chmod, octal, umask. Relevant for Linux+, RHCSA, Security+.
File permissions are three digits that everyone half-knows. Subbing makes them automatic: octal to symbolic and back, what a new file gets under a given umask, what chmod g+w does to a mode, which special bit a leading 4, 2 or 1 sets, and whether a given user can read, write or execute a file.
Generated by the same code as the app. They change with every release; the app never runs out.
Linux file permissions: Read and write Linux permission modes: the three digits, rwx triplets, umask arithmetic, symbolic chmod, setuid, setgid and the sticky bit.
rwxr-xr-x. The owner gets read, write and execute (4+2+1), the group and others get read and execute (4+1).
It removes write permission for group and others from the default. New files start at 666 and become 644; new directories start at 777 and become 755.
Shown as a t in the others execute slot (for example rwxrwxrwt on /tmp). In a directory it means only a file's owner can delete or rename it, even though everyone can write to the directory.