sed for substitution in scripts in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to replace 'apple' with 'orange' in the file 'fruits.txt'.

Bash Scripting

sed '[1]' fruits.txt

Drag options to blanks, or click blank then click option'

As/apple/orange/g

Bd/apple/orange/g

Cr/apple/orange/g

Dx/apple/orange/g

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'r' instead of 's' for substitution.

Forgetting the 'g' flag to replace all occurrences.

Using incorrect command letters like 'd' or 'x'.

The 's' command in sed is used for substitution. 's/apple/orange/g' replaces all occurrences of 'apple' with 'orange'.

2fill in blank

medium

Complete the code to replace only the first occurrence of 'cat' with 'dog' in each line of 'pets.txt'.

Bash Scripting

sed '[1]' pets.txt

Drag options to blanks, or click blank then click option'

As/cat/dog/g

Bs/cat/dog/1

Cs/cat/dog/

Ds/cat/dog/2

Attempts:

3 left

💡 Hint

Common Mistakes

Adding 'g' flag replaces all occurrences, not just the first.

Using numbers like '/1' or '/2' which are not valid flags here.

Without the 'g' flag, sed replaces only the first occurrence per line. So 's/cat/dog/' replaces the first 'cat' with 'dog'.

3fill in blank

hard

Fix the error in the sed command to replace 'blue' with 'red' in 'colors.txt'.

Bash Scripting

sed '[1]' colors.txt

Drag options to blanks, or click blank then click option'

As/blue/red

Bs/blue/red/g

Cs/blue/red/g;

Ds/blue/red/g//

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a semicolon at the end causes syntax error.

Adding extra slashes after 'g' causes errors.

Omitting the 'g' flag replaces only first occurrence.

The correct sed substitution command is 's/blue/red/g' without extra characters. It replaces all 'blue' with 'red'.

4fill in blank

hard

Fill both blanks to create a sed command that replaces 'dog' with 'cat' only on lines containing 'pet'.

Bash Scripting

sed -n '/[1]/ s/[2]/cat/gp' animals.txt

Drag options to blanks, or click blank then click option'

Apet

Bdog

Ccat

Danimal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'cat' as the pattern to match lines instead of 'pet'.

Replacing 'cat' with 'dog' instead of the other way around.

Omitting the 'p' flag so lines are not printed.

The command '/pet/ s/dog/cat/gp' means: on lines matching 'pet', substitute 'dog' with 'cat' globally and print those lines.

5fill in blank

hard

Fill all three blanks to create a sed command that replaces 'apple' with 'banana' only on lines containing 'fruit' and saves output to 'new.txt'.

Bash Scripting

sed -n '/[1]/ s/[2]/[3]/gp' fruits.txt > new.txt

Drag options to blanks, or click blank then click option'

Afruit

Bapple

Cbanana

Dorange

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong pattern to match lines.

Swapping 'apple' and 'banana' in substitution.

Forgetting to redirect output to 'new.txt'.

The command '/fruit/ s/apple/banana/gp' means: on lines with 'fruit', replace 'apple' with 'banana' globally and print those lines. Redirect output to 'new.txt'.

Practice

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

easy

A. Prints lines containing 'cat' from file.txt

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

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

D. Deletes all lines containing 'cat' from file.txt

Solution

1. Step 1: Understand the sed substitution syntax

   The command sed 's/cat/dog/' file.txt uses s/old/new/ which replaces only the first match per line.

2. Step 2: Apply to the file content

   Each line in file.txt will have its first 'cat' replaced by 'dog', other occurrences remain unchanged.

3. Final Answer:

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

4. Quick Check:

   sed s/old/new/ replaces first match per line [OK]

Hint: Remember: no 'g' means only first match replaced per line [OK]

Common Mistakes:

• Thinking it replaces all matches 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' in a file using sed?

easy

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

B. sed 'replace apple with orange' file.txt

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

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

Solution

1. Step 1: Identify the global flag usage

   To replace all occurrences on each line, the 'g' flag is needed after the substitution pattern.

2. Step 2: Check each option

   sed 's/apple/orange/g' file.txt uses s/apple/orange/g which correctly replaces all 'apple' with 'orange'. sed 's/apple/orange/' file.txt replaces only first occurrence. sed 'replace apple with orange' file.txt is invalid syntax. sed 's/apple/orange//g' file.txt has extra slashes causing syntax error.

3. Final Answer:

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

4. Quick Check:

   Use 'g' flag for all matches [OK]

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

Common Mistakes:

• Omitting the 'g' flag to replace all matches
• Adding extra slashes causing syntax errors
• Using incorrect command words like 'replace'

3. What will be the output of the following command if the input file contains the line apple apple apple?

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

medium

A. orange orange orange

B. orange apple apple

C. apple orange apple

D. apple apple orange

Solution

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

   Without the 'g' flag, sed replaces only the first occurrence of 'apple' per line.

2. Step 2: Apply substitution to the input line

   The first 'apple' becomes 'orange', the rest remain unchanged, resulting in 'orange apple apple'.

3. Final Answer:

   orange apple apple -> Option B

4. Quick Check:

   First match replaced only without 'g' [OK]

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

Common Mistakes:

• Assuming all occurrences change without 'g'
• Confusing order of words after substitution
• Thinking sed changes input file by default

4. You wrote this sed command to replace 'foo' with 'bar' globally: sed 's/foo/bar/' file.txt. But only the first 'foo' per line is replaced. What is the fix?

medium

A. Add the 'g' flag: sed 's/foo/bar/g' file.txt

B. Use double quotes instead of single quotes

C. Add a space after 'bar/'

D. Use sed -i option to edit in place

Solution

1. Step 1: Identify missing global flag

   The command lacks the 'g' flag, so only first occurrence per line is replaced.

2. Step 2: Add the 'g' flag to fix

   Adding 'g' after the substitution pattern makes sed replace all occurrences per line.

3. Final Answer:

   Add the 'g' flag: sed 's/foo/bar/g' file.txt -> Option A

4. Quick Check:

   Global flag 'g' replaces all matches [OK]

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

Common Mistakes:

• Thinking quotes affect substitution behavior
• Adding spaces inside substitution pattern
• Confusing in-place edit with substitution flags

5. You want to replace the word 'error' with 'warning' only on lines containing 'log' in a file. Which sed command achieves this?

hard

A. sed '/log/ s/error/warning/g' file.txt

B. sed 's/error/warning/g /log/' file.txt

C. sed 's/error/warning/g if /log/' file.txt

D. sed 's/error/warning/g' file.txt | grep log

Solution

1. Step 1: Understand conditional substitution in sed

   To apply substitution only on lines matching a pattern, use /pattern/ s/old/new/flags syntax.

2. Step 2: Analyze options

   sed '/log/ s/error/warning/g' file.txt correctly applies substitution only on lines containing 'log'. Options A and C have invalid syntax. sed 's/error/warning/g' file.txt | grep log replaces everywhere then filters lines, which does not limit substitution.

3. Final Answer:

   sed '/log/ s/error/warning/g' file.txt -> Option A

4. Quick Check:

   Use address before s/// to limit lines [OK]

Hint: Put pattern before s/// to restrict substitution lines [OK]

Common Mistakes:

• Placing substitution after pattern incorrectly
• Trying to use if statements inside sed
• Filtering lines after substitution instead of limiting it