Mastering the Sed Command in Linux

Introduction

Sed (Stream EDitor) is a powerful text transformation tool that processes text line-by-line using a compact scripting language. It’s particularly useful for making batch edits to files and filtering text streams.

Why Learn sed?

• Perform non-interactive text editing
• Edit files without opening them
• Process large files efficiently
• Use regular expressions for pattern matching
• Automate repetitive text transformations

Basic Syntax

The basic syntax for sed is:

sed [options] 'script' input-file

sed Cheat Sheet

Sample Files Setup

Create test files for our examples:

echo "Hello World
Welcome to Linux
The quick brown fox
jumps over the lazy dog
SED is awesome
12345
Goodbye World" > poem.txt

echo "John,25,New York
Alice,30,London
Bob,22,Paris
Carol,35,Tokyo" > data.csv

Basic Examples

Example 1: Simple Substitution (s command)

sed 's/World/UNIX/' poem.txt

Example 2: Delete Lines (d command)

sed '3,5d' poem.txt

Example 3: Print Specific Lines (p command)

sed -n '2,4p' poem.txt

Intermediate Examples

Example 4: Multiple Commands (-e option)

sed -e 's/Linux/Unix/' -e '/dog/s/lazy/active/' poem.txt

Example 5: In-place Editing (-i option)

cp poem.txt poem-backup.txt
sed -i 's/World/Planet/' poem.txt
cat poem.txt

Example 6: Using Regular Expressions

sed '/^[A-Z]/d' poem.txt

Advanced Examples

Example 7: Group Replacement (& and \1)

sed -r 's/([0-9]+)/Number: \1/' poem.txt

Example 8: Append/Insert Text (a and i commands)

sed '3a\--- Add new line after line 3 ---' poem.txt

Example 9: Complex Transformation (CSV to HTML)

sed -e '1i\<table>' \
    -e 's/^/<tr><td>/' \
    -e 's/,/<\/td><td>/g' \
    -e 's/$/<\/td><\/tr>/' \
    -e '$a\</table>' \
    data.csv

Regular Expressions with sed

1. Address Ranges

sed '2,4s/o/O/g' poem.txt

2. Start/End Anchors

sed '/^SED/,/^Goodbye/d' poem.txt

Pro Tips

1. Combine with other commands:

grep 'Linux' poem.txt | sed 's/Linux/Unix/'

Welcome to Unix

1. Use sed scripts for complex tasks:

echo "s/World/Planet/
s/Linux/Unix/" > script.sed
sed -f script.sed poem.txt

1. Handle whitespace carefully:

sed -E 's/\s+/ /g' poem.txt  # Collapse multiple spaces