LINUX FIELD NOTES

Practice grep with a small log file

Find useful lines, ignore letter case, invert a match, and count results with a pipeline.

Search the contents, not the name

grep reads text and selects matching lines. Put the pattern before the filename. The example below keeps lines containing ERROR in app.log.

grep ERROR app.log

Match case on purpose

Use -i when capitalization should not change the result. Searching for error with -i also finds ERROR. In a real Linux shell, grep supports regular expressions; our teaching simulation currently treats patterns as literal text.

grep -i error app.log

Invert the question

Sometimes you want everything except the matching lines. -v keeps nonmatching lines. This is useful for excluding a known class of entries while you inspect the rest.

grep -v ERROR app.log

Turn matching lines into a count

The pipe sends the selected lines to wc. The -l option counts newline characters. A count can help you scope a problem before reading each error in detail.

grep ERROR app.log | wc -l

Make it stick.

Try the matching guided exercise in your browser. The first 10 exercises are free; later exercises require a plan.

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