Phase 0: Command Line & OS Literacy · ~35 minutes · Terminal · grep · sort

Reading, Filtering, and Combining Text

grep "error" on a file full of ERROR returns nothing. No error message. Just silence — and a wrong conclusion, if you don't know to expect it.

Hiring signal: Composes small tools with pipes instead of reaching for a script, and knows exactly why a pipe returned nothing

What you will learn

Use cat/head/tail/grep/sort/uniq as small, composable tools
Chain commands with pipes to answer a question no single command answers alone
Explain why pipes pass plain text, not structured data, and what that implies
Diagnose a pipe that silently returns nothing due to a near-miss match

Introduction

Type: Learn Languages: Bash / Zsh Prerequisites: Lesson 01 (Navigating Without a Mouse) Time: ~35 minutes

Objective

Learning objectives

Use cat/head/tail/grep/sort/uniq as small, composable tools
Chain commands with pipes to answer a question no single command answers alone
Explain why pipes pass plain text, not structured data, and what that implies
Diagnose a pipe that silently returns nothing due to a near-miss match

What you're building

Using a real (synthetic is fine) server log file with at least 30 lines, a mix of INFO/WARNING/ERROR levels, and at least 2 different dates:

Answer "how many errors happened on [a specific date in your log]" using only a piped command chain
Answer "what are the 3 most common non-INFO messages" using grep, cut, sort, and uniq -c chained together
Deliberately reproduce this lesson's case-sensitivity trap: run a grep that should match but doesn't due to case, show the empty result, then fix it with -i
Save all three commands and their real output in log_analysis.txt

A command grep "Timeout" server.log | wc -l returns 0, but you can see with your own eyes that the file contains lines with the word timeout (lowercase). What's the most likely explanation and correct next step?

This is exactly this lesson's central trap, applied without the answer already given. grep "Timeout" (capital T) does not match lines containing lowercase timeout — no error, just a silently empty result, identical in appearance to 'this genuinely isn't in the file.' The fix demonstrated in this lesson's predict block is grep -i, which matches regardless of case. Nothing here indicates file corruption or a broken wc — the pipeline is working exactly as literally instructed, which is the whole point: the tools aren't wrong, the assumption about matching was.

Unlock the full lesson

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