Send command output through a pipe

A pipe | takes the output of the command on the left and uses it as input on the right. You stop saving throwaway files just to filter them.

ls prints names. grep keeps the lines you care about. Together:

Keep the text files:

> writes to a file. | writes to another command. Mixing them up is the usual first mistake.

You can pipe more than once. Count matching lines without opening the log:

printf '%s\n' 'info: started' 'warn: slow' 'warn: timeout' > log.txt
cat log.txt | grep warn | wc -l

cat prints the file, grep keeps warning lines, wc -l counts them.

Build pipes left to right: produce data, filter it, then summarize it.