755Chmod Calculator

About the Chmod Calculator

A Unix file mode is a set of bits. Three groups of three, for the owner, the group and everyone else, say whether each may read (4), write (2) or execute (1), so 7 is read, write and execute, 5 is read and execute, and 755 means the owner has full access while everyone else can read and run. A fourth digit in front holds three special bits: setuid (4), which runs a program with its owner's rights, setgid (2), which does the same for the group and makes new files in a directory inherit its group, and the sticky bit (1), which on a shared directory such as /tmp stops users from deleting each other's files. In ls -l these appear as s, S, t and T, where a capital means the bit is set without execute. This calculator converts between all the forms and checks them against each other. The symbolic input follows the GNU chmod rules: who is u, g, o or a; the operator is + to add, - to remove or = to set exactly, and = clears the bits it mentions; permissions are r, w, x, s, t, X (execute only for a directory or a file that is already executable) or u, g, o to copy another class's bits. A clause with no who letters leaves the umask bits alone, and the sticky bit is changed through o or a. The umask section shows the permissions a new file (starting from 666) and directory (starting from 777) get after the mask is removed, so a umask of 022 gives 644 and 755. Permissions control who can use a file on the system, not whether its contents are safe, and a mode that allows everyone to write is rarely what you want.