chmod 400
chmod 400 is r--------.
What 400 grants
| Who | Digit | Permissions | Adds up as |
|---|---|---|---|
| Owner | 4 | read | 4 = 4 |
| Group | 0 | no access | 0 |
| Others | 0 | no access | 0 |
When 400 is the right answer
Read-only for the owner and nothing for anyone else. This is what an SSH private key should be, and OpenSSH will refuse to use a key file that is more permissive.
The command
chmod 400 filename
chmod -R 400 directory/Note that nobody has the execute bit here, so this value cannot be used on a directory that anyone needs to enter. On a directory, execute grants traversal rather than the right to run something.
How it is written elsewhere
| Octal | 400 |
| Symbolic | r-------- |
As a file in ls -l | -r-------- |
As a directory in ls -l | dr-------- |
| Binary | 100 000 000 |
| World-writable | No |
Frequently asked questions
What does chmod 400 mean?
It sets owner to read, group to no access, others to no access. In symbolic notation that is r--------.
How do I apply 400?
Run chmod 400 filename. Add -R to apply it to a directory and everything inside, though be careful: a recursive numeric chmod marks every file the same way, which is rarely correct for a mixed tree of files and directories.
Is 400 safe?
It grants no write access outside the owner, so it is a reasonable setting as long as the owner is the account you intend.
What is 400 in symbolic notation?
r--------. Written as ls -l shows it, with a leading file-type character, an ordinary file would appear as -r-------- and a directory as dr--------.