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Pipes & Filters

Combine small commands with pipes and redirection — the core idea that makes the shell powerful.

The real power of the shell is composition: each command does one small thing, and you wire them together. A pipe (|) sends the output of one command straight into the next as its input.

Run cat data/fruits.txt | sort | uniq below and watch three tiny tools combine into “the distinct fruits, in order.”

cs shell — type "help" to see commands.

learner@cs:~$

A simulated shell — supports pipes ( | ) and redirection ( > , >> ). Use ↑/↓ for history.

The Unix philosophy

Programs that “do one thing well” and read/write plain text can be chained endlessly. Useful filters read input and transform it:

grep <pattern> — keep only lines that match. grep apple data/fruits.txt.
sort — sort lines; uniq — collapse adjacent duplicates (so usually sort | uniq).
wc — count: wc -l for lines, -w words, -c characters.
head / tail — first / last lines (head -n 3).

Each works on a file argument or on piped input, so they drop into a pipeline anywhere.

Redirection

A pipe feeds another command; redirection sends output to a file:

> writes (overwrites): echo hello > greeting.txt.
>> appends: echo again >> greeting.txt.

Then cat greeting.txt to see the result. Combine freely: cat data/fruits.txt | sort | uniq > unique.txt.

Building a pipeline

Read a pipeline left to right as a flow of text: source → filter → filter → destination. cat numbers.txt | sort | head -n 3 means “take the numbers, sort them, keep the first three.” Small, composable tools beat one big program.

Takeaways

| pipes one command’s output into the next; chains do a lot with a little.
Filters (grep, sort, uniq, wc, head, tail) work on files or stdin.
> and >> redirect output into files (overwrite vs append).