String suffix removal (${var%pattern}) in Bash Scripting - Step-by-Step Execution

Concept Flow - String suffix removal (${var%pattern})

Start with variable var

↓

Apply suffix pattern match

↓

Remove shortest matching suffix

↓

Return modified string

↓

End

The shell checks the variable's value, finds the shortest suffix matching the pattern, removes it, and returns the result.

Execution Sample

Bash Scripting

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

Removes the shortest suffix '.txt' from the variable 'filename' and prints the result.

Execution Table

Step	Variable 'filename'	Pattern	Action	Resulting Value	Output
1	report.txt	%.txt	Check if suffix '.txt' matches	Matches suffix '.txt'
2	report.txt	%.txt	Remove shortest matching suffix '.txt'	report
3	report	N/A	Echo the result	N/A	report
4	report	N/A	End of script	N/A

💡 Suffix '.txt' removed; no more matches; script ends.

Variable Tracker

Variable	Start	After suffix removal	Final
filename	report.txt	report.txt	report.txt
result	N/A	report	report

Key Moments - 2 Insights

Why does only the shortest suffix matching the pattern get removed?

Because the '%' operator removes the shortest suffix matching the pattern, as shown in execution_table step 2 where only '.txt' is removed, not more.

What happens if the pattern does not match the suffix?

No removal happens; the variable stays the same. This is implied because the action only removes when a match is found (step 1 and 2).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'result' after suffix removal?

A .txt

B report

C report.txt

D report.tx

💡 Hint

Check execution_table row 2 under 'Resulting Value' column.

At which step does the script output the final result?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at execution_table row 3 under 'Output' column.

If the variable was 'archive.tar.gz' and pattern was '%.gz', what would be the result after removal?

A archive.tar

B archive

C archive.tar.gz

D tar.gz

💡 Hint

Remember '%' removes shortest matching suffix; see concept_flow and execution_table logic.

Concept Snapshot

Syntax: ${var%pattern}
Removes shortest suffix matching pattern from var's value.
Useful to strip file extensions or suffixes.
Only removes if pattern matches suffix.
Does not change original variable unless reassigned.

Full Transcript

This example shows how to remove a suffix from a string variable in bash using ${var%pattern}. The shell looks at the variable's value, finds the shortest suffix matching the pattern, removes it, and returns the modified string. For example, with filename="report.txt" and pattern '%.txt', the suffix '.txt' is removed, resulting in 'report'. The script then echoes this result. If the pattern does not match, the string remains unchanged. This is a simple way to strip file extensions or other suffixes in bash scripting.

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