chmod Calculator
Calculate Unix permissions in octal, ls -l notation and check boxes, with setuid, setgid and Sticky bits and the exact chmod command for a file or a directory.
Set the permissions of a script, a shared directory or a private file: tick the permissions or type a mode, then copy the chmod command that matches the chosen type.
Input
Enable JavaScript to change the permissions. The result shown is mode 644 for a file.
Results
Mode 0644, -rw-r--r--.
- Octal mode
0644- ls -l notation
-rw-r--r--- Octal chmod command
chmod 644 file- Symbolic chmod command
chmod u=rw,g=r,o=r,a-t file
Commands checked with GNU coreutils chmod (Linux) and macOS chmod. For a directory, the five-digit octal form and the added u-s and g-s explicitly clear setuid and setgid, which GNU would otherwise keep. ACLs and extended attributes are not taken into account.
Warnings
What this mode allows
- The owner: read the content and modify the content.
- The group: read the content.
- Others: read the content.
Understanding Unix permissions
Permissions and classes
Every file has three classes: owner (u), group (g) and others (o). Each gets read (r = 4), write (w = 2) and execute (x = 1); the octal digit of a class is their sum. 754 gives rwx to the owner, r-x to the group and r-- to others.
Special bits
A fourth digit, in front, adds setuid (4), setgid (2) and the Sticky bit (1). They are shown in place of execute: s or t when execute is granted, S or T otherwise.
Generated commands
The octal command sets the whole mode. The symbolic command assigns each class (u=, g=, o=) then sets the special bits explicitly. For a directory, the five-digit form (00755) and the added u-s and g-s clear setuid and setgid, which GNU/Linux chmod would otherwise keep; a+t and a-t set the Sticky bit the same way on Linux and macOS.
Precautions and limits
Do not grant write access to others without a reason: 777 or 666 let any user modify the content. Keep setuid and setgid for programs designed for them and check the result with ls -l. Commands checked with GNU coreutils chmod 9.1 and macOS chmod; ACLs, extended attributes and mount options such as nosuid may change the actual effect.
Frequently asked questions
What does chmod 755 mean?
7 for the owner (4 + 2 + 1: read, write, execute), 5 for the group and others (4 + 1: read and execute), that is rwxr-xr-x. A common mode for a script or a directory everyone can read.
What is the difference between chmod 644 and 755?
644 (rw-r--r--) grants execute to nobody: it suits data files. 755 adds execute for everyone: it suits programs, scripts and directories, which execute lets you enter.
How do I make a script executable?
Grant execute to the owner, for example 744 or 755. The generated commands set the whole mode; chmod u+x script only adds execute for the owner and leaves the rest unchanged.
What does chmod 777 mean and why avoid it?
Every permission for everyone: any user can modify or replace the file. Prefer 755 or 644, and 700 or 600 for private content; for a shared directory, 1777 adds the Sticky bit.
What are setuid, setgid and the Sticky bit for?
setuid and setgid run a program with the rights of its owner or group; on a directory, setgid makes new entries inherit its group. The Sticky bit lets only the owner of each entry delete it, as in /tmp.
Why do directory permissions behave differently?
On a directory, r lists the names, x lets you enter it and open its entries, w lets you create, rename or delete entries when combined with x. A directory in 644 therefore cannot be entered.