Mastering the Basics of Sed Command: A Practical Guide with Examples

Introduction to Sed
Sed (Stream Editor) is a lightweight yet powerful Unix/Linux command-line tool for parsing and transforming text. It processes input line-by-line, making it ideal for automated editing tasks without manual file manipulation. Think of it as a “search-and-replace wizard” for your terminal!

🔧 Basic Syntax

sed [options] 'command' filename

• options: Flags like -i (in-place edit), -n (suppress output)
• command: Operations like substitution (s), deletion (d), etc.

✨ Core Operations & Examples

1️⃣ Substitution (s command)

Replace text patterns. Syntax: s/pattern/replacement/flags
Example 1: Replace first occurrence per line

echo "Hello World" | sed 's/World/Linux/'
# Output: Hello Linux

Example 2: Global replacement (all occurrences)

echo "apple apple" | sed 's/apple/orange/g'
# Output: orange orange

Example 3: Case-insensitive replacement

echo "Cat CAT cat" | sed 's/cat/dog/gi'
# Output: dog dog dog

2️⃣ Deletion (d command)

Remove lines matching a pattern.
Example 1: Delete lines containing “error”

sed '/error/d' logfile.txt

Example 2: Delete lines 3-5

sed '3,5d' data.txt

3️⃣ Insertion/Appending (i and a commands)

Add text before (i) or after (a) a line.
Example: Add header before line 1

sed '1i\---HEADER---' notes.txt

Example: Append footer after last line

sed '$a\---END---' report.txt

4️⃣ Printing Specific Lines (p command)

Use with -n to suppress default output.
Example: Print lines 5-10

sed -n '5,10p' document.txt

🚀 Advanced Tips

• Multiple Commands: Separate with ;

  sed 's/foo/bar/; s/baz/qux/' file.txt

• Edit Files In-Place: Use -i (backup with -i.bak)

  sed -i.bak 's/old/new/g' file.txt  # Creates backup file.txt.bak

• Regex Power: Use .* (any character), ^ (start), $ (end)

  sed '/^#/d' script.sh  # Delete all comment lines

⚠️ Important Notes

1. Sed processes lines sequentially – commands apply to each line individually.
2. Original files remain unchanged unless -i is used.
3. Delimiters (/) can be replaced (e.g., s|old|new| if text contains /).

💡 Practice Exercise

Try replacing all “2023” dates with “2024” in dates.txt and save changes:

sed -i 's/2023/2024/g' dates.txt

Conclusion
Sed is indispensable for quick text transformations. Start with simple substitutions (s/old/new/), experiment with deletion/insertion, and leverage regex for complex tasks. Once mastered, you’ll handle log files, configs, and data like a pro! 🐧