Chmod Calculator

How to use the chmod calculator

1. Start from any representation: tick the boxes in the Permissions table, type a mode in Octal (e.g. 755 or 4755), or paste a string such as rwxr-xr-x or drwxr-sr-x (straight from ls -l) in Symbolic (e.g. rwxr-xr-x). The other views follow instantly.
2. Set the Special bits if you need them: Setuid (4000), Setgid (2000) or Sticky (1000). The octal mode then has four digits.
3. Enter the File or directory name, tick It is a directory and Recursive (-R) as needed, and copy the Numeric or Symbolic command.
4. To test a relative change, type it in Symbolic change (e.g. u+x,go-w) and click Apply change.

The Common presets buttons load typical modes such as 644, 600, 755 or 2775. The Relative change from … line shows the chmod expression that turns the last preset or typed mode into the current one, for example a+x from 644 to 755.

Reading Unix permissions

Every file has three classes of users: the owner (u), the file’s group (g) and others (o), meaning everyone else. Each class gets three permissions, each worth a number: read = 4, write = 2, execute = 1. Adding them gives one octal digit per class: rwx = 7, rw- = 6, r-x = 5, r– = 4. That is why rwxr-xr-x is 755.

On a directory the same letters mean something slightly different, which the Who can do what list spells out: r lets you list the entries, w lets you create, rename and delete entries (together with x), and x lets you enter the directory and reach the files inside.

Special bits

• Setuid (4000) on an executable makes it run with the rights of the file’s owner. passwd uses it to write the password database. It shows as s in the owner’s execute position, or S if execute is missing.
• Setgid (2000) on an executable runs it with the group’s rights. On a directory, new files inherit the directory’s group, which keeps shared project folders consistent (2775).
• Sticky (1000) on a directory lets only a file’s owner delete or rename it, even when everyone can write there. /tmp is 1777. It shows as t (or T) in the others’ execute position.

Symbolic changes

A symbolic expression is “who, operator, permissions”: u+x adds execute for the owner, go-w removes write for group and others, a=r sets read-only for all, g=u copies the owner’s rights to the group. Separate clauses with commas. The capital X adds execute only for directories or files that already have an execute bit, which makes chmod -R a+X safe on a tree of mixed files. When “who” is omitted, this tool applies the change to all classes, as GNU chmod does with a umask of 0.

Security tips

Avoid 777. It lets any account on the machine, including a compromised web server process, modify or replace the file. Typical safe choices are 644 for files, 755 for directories and scripts, 600 for private keys and 700 for private directories. The calculator warns about world-writable modes, 777 and setuid programs.