Grep Syntax
$ grep [OPTIONS] PATTERN [FILE...]
$ grep [OPTIONS] -e PATTERN ... [FILE...]
$ grep [OPTIONS] -f PATTERN_FILE ... [FILE...]Grep searches each FILE or standard input for PATTERN. By default, grep prints the matching lines. Essential for Linux system administration and VPS server troubleshooting.
Table of Contents
Common Grep Options
| Option | Description | Example |
|---|---|---|
| -i | Ignore case distinctions | $ grep -i'pattern' file.txt |
| -v | Invert match (select non-matching lines) | $ grep -v'pattern' file.txt |
| -n | Print line numbers with output | $ grep -n'pattern' file.txt |
| -c | Count matching lines | $ grep -c'pattern' file.txt |
| -l | List filenames with matches | $ grep -l'pattern' *.txt |
| -L | List filenames without matches | $ grep -L'pattern' *.txt |
| -r or -R | Recursive search in directories | $ grep -r'pattern' /path/to/dir/ |
| -w | Match whole words only | $ grep -w'word' file.txt |
| -x | Match whole lines only | $ grep -x'exact line' file.txt |
| -o | Show only matching part of line | $ grep -o'pattern' file.txt |
| -A NUM | Print NUM lines after match | $ grep -A 3'pattern' file.txt |
| -B NUM | Print NUM lines before match | $ grep -B 2'pattern' file.txt |
| -C NUM | Print NUM lines before and after | $ grep -C 2'pattern' file.txt |
| -e | Specify multiple patterns | $ grep -e'pat1' -e'pat2' file.txt |
| -f FILE | Read patterns from file | $ grep -f patterns.txt file.txt |
| -E | Extended regex (egrep) | $ grep -E'pat1|pat2' file.txt |
| -F | Fixed strings (fgrep) | $ grep -F'literal$tring' file.txt |
| -P | Perl-compatible regex | $ grep -P'\d{3}-\d{3}' file.txt |
| -m NUM | Stop after NUM matches | $ grep -m 5'pattern' file.txt |
| --color | Highlight matches in color | $ grep --color'pattern' file.txt |
Common Use Cases
Basic Text Search
$ grep'search_term' filenameSearch for a simple text pattern in a file
Case-Insensitive Search
$ grep -i'error' /var/log/syslogFind all occurrences regardless of case (Error, ERROR, error). Essential for analyzing server logs and debugging issues.
Recursive Directory Search
$ grep -r'function_name' /path/to/project/Search through all files in a directory recursively. Combine with Linux commands for powerful file operations.
Count Occurrences
$ grep -c'pattern' file.txtCount how many lines contain the pattern
Show Line Numbers
$ grep -n'TODO' *.phpDisplay line numbers where pattern appears
Context Lines (Before/After/Around)
$ grep -A 3 -B 2'error' log.txt # 2 before, 3 after
$ grep -C 5'exception' log.txt # 5 lines before and afterShow surrounding context around matches
Multiple Patterns (OR Logic)
$ grep -E'error|warning|critical' /var/log/syslog
$ grep -e'pattern1' -e'pattern2' file.txtSearch for multiple patterns simultaneously
Exclude/Invert Match
$ grep -v'comment' code.js # Show lines NOT containing'comment'
$ grep -v'^#' config.conf # Exclude comment linesDisplay lines that don't match the pattern
Search by File Type
$ grep -r --include="*.php"'class' /var/www/
$ grep -r --exclude="*.min.js"'function' ./src/Filter search by file extension. Perfect for searching web applications on your VPS hosting.
List Only Filenames
$ grep -l'pattern' *.txt # Files with matches
$ grep -L'pattern' *.txt # Files without matchesShow only filenames instead of matching lines
Whole Word Match
$ grep -w'cat' file.txt # Matches'cat' but not'category'Match complete words only
Pipe with Other Commands
$ ps aux | grep nginx
$ cat access.log | grep'404' | wc -lCombine grep with other command outputs. Use Port Scanner and DNS Lookup tools for network diagnostics.
