chmod Command Guide

Understanding Permissions

Linux file permissions are divided into three categories for three groups:

Permission Types

r (read)	View file contents or list directory
w (write)	Modify file or create/delete in directory
x (execute)	Run file or access directory

Permission Groups

u (user)	The file owner
g (group)	Users in the file group
o (others)	All other users
a (all)	All users (u, g, and o)

View current permissions with ls -l:

bash

ls -l file.txt
-rw-r--r-- 1 user group 1024 Jan 15 10:30 file.txt

Symbolic Mode

Symbolic mode uses letters to modify permissions.

Add permission (+)

bash

# Add execute for owner
chmod u+x script.sh

# Add read for group
chmod g+r file.txt

# Add write for others
chmod o+w file.txt

Remove permission (-)

bash

# Remove write from group
chmod g-w file.txt

# Remove all permissions for others
chmod o-rwx file.txt

Set exact permission (=)

bash

# Set exact permissions for user
chmod u=rwx file.txt

# Set read-only for all
chmod a=r file.txt

Combined operations

bash

# Multiple changes at once
chmod u+x,g-w,o-rwx file.txt

# Same permission for multiple groups
chmod ug+x file.txt

Octal (Numeric) Mode

Octal mode uses numbers to set all permissions at once.

Octal Values

4	Read (r)
2	Write (w)
1	Execute (x)
0	No permission

Add the values for each permission group (owner, group, others):

bash

# 755 = rwxr-xr-x (owner: all, group/others: read+execute)
chmod 755 script.sh

# 644 = rw-r--r-- (owner: read+write, group/others: read)
chmod 644 file.txt

# 700 = rwx------ (owner: all, group/others: none)
chmod 700 private.txt

# 777 = rwxrwxrwx (everyone: all) - Use with caution!
chmod 777 file.txt

Common Octal Permissions

755	rwxr-xr-x - Standard for executables
644	rw-r--r-- - Standard for files
700	rwx------ - Private executables
600	rw------- - Private files
775	rwxrwxr-x - Group writable
664	rw-rw-r-- - Group writable files

Warning

Avoid using chmod 777 as it gives everyone full access. Use the minimum permissions necessary.

Recursive Changes

Use -R to change permissions recursively.

bash

# Change all files in a directory
chmod -R 755 /path/to/directory

# Make all scripts executable
chmod -R u+x scripts/

Tip

When applying permissions recursively, consider using find to target only files or directories:

bash

# Set 755 for directories only
find /path -type d -exec chmod 755 {} \;

# Set 644 for files only
find /path -type f -exec chmod 644 {} \;

Practical Examples

Make a script executable

bash

chmod +x script.sh
# or
chmod 755 script.sh

Protect sensitive files

bash

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

Web server permissions

bash

# HTML files readable by all
chmod 644 /var/www/html/*.html

# CGI scripts executable
chmod 755 /var/www/cgi-bin/*.cgi

Shared directory for a group

bash

chmod 775 /shared/project
chmod g+s /shared/project  # Set group ID bit

Special Permissions

Special Permission Bits

setuid (4000)	Run as file owner
setgid (2000)	Run as file group
sticky (1000)	Only owner can delete

bash

# Set setuid
chmod u+s file
chmod 4755 file

# Set setgid on directory
chmod g+s directory
chmod 2755 directory

# Set sticky bit (common on /tmp)
chmod +t directory
chmod 1755 directory

Info

The sticky bit on a directory prevents users from deleting files they don't own, even if they have write permission on the directory.

Summary

Understanding chmod is essential for Linux system administration. Key takeaways:

Use symbolic mode (u+x) for simple changes
Use octal mode (755) for setting complete permissions
Common patterns: 755 for scripts, 644 for files, 700/600 for private
Use -R for recursive changes
Always use minimum necessary permissions for security

Quick Reference

Symbolic

Octal

Common

Special

Downloadable Image Preview

Understanding Permissions

Permission Types

Permission Groups

Symbolic Mode

Add permission (+)

Remove permission (-)

Set exact permission (=)

Combined operations

Octal (Numeric) Mode

Octal Values

Common Octal Permissions

Recursive Changes

Practical Examples

Make a script executable

Protect sensitive files

Web server permissions

Shared directory for a group

Special Permissions

Special Permission Bits

Summary

Related Articles

ls Command Guide

find Command Guide