chmod 400 (r--------)
What chmod 400 does, who gets which permissions, and when to use it — with the exact commands.
| Read (4) | Write (2) | Execute (1) | |
|---|---|---|---|
| Owner | ✔ read | — | — |
| Group | — | — | — |
| Others | — | — | — |
Notes
When you'd use 400: Read-only for the owner, invisible to everyone else — the permission AWS requires on EC2 .pem key files (ssh refuses keys that are readable by others).
Unix permissions are octal digits — owner, group, others — where read=4, write=2, execute=1, summed per digit. An optional leading digit holds setuid (4), setgid (2) and the sticky bit (1). On a directory, "execute" means permission to enter it and reach files inside.
Other values: 444 · 500 · 555 · 600 · 640 · 644 · 664 · 666 · 700 · 711 · 750 · 755 · 770 · 775 · 777 · 1777 · 2755 · 4755
Frequently asked questions
What does chmod 400 mean?
chmod 400 sets the permission r--------. The owner can read; Group members can do nothing; Everyone else can do nothing.
What is the symbolic notation for chmod 400?
r-------- — three triplets (owner, group, others) of read/write/execute flags.
How do I apply chmod 400 to a whole directory?
Run chmod -R 400 yourdir to apply it recursively. Be careful: files and directories usually should not share the same mode (directories need execute to be entered, regular files usually should not be executable).