Pipes: connect commands with |

Linux commands are small, and each does one job well. The pipe, |, joins them: the output of the command on the left becomes the input of the command on the right.

ls photos prints one name per line. Instead of reaching the screen, those lines flow into wc -l (word count, counting lines), which prints a single number: how many files there are.

Building a pipeline

Pipelines are built one step at a time. Run the first command, look at what it prints, then add the next:

Commands that read their input and print a changed version are called filters. You already know some:

• sort puts lines in order (-r reverses it, -n sorts by number),
• head and tail keep the start or the end,
• wc -l counts lines,
• grep keeps matching lines (next lesson).

You can chain as many as you like: sort names.txt | tail -n 5 | sort -r.

A pipe passes output, not files. wc -l photos fails because photos is a directory; ls photos | wc -l counts what ls printed.