String replacement (${var/old/new}) in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to replace the first occurrence of 'cat' with 'dog' in the variable.

Bash Scripting

animal="cat and cat"
new_animal=${animal/$[1]/dog}
echo "$new_animal"

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

Aanimal

Bdog

Cand

Dcat

Attempts:

3 left

💡 Hint

Common Mistakes

Using double slashes // which replace all occurrences instead of one.

Replacing the wrong word like 'dog' instead of 'cat'.

The syntax ${var/old/new} replaces the first occurrence of 'old' with 'new' in the variable 'var'. Here, 'cat' is replaced by 'dog'.

2fill in blank

medium

Complete the code to replace all occurrences of 'apple' with 'orange' in the variable.

Bash Scripting

fruits="apple banana apple"
new_fruits=${fruits//$[1]/orange}
echo "$new_fruits"

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

Aapple

C/apple

Dbanana

Attempts:

3 left

💡 Hint

Common Mistakes

Using single slash / which replaces only the first occurrence.

Forgetting the double slashes and causing no replacement.

Using double slashes // in ${var//old/new} replaces all occurrences of 'old' with 'new'.

3fill in blank

hard

Fix the error in the code to replace 'red' with 'blue' in the variable.

Bash Scripting

color="red green red"
new_color=${color$[1]red/blue}
echo "$new_color"

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

A//

Bred/

C/

D/red/

Attempts:

3 left

💡 Hint

Common Mistakes

Using no slash or wrong placement of slashes.

Adding extra slashes causing syntax errors.

The correct syntax is ${var/old/new} with a single slash to replace the first occurrence of 'red' with 'blue'.

4fill in blank

hard

Fill both blanks to replace the first occurrence of 'cat' with 'dog' and then all occurrences of 'dog' with 'wolf'.

Bash Scripting

text="cat dog cat dog"
step1=${text$[1]cat/dog}
step2=${step1$[2]dog/wolf}
echo "$step2"

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

A/

B//

C/dog

D//cat

Attempts:

3 left

💡 Hint

Common Mistakes

Using double slashes for first replacement.

Using single slash for replacing all occurrences.

Use single slash / to replace first occurrence and double slash // to replace all occurrences.

5fill in blank

hard

Fill all three blanks to replace 'apple' with 'orange', then 'banana' with 'grape', and finally 'grape' with 'melon' in the variable.

Bash Scripting

basket="apple banana banana apple"
step1=${basket$[1]apple/orange}
step2=${step1$[2]banana/grape}
step3=${step2$[3]grape/melon}
echo "$step3"

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

A//

B/

Attempts:

3 left

💡 Hint

Common Mistakes

Using double slash for first replacement causing all to change.

Using single slash for later replacements causing only first to change.

Use single slash / to replace first occurrence and double slash // to replace all occurrences. Here, first replacement is single, next two are all occurrences.

Practice

1. What does the expression ${var/old/new} do in bash scripting?

easy

A. Appends 'new' to the variable 'var' if it contains 'old'.

B. Replaces the first occurrence of 'old' with 'new' in the variable 'var'.

C. Deletes the variable 'var' if it contains 'old'.

D. Replaces all occurrences of 'old' with 'new' in the variable 'var'.

Solution

1. Step 1: Understand the syntax ${var/old/new}

   This syntax replaces only the first occurrence of the substring 'old' with 'new' inside the variable 'var'.

2. Step 2: Differentiate from ${var//old/new}

   The double slash version replaces all occurrences, but the single slash replaces only the first one.

3. Final Answer:

   Replaces the first occurrence of 'old' with 'new' in the variable 'var'. -> Option B

4. Quick Check:

   Single slash means first occurrence only [OK]

Hint: Single slash replaces first match only, double slash replaces all [OK]

Common Mistakes:

• Confusing single slash with double slash for all replacements
• Thinking it deletes or appends instead of replacing
• Assuming it works on strings outside variables

2. Which of the following is the correct syntax to replace the first occurrence of 'cat' with 'dog' in variable animal?

easy

A. ${animal/cat/dog}

B. ${animal//cat/dog}

C. ${animal/cat|dog}

D. ${animal:cat:dog}

Solution

1. Step 1: Identify correct replacement syntax

   The correct syntax for replacing the first occurrence is ${variable/old/new}, so ${animal/cat/dog} is correct.

