Unix/Linux file permissions explained

Every file and directory on Unix-like systems carries permission bits for three audiences: the owner (user), the owning group, and others. Each audience may have read (r), write (w), and execute (x). Directories treat “execute” as permission to traverse — enter the folder and access entries if you know their names.

Symbolic notation

ls -l might show -rw-r--r-- for a regular file: owner read+write, group read-only, others read-only. A leading d marks a directory; l a symlink. Our Unix/Linux File Permissions tool toggles bits and shows both symbolic and octal output.

Octal (chmod) values

Each triplet maps to a digit 0–7: r=4, w=2, x=1, summed per class. Examples:

• 644 — rw-r--r-- (common for web files)
• 755 — rwxr-xr-x (executable or directory)
• 600 — rw------- (private file)

Worked example

You want a script runnable only by you: user rwx, group none, others none → 4+2+1, 0, 0 = 700 (chmod 700 deploy.sh). A world-readable config with write restricted to owner: 644.

Special bits (brief)

Setuid, setgid, and sticky bits extend the model (fourth digit in some chmod forms). The calculator on this site focuses on the standard rwx triplets; consult your system’s chmod manual for setuid on executables or the sticky bit on /tmp.

FAQs

Why can I delete a file I cannot write?

Directory write permission controls creating and removing names; file write is separate.

umask?

Processes subtract umask from default creation modes (often 022 → files 644, dirs 755).

ACLs?

Extended ACLs can grant per-user rights beyond the nine bits; ls may show a +.

Sources

POSIX file permission model; GNU coreutils chmod documentation.