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Chmod Calculator

Convert Linux file permissions between checkboxes, octal (755) and symbolic (rwxr-xr-x) notation. Edit any one and the others update instantly.

Who Read (4) Write (2) Execute (1)
Owner
Group
Others
Special bits (setuid, setgid, sticky)
Presets:

How the numbers work

A Unix permission is three groups - owner, group, others - and each group has three switches: read, write and execute. Give each switch a value (read is 4, write is 2, execute is 1) and add up the ones that are on. That sum, 0 to 7, is one octal digit. Three digits describe the whole file: 755 is owner 7 (rwx), group 5 (r-x), others 5 (r-x). The symbolic form rwxr-xr-x spells out the same thing with letters, which is what ls -l prints.

The permissions you actually use

OctalSymbolicTypical use
644rw-r--r--Regular files: owner edits, others read
755rwxr-xr-xDirectories and executable scripts
600rw-------Private keys and secrets: owner only
700rwx------Private directories
777rwxrwxrwxAvoid: world-writable, a security hole

Special bits

A fourth leading digit sets the special bits. Setuid (4) runs an executable as its owner, setgid (2) runs it as its group or makes a directory hand its group to new files, and the sticky bit (1) on a shared directory like /tmp stops users deleting files they do not own. In symbolic form these replace the execute letter: 4755 shows as rwsr-xr-x and the classic 1777 on /tmp is rwxrwxrwt.

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Frequently asked questions

How does octal map to rwx permissions?
Each digit is a sum of three values: read is 4, write is 2, execute is 1. Add the ones you want. 7 is 4+2+1 = rwx, 6 is 4+2 = rw-, 5 is 4+1 = r-x, 4 is r--. A permission like 755 means owner 7 (rwx), group 5 (r-x), others 5 (r-x).
What is the difference between 644 and 755?
644 (rw-r--r--) is the standard for regular files: the owner can read and write, everyone else can only read. 755 (rwxr-xr-x) adds the execute bit for everyone, which is what directories and scripts need - you cannot enter a directory or run a script without execute permission.
Why is 777 dangerous?
777 (rwxrwxrwx) lets any user on the system read, write and execute the file. On a shared or internet-facing server that means anyone can overwrite your code or replace a script with malware. It is almost never the right answer; 755 or 644 usually is. Fix the actual owner or group instead of opening a file to the world.
What are setuid, setgid and the sticky bit?
They are the special permission bits, shown as a fourth leading octal digit. Setuid (4) makes an executable run as its owner rather than the caller. Setgid (2) does the same for the group, or on a directory forces new files to inherit the group. The sticky bit (1) on a directory, like /tmp, stops users deleting each others files. In symbolic form they appear as s, s and t.
What does the s or t appear as in symbolic notation?
When a special bit is set, it replaces the execute character. Setuid shows as s in the owner execute slot (S if execute is off), setgid as s in the group slot, and the sticky bit as t in the others slot (T if execute is off). So 1777 is rwxrwxrwt and 4755 is rwsr-xr-x.

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