Mastering Sed: Essential Commands and Real‑World Examples for Powerful Text Processing

Introduction

Sed (Stream Editor) is a non‑interactive text‑processing tool available on Linux/Unix systems. It can perform complex editing operations—search, replace, delete, insert—directly on text streams without modifying files manually.

Basic Syntax

The general command format is: sed [options] 'operation' input_file Options : optional flags, e.g., -n suppresses automatic printing of each line.

Operation : a combination of an address (or range) and a command that defines what to do.

Input file : the file to process, or standard input.

Address Ranges

Addresses specify which lines the operation applies to. They can be a line number, a regular expression, or a combination:

Single line number (e.g., 3) – affect only that line.

Range of lines (e.g., 3,5) – affect lines 3 through 5.

Regular expression (e.g., /pattern/) – affect all lines matching the pattern.

Common Sed Commands

Substitution (replace)

Replace text using the s command:

sed 's/old/new/g' file.txt

s

stands for substitute; g makes the replacement global on each line.

Delete lines

sed '/pattern/d' file.txt

Deletes every line that matches pattern.

Insert line before

sed '/pattern/i\New line before' file.txt

Inserts the specified text before each matching line.

Append line after

sed '/pattern/a\New line after' file.txt

Appends the specified text after each matching line.

Show line numbers

Use = to print line numbers, or p to print the current line.

sed '=' file.txt

Practical Applications

Example 1 – Bulk domain replacement

Replace all occurrences of www.oldsite.com with www.newsite.com in a configuration file:

sed -i 's/www.oldsite.com/www.newsite.com/g' config.txt

The -i option edits the file in place.

Example 2 – Delete comment lines

Remove every line that starts with # from a script:

sed '/^#/d' script.sh

Example 3 – Add copyright header

Insert a copyright notice at the top of each source file:

sed -i '1i\/*
 * Copyright (c) 2024. All rights reserved.
 */' *.c

Example 4 – Extract a column (simple case)

Although awk is better for column handling, sed can clean up the result: cut -d',' -f1 data.csv | sed 's/,//g' This extracts the first column from a CSV file and removes any stray commas.

Conclusion

Sed’s flexibility and efficiency make it indispensable for text processing tasks. Mastering its basic commands and address syntax can dramatically speed up routine operations and development workflows. The examples above cover only a fraction of its capabilities; users are encouraged to explore advanced features such as branching, loops, and label jumps for more complex scenarios.