chmod calculator

Each digit is one class: owner, group, then others. Read counts 4, write 2 and execute 1, added up: 7 is all three, 5 is read and execute, 4 is read only. So 755 is rwxr-xr-x: the owner can do everything, everyone else can read and run.

For a file, execute lets it run as a program. For a folder it lets you enter the folder and reach the files inside; read lets you list their names. A folder usually needs both, which is why folders are 755 and plain files 644.

A fourth digit in front. Setuid (4) makes a program run as its owner, whoever starts it; setgid (2) runs it as its group, and in a folder gives new files the folder’s group; sticky (1), on a shared folder such as /tmp (1777), stops people deleting each other’s files. In symbolic form they show as s or t in the execute place: 4755 is rwsr-xr-x; a capital S or T means the bit is set without execute.

It gives everyone every permission (others can read, write and run), so any user or process on the machine can change or replace the file. It’s rarely needed; for a folder that several people share, 1777 or a shared group with 775 is the usual answer.

Those change permissions relative to the file’s current ones, which this page can’t see. Start from the mode the file has now (ls -l shows it, and you can paste that line here), tick what you want, and use the command it gives.

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