Reading long files: tail and grep

Log files run to thousands of lines, and usually only the end or certain lines concern you. cat dumps the lot; there are better tools: head shows a few lines from the start and tail from the end, and grep picks the lines that contain a word.

1head -n 5 access.log
2tail -n 20 access.log
3grep WARN app.log
4grep -i timeout app.log
5grep -c WARN app.log

-n says how many lines you want. grep word file writes the lines where the word occurs; -i ignores upper and lower case, -c counts the lines instead of writing them, -v gives the lines where the word does not occur.

grep finds the word wherever it is on the line, and what you look for may also be part of a word: grep ERR brings both the ERROR and the STDERR lines. If the text you search for has a space, put it in quotes: grep "disk full" app.log.

Tasks