Chmod Calculator

What is chmod?

chmod ("change mode") is a Unix command (POSIX standard, originally Bell Labs Unix) that sets file and directory permissions. Unix permissions control three actions — read, write, execute — for three classes of user: owner, group, and others (everyone else).

The 9 permission bits

Every file has 9 permission bits arranged as 3 sets of (r, w, x):

-rwxr-xr-x
 ↑↑↑↑↑↑↑↑↑
 │└─┘└─┘└─┘
 │ │  │  └─ others (4=r, 2=w, 1=x)
 │ │  └──── group  (4=r, 2=w, 1=x)
 │ └─────── owner  (4=r, 2=w, 1=x)
 └───────── file type (- regular, d directory, l symlink)

Numeric vs symbolic notation

Two equivalent ways to express the same permissions:

• Numeric (octal): Each user class becomes one digit (4 + 2 + 1). chmod 755 file
• Symbolic: Class + operator + permissions. chmod u=rwx,g=rx,o=rx file
• Symbolic relative: chmod +x file (add execute for everyone), chmod g-w file (remove group write)

Special bits: setuid, setgid, sticky

An optional 4th leading digit controls three special bits: setuid (4) — execute as the file's owner; setgid (2) — execute as the file's group, or new files in a directory inherit the group; sticky (1) — only the file's owner can delete it (used on /tmp). chmod 4755 /usr/bin/passwd gives passwd setuid root so any user can change their password.