chmod

What it does

chmod ("change mode") changes the permission bits on a file or directory — who can read, write, or execute it. Permissions can be set symbolically (u+x, g-w) or numerically (755, 644).

Beginner examples

• chmod +x script.sh — make a file executable for everyone
• chmod 644 file.txt — owner can read/write, everyone else read-only
• chmod u+w file.txt — add write permission for the owner
• chmod -R 755 mydir/ — apply recursively to a directory tree

chmod 644 config.yaml

Advanced examples

• Remove execute permission for group and others: chmod go-x file
• Set permissions symbolically for multiple classes at once: chmod u=rwx,g=rx,o=r file
• Apply the same permissions as another file: chmod --reference=other_file target_file
• Set the setuid bit: chmod u+s /usr/bin/some-binary (use with caution — security sensitive)

find . -type f -exec chmod 644 {} \;

Common mistakes

• Running chmod 777 on a file "just to make it work" — this grants write access to everyone, a common source of security vulnerabilities.
• Forgetting -R when trying to change permissions on an entire directory tree, and wondering why subdirectories are unaffected.
• Confusing numeric modes: 755 is rwxr-xr-x, not "rwx-r-x-r-x" — each digit is one octal value covering owner, group, other in that order.
• Making a file executable (chmod +x) when the actual problem was a missing shebang line or wrong interpreter path.

Tips

• Use chmod -R together with find and -type f/-type d when files and directories need different permissions (e.g. files 644, directories 755).
• chmod +x is shorthand for adding execute to owner, group, and other — use u+x if you only want the owner to be able to execute it.
• Check current permissions with ls -l before and after to confirm the change did what you expected.

Best practices

• Avoid 777 permissions in production; grant the minimum access actually needed (principle of least privilege).
• Prefer numeric mode (644, 755) for scripts and automation — it's unambiguous and idempotent, unlike relative symbolic changes (+x).
• Be especially careful with setuid/setgid bits (chmod u+s, chmod g+s) — they can be a security risk if applied to writable or untrusted binaries.

Real-world use cases

• Making a downloaded shell script executable before running it: chmod +x install.sh.
• Locking down a secrets file so only the owner can read it: chmod 600 .env.
• Fixing "permission denied" errors on a deployed web app's file-uploads directory.

Common interview questions

• What does chmod 755 mean? Owner gets read/write/execute (7), group and others get read/execute (5 each) — rwxr-xr-x.
• What's the difference between symbolic and numeric modes? Symbolic (u+x, g-w) modifies existing permissions relative to their current state; numeric (755) sets the absolute permission bits directly.
• Why is chmod 777 considered bad practice? It gives every user on the system read, write, and execute access, removing any access control and creating a security risk.