Unix Permissions Calculator

How to use the unix permissions calculator

1. Tick the permissions you want.
2. Read the octal value.
3. Copy the chmod command.

About this tool

Each digit of a chmod value is read, write and execute added together — 4, 2 and 1 — for the owner, the group and everyone else in turn. Because those three numbers are powers of two, every combination produces exactly one digit, which is why 755 and 644 are the values you see everywhere.

The part that trips people up is execute on a directory. On a file it means the file can be run. On a directory it means the directory can be entered and its contents reached — so a directory without execute is unusable even if it is readable. That is the whole reason 755 is normal for a folder while 644 is normal for a file.

777 deserves its reputation. It grants everyone full control, and it is what people reach for when a permission problem is really an ownership problem. It rarely fixes the actual cause and it removes every protection while failing to.

Common uses

• Working out the right chmod value for a script or folder.
• Understanding what an existing permission grants.
• Fixing permissions after copying files between systems.

Frequently asked questions

What does 755 mean?

Owner can read, write and execute; group and others can read and execute. It is the usual setting for directories and executable scripts.

Why does a directory need execute permission?

On a directory, execute means the right to enter it and reach what is inside. Without it the directory cannot be used even if it is readable.

Is chmod 777 ever right?

Almost never. It grants everyone full control and is usually applied to a problem that is really about ownership, which it does not fix.