Lion Cipher Lab

Understand the umask and default file permissions

• Foundations
• Beginner
• ~25 min
• Free · no account required

Why it matters

Every file you create on Linux is born with a set of permissions you never typed — and the umask is the quiet rule that decided them. Get it wrong and a config file full of secrets lands world-readable, or a shared-group folder silently blocks your teammates. This is one of the first things a defender checks on a hardened host and one of the first things an attacker checks on a sloppy one, so understanding it turns "permissions are mysterious" into "I can predict and control exactly who can read my files."

What you'll learn

• Reading permission bits with `ls -l` and `stat -c '%a'`
• Predicting a new file's or directory's mode from the umask
• Setting and resetting the umask for a shell session
• Understanding the base-permission rule (666 for files, 777 for directories)
• Knowing the limits of umask (existing files, per-session scope, setuid/setgid)

Steps in this lab

1. What the umask is, in one breath
2. Open a plain Linux shell
3. See your current umask and create a baseline file
4. Tighten the mask to 077 and watch files lock down
5. Loosen it to 002 for a shared-group workflow
6. Honest limit #1: the umask never touches files that already exist
7. Honest limit #2: per-session scope, and the program still gets a say
8. What you just proved — and why it matters

Open the full interactive lab →