File Permissions

Overview

Every file and directory on a Unix-like system has permissions that control who can read, write, or execute it. Permissions apply to three groups: the owner, the group, and everyone else. Understanding them is essential for security and for making scripts runnable.

Syntax / Usage

Use ls -l to view permissions and chmod to change them. Permissions can be set with symbols or octal numbers.

ls -l script.sh              # view permissions, e.g. -rwxr-xr--

chmod +x script.sh           # add execute for everyone
chmod u+x script.sh          # add execute for the owner only
chmod 755 script.sh          # rwx for owner, r-x for group and others
chmod 644 notes.txt          # rw for owner, r for group and others

chown alice notes.txt        # change the file's owner

Examples

Make a shell script executable so you can run it directly:

chmod +x deploy.sh
./deploy.sh

Lock down a private key so only you can read it:

chmod 600 ~/.ssh/id_rsa
ls -l ~/.ssh/id_rsa

Common Mistakes

• Using chmod 777 "to make it work" — this makes files world-writable and is a security risk
• Confusing the octal digits: 4 = read, 2 = write, 1 = execute, added together
• Forgetting that a directory needs execute (x) permission to be entered
• Editing permissions on a file you don't own and getting "Operation not permitted"
• Mixing up chmod (permissions) with chown (ownership)

See Also

command-line-files-and-directories command-line-shell-scripting