Linux File-Permissions

Linux (or other UNIX-ish systems) use 4 groups of 3 bits to represent basic file permissions. The first group sets the flags set UID, set GID, and sticky. The last three groups set the file-permissions for the owner user, the owner group, and everyone else in that order. Here the 3 bits stand for read, write, and execute. On directories execute stands for open / navigate into.