Processing Text with grep

1. Searching with Basic Patterns

Pattern	Description	Example	Result
abc	Literal string search	`grep "abc" file.txt`	Matches lines containing "abc"
^abc	Start of line	`grep "^abc" file.txt`	Matches lines starting with "abc"
abc$	End of line	`grep "abc$" file.txt`	Matches lines ending with "abc"
.	Any single character (except newline)	`grep "a.c" file.txt`	Matches "abc", "axc", etc.
^.*$	Any line (equivalent to no pattern)	`grep "^.*$" file.txt`	Matches all lines in the file

Example: Basic Pattern Search

grep "error" /var/log/syslog

Example: Start of Line Match

grep "^ERROR" /var/log/syslog

2. Case-Insensitive Search

Option	Values	Description
-i	`grep -i`	Perform case-insensitive matching
--ignore-case	`grep --ignore-case`	Alternative long form of case-insensitive matching

Example: Case-Insensitive Search

grep -i "hello" file.txt

Note: The -i flag makes grep match patterns regardless of case, useful for searching logs or documentation where capitalization may vary.

3. Recursive Search

Use grep -r to search for patterns recursively in files within directories.

Option	Description	Example
-r	Recursively search subdirectories	`grep -r "pattern" /path/to/dir`
-R	Alias for `-r`	`grep -R "pattern" /path/to/dir`
--include	Include only files matching a pattern	`grep -r --include="*.txt" "pattern" /path/to/dir`
--exclude	Exclude files matching a pattern	`grep -r --exclude="*.log" "pattern" /path/to/dir`

Example: Recursive search in a directory

grep -r "error" /var/log/

Example: Recursive search with file type filter

grep -r --include="*.py" "def " /home/user/project/

4. Showing Line Numbers

Option	Values	Description
-n	`grep -n "pattern" file.txt`	Show line numbers with matches
--line-number	`grep --line-number "pattern" file.txt`	Alternative long form for showing line numbers

Example: Display line numbers for matches

grep -n "error" log.txt
1:ERROR: Failed to connect
5:ERROR: Invalid input

5. Counting Matches

Option	Values	Description
-c	`grep -c pattern file`	Count the number of lines matching the pattern
-c -v	`grep -c -v pattern file`	Count lines that do NOT match the pattern
-c -l	`grep -c -l pattern file`	Count how many files contain matches (with filenames)

Example: Counting Matches in a File

grep -c "error" /var/log/syslog

Example: Counting Non-Matching Lines

grep -c -v "debug" /var/log/app.log

Note: When using -c, grep outputs only a count number, not the actual matching lines.

6. Inverting Match

Option	Values	Description
-v	`grep -v pattern file`	Invert the match to show lines that do NOT contain the pattern
--invert-match	`grep --invert-match pattern file`	Alternative long form of the invert match option

Example: Inverting Match with grep

# Sample file content
echo -e "apple\nbanana\ncherry" &gt; fruits.txt

# Show lines that do NOT contain "banana"
grep -v "banana" fruits.txt
# Output:
# apple
# cherry

Note: The -v flag is useful for filtering out unwanted lines from a dataset.

7. Matching Whole Words

Option	Values	Description
-w	`grep -w "word"`	Match the whole word only

Example: Matching whole words

grep -w "cat" file.txt
# Matches "cat" as a whole word, not "catch" or "concatenate"

Note: The -w option ensures that the pattern matches only at word boundaries, preventing partial matches within larger words.

8. Searching with Context

Option	Values	Description
-A NUM	`5`	Print NUM lines after match
-B NUM	`3`	Print NUM lines before match
-C NUM	`2`	Print NUM lines before and after match

Example: Show 2 lines before and after each match

grep -C 2 "error" /var/log/syslog

Example: Show 3 lines after each match

grep -A 3 "failed" /var/log/auth.log

9. Using Extended Regex

Feature	Description	Example
-E	Enable extended regular expressions	`grep -E 'cat\|dog' file.txt`
+	One or more occurrences of the preceding element	`grep -E 'a+' file.txt`
?	Zero or one occurrence of the preceding element	`grep -E 'colou?r' file.txt`
()	Grouping expressions for reuse or precedence	`grep -E '(ab)+' file.txt`
\|	Alternation (OR operator)	`grep -E 'start\|end' file.txt`
.	Any character except newline	`grep -E 'a.c' file.txt`
^	Start of line anchor	`grep -E '^hello' file.txt`
$	End of line anchor	`grep -E 'world$' file.txt`

Example: Matching IP Addresses

grep -E '([0-9]{1,3}\.){3}[0-9]{1,3}' file.txt

Example: Using Groups for Capture

grep -E '([a-z]+)@\1' file.txt

10. Using Perl-Compatible Regex

Option	Values	Description
-P	`perl-compatible-regex`	Enable Perl-compatible regular expressions (PCRE)
-pcre	`perl-compatible-regex`	Alias for `-P`

Example: Using PCRE to match word boundaries

grep -P '\b\w+ing\b' file.txt

Example: Using PCRE to match multiline patterns

grep -P 'start.*?end' file.txt

Note: PCRE support requires grep to be compiled with PCRE support. Not all systems have this enabled by default.

Warning: PCRE regex syntax is more complex than POSIX basic/extended regex and may not be portable across different systems.

11. Listing Only Filenames

Option	Values	Description
-l	`grep -l`	List only filenames that contain matches

Example: Using -l to list filenames

grep -l "error" /var/log/*.log

Note: The -l option suppresses the actual matching lines and outputs only the names of files containing the pattern.

12. Matching Multiple Patterns

Pattern	Description	Example	Notes
grep 'pattern1\\|pattern2'	Matches lines containing either pattern1 or pattern2	`grep "error\\|warning" logfile.txt`	Uses `\|` as OR operator in extended regex mode
grep -E 'pattern1\|pattern2'	Extended regex for multiple patterns	`grep -E "cat\|dog" animals.txt`	Requires `-E` flag for extended regex
grep -f patterns.txt file.txt	Reads multiple patterns from a file	`grep -f keywords.txt document.txt`	Each pattern on a separate line in the file
grep -E 'pattern1.*pattern2'	Matches lines with pattern1 followed by pattern2	`grep -E "start.*end" log.txt`	Matches any characters between patterns

Note: When using | in basic regex, it must be escaped as \| to avoid interpreting it as a special character.