# grep Quick Reference

*Pattern matching, regex, recursive search, context, filtering*

> Source: GNU grep Manual (gnu.org/software/grep) · MIT

## 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

| Command | Description |
|---------|-------------|
| `-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)

| Command | Description |
|---------|-------------|
| `.` | 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)

| Command | Description |
|---------|-------------|
| `+` | 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

| Command | Description |
|---------|-------------|
| `-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

| Command | Description |
|---------|-------------|
| `-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

| Command | Description |
|---------|-------------|
| `-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

| Command | Description |
|---------|-------------|
| `-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

| Command | Description |
|---------|-------------|
| `-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

| Command | Description |
|---------|-------------|
| `-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

| Command | Description |
|---------|-------------|
| `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` |