sed substitution in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the sed command do in Linux?

sed is a stream editor used to perform basic text transformations on an input stream (a file or input from a pipeline).

beginner

Explain the basic syntax of a sed substitution command.

The basic syntax is sed 's/pattern/replacement/' filename. It replaces the first occurrence of pattern with replacement on each line.

beginner

How do you replace all occurrences of a pattern in each line using sed?

Add the g flag at the end: sed 's/pattern/replacement/g' filename. This replaces every match on each line.

beginner

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

It replaces every occurrence of the word cat with dog in each line of file.txt, printing the result to the screen.

intermediate

How can you save the changes made by sed substitution back to the original file?

Use the -i option for in-place editing: sed -i 's/old/new/g' filename. This updates the file directly.

What does the g flag do in a sed substitution?

A Replaces only the first occurrence in the entire file

B Deletes the matching lines

C Replaces all occurrences of the pattern in each line

D Prints the matching lines only

The g flag means global replacement on each line, changing all matches, not just the first.

Which command replaces the first occurrence of 'apple' with 'orange' in each line of fruits.txt?

A sed 's/apple/orange/' fruits.txt

B sed 's/apple/orange/g' fruits.txt

C sed -i 's/apple/orange/g' fruits.txt

D sed 's/apple/orange/1' fruits.txt

Without the g flag, only the first occurrence per line is replaced.

How do you apply sed substitution and save changes directly to the file?

A sed -i 's/old/new/g' filename

B sed 's/old/new/g' filename > filename

C sed 's/old/new/g' filename

D sed -e 's/old/new/g' filename

The -i option edits the file in place, saving changes directly.

What will this command do? sed 's/blue/red/2' colors.txt

A Replace the second line containing 'blue' with 'red'

B Replace all occurrences of 'blue' with 'red'

C Replace the first occurrence of 'blue' with 'red'

D Replace the second occurrence of 'blue' with 'red' on each line

The number after the substitution command specifies which occurrence to replace on each line.

Which delimiter can you use instead of / in sed substitution?

A *

B #

C &

D %

You can use other delimiters like # to avoid escaping slashes in patterns.

Describe how to use sed to replace all occurrences of a word in a file and save the changes.

Think about the command structure and flags needed.

Explain the difference between using sed 's/pattern/replacement/' and sed 's/pattern/replacement/g'.

Focus on how many matches get replaced per line.

Practice

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

easy

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

B. Replaces all occurrences of 'cat' with 'dog' in the entire file.txt

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

D. Prints lines containing 'dog' from file.txt

Solution

1. Step 1: Understand the sed substitution syntax

   The command sed 's/cat/dog/' file.txt uses s/pattern/replacement/ which replaces the first match per line.

2. Step 2: Apply the command to the file

   It replaces only the first 'cat' found on each line with 'dog', not all occurrences or lines.

3. Final Answer:

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

4. Quick Check:

   sed s/pattern/replacement/ replaces first match per line [OK]

Hint: No 'g' flag means only first match per line replaced [OK]

Common Mistakes:

• Thinking all matches are replaced without 'g'
• Confusing substitution with deletion
• Assuming it prints only matching lines

2. Which of the following is the correct syntax to replace all occurrences of 'apple' with 'orange' using sed?

easy

A. sed 's/apple/orange/' file.txt

B. sed 's/apple/orange/g' file.txt

C. sed 's/apple/orange/g;' file.txt

D. sed 's/apple/orange/gp' file.txt

Solution

1. Step 1: Identify the flag for global replacement

   The 'g' flag after the substitution pattern tells sed to replace all occurrences on each line.

2. Step 2: Check syntax correctness

   sed 's/apple/orange/g' file.txt uses sed 's/apple/orange/g' which is the correct syntax without extra characters.

3. Final Answer:

   sed 's/apple/orange/g' file.txt -> Option B

4. Quick Check:

   'g' flag means replace all matches per line [OK]

Hint: Add 'g' after last slash to replace all matches [OK]

Common Mistakes:

• Omitting the 'g' flag for all replacements
• Adding extra semicolons inside quotes
• Using 'gp' which is invalid here

3. What is the output of this command if input.txt contains:

apple apple banana
banana apple apple

sed 's/apple/orange/' input.txt?

medium

A. orange orange banana
banana orange orange

B. orange apple banana
banana apple apple

C. orange apple banana
banana orange apple

D. apple apple banana
banana apple apple

Solution

1. Step 1: Understand sed substitution without 'g'

   Without 'g', only the first 'apple' on each line is replaced by 'orange'.

2. Step 2: Apply substitution line by line

   Line 1: 'apple apple banana' becomes 'orange apple banana'. Line 2: 'banana apple apple' becomes 'banana orange apple'.

3. Final Answer:

   orange apple banana banana orange apple -> Option C

4. Quick Check:

   First match per line replaced only [OK]

Hint: No 'g' means only first match per line changes [OK]

Common Mistakes:

• Replacing all 'apple' without 'g'
• Replacing none due to misunderstanding
• Confusing line order or content

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

medium

A. File name should come before the command

B. Incorrect delimiter used

C. Missing 'g' flag for global replacement

D. Missing closing quote after 'g'

Solution

1. Step 1: Check the quotes in the command

   The command has an opening single quote before s/hello/world/g but no closing quote after 'g'.

2. Step 2: Confirm correct placement of quotes and arguments

   The file name should be outside the quotes. The missing closing quote causes syntax error.

3. Final Answer:

   Missing closing quote after 'g' -> Option D

4. Quick Check:

   Quotes must enclose entire sed expression [OK]

Hint: Always close quotes around sed expression [OK]

Common Mistakes:

• Placing file name inside quotes
• Forgetting closing quote
• Confusing delimiter with quotes

5. You want to replace all occurrences of 'foo' with 'bar' only on lines containing 'baz' in data.txt. Which command achieves this?

hard

A. sed '/baz/ s/foo/bar/g' data.txt

B. sed 's/foo/bar/g /baz/' data.txt

C. sed 's/foo/bar/g' data.txt | grep baz

D. sed '/foo/ s/baz/bar/g' data.txt

Solution

1. Step 1: Understand address restriction in sed

   Using /baz/ s/foo/bar/g applies substitution only on lines matching 'baz'.

2. Step 2: Verify command correctness

   sed '/baz/ s/foo/bar/g' data.txt correctly restricts substitution to lines with 'baz' and replaces all 'foo' with 'bar'.

3. Final Answer:

   sed '/baz/ s/foo/bar/g' data.txt -> Option A

4. Quick Check:

   Address before command limits lines affected [OK]

Hint: Use /pattern/ before s/// to limit lines [OK]

Common Mistakes:

• Placing substitution after file name
• Using grep after sed instead of address
• Mixing patterns in wrong order