Grep

grep is a handy utility to search lines by patterns.

There are some variations.

egrep can read the patterns as extended regex.

fgrep is the most performant, but it does not understand regex.

To grep compressed file, there are zgrep, zegrep, and zfgrep accordingly.

Useful Command Options

Turn grep into egrep

grep -E <PATTERN> <FILE>

Print Only The Matching Parts

grep -o <PATTERN> <FILE>

# [EXAMPLE]
# Command: echo 'INFO 2020-01-01 Hello World' | grep -Eo '^[A-Z]+'
# Output:  INFO

Print The N Lines Around Each Match

# Print the matching line, and the N lines AFTER.
grep -A <N> <PATTERN> <FILE>

# Print the matching line, and the N lines BEFORE.
grep -B <N> <PATTERN> <FILE>

# Print the matching line, and the N lines AROUND.
grep -C <N> <PATTERN> <FILE>

Print The Mismatching Lines

grep -v <PATTERN> <FILE>