Overview

chmod changes file modes. Use it to grant or remove read, write, and execute permissions.

Syntax

chmod [options] mode file...

Arguments

• mode: Symbolic (u+rw) or numeric (640) permissions.
• file: One or more targets.

Options Table

Examples

chmod 640 app.log
chmod -R u=rwX,g=rX,o= /srv/app

Real-world Usage

Set logs to read-only for developers while keeping write access for operators.

Common Mistakes

• Using chmod -R without verifying the directory scope.
• Setting execute bits on data files.

Performance Notes

Recursive operations can be slow on large trees. Use targeted paths.

Security Considerations

Avoid world-writable permissions on production systems.

Related Commands

chown, umask, stat

Practice Exercises

1. Set a directory to 750 and explain who can access it.
2. Copy permissions from one file to another using --reference.