Regular Expression Wildcards
| Wildcard | Description | Example |
|---|---|---|
| . | Match any single character except newline | $ grep'c.t' file.txt |
| ? | Match 0 or 1 of the preceding character | $ grep -E'colou?r' file.txt |
| * | Match 0 or more of the preceding character | $ grep'ab*c' file.txt |
| + | Match 1 or more of the preceding character | $ grep -E'ab+c' file.txt |
Character Classes
| Class | Description | Example |
|---|---|---|
| [A-Za-z] | Any lowercase or uppercase letter | $ grep'[A-Z]' file.txt |
| [0-9] | Any digit (number) | $ grep'[0-9]' file.txt |
| [0-9A-Za-z] | Any lowercase or uppercase letter or digit | $ grep'[0-9A-Za-z]' file.txt |
| [^...] | Negation - match anything NOT in the brackets | $ grep'[^0-9]' file.txt |
Quantifiers
| Quantifier | Description | Example |
|---|---|---|
| {n} | Matches exactly n times | $ grep -E'[0-9]{3}' file.txt |
| {n,} | Matches n or more times | $ grep -E'[a-z]{5,}' file.txt |
| {,m} | Matches up to m times (maximum) | $ grep -E'[0-9]{,3}' file.txt |
| {n,m} | Matches between n and m times (minimum, maximum) | $ grep -E'[a-z]{3,6}' file.txt |
POSIX Character Classes
| Class | Description | Example |
|---|---|---|
| [:alpha:] | Matches any alphabetical character (either upper or lower case) | $ grep'[[:alpha:]]' file.txt |
| [:alnum:] | Matches any alphanumeric character (a-z, A-Z, 0-9) | $ grep'[[:alnum:]]' file.txt |
| [:digit:] | Matches any numerical digit (0-9) | $ grep'[[:digit:]]' file.txt |
| [:lower:] | Matches any lowercase alphabetical character (a-z) | $ grep'[[:lower:]]' file.txt |
| [:upper:] | Matches any uppercase alphabetical character (A-Z) | $ grep'[[:upper:]]' file.txt |
| [:punct:] | Matches any punctuation character | $ grep'[[:punct:]]' file.txt |
| [:space:] | Matches any whitespace character (space, tab, newline, etc.) | $ grep'[[:space:]]' file.txt |
| [:print:] | Matches any printable character | $ grep'[[:print:]]' file.txt |
| [:blank:] | Matches space or tab characters | $ grep'[[:blank:]]' file.txt |
Position Anchors
| Anchor | Description | Example |
|---|---|---|
| ^ | Beginning of line | $ grep'^Error' log.txt |
| $ | End of line | $ grep'done$' log.txt |
| ^$ | Empty line (start and end with nothing between) | $ grep'^$' file.txt |
| \< | Start of word | $ grep'\<word' file.txt |
| \> | End of word | $ grep'word\>' file.txt |
| \b | Word boundary (start or end of word) | $ grep'\bword\b' file.txt |
BRE, ERE & PCRE
Basic Regular Expressions (BRE)
Default grep behavior. Unless preceded by a backslash, these characters have no special meaning. Ideal for simple pattern matching in log files.
$ grep'pattern' file.txt
$ echo"hello world" | grep'h.*o'Characters: ^ $ . [ ] * \ ( ) { }
Extended Regular Expressions (ERE)
Activated with grep -E or egrep. ERE gives each of these characters a unique meaning:
$ grep -E'pattern1|pattern2' file.txt
$ egrep'(color|colour)' file.txt
$ grep -E'[0-9]{3}-[0-9]{3}-[0-9]{4}' phonebook.txtAdditional characters: ? + | ( )
Perl Compatible Regular Expressions (PCRE)
Activated with grep -P. Additional anchors, character classes, lookahead, lookbehind, conditional expressions, comments, and other features are available in PCRE. Powerful for complex server monitoring tasks.
$ grep -P'\d{3}-\d{3}-\d{4}' phonebook.txt
$ grep -P'(?<=@)\w+' emails.txt
$ grep -P'\b\w{5}\b' file.txtFeatures: \d \w \s \D \W \S (?=...) (?!...) (?<=...) (?<!...)
