TopolearnContext lines make logs readable, and grep's exit status is the idiomatic 'does this file contain X?' test in shell scripts.
When reading logs you usually want the lines around a match. -A 3 prints three lines after each match, -B 3 three before, -C 3 both. Groups of context are separated by -- lines.
Other output controls:
-m Nstops after N matching lines (per file), which is fast on huge files.-oprints only the matched parts, one per line; combine with-Eto extract IDs, IPs or URLs.--color=autohighlights matches on a terminal.-Z/--nullends file names with a NUL byte for safe use withxargs -0.
grep's exit status is designed for scripts: 0 if any line matched, 1 if none matched, 2 on an error such as a missing file. With -q (quiet) grep prints nothing and exits on the first match, so it is the standard way to test for presence:
if grep -q '^backup:' /etc/passwd; then echo "user exists"; fiBe careful in scripts using set -e or pipefail: a grep that finds nothing returns 1, which counts as a failure. If "no matches" is fine, write grep ... || true.
-s suppresses messages about unreadable files, useful with broad recursive searches.
Example
printf 'start\nload config\nERROR disk full\nretry\nok\nERROR timeout\n' > app.log
grep -B1 -A1 ERROR app.log
# load config
# ERROR disk full
# retry
# --
# ok
# ERROR timeout
grep -m1 ERROR app.log # ERROR disk full
grep -oE 'ERROR [a-z]+' app.log # ERROR disk / ERROR timeout
if grep -q 'timeout' app.log; then
echo "saw a timeout" # saw a timeout
fi
grep -q 'panic' app.log; echo "status $?" # status 1Remember
- -A/-B/-C add context lines after, before or around matches
- Exit status: 0 = matched, 1 = no match, 2 = error
- grep -q in an if is the idiomatic presence test
Watch out for
- A grep with no matches failing a set -e script; add || true when an empty result is acceptable.