Linux grep Command: Search Inside Files

Basic searches

grep "error" app.log

Shows every line in app.log that contains error. The search is case sensitive.

grep -i "error" app.log

Ignores case, so Error and ERROR match too.

grep -n "error" app.log

Shows the line number in front of each match.

grep -c "error" app.log

Counts the matching lines instead of showing them.

grep -v "^#" config.conf

Shows the lines that do not match. Here it hides comment lines starting with #.

Searching many files

grep -r "TODO" .

Searches every file in the current directory and below.

grep -rl "TODO" .

Lists only the names of the files that contain a match.

grep -r --include="*.php" "TODO" .

Searches only files ending in .php.

Better matching

grep -w "cat" pets.txt

Matches cat as a whole word only, so category and concatenate are skipped.

grep -E "error|warning" app.log

Matches lines containing error or warning. -E turns on extended patterns.

grep -A 2 -B 1 "error" app.log

Shows 1 line before and 2 lines after each match, so you can see what happened around it.

Combining with other commands

grep is often used after a pipe (|) to filter the output of another command.

ls /etc | grep -i "conf"

Lists only the entries in /etc with conf in the name.

grep -oE "^[0-9.]+" access.log | sort | uniq -c | sort -rn | head

Counts the visits from each IP address in a web server log, busiest first.

Quick reference