Chmod Calculator

This tool converts between the octal form of a Unix permission, such as 755, and the symbolic form, such as rwxr-xr-x. Change either form and the other updates.

Each digit covers one group: the owner, the group, and everybody else. The digit is the sum of read (4), write (2), and execute (1). So 7 is read, write, and execute, and 5 is read and execute.

Use 644 for a normal file and 755 for a directory or a script. Never use 777, because it lets any user on the machine change the file. An SSH private key needs 600, or the client refuses to use it.