Boneyard Tools

Octal and Unix file permissions

Why chmod uses base-8 numbers, how each octal digit maps to read, write and execute bits, and how to read modes like 755 and 644 at a glance.

Open the Decimal to Octal →

Why permissions are written in octal

A Unix file stores permissions as nine bits, split into three groups of three for the owner, the group and everyone else. Three bits happen to hold exactly one octal digit, from 0 to 7, which is why a mode fits neatly into three octal figures. That tidy mapping is the reason chmod and ls settled on base 8 rather than decimal or hex. Each digit summarises one group's read, write and execute rights in a single character.

Reading the three permission bits

Within each group the read bit is worth 4, the write bit is worth 2 and the execute bit is worth 1. You add up the bits that are set to get the octal digit, so read and write together make 6, while read, write and execute make 7. A digit of 5 means read and execute but not write, the classic setting for a program a user may run but not change. Because the values 4, 2 and 1 never overlap, every combination has one unique digit.

Decoding common modes

Mode 755 gives the owner full read, write and execute, and read plus execute to the group and others, which suits scripts and directories. Mode 644 grants the owner read and write while everyone else gets read only, the usual choice for ordinary files. Convert those octal modes back to decimal and you get 493 and 420, the numbers some tools and APIs store internally. Pasting either form into this converter lets you move between the two views quickly.

From decimal back to a mode

Sometimes a config file, log or programming language reports a permission as a plain decimal integer rather than the familiar three-digit octal. Converting it back to octal reveals the real mode: 511 becomes 777, wide-open access, and 448 becomes 700, private to the owner. Doing the conversion by hand means dividing by 8 twice and collecting remainders, but this tool does it instantly. That makes it easy to audit a permission that some system printed in an unexpected base.

Frequently asked questions

What does chmod 777 actually allow?

Octal 777 sets read, write and execute for the owner, the group and everyone else. It is rarely a good idea because it lets any user modify the file, so tighter modes like 755 or 644 are usually safer.

Is a leading zero part of the octal value?

In source code a leading zero often marks a number as octal, but this converter returns the digits without one. When you paste a mode back into a shell or language, add the leading zero yourself if that syntax requires it.