The Pipe: |

The pipe (|) is one of Linux's most powerful ideas. It takes the output of one command and feeds it directly as input to another command, no intermediate file needed.

The pattern is: COMMAND1 | COMMAND2.

cat system.log | grep error

error: sensor 3 offline
error: low coolant

Here cat outputs the file, and grep receives that output and filters it. The pipe connects them in real time.

Why not just grep error system.log?

Both work! But pipes shine when you chain commands that cannot take a filename as an argument, or when you build longer chains (next lesson). The pipe makes every command composable: each one does one small job, and you snap them together like modules on the station.

Another example

ls /usr/bin | wc -l

This counts how many programs are in /usr/bin: ls lists them, wc -l counts the lines.

Mission: Distress Signal Filter

The station's comms officer has flagged transmissions.log: it contains routine chatter mixed with distress calls. Command needs the emergencies isolated and filed.

1. Use a pipe to send the output of cat transmissions.log into grep and keep only the lines containing mayday.
2. Send that filtered stream into a new file called distress.txt instead of the screen.
3. Create a folder called command-center and move distress.txt into it.
4. Count how many distress calls the file holds.

Expected result

command-center/distress.txt holds the three mayday: lines.