chmod

Overview

The chmod command changes file and directory permissions in Linux. It controls read, write, and execute permissions for owner, group, and others.

Syntax

chmod [options] mode file...
chmod [options] octal-mode file...
chmod [options] --reference=rfile file...

Common Options

Option Description
-R Recursive operation
-v Verbose output
-c Report changes only
-f Suppress error messages
--reference=file Use file’s permissions
--preserve-root Protect root directory

Permission Modes

Symbol Meaning
r Read permission (4)
w Write permission (2)
x Execute permission (1)
u User/owner
g Group
o Others
a All (u+g+o)

Octal Notation

Octal Binary Permissions
0 000 —
1 001 –x
2 010 -w-
3 011 -wx
4 100 r–
5 101 r-x
6 110 rw-
7 111 rwx

Key Use Cases

  1. Set file permissions
  2. Make files executable
  3. Secure sensitive files
  4. Configure directory access
  5. Batch permission changes

Examples with Explanations

Example 1: Make File Executable

chmod +x script.sh

Adds execute permission for all users

Example 2: Set Specific Permissions

chmod 755 file.txt

Sets rwxr-xr-x permissions (owner: rwx, group/others: r-x)

Example 3: Recursive Directory Permissions

chmod -R 644 /path/to/directory

Sets rw-r–r– permissions recursively

Understanding Permission Strings

Format: drwxrwxrwx - First character: file type (d=directory, -=file, l=link) - Next 3: owner permissions (rwx) - Next 3: group permissions (rwx) - Last 3: other permissions (rwx)

Common Usage Patterns

  1. Secure private file:

    chmod 600 private.txt

  2. Public readable directory:

    chmod 755 public_dir/

  3. Remove all permissions:

    chmod 000 restricted_file

Special Permissions

Permission Octal Description
Setuid 4000 Run as owner
Setgid 2000 Run as group
Sticky bit 1000 Restrict deletion

Performance Analysis

  • Minimal performance impact
  • Recursive operations can be slow on large directories
  • Use find with -exec for complex permission changes
  • Consider using parallel processing for large datasets

Additional Resources

Best Practices

  1. Use least privilege principle
  2. Be careful with recursive operations
  3. Test permissions before applying
  4. Document permission requirements
  5. Use symbolic notation for clarity

Security Considerations

  1. Avoid 777 permissions
  2. Protect sensitive files (600/700)
  3. Use setuid/setgid carefully
  4. Regular permission audits
  5. Monitor permission changes