String suffix removal (${var%pattern}) in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the syntax ${var%pattern} do in bash scripting?

It removes the shortest matching suffix pattern from the value of the variable var. This means it deletes the smallest part at the end of the string that matches pattern.

intermediate

How does ${var%%pattern} differ from ${var%pattern}?

${var%%pattern} removes the longest matching suffix pattern, while ${var%pattern} removes the shortest matching suffix pattern from the variable's value.

beginner

Example: If file="report.txt.bak", what is the result of ${file%.bak}?

The result is report.txt. It removes the shortest suffix matching .bak from the variable file.

intermediate

Why use string suffix removal instead of external commands like sed or cut?

Because string suffix removal is faster and uses built-in shell features, so it avoids starting extra programs. It is simpler and more efficient for small string changes.

beginner

What happens if the pattern does not match any suffix in ${var%pattern}?

If the pattern does not match the end of the string, the variable's value remains unchanged.

What does ${filename%.txt} do if filename="notes.txt"?

A Does nothing

B Removes the prefix '.txt' from filename

C Removes all '.txt' occurrences in filename

D Removes the suffix '.txt' from filename

It removes the shortest suffix matching '.txt' from the variable's value.

Which syntax removes the longest matching suffix pattern in bash?

A ${var%%pattern}

B ${var#pattern}

C ${var%pattern}

D ${var##pattern}

${var%%pattern} removes the longest matching suffix pattern.

If path="folder/file.tar.gz", what does ${path%.gz} return?

A folder/file.tar.gz

B folder/file.tar.g

C folder/file.tar

D folder/file.tar.gz.gz

It removes the shortest suffix '.gz' from the string.

What happens if the pattern does not match the suffix in ${var%pattern}?

A The variable is emptied

B The variable value is unchanged

C An error occurs

D The entire string is removed

If no match, the original string remains unchanged.

Why is using ${var%pattern} preferred over external tools for suffix removal?

A It is built-in and faster

B It requires installing extra software

C It is slower but more readable

D It works only on Linux

Built-in string manipulation is faster and does not require extra programs.

Explain how to remove a file extension suffix from a filename variable in bash using string suffix removal.

Think about removing the shortest suffix matching the extension.

Describe the difference between ${var%pattern} and ${var%%pattern} in bash string manipulation.

One removes the smallest matching suffix, the other the largest.

Practice

1. What does the Bash expression ${filename%.txt} do to the variable filename?

easy

A. Removes the shortest prefix matching '.txt' from the start of filename

B. Removes the longest suffix matching '.txt' from the end of filename

C. Removes the shortest suffix matching '.txt' from the end of filename

D. Removes the longest prefix matching '.txt' from the start of filename

Solution

1. Step 1: Understand the % operator in Bash

   The single % operator removes the shortest matching suffix pattern from the variable's value.

2. Step 2: Apply to the pattern '.txt'

   It removes '.txt' only if it appears at the end of the string, trimming that suffix.

3. Final Answer:

   Removes the shortest suffix matching '.txt' from the end of filename -> Option C

4. Quick Check:

   ${var%pattern} removes shortest suffix [OK]

Hint: Single % removes shortest suffix, double %% removes longest [OK]

Common Mistakes:

• Confusing % with prefix removal operators
• Using %% instead of %
• Thinking it removes prefix instead of suffix

2. Which of the following is the correct syntax to remove the shortest suffix '.log' from the variable file in Bash?

easy

A. ${file%%.log}

B. ${file%.log}

C. ${file#log}

D. ${file%log}

Solution

1. Step 1: Identify suffix removal syntax

   To remove the shortest suffix matching a pattern, use single % with the pattern including the dot: %.log

2. Step 2: Check each option

   A uses %% which removes longest suffix, C uses # which removes prefix, D misses the dot before 'log'. Only B is correct syntax.

3. Final Answer:

   ${file%.log} -> Option B

4. Quick Check:

   Use single % with pattern for shortest suffix removal [OK]

Hint: Use single % with exact suffix pattern to remove shortest suffix [OK]

Common Mistakes:

• Using %% instead of %
• Confusing % with # for prefix removal
• Omitting the dot in the pattern

3. Given the Bash code:

filename="report.final.txt"
echo ${filename%.txt}

What is the output?

medium

A. report.final.tx

B. report

C. report.final.txt

D. report.final

Solution

1. Step 1: Understand the variable value

   The variable filename contains 'report.final.txt'.

2. Step 2: Apply ${filename%.txt}

   This removes the shortest suffix matching '.txt' from the end, leaving 'report.final'.

3. Final Answer:

   report.final -> Option D

4. Quick Check:

   Suffix '.txt' removed once from end [OK]

Hint: Remove suffix by matching pattern at string end with % [OK]

Common Mistakes:

• Removing more than '.txt' suffix
• Confusing prefix removal with suffix removal
• Expecting the full string unchanged

4. Consider this Bash snippet:

file="archive.tar.gz"
echo ${file%*.gz}

It outputs archive.tar. You want to remove the entire suffix .tar.gz. What is the fix?

medium

A. Use ${file%%*.gz} to remove the longest matching suffix

B. Use ${file%.gz} to remove only '.gz' suffix

C. Use ${file#*.tar.gz} to remove prefix instead

D. Use ${file%.tar} to remove only '.tar' suffix

Solution

1. Step 1: Understand why ${file%*.gz} removes only '.gz'

   Single % removes shortest suffix matching *.gz pattern, so it matches '.gz' (with * matching empty).

2. Step 2: Use %% to remove longest suffix

   Double %% removes longest matching suffix, so ${file%%*.gz} removes entire '.tar.gz'.

3. Final Answer:

   Use ${file%%*.gz} to remove the longest matching suffix -> Option A

4. Quick Check:

   %% removes longest suffix, % removes shortest [OK]

Hint: Use %% to remove longest suffix pattern [OK]

Common Mistakes:

• Using single % when longest suffix removal needed
• Confusing prefix removal (#) with suffix removal (%)
• Trying to remove partial suffix only

5. You have a list of filenames:

files=("data1.csv" "data2.csv" "notes.txt" "summary.csv")

You want to remove the '.csv' suffix only from files that end with '.csv' using Bash parameter expansion. Which command correctly prints the filenames with '.csv' removed where applicable?

hard

A. for f in "${files[@]}"; do echo ${f%.csv}; done

B. for f in "${files[@]}"; do echo ${f%%csv}; done

C. for f in "${files[@]}"; do echo ${f%csv}; done

D. for f in "${files[@]}"; do echo ${f#%.csv}; done

Solution

1. Step 1: Understand the goal

   Remove '.csv' suffix only if it exists, leave other filenames unchanged.

2. Step 2: Check each option's effect

   A uses single % with '.csv' pattern, removing suffix if present. B uses %%csv missing the dot, incorrect. C misses the dot, incorrect. D uses # which removes prefix, wrong direction.

3. Step 3: Choose best option

   for f in "${files[@]}"; do echo ${f%.csv}; done is correct and simplest for shortest suffix removal of '.csv'.

4. Final Answer:

   for f in "${files[@]}"; do echo ${f%.csv}; done -> Option A

5. Quick Check:

   Use ${var%.csv} to remove '.csv' suffix if present [OK]

Hint: Use ${var%.ext} in loop to trim suffix conditionally [OK]

Common Mistakes:

• Using prefix removal (#) instead of suffix removal (%)
• Omitting the dot in suffix pattern
• Using %% unnecessarily when % suffices