File Locking on Linux: flock, fcntl, and the Lock You Thought You Had

You write a script that locks a file before writing to it, so two copies don't trample each other. It works in testing. In production, two copies sometimes write at once anyway. The cause is that Linux has *four* different locking mechanisms with subtly incompatible semantics, and you picked the wrong one. Here's the working knowledge to pick correctly.