Changing Permissions (chmod)

The chmod command changes file permissions. It's essential for controlling access to files and making scripts executable.

Basic Usage

chmod mode filename

Two Modes: Symbolic and Numeric

Symbolic Mode

Uses letters and symbols:

chmod u+x file.sh       # Add execute for user
chmod g-w file.txt      # Remove write for group
chmod o=r file.txt      # Set others to read only
chmod a+x script.sh     # Add execute for all

Symbol	Meaning
u	User (owner)
g	Group
o	Others
a	All (u+g+o)
+	Add permission
-	Remove permission
=	Set exactly

Numeric Mode

Uses octal numbers:

chmod 755 script.sh     # rwxr-xr-x
chmod 644 file.txt      # rw-r--r--
chmod 700 private/      # rwx------
chmod 600 secret.txt    # rw-------

Common Permission Sets

Mode	Permission	Use Case
755	rwxr-xr-x	Executables, directories
644	rw-r--r--	Regular files
700	rwx------	Private directories
600	rw-------	Private files
777	rwxrwxrwx	Open to all (avoid!)
400	r--------	Read-only for owner

Exercise: Make Script Executable

Make a script executable:

Recursive Changes

Change permissions on directory and contents:

chmod -R 755 directory/      # All files and subdirs
chmod -R u+w project/        # Add write to all

chmod Options

Option	Description
-R	Recursive (directories)
-v	Verbose (show changes)
-c	Report only changes
--reference	Copy from another file

Symbolic Examples

# Make executable
chmod +x script.sh          # For all
chmod u+x script.sh         # Owner only

# Remove permissions
chmod go-w file.txt         # Remove write from group/others
chmod o-rwx private.txt     # Remove all from others

# Set specific permissions
chmod u=rwx,g=rx,o=r file   # Specific for each

Numeric Examples

# Calculate: r=4, w=2, x=1
chmod 754 file    # rwxr-xr--
#         7 = 4+2+1 = rwx (user)
#         5 = 4+0+1 = r-x (group)
#         4 = 4+0+0 = r-- (others)

Common Mistakes

# Too permissive (security risk)
chmod 777 file              # Everyone can do everything

# Script won't run
./script.sh                 # Permission denied
chmod +x script.sh          # Fix: add execute

# Can't enter directory
chmod 644 mydir/            # No x on directory
chmod 755 mydir/            # Fix: add execute

Key Takeaways

• chmod changes file permissions
• Symbolic: chmod u+x (letters and operators)
• Numeric: chmod 755 (octal numbers)
• Use -R for recursive changes
• 755 for executables/directories, 644 for files