chmod Calculator (Linux File Permissions)

Linux file permissions look cryptic at first glance: 755, rwxr-xr-x, chmod u+x file — three different ways to say the same thing. This tool lets you toggle the nine permission bits (owner / group / other × read / write / execute) plus the special setuid / setgid / sticky bits, and see all representations side-by-side. Also runs in reverse — type an octal value and the checkboxes light up.

How to use the chmod Calculator (Linux File Permissions)

Click the checkboxes to toggle each permission. Owner = the file's user owner; Group = the file's group owner; Other = everyone else. Read (r) lets you view file contents or list a directory; Write (w) lets you modify; Execute (x) lets you run a binary or enter a directory.

The octal value updates live. To go in reverse, type any 3- or 4-digit octal in the "Or type octal" field — the checkboxes match. Octal digits are sums: read = 4, write = 2, execute = 1; so 7 = read + write + execute, 5 = read + execute, 6 = read + write.

About chmod Calculator (Linux File Permissions)

Unix file permissions encode nine bits across three classes (user / group / other) × three permission types (read / write / execute). These nine bits are conventionally displayed as three octal digits (each digit = three bits = sum of 4+2+1) or as a 9-character string of r/w/x letters.

Three additional bits exist in the high octal digit:

• setuid (4000) — when set on an executable, the program runs with the file owner's privileges instead of the calling user. Used by tools like sudo, passwd, ping. Security-sensitive; the file owner is effectively a privilege boundary.
• setgid (2000) — same idea but for group. On a directory, setgid means new files inherit the directory's group ownership (useful for shared project dirs).
• sticky (1000) — on a directory, only the file owner can delete their files (even if the directory is world-writable). Used on /tmp.

The chmod command accepts both octal (chmod 755 file) and symbolic (chmod u+x file = add execute for user) forms. Symbolic is better for incremental changes ("add execute"); octal is better for setting absolute permissions ("set to 755").

For directories: r = list contents; w = create/delete files in it; x = enter / use as cwd / look up files inside. Note: you can have r without x on a directory and lose useful access (you can list names but not stat or open files).

Common use cases

• Setting SSH key permissions — chmod 600 ~/.ssh/id_rsa (most SSH clients refuse to use keys with looser permissions).
• Web server file deploys — typically 644 for files, 755 for directories.
• Sharing a project directory — 770 + setgid so team members can edit each other's files.
• Reading mysterious chmod commands in shell scripts — quickly decode chmod 2755 (setgid + standard executable).
• Debugging permission-denied errors — see exactly what bits are set vs what's needed.