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umask is the Unix mechanism for setting default permissions on newly created files — a bitmask that's subtracted from the maximum permissions a new file could have. Set it right and your files emerge with sensible permissions; set it wrong and you're creating world-writable files in your home directory without...

The finger protocol, dating from 1971, allowed anyone on the internet to query a server for information about its users — real name, last login, current location, even the contents of a .plan file the user could update at will. It was used by researchers to share project updates, by...

lsof — "list open files" — was written by Vic Abell in the early 1990s and does exactly what it says, which turns out to be an enormous amount. Since everything on Unix is a file, lsof reveals open regular files, sockets, pipes, devices, and network connections for every process...

Stephen Bourne wrote the Bourne shell (sh) at Bell Labs in 1979, replacing the earlier Thompson shell with something that had variables, control flow, and a vaguely programmable syntax. It became the standard Unix shell and the ancestor of essentially every POSIX-compatible shell since. The C shell (csh) arrived from...