System Administration

Grep Command Cheatsheet

Comprehensive guide to grep commands covering basic options, regular expressions, wildcards, character classes, quantifiers, POSIX classes, and advanced pattern matching for efficient text search in Linux. Master grep for log analysis, text processing with sed, and VPS server management.

#grep #text-search #regex #linux #pattern-matching #sysadmin

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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.

Options

Flags like -i, -r, -n

Pattern

Text or regex to search

File

Target file(s) to search

1. Common Grep Options 2. Common Use Cases 3. Regular Expression Wildcards 4. Character Classes 5. Quantifiers 6. POSIX Character Classes 7. Position Anchors 8. BRE, ERE & PCRE

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' filename

Search for a simple text pattern in a file

Case-Insensitive Search

$ grep -i 'error' /var/log/syslog

Find 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.txt

Count how many lines contain the pattern

Show Line Numbers

$ grep -n 'TODO' *.php

Display 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 after

Show surrounding context around matches

Multiple Patterns (OR Logic)

$ grep -E 'error|warning|critical' /var/log/syslog
$ grep -e 'pattern1' -e 'pattern2' file.txt

Search for multiple patterns simultaneously

Exclude/Invert Match

$ grep -v 'comment' code.js          # Show lines NOT containing 'comment'
$ grep -v '^#' config.conf           # Exclude comment lines

Display 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 matches

Show 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 -l

Combine 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.txt

Additional 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.txt

Features: \d \w \s \D \W \S (?=...) (?!...) (?<=...) (?<!...)

Master Grep on High-Performance VPS

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