Last updated: 2026-06-28

Chmod Permissions

POSIX file permissions control which users can read, write, or execute a file on Unix and Linux systems. Every file and directory carries three permission triads — one each for the owning user (u), the owning group (g), and all others (o) — and chmod is the command that sets them. This calculator converts between the compact octal notation (e.g., 755) and the human-readable symbolic form (e.g., rwxr-xr-x) in both directions.

How permission bits work

Each triad holds three bits: read (r), write (w), and execute (x). A bit is either set (the permission is granted) or unset (it is denied, shown as a hyphen). The nine-character symbolic string follows the order:

r w x r - x r - x
└─┬─┘ └─┬─┘ └─┬─┘
 owner  group others

The first three characters belong to the owner, the next three to the group, and the last three to others.

Octal encoding

Each triad converts to a single octal digit (0–7) by assigning fixed weights to the three bits:

A triad's digit equals the sum of the weights of its set bits. For example, read + execute = 4 + 1 = 5, and read + write + execute = 4 + 2 + 1 = 7. The three digits are written in order — owner, group, others — giving a three-digit octal code.

Worked example: 755

Result: 755 → rwxr-xr-x. The owner can read, write, and execute the file; members of the owning group and all other users can read and execute it but cannot write to it. This is the conventional permission for executables and public directories.

Common permission patterns

Applying permissions with chmod

Once you have the octal code, apply it with:

chmod 755 myscript.sh
chmod -R 644 public/

The -R flag applies the mode recursively to all files and subdirectories. Note that chmod -R 644 on a directory removes execute, which prevents navigating into it — use find with separate -type f and -type d branches when you need different modes for files and directories:

find . -type f -exec chmod 644 {} +
find . -type d -exec chmod 755 {} +

Special permission bits

Beyond the nine standard bits, POSIX defines three additional bits encoded as an optional leading digit:

• Setuid (4xxx) — when set on an executable, the process runs with the file owner's effective user ID rather than the caller's. Common on system utilities like passwd.
• Setgid (2xxx) — on a file, the process inherits the file's group. On a directory, new files created inside inherit the directory's group rather than the creator's primary group.
• Sticky bit (1xxx) — on a directory, only the file owner, directory owner, or root can delete or rename files within it. Applied to /tmp on most Unix systems.

These bits appear before the three-digit mode, e.g., chmod 4755 binary sets setuid on an executable with 755 permissions.