Shell Pipes Explained: How `|` Works in the Command Line

The pipe operator (|) is one of the most powerful ideas in the Unix shell. It connects the output of one command directly into the input of the next command, enabling quick, composable workflows.

What a Pipe Does

In plain terms:

commandA | commandB

means:

commandA writes to stdout (standard output)
commandB reads from stdin (standard input)
the pipe connects them

This lets you chain small tools together without temporary files.

Simple Examples

List files and filter by keyword:

ls -la | grep "log"

Count how many matches you have:

rg "error" app.log | wc -l

Sort processes by memory usage:

ps aux | sort -k 4 -r | head -n 5

Why Pipes Are Powerful

Composability: build complex workflows from small tools
Speed: no intermediate files
Clarity: each command does one thing well

Unix tools are designed to work like LEGO: the pipe is the connector.

Common Patterns

Search → Filter → Act

rg "TODO" -n | fzf | awk -F: '{print $1\":\"$2}' | xargs -I{} ${EDITOR:-vim} {}

Produce → Reduce

du -sh * | sort -h | tail -n 5

Generate → Transform

printf "%s\n" one two three | tr 'a-z' 'A-Z'

Pipes vs Redirects

Pipes are not the same as redirects:

> writes output to a file: ls > files.txt
| sends output to another command: ls | grep "src"

You can combine them:

rg "error" app.log | tee errors.txt | wc -l

Here tee writes to a file and passes the stream along.

Exit Codes and Failures

By default, only the last command's exit status is returned. In Bash, you can inspect all statuses with:

echo ${PIPESTATUS[@]}

In Zsh:

echo $pipestatus

This matters when you need to know if an earlier command failed.

Practical Recipes

Find and open a file:

rg --files | fzf | xargs -I{} ${EDITOR:-vim} {}

Kill a selected process:

ps aux | fzf -m | awk '{print $2}' | xargs kill

Extract columns from CSV:

cat data.csv | cut -d, -f1,3 | column -t -s,

Takeaway

The pipe is the glue of the Unix philosophy: small tools connected into powerful pipelines. Once you get comfortable with |, your command line becomes a programmable toolkit rather than a list of isolated commands.

Shell Pipes Explained: How | Works in the Command Line