sed (stream editor) basics in Linux CLI - Mini Project: Build & Apply

sed (stream editor) basics

📖 Scenario: You have a text file with a list of fruits and their quantities. You want to practice using sed to edit this file's content without opening a text editor.

🎯 Goal: Learn how to use sed commands to replace text, delete lines, and print specific lines from a file.

📋 What You'll Learn

Use sed commands to modify text

Replace specific words in the file

Delete a line containing a certain word

Print only a specific line from the file

💡 Why This Matters

🌍 Real World

Editing configuration files or logs quickly without opening an editor is common in system administration and automation.

💼 Career

Knowing sed helps automate text processing tasks, saving time and reducing errors in many IT and development roles.

1

Create the initial text file

Create a file named fruits.txt with these exact lines:
apple 10
banana 5
cherry 20
date 7
elderberry 3

Linux CLI

# Create the file fruits.txt with the given lines
# Your code here

Hint

Use echo -e with newline characters \n and redirect output to fruits.txt.

2

Set the word to replace

Create a variable called old_word and set it to banana.
Create another variable called new_word and set it to blueberry.

Linux CLI

echo -e "apple 10\nbanana 5\ncherry 20\ndate 7\nelderberry 3" > fruits.txt
# Create variables old_word and new_word
# Your code here

Hint

Use simple variable assignment like old_word=banana.

3

Replace the word using sed

Use sed with the s command to replace all occurrences of $old_word with $new_word in fruits.txt and save the output to a new file called fruits_updated.txt.

Linux CLI

echo -e "apple 10\nbanana 5\ncherry 20\ndate 7\nelderberry 3" > fruits.txt
old_word=banana
new_word=blueberry
# Use sed to replace old_word with new_word and save to fruits_updated.txt
# Your code here

Hint

Use sed "s/$old_word/$new_word/g" fruits.txt > fruits_updated.txt to replace all occurrences.

4

Print the updated file content

Use cat to display the contents of fruits_updated.txt.

Linux CLI

echo -e "apple 10\nbanana 5\ncherry 20\ndate 7\nelderberry 3" > fruits.txt
old_word=banana
new_word=blueberry
sed "s/$old_word/$new_word/g" fruits.txt > fruits_updated.txt
# Display the contents of fruits_updated.txt
# Your code here

Hint

Use cat fruits_updated.txt to see the updated file.

Practice

1. What does the sed command sed 's/cat/dog/' file.txt do?

easy

A. Deletes all lines containing 'cat' in file.txt

B. Replaces the first occurrence of 'cat' with 'dog' on each line of file.txt

C. Prints only lines containing 'dog' from file.txt

D. Appends 'dog' at the end of each line containing 'cat' in file.txt

Solution

1. Step 1: Understand the substitution command

   The s/pattern/replacement/ syntax in sed replaces the first match of 'pattern' with 'replacement' on each line.

2. Step 2: Apply to the given command

   The command replaces the first occurrence of 'cat' with 'dog' on every line of file.txt.

3. Final Answer:

   Replaces the first occurrence of 'cat' with 'dog' on each line of file.txt -> Option B

4. Quick Check:

   sed substitution = replace first match [OK]

Hint: Remember: s/pattern/replacement/ changes first match per line [OK]

Common Mistakes:

• Thinking it deletes lines instead of substituting
• Assuming it replaces all occurrences without 'g' flag
• Confusing printing lines with substitution

2. Which of the following is the correct syntax to delete all lines containing the word 'error' using sed?

easy

A. sed '/error/d' filename

B. sed 'd/error/' filename

C. sed 's/error/d/' filename

D. sed 'delete /error/' filename

Solution

1. Step 1: Identify the delete command syntax in sed

   To delete lines matching a pattern, the syntax is sed '/pattern/d'.

2. Step 2: Match the correct option

   sed '/error/d' filename uses sed '/error/d', which deletes lines containing 'error'. Other options have incorrect syntax.

3. Final Answer:

   sed '/error/d' filename -> Option A

4. Quick Check:

   Delete lines with pattern = /pattern/d [OK]

Hint: Use '/pattern/d' to delete matching lines in sed [OK]

Common Mistakes:

• Placing 'd' before the pattern
• Using substitution syntax instead of delete
• Writing 'delete' instead of 'd'

3. What is the output of the command echo 'apple banana apple' | sed 's/apple/orange/g'?

medium

A. orange banana orange

B. orange banana apple

C. apple banana apple

D. apple banana orange

Solution

1. Step 1: Understand the 'g' flag in substitution

   The 'g' flag means replace all occurrences of the pattern on each line, not just the first.

2. Step 2: Apply substitution to the input string

   All 'apple' words are replaced by 'orange', so 'apple banana apple' becomes 'orange banana orange'.

3. Final Answer:

   orange banana orange -> Option A

4. Quick Check:

   Substitute all matches with 'g' flag = replaced all [OK]

Hint: Add 'g' to replace all matches, not just first [OK]

Common Mistakes:

• Forgetting the 'g' flag replaces only first occurrence
• Confusing output with original input
• Misreading the pipe input

4. Identify the error in this sed command: sed 's/hello/goodbye' file.txt

medium

A. No error; command is correct

B. Incorrect command; 's' cannot be used with text files

C. The pattern 'hello' is invalid

D. Missing delimiter after replacement text

Solution

1. Step 1: Check the substitution syntax

   The substitution command requires delimiters: s/pattern/replacement/. The final '/' is missing here.

2. Step 2: Identify the missing delimiter

   The command ends with 's/hello/goodbye' missing the closing '/' delimiter after 'goodbye'.

3. Final Answer:

   Missing delimiter after replacement text -> Option D

4. Quick Check:

   Substitution needs closing '/' delimiter [OK]

Hint: Check that substitution ends with '/' delimiter [OK]

Common Mistakes:

• Omitting the last '/' in substitution
• Assuming 's' can't be used on files
• Thinking pattern must be a regex error

5. You want to replace the word 'cat' with 'dog' only on lines 2 to 4 in a file named pets.txt. Which sed command achieves this?

hard

A. sed 's/cat/dog/ 2,4' pets.txt

B. sed '/2,4 s/cat/dog/' pets.txt

C. sed '2,4s/cat/dog/' pets.txt

D. sed 's/cat/dog/ 2-4' pets.txt

Solution

1. Step 1: Understand address range in sed

   To apply a command only on specific lines, use start,endcommand syntax, e.g., 2,4s/pattern/replacement/.

2. Step 2: Match the correct syntax

   sed '2,4s/cat/dog/' pets.txt correctly uses 2,4s/cat/dog/ to replace 'cat' with 'dog' on lines 2 to 4.

3. Final Answer:

   sed '2,4s/cat/dog/' pets.txt -> Option C

4. Quick Check:

   Use line range before command to limit sed action [OK]

Hint: Use 'start,end' before 's' to limit substitution lines [OK]

Common Mistakes:

• Placing line numbers after substitution command
• Using incorrect range syntax like '2-4'
• Adding slashes around line numbers