How to grep multiline search patterns in Linux?

In Linux, searching for multiline patterns requires special techniques beyond the basic grep command. While grep is designed primarily for single-line pattern matching, we can use various approaches and tools to search across multiple lines effectively.

The grep command in Linux is used to filter searches in a file for a particular pattern of characters. It displays lines that contain the pattern we are trying to search, where the pattern is referred to as a regular expression.

Basic Grep Syntax

grep [options] pattern [files]

Common options include −

-c : Count of lines that match the pattern
-h : Display matched lines only
-i : Ignore case for matching
-l : Print filenames only
-n : Display matched lines with line numbers
-v : Print lines that do not match the pattern
-r : Recursively search subdirectories

Single Directory Search

To find a pattern in all files within a directory −

grep -rni "func main()" *

This command searches for "func main()" in all files recursively, ignoring case and showing line numbers.

main.go:120:func main() {}

To search only in the current directory (not subdirectories) −

grep -s "func main()" *

Multiline Pattern Search Methods

Consider this sample file sample.txt −

$ cat sample.txt
blabla
blabla
foofoo
here
is the
text2strike
Just a pattern
bar
blabla
blabla

Method 1: Using grep with Perl Regex (-P)

grep -Pzo "foofoo[\s\S]*?bar" sample.txt

Method 2: Using pcregrep for Multiline

pcregrep -M "foofoo.*?\nbar" sample.txt

Method 3: Using sed for Range Matching

sed -n '/foofoo/,/bar/p' sample.txt

foofoo
here
is the
text2strike
Just a pattern
bar

Method 4: Using awk for Complex Patterns

awk '/foofoo/,/bar/' sample.txt

Advanced Techniques

Key Points

• Standard grep works line-by-line; use -P flag for multiline with Perl regex

• The -z option treats input as null-separated records instead of newline-separated

• pcregrep provides more advanced multiline capabilities than standard grep

• sed and awk are excellent alternatives for range-based multiline searches

Conclusion

Multiline pattern searching in Linux requires tools beyond basic grep, such as grep with Perl regex, pcregrep, sed, or awk. Each method has its strengths − choose based on pattern complexity and performance requirements. The sed range method is often the most readable for simple start-to-end pattern matching.