String suffix removal (${var%pattern}) in Bash Scripting - Time & Space Complexity

Time Complexity: String suffix removal (${var%pattern})

O(n)

Understanding Time Complexity

We want to understand how long it takes to remove a suffix from a string using bash's ${var%pattern} syntax.

Specifically, how does the time needed change as the string or pattern size grows?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

string="filename_backup_2024.txt"
pattern="*.txt"
result=${string%$pattern}
echo "$result"
    

This code removes the shortest suffix matching the pattern from the string.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Matching the pattern against the string suffix.
• How many times: The shell checks suffixes from the end of the string, moving backward until a match is found.

How Execution Grows With Input

As the string length grows, the shell tries more suffixes to find a match.

Input Size (n)	Approx. Operations
10	Up to 10 suffix checks
100	Up to 100 suffix checks
1000	Up to 1000 suffix checks

Pattern observation: The number of checks grows roughly in direct proportion to the string length.

Final Time Complexity

Time Complexity: O(n)

This means the time to remove the suffix grows linearly with the string length.

Common Mistake

[X] Wrong: "The suffix removal happens instantly no matter how long the string is."

[OK] Correct: The shell must check suffixes step by step, so longer strings take more time.

Interview Connect

Understanding how string operations scale helps you write efficient scripts and explain your reasoning clearly in interviews.

Self-Check

"What if we used the longest suffix removal syntax ${var%%pattern} instead? How would the time complexity change?"

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