Basic Usage

Running grep

grep "pattern" file.txt            # search in file
grep "error" *.log                 # search multiple files
grep "hello" file1.txt file2.txt   # explicit file list
cat file.txt | grep "pattern"      # pipe input
dmesg | grep -i "usb"              # filter command output

Common Flags

-i	Case-insensitive matching
-v	Invert match — print non-matching lines
-c	Print count of matching lines
-n	Show line numbers
-l	List filenames with matches only
-L	List filenames without matches
-w	Match whole words only
-x	Match whole lines only

Regex Patterns

Basic Regular Expressions (BRE)

.	Any single character
*	Zero or more of preceding element
^	Start of line
$	End of line
[abc]	Character class — any of a, b, c
[^abc]	Negated class — anything except a, b, c
[a-z]	Range — lowercase letters
\<, \>	Word boundaries (GNU)
, \1	Capture group and back-reference

BRE Examples

grep '^#' file.conf                # lines starting with #
grep 'error$' file.log             # lines ending with error
grep '^$' file.txt                 # blank lines
grep 'col[ou]r' file.txt           # match color or colour

Extended Regex

Extended Regular Expressions (ERE)

+	One or more of preceding element
?	Zero or one of preceding element
{n}	Exactly n repetitions
{n,m}	Between n and m repetitions
(a\|b)	Alternation — match a or b
( )	Grouping (no backslash needed)

ERE Examples

grep -E '[0-9]{3}-[0-9]{4}' f     # phone number pattern
grep -E '(error|warn|fatal)' f     # multiple patterns
grep -E '^[A-Z][a-z]+' f           # capitalized words
grep -P '\d{1,3}\.\d{1,3}' f      # Perl regex: IP fragments

Context Lines

Context Examples

grep -B 3 "error" app.log          # 3 lines before match
grep -A 5 "FAIL" test.log          # 5 lines after match
grep -C 2 "crash" kern.log         # 2 lines before and after
grep --group-separator="---" -C 1 "err" f  # custom separator

Context Flags

-B N	Show N lines before each match
-A N	Show N lines after each match
-C N	Show N lines before and after (context)
--group-separator=str	Separator between match groups (default `--`)
--color=auto	Highlight matches in terminal

Recursive Search

Recursive Examples

grep -r "TODO" .                   # recursive from current dir
grep -rn "FIXME" src/              # recursive with line numbers
grep -r --include="*.py" "import" .  # only .py files
grep -r --exclude="*.log" "error" .  # skip .log files
grep -r --exclude-dir=node_modules "require" .

Recursive Flags

-r / --recursive	Search directories recursively
-R	Like `-r` but follows symlinks
--include=glob	Search only files matching glob
--exclude=glob	Skip files matching glob
--exclude-dir=dir	Skip directories matching name
--include-dir=dir	Only search directories matching name

Counting & Listing

Count & List Examples

grep -c "error" *.log              # count matches per file
grep -l "TODO" src/*.py            # list files with TODOs
grep -L "test" src/*.py            # files missing "test"
grep -o "http[^ ]*" page.html      # extract matching parts only
grep -c '' file.txt                # count total lines (like wc -l)

Output Flags

-c	Print count of matching lines per file
-l	Print only names of files with matches
-L	Print only names of files without matches
-o	Print only the matched parts of lines
-H / -h	Show / hide filename prefix
-Z	Null-delimited output (for xargs -0)

Inverse Match

Invert & Exclude

grep -v "^#" config.conf           # remove comment lines
grep -v "^$" file.txt              # remove blank lines
grep -v -e "debug" -e "trace" app.log  # exclude two patterns
grep -v "pattern" f | grep "other" # chain: NOT A, then B

Filtering Strategies

-v	Invert match — select non-matching lines
-v with -e	Exclude multiple patterns
pipe chain	Chain grep calls for complex filtering
grep -v '^$' \| grep -v '^#'	Remove blanks and comments
-v with -c	Count non-matching lines

Multiple Patterns

Multiple Pattern Examples

grep -e "error" -e "warning" app.log
grep -E "error|warning|fatal" app.log
grep -f patterns.txt file.txt       # patterns from file
grep -w -e "GET" -e "POST" access.log

Pattern Options

-e pattern	Specify a pattern (use multiple times)
-f file	Read patterns from file (one per line)
-E 'a\|b\|c'	ERE alternation for multiple patterns
-F	Fixed strings — no regex, faster matching
-G	Basic regex (default mode)
-P	Perl-compatible regex (PCRE)

Performance

Performance Tips

-F (fgrep)	Fixed-string mode — fastest for literal strings
LC_ALL=C grep	Bypass locale for 2-10x speedup on ASCII data
--include/--exclude	Reduce files searched before opening
-m N	Stop after N matches per file
-q	Quiet mode — exit on first match (for scripts)
ripgrep (rg)	Drop-in replacement; faster on large repos

Performance Examples

LC_ALL=C grep -F "exact string" huge.log
grep -r -m 1 "needle" /var/log/     # stop after first hit
grep -rq "pattern" . && echo "found" # boolean test
grep -r --include="*.go" "func main" .

Common Patterns

One-Liners

grep -rn "TODO\|FIXME\|HACK" src/      # find code markers
grep -oP '(?<=")[^"]+(?=")' f           # extract quoted strings
grep -E '^\s*$' f | wc -l              # count blank lines
grep -c '' *.py | sort -t: -k2 -rn     # sort files by line count
grep -rn --include="*.yaml" "password" . # audit for secrets

Recipes

IP addresses	`grep -oE '[0-9]{1,3}(\.[0-9]{1,3}){3}'`
Email addresses	`grep -oE '[a-zA-Z0-9._%+-]+@[a-z.-]+'`
URLs	`grep -oE 'https?://[^ ]+'`
Lines between markers	`grep -A999 'START' f \| grep -B999 'END'`
Unique matches	`grep -oE 'pattern' f \| sort -u`
Count per pattern	`grep -c 'pat1' f; grep -c 'pat2' f`

GREP QUICK REFERENCE