyone full access. Almost never what you want. Use the minimum permissions needed.

Recursive on wrong directory:

chmod -R 755 /

Would break your system. Always double-check paths before -R.

Fixing Common Permission Issues

"Permission denied" running a script:

chmod +x script.sh

Can't edit a file you own:

chmod u+w file.txt

Directory won't let you in:

chmod +x directory/

chown vs chmod

Command Changes
chmod Permissions (who can do what)
chown Ownership (who owns it) 
chmod 755 file          # Change permissions
chown john:staff file   # Change owner to john, group to staff

Quick Reference

Command Result
chmod 755 file rwxr-xr-x
chmod 644 file rw-r--r--
chmod 600 file rw-------
chmod +x file Add execute
chmod -w file Remove write
chmod -R 755 dir/ Recursive

Keep Learning

Permissions are fundamental to Unix security. The free course covers this and other essential concepts.

Check it out at Mac Terminal for Humans.