Inserting a Line Before the Last String in a Specific Pattern Using sed
Problem: You have a file with a specific pattern, and you need to insert a line before the last occurrence of a string within that pattern.
Simplified: Imagine you have a recipe where ingredients are listed in groups, and you need to add a new ingredient before the last one in each group.
Let's see an example:
Suppose you have a file named ingredients.txt containing:
## Vegetables
Carrot
Onion
Tomato
## Fruits
Apple
Banana
Mango
You want to insert the line "Garlic" before "Tomato" in the "Vegetables" group, and "Strawberry" before "Mango" in the "Fruits" group.
Here's the original code using sed:
sed '/^## /{ :a; N; $!ba; s/\(.*\)\(Tomato\)/\1\nGarlic\2/g; s/\(.*\)\(Mango\)/\1\nStrawberry\2/g; }' ingredients.txt
Let's break down the code:
/^## /{ ... }: This part targets lines starting with "##", indicating the beginning of a new group.:a; N; $!ba;: This loop reads the next line (N) until the end of the file ($!ba) and stores them in the pattern space.s/\(.*\)\(Tomato\)/\1\nGarlic\2/g: This command searches for the pattern "Tomato" and inserts the line "Garlic" before it.s/\(.*\)\(Mango\)/\1\nStrawberry\2/g: Similar to the previous command, this inserts "Strawberry" before "Mango".
Understanding the Key Concepts:
- Pattern Space:
sedworks by storing a line of text (the pattern space) and applying commands to it. - Address: The
/^## /part is an address that specifies the lines to be processed. - Hold Space: The
Ncommand reads the next line and stores it in the hold space, effectively combining lines. - Backreferences: The
\1and\2refer to captured groups in the search pattern, allowing us to insert the line before the desired string.
Important Considerations:
- File Handling: The original code provides output, but does not modify the original file. To save the changes, use
sed -i '...' ingredients.txt. - Multiple Replacements: You can add multiple
s/.../.../gcommands to insert lines before different strings within the pattern.
Additional Value:
This method can be applied to various scenarios:
- Adding comments before specific lines in a code file.
- Inserting metadata before sections in a document.
- Adding pre-processing instructions before certain parts of a data file.
References:
This article provides a practical guide on using sed to insert lines before specific occurrences within a pattern. By understanding the core concepts and implementing the code, you can confidently manipulate text files and automate your workflows.
