10 Pipes: one command into the next

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Goal

Print how many lines in logs/app.log contain ERROR.

The | (pipe) takes everything one command prints and feeds it to the next command as its input. Small tools, each doing one thing, snap together into something none of them could do alone.

grep ERROR file prints only the lines containing ERROR. Piped into wc -l, those lines become a count.

Try it

Run the grep on its own first, then add the pipe. Watch the lines flow through each stage below the tree — and how many survive.

Go to the terminal

Hint

grep ERROR logs/app.log | wc -l — grep keeps the matching lines, wc counts them.

Show a solution

grep ERROR logs/app.log | wc -l

This interactive environment needs JavaScript. Everything you need to read is on this page, and the solution works the same way for real.

Command line

Starting Command line…