chmod - Change File Mode Bits

Synopsis

chmod [OPTION]... MODE[,MODE]... FILE...
chmod [OPTION]... OCTAL-MODE FILE...
chmod [OPTION]... --reference=RFILE FILE...

Change the file mode bits of each FILE to MODE. MODE can be symbolic or octal.

Flags

Permission Modes

Octal Mode

Each digit represents user/group/others: read=4, write=2, execute=1.

Symbolic Mode

Format: [ugoa][+-=][rwxXstugo]

Examples

# Make script executable
chmod +x script.sh

# Set standard file permissions
chmod 644 file.txt

# Set standard directory permissions
chmod 755 dir/

# Recursive permission change
chmod -R 755 dir/

# Add execute for directories only (capital X)
chmod -R u=rwX,g=rX,o=rX dir/

# Remove write permission for group and others
chmod go-w file.txt

# Set exact permissions for user, add read for group
chmod u=rwx,g+r file.txt

# Copy permissions from another file
chmod --reference=source.txt target.txt

# Verbose output
chmod -v 644 file.txt

# Set setuid bit
chmod u+s /usr/local/bin/program

# Set sticky bit on directory
chmod +t /tmp/shared/

# Make files read-only
chmod a-w file.txt

Gotchas

• Never use 777 in production. It gives everyone full access.
• chmod -R 755 sets execute on ALL files. Use find with -type to differentiate:

  find dir/ -type d -exec chmod 755 {} +
  find dir/ -type f -exec chmod 644 {} +

• Capital X is crucial for recursive operations: it adds execute only to directories and files that already have execute, avoiding making every file executable.
• chmod does not follow symlinks by default. The permissions of a symlink itself are typically ignored; the target's permissions matter.
• Setuid/setgid (chmod u+s, g+s) on scripts is ignored by most modern kernels for security reasons. It only works on compiled binaries.