Explain grep code

What this grep sample shows

-i and -n

grep prints every line that contains the pattern. -i ignores case and -n adds line numbers: on a test log it printed 2:09:00:05 ERROR could not reach database and 5:09:02:30 Error: disk almost full.

-c and -w

-c prints how many lines matched instead of the lines: 2 for timeout. -w only matches the whole word, so -w "fail" printed only WARN fail count 3 and skipped the lines with failed and failure.

-v, -r and -l

-v turns the search around and prints the lines that don't match: the access log came back without its two health checks. -r searches every file in a folder and its subfolders, and -l prints only the names of files with a match: src/cart.js and src/lib/retry.py, but not src/price.js.

-o and -E

-E allows extended patterns such as {2} for exactly two, and -o prints only the matched part. [45][0-9]{2} is a 4 or 5 followed by two digits, so the command printed 500 and 404 and left out the 200s.

Good to know

• Run with GNU grep 3.12 on a test log, access log and source folder.
• grep's exit code says what happened: 0 when something matched, 1 when nothing did, and 2 for an error such as nope.log: No such file or directory.
• In a script with set -e, a search that finds nothing ends the script. grep -c "panic" printed 0, exited with 1, and the next line never ran.

Watch out: A . in the pattern matches any character, not just a dot. grep "10.0.0.5" also matched a line with 10a0b0c5. Use grep -F to search for the text exactly as written, or write \. for a dot.