Bash by Example: Change Permissions
Modifying file access permissions using chmod command with both symbolic notation for human-readable changes and octal numeric notation for precise control, including common permission patterns for scripts, configuration files, and security-sensitive data.
Code
#!/bin/bash
touch script.sh
echo "1. Symbolic Mode"
# Add execute permission for Owner
chmod u+x script.sh
ls -l script.sh
# Remove write permission for Group and Others
chmod go-w script.sh
# Add read/write for Everyone
chmod a+rw script.sh
echo "2. Octal Mode"
# 755: rwx (7) for owner, rx (5) for group/others
chmod 755 script.sh
ls -l script.sh
# 644: rw- (6) for owner, r-- (4) for group/others
# Standard for regular files
chmod 644 script.sh
# 600: rw- (6) for owner, --- (0) for others
# Private file (SSH keys, secrets)
chmod 600 script.sh
rm script.shExplanation
The chmod (change mode) command is the primary tool for modifying file and directory permissions in Unix-like systems. It offers two distinct syntaxes for specifying permissions: symbolic notation and octal (numeric) notation. Symbolic mode is more intuitive for making incremental changes, while octal mode provides a concise way to set all permissions at once.
Symbolic Mode uses letters to represent who is affected and what changes to make:
u(user/owner),g(group),o(others),a(all three categories)- Operators:
+adds permissions,-removes them,=sets exactly - Permissions:
r(read),w(write),x(execute) - Example:
chmod u+x,go-w fileadds execute for owner and removes write for group and others
Octal Mode uses three-digit numbers where each digit is the sum of permission values. Each digit represents one category (owner, group, others), and each permission type has a numeric value: Read=4, Write=2, Execute=1. For example, chmod 755 file means owner gets 7 (4+2+1=rwx), while group and others get 5 (4+1=r-x). Common patterns include 644 for regular files (owner can read/write, others can only read), 755 for executable files and directories, and 600 for private files like SSH keys.
Code Breakdown
chmod u+x adds execute permission only for the owner. This is safer than chmod +x (or a+x) which would grant execute to everyone.chmod go-w removes write permission from group and others, a common security hardening step.chmod 755 is the standard permission pattern for executable scripts and directories. Owner has full control (rwx), while group and others can read and execute (r-x).644 for regular files, 600 for private files (SSH keys, credentials). The more restrictive, the more secure.