Pipes

Core Idea

A pipe connects stdout from one process to stdin of another process.

The pipe character | is shell syntax, not a command. Bash creates the pipe, starts both commands, and connects their file descriptors.

First Pipeline

history | grep ssh

Read it left to right:

• history prints text to stdout.
• | connects that stdout to the next command’s stdin.
• grep ssh reads stdin and prints matching lines.

Build Pipelines Slowly

history
history | grep ssh
history | grep ssh | wc -l

Add one stage only after the previous stage makes sense.

What Crosses The Pipe

Bytes cross the pipe. Usually those bytes are text lines, but the shell does not care. Tools such as grep, sort, uniq, cut, and wc cooperate because they can read stdin and write stdout.

Common Patterns

Count matches:

grep -R makers ~/src/pages | wc -l

Sort and deduplicate:

grep -R '^#' ~/src/pages | sort | uniq

Keep a copy while continuing the pipeline:

history | tee ~/playground/history.txt | grep ssh

Watch Out

• A pipeline can hide which stage failed. Test stages separately.
• Some commands wait for stdin. Press Ctrl-C if a pipeline hangs.
• Pipeline exit status has details; set -o pipefail makes script failures louder.
• Do not use cat file | command when command file works, unless you are teaching stdin deliberately.

Proof Check

Run a three-stage pipeline and explain exactly what each stage reads and writes.

Docs Pointers

• Run man bash and search for Pipelines.
• Read I/O, file descriptors, stream redirection, one-liners, regular expressions, sort, and uniq.