String replacement (${var/old/new}) in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this string replacement?

Given the following bash script, what will be printed?

Bash Scripting

text="hello world"
echo ${text/world/universe}

A hello universe

B hello world

C universe world

D hello

💡 Hint

The syntax ${var/old/new} replaces the first occurrence of 'old' with 'new' in the variable's value.

The variable 'text' contains 'hello world'. The expression ${text/world/universe} replaces the first 'world' with 'universe', so the output is 'hello universe'.

💻 Command Output

intermediate

What does this double slash replacement output?

What will this bash command output? text="banana bandana" echo ${text//ana/123}

Bash Scripting

text="banana bandana"
echo ${text//ana/123}

A b123 b123d123

B b123na band123

C banana bandana

D b123na b123d123

💡 Hint

Using double slashes replaces all occurrences, not just the first.

The expression ${text//ana/123} replaces all 'ana' substrings with '123'. So 'banana' becomes 'b123na' and 'bandana' becomes 'band123'.

💻 Command Output

advanced

What is the output when replacing a substring that does not exist?

Consider this bash code: text="apple orange" echo ${text/applee/fruit}

Bash Scripting

text="apple orange"
echo ${text/applee/fruit}

A Syntax error

B fruit orange

C orange

D apple orange

💡 Hint

If the substring to replace is not found, the original string remains unchanged.

Since 'applee' is not found in 'apple orange', no replacement occurs, so the output is the original string.

💻 Command Output

advanced

What is the output of this nested replacement?

What will this bash script print? text="foo bar foo" echo ${text/foo/bar} echo ${text//foo/bar}

Bash Scripting

text="foo bar foo"
echo ${text/foo/bar}
echo ${text//foo/bar}

A

bar bar foo
bar bar bar

B

bar bar foo
foo bar foo

C

foo bar foo
bar bar bar

D

bar bar foo
bar foo bar

💡 Hint

Single slash replaces first occurrence; double slash replaces all occurrences.

The first echo replaces only the first 'foo' with 'bar', resulting in 'bar bar foo'. The second echo replaces all 'foo' with 'bar', resulting in 'bar bar bar'.

💻 Command Output

expert

What is the output when using replacement with special characters?

Given this bash code, what will be the output? text="path/to/file.txt" echo ${text//./_}

Bash Scripting

text="path/to/file.txt"
echo ${text//./_}

A txt_elif/ot/htap

B path/to/file_txt_

C path/to/file_txt

D ath/to/file_txt

💡 Hint

The dot '.' is replaced by underscore '_' everywhere in the string.

The expression replaces all '.' characters with '_'. Since there is only one dot before 'txt', it becomes 'path/to/file_txt'.

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