Pipe

A pipe is a unidirectional communication channel between two processes, one of the oldest and simplest inter-process communication mechanisms in Unix and Unix-like operating systems. Data written to the write end of a pipe is buffered by the kernel and can be read from the read end in the order it was written, enforcing a strict first-in-first-out discipline. Pipes are the foundation of the Unix philosophy: small programs connected by streams, each doing one thing well and passing its output to the next.

The pipe's simplicity is its power and its limitation. It is unidirectional: if two processes need bidirectional communication, two pipes are required. It is anonymous: the processes must be related — typically parent and child — for the pipe to be established. Named pipes, or FIFOs, relax this constraint by existing in the filesystem, but they inherit the same semantics and the same blocking behavior.

The pipe embodies a specific philosophy of coordination: producer and consumer, sequential and ordered. It does not support random access, parallel readers, or complex querying. These limitations are not oversights; they are constraints that make the pipe predictable, composable, and amenable to reasoning. In a world of shared memory complexity, the pipe is a refusal to share — a message-passing boundary that keeps processes honest.