How to use
- Tick the permissions for owner, group and others, or type an octal value or a symbolic string.
- Add setuid, setgid or the sticky bit if you need them.
- Copy the chmod command in numeric or symbolic form.
Options explained
- Octal
- Each digit adds read (4), write (2) and execute (1) for owner, group and others. 755 = rwx for the owner, r-x for group and others. A fourth leading digit sets the special bits.
- Special bits
- setuid (4) runs a program as its owner, setgid (2) runs it as its group or makes new files in a directory inherit the group, and the sticky bit (1) on a directory such as /tmp lets only owners delete their files.
Examples
| Octal | Symbolic | Typical use |
|---|---|---|
| 600 | rw—–– | Private keys, secrets |
| 644 | rw-r–r– | Regular files |
| 755 | rwxr-xr-x | Directories, scripts |
| 1777 | rwxrwxrwt | /tmp |
Frequently asked questions
What do 644 and 755 mean?
644 (rw-r--r--) is typical for files — the owner can edit, everyone can read. 755 (rwxr-xr-x) is typical for directories and scripts — everyone can read and enter or run, only the owner can change.
Why should I avoid 777?
It lets every user on the system change or replace the file. Web servers and SSH refuse or warn about world-writable files for this reason.
What does a capital S or T mean in ls output?
The special bit is set but the matching execute bit is not, which is usually a mistake.
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