The pipe

a | b sends what a prints into b as its input. Small tools joined by pipes do what a program would take an hour to write.

Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.

What it is for

Count the errors in a log, find the biggest files, list the words in a file by frequency: each is a pipe of two or three commands.

Build a pipe left to right, running it after each step to see what the next command will receive.

printf 'INFO ok\nERROR one\nERROR two\n' > log.txt

A log with two errors.

cat log.txt | grep ERROR

The two ERROR lines.

grep ERROR log.txt | wc -l

2: the count, by adding one more tool to the chain.

Count the lines containing WARN in app.log.

grep WARN app.log  wc -l

Piping into a command that takes file names, like rm or cp. They do not read input; xargs is the bridge.

Practise the pipe | on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.