Pipes & filters
This is the single most important idea on the command line, and the one that makes Unix feel like a superpower once it clicks: instead of one giant program with a hundred options, you have many small tools that each do one thing well, and you snap them together to solve the problem in front of you.
You'll build that up in three short steps, on a web server's access.log: count its
requests, pick out the failures, then join the two with a pipe to count just the
failures. Each step is one command typed into a real terminal.
linuxclipipes
Interview prep
Pipelines are the Unix mindset interviewers look for.
How would you count how many times an error appears in a log?
Pipe a filter into a counter: grep ERROR app.log | wc -l. Each tool does one job; the pipe (|) feeds one's stdout into the next's stdin.