Sed Command

  

🔥 What is sed?

sed stands for Stream Editor — it's a powerful Unix tool to search, find and replace, insert, and delete text in files or streams non-interactively.

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🧠 sed Syntax

sed [OPTIONS] 'COMMAND' file

Breakdown:

• COMMAND: Script-like instructions, e.g., 's/old/new/g'
  
  

• file: Input text file or stream (can also be piped)
  
  

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📘 SECTION 1: BASIC COMMANDS

1️⃣ Print a file

sed '' filename

2️⃣ Delete lines

sed '2d' file.txt        # Delete 2nd line sed '2,4d' file.txt      # Delete lines 2 to 4

sed '/^$/d' file.txt     # Delete all empty lines

3️⃣ Find and replace

sed 's/old/new/' file.txt            # Replace first 'old' per line sed 's/old/new/g' file.txt           # Replace ALL 'old' per line sed 's/foo/bar/2' file.txt           # Replace 2nd occurrence of 'foo' per line

4️⃣ Case insensitive replace

sed 's/apple/orange/I' file.txt      # 'I' for case insensitive

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📘 SECTION 2: INTERMEDIATE USAGE

5️⃣ In-place edit with -i

sed -i 's/foo/bar/g' file.txt        # Linux sed -i '' 's/foo/bar/g' file.txt     # macOS (BSD `sed`)

6️⃣ Insert / Append text

sed '3i\This is inserted above line 3' file.txt sed '3a\This is added below line 3' file.txt

7️⃣ Replace on a specific line

sed '5s/error/ERROR/' file.txt       # Replace only on line 5

8️⃣ Replace using regex patterns

sed 's/[0-9][0-9]*/[number]/g' file.txt

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📘 SECTION 3: ADVANCED USE CASES

9️⃣ Replace text between two markers

sed '/START/,/END/s/foo/bar/g' file.txt

🔟 Remove comments from config files

sed '/^\s*#/d;/^$/d' config.txt

11️⃣ Change multiple patterns

sed -e 's/apple/banana/g' -e 's/orange/mango/g' fruits.txt

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⚙️ Real-Time Use Cases (DevOps/Data)

✅ Clean log files

sed '/DEBUG/d' app.log > cleaned.log

✅ Mask sensitive info

sed -E 's/(password\s*=\s*).*/\1*****/' config.ini

✅ Change URL in CI/CD pipeline YAML

sed -i 's#https://old-repo#https://new-repo#g' .gitlab-ci.yml

✅ Parse output from commands

ps aux | sed -n '/nginx/p'

Note: with print always using -n

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❓ FAANG-Level Interview Questions with Answers

Q1: How would you replace the 3rd occurrence of a word in a line using sed?

sed 's/foo/bar/3' file.txt

Q2: Delete all blank lines and lines starting with #

sed '/^\s*#/d;/^$/d' config.txt

Q3: Use sed to replace only within a function in a file (e.g., between { and })

sed '/function_name {/,/}/s/foo/bar/' file.sh

Q4: What does sed -n '5p' file.txt do?

A: It prints only the 5th line (-n suppresses automatic output).

Q5: How can you extract everything before a delimiter (:)?

sed 's/:.*//' file.txt

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🧪 Practice Exercises

1. Mask all email addresses in a file
   
   

2. Remove HTML tags from a file
   
   

3. Replace tabs with commas
   
   

4. Swap two columns (use sed + awk)
   
   

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📌 Tips

Tip	Description
Use -n + p	For printing specific lines
Always quote patterns	's/foo/bar/' not s/foo/bar/
Test before -i	Avoid data loss
Combine with grep/awk	For powerful pipelines

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📺 Recommended YouTube Channels

1. Linux Leech – deep dive into sed and shell scripting
   
   

2. The Net Ninja – Bash scripting series
   
   

3. Chris Titus Tech – Linux command mastery
   
   

4. TechTFQ – Unix interview questions
   
   

Dave Jones (eLinux) – sed/awk/grep tutorials

Here's a focused and clear tutorial on how to find lines using sed and then insert or append lines before or after the match, including FAANG-level examples.

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🔍 PART 1: FIND and INSERT / APPEND in sed

🔸 Syntax Overview

Operation	Syntax
Insert line before match	sed '/pattern/i\new line' file.txt
Append line after match	sed '/pattern/a\new line' file.txt

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🧪 PRACTICAL EXAMPLES

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✅ 1. Insert line before a match

sed '/server_name/i\# Inserted line before server_name' nginx.conf

📌 Before:

server {   server_name example.com; }

📌 After:

server {   # Inserted line before server_name   server_name example.com; }

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✅ 2. Append line after a match

sed '/server_name/a\# Appended line after server_name' nginx.conf

📌 After:

server {   server_name example.com;   # Appended line after server_name }

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✅ 3. Insert a line before every line matching a keyword (e.g., ERROR)

sed '/ERROR/i\--- ERROR FOUND ---' logs.txt

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✅ 4. Append a blank line after each } (useful in code formatting)

sed '/}/a\\' script.sh

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✅ 5. Insert multiple lines before match

sed '/pattern/i\ Line one\ Line two' file.txt

(Each new line ends with \, escape newline)

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✅ 6. Append content from a file after a pattern

sed '/pattern/r file_to_append.txt' main.txt

• Appends file_to_append.txt after every line that matches pattern.
  
  

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📘 ADVANCED COMBINED EXAMPLE

🧠 FAANG-style: Add a comment before every function and a closing marker after it.

sed '/^def /i\# START OF FUNCTION' script.py | sed '/^def /,/^$/a\# END OF FUNCTION'

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⚙️ Real-Time Use Case

DevOps: Insert a new cron job into crontab file before the daily job

sed '/0 2 \* \* \*/i\15 1 * * * /usr/bin/cleanup.sh' crontab.txt

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🔄 Alternative: Insert line using line number (not pattern)

sed '3i\Inserted before line 3' file.txt sed '5a\Appended after line 5' file.txt

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❓ Quick Interview Questions

Q1. How do you insert a header at the top of every config file?

sed '1i\# Configuration File - Do not edit manually' config.ini

Q2. How to add a logging line after every function definition in Python?

sed '/^def /a\    print("Entering function")' script.py