2. Step 2: Check other options for errors

   ${animal//cat/dog} uses double slash which replaces all occurrences, not just the first. Options C and D use invalid syntax.

3. Final Answer:

   ${animal/cat/dog} -> Option A

4. Quick Check:

   Single slash correct for first occurrence [OK]

Hint: Use single slash for first replacement, double slash for all [OK]

Common Mistakes:

• Using double slash when only first replacement is needed
• Using wrong delimiters like | or : instead of /
• Confusing syntax with substring extraction

3. Given text="hello world world", what is the output of echo ${text/world/earth}?

medium

A. hello earth earth

B. hello world earth

C. hello earth world

D. hello world world

Solution

1. Step 1: Understand the variable content

   The variable text contains "hello world world" with two 'world' words.

2. Step 2: Apply ${text/world/earth}

   This replaces only the first occurrence of 'world' with 'earth', so the result is "hello earth world".

3. Final Answer:

   hello earth world -> Option C

4. Quick Check:

   Single slash replaces first match only [OK]

Hint: Single slash replaces only first match, so second 'world' stays [OK]

Common Mistakes:

• Expecting all 'world' to change to 'earth'
• Confusing output with original variable
• Misreading variable content

4. What is wrong with this code snippet?

file="data.txt"
newfile=${file/data/txt}
echo $newfile

medium

A. It replaces 'data' with 'txt' correctly.

B. Syntax error due to missing slashes in replacement.

C. It replaces 'data' with 'txt' but misses the dot.

D. It replaces 'data' with 'txt' but changes the file extension incorrectly.

Solution

1. Step 1: Check the replacement syntax

   The replacement syntax must be ${var/old/new} with slashes separating parts. Here, ${file/data/txt} is correct syntax.

2. Step 2: Analyze the replacement effect

   The code replaces 'data' with 'txt', so 'data.txt' becomes 'txt.txt'. There is no syntax error.

3. Step 3: Identify the logical issue

   While the code executes correctly, the replacement of 'data' with 'txt' produces 'txt.txt', which alters the filename in a way that incorrectly impacts the intended file extension handling.

4. Final Answer:

   It replaces 'data' with 'txt' but changes the file extension incorrectly. -> Option D

5. Quick Check:

   Replacement changes 'data' to 'txt', affecting filename [OK]

Hint: Check slashes and replaced parts carefully for syntax and effect [OK]

Common Mistakes:

• Assuming syntax error when slashes are correct
• Ignoring that replacement changes filename parts
• Confusing replacement with substring extraction

5. You have a variable sentence="apple banana apple cherry apple". How do you replace only the second occurrence of 'apple' with 'orange' using bash string replacement?

hard

A. Use parameter expansion with substring and manual replacement.

B. Use ${sentence//apple/orange} to replace all.

C. Use ${sentence/apple/orange} twice.

D. Use ${sentence/apple/orange} once; it replaces second occurrence automatically.

Solution

1. Step 1: Understand limitation of ${var/old/new}

   This syntax replaces only the first occurrence, no direct way to replace the second occurrence.

2. Step 2: Consider manual approach

   To replace the second occurrence, you must split or manipulate the string manually using substring extraction and concatenation.

3. Step 3: Confirm the approach

   Bash parameter expansion with single or double slashes cannot target specific non-first occurrences; manual string manipulation using substrings is required.

4. Final Answer:

   Use parameter expansion with substring and manual replacement. -> Option A

5. Quick Check:

   Only manual substring methods can target second occurrence [OK]

Hint: Single slash replaces first only; second needs manual substring handling [OK]

Common Mistakes:

• Assuming single replacement syntax can target second occurrence
• Using double slash to replace all instead of just second
• Expecting repeated use of ${var/old/new} to replace second occurrence