Piping

Objectives

By the end of this chapter, you should be able to:

• Explain what head, tail, sort, uniq, wc, and grep do
• Define what piping is
• Chain commands together with | to solve problems no single command handles alone

💡 Why this matters: Redirection connects a command to a file. Piping connects a command directly to another command — that’s the difference between typing one command at a time and building a small pipeline, which is most of what real command-line work actually looks like.

A Few Commands Worth Knowing First

• head — prints the first lines of a file (-n picks how many)
• tail — prints the last lines of a file (-n picks how many)
• sort — sorts a file’s lines
• uniq — removes duplicate lines, but only if they’re already sorted and adjacent
• wc — counts words, lines, characters, and bytes

Combining Files with cat

Given first.txt:

First
Second
Third

and second.txt:

Fourth
Fifth
Sixth

cat first.txt second.txt concatenates them — prints both, one after the other, as if they were a single file.

Piping

What if you want to concatenate two files and then count the words? Or concatenate and then sort? You need a way to feed one command’s output straight into another — that’s what a pipe (|) does. Think of it as a hose connecting the output of the command on the left to the input of the command on the right:

cat first.txt second.txt | sort

Pipes chain as many times as you want:

cat first.txt second.txt | sort | head -n 2

Try reasoning through this one before running it:

cat first.txt second.txt | sort | tail -n 3 | head -n 1

Step by step: concatenate the two files, sort the result, keep the last three lines, then keep just the first of those three. Net effect: the third-from-last line of the combined, sorted output — without ever knowing how many lines the file has.

Searching with grep

grep searches text for a pattern, and it’s most useful piped in after something else. cat first.txt | grep First prints First — the one line that matched. No match means no output at all; multiple matches means every one gets printed. Try:

cat first.txt second.txt | grep Nope
cat first.txt second.txt | grep th

Putting It Together

Say petnames.txt contains:

Lassie
Moxie
Whiskey
Fido
Lassie
Moxie

Plenty of duplicates. Running uniq petnames.txt directly doesn’t remove them, because uniq only catches duplicates that are already next to each other — the file needs sorting first:

sort petnames.txt | uniq

Fido
Lassie
Moxie
Whiskey

Add redirection on top and you can send that straight to a new file instead of just printing it:

sort petnames.txt | uniq > petnames_sorted.txt

Quick Reference

Try It

Pipes run against real files, so try these yourself:

1. Create two short text files and use cat and | to combine and sort them in one line.
2. Use wc piped after something else to count how many lines contain a specific word.
3. Recreate the petnames.txt example with your own list of duplicated words, then sort, dedupe, and redirect the result to a new file — all in one line.

Recap

• | connects the output of one command to the input of the next, and you can chain several together.
• head/tail grab lines from one end of a file, sort/uniq clean up ordering and duplicates, wc counts, and grep filters by pattern.
• Piping and redirection combine freely — a pipeline can end by printing to your screen or writing to a file.

Next module: environment variables, the PATH, and how to find and manage running processes.