String prefix 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 prefix pattern from the value of the variable var. This means it deletes the smallest part at the start of the string that matches pattern.

intermediate

How is ${var##pattern} different from ${var#pattern}?

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

beginner

Example: Given file="/home/user/docs/report.txt", what is the result of ${file#*/}?

It removes the shortest prefix matching */, which is / up to and including the first slash. Result: home/user/docs/report.txt.

beginner

Why use string prefix removal in bash scripts?

It helps to extract parts of strings by cutting off unwanted beginnings, like removing directory paths or prefixes, making scripts cleaner and easier to read.

beginner

What happens if the pattern does not match any prefix in ${var#pattern}?

The variable's value remains unchanged because no matching prefix is found to remove.

What does ${var#*/} do to the string path="/usr/local/bin"?

A Removes the longest prefix ending with the last slash, resulting in <code>bin</code>

B Removes the shortest prefix ending with the first slash, resulting in <code>usr/local/bin</code>

C Removes all slashes from the string

D Removes the first character only

${var#*/} removes the shortest prefix matching */, which is everything up to and including the first slash.

Which syntax removes the longest matching prefix pattern in bash?

A ${var%pattern}

B ${var#pattern}

C ${var##pattern}

D ${var%%pattern}

${var##pattern} removes the longest matching prefix, while ${var#pattern} removes the shortest.

If var="abc123abc", what is the result of ${var#abc}?

A 123abc

B abc123abc

C 123

D bc123abc

The shortest prefix matching abc is removed, leaving 123abc.

What happens if the pattern does not match the start of the string in ${var#pattern}?

A The pattern is removed from anywhere in the string

B The entire string is removed

C An error is thrown

D The string remains unchanged

If the pattern does not match the start, no removal happens and the string stays the same.

Which of these is a practical use of ${var#pattern} in scripts?

A Extracting filename from a full path

B Replacing text inside a string

C Removing file extensions

D Counting characters in a string

Removing the shortest prefix can extract the filename by cutting off the directory path.

Explain how ${var#pattern} works in bash scripting and give a simple example.

Think about cutting off the start of a string that matches a pattern.

Describe the difference between ${var#pattern} and ${var##pattern} and when you might use each.

One removes the smallest match, the other the biggest match at the start.

Practice

1. What does the Bash expression ${var#pattern} do?

easy

A. Removes the shortest matching prefix pattern from the variable's value

B. Removes the longest matching suffix pattern from the variable's value

C. Removes the shortest matching suffix pattern from the variable's value

D. Removes the longest matching prefix pattern from the variable's value

Solution

1. Step 1: Understand the syntax

   The syntax ${var#pattern} is used in Bash to remove a prefix pattern from the start of the variable's value.

2. Step 2: Identify shortest prefix removal

   The single # removes the shortest matching prefix, unlike ## which removes the longest.

3. Final Answer:

   Removes the shortest matching prefix pattern from the variable's value -> Option A

4. Quick Check:

   Prefix removal = shortest match = A [OK]

Hint: Single # removes shortest prefix, double ## removes longest [OK]

Common Mistakes:

• Confusing # with ## for prefix length
• Mixing prefix removal with suffix removal
• Thinking it removes suffix instead of prefix

2. Which of the following is the correct syntax to remove the shortest prefix matching abc from variable var in Bash?

easy

A. ${var#abc}

B. ${var##abc}

C. ${var%abc}

D. ${var%%abc}

Solution

1. Step 1: Recall prefix removal syntax

   Single # removes shortest prefix, double ## removes longest prefix. % and %% remove suffixes.

2. Step 2: Match the pattern

   To remove shortest prefix matching 'abc', use ${var#abc}.

3. Final Answer:

   ${var#abc} -> Option A

4. Quick Check:

   Shortest prefix removal = single # = B [OK]

Hint: Use single # for shortest prefix removal syntax [OK]

Common Mistakes:

• Using ## instead of # for shortest prefix
• Using % or %% which remove suffixes
• Confusing prefix and suffix removal syntax

3. Given var="folder/subfolder/file.txt", what is the output of echo ${var#*/}?

medium

A. folder/subfolder/file.txt

B. subfolder/file.txt

C. file.txt

D. folder

Solution

1. Step 1: Understand the pattern */

   The pattern */ matches the shortest prefix ending with a slash.

2. Step 2: Remove shortest prefix matching */

   From "folder/subfolder/file.txt", the shortest prefix matching */ is "folder/". Removing it leaves "subfolder/file.txt".

3. Final Answer:

   subfolder/file.txt -> Option B

4. Quick Check:

   Remove shortest prefix ending with / = subfolder/file.txt [OK]

Hint: Pattern */ removes up to first slash [OK]

Common Mistakes:

• Removing longest prefix with ## instead of #
• Expecting only filename instead of subfolder/file.txt
• Confusing prefix removal with suffix removal

4. Consider the script snippet:

var="abc123abc456"
echo ${var##abc*}

It outputs an empty line. Why?

medium

A. Because the pattern is invalid and causes an error

B. Because abc* matches the entire string as shortest prefix

C. Because abc* matches the longest prefix, so nothing remains

D. Because the variable is empty

Solution

1. Step 1: Analyze the pattern abc*

   The pattern abc* matches 'abc' followed by zero or more any characters.

2. Step 2: Understand longest prefix removal

   With ##, it removes the longest matching prefix. The * matches greedily as many characters as possible after 'abc', so the match covers the entire string.

3. Final Answer:

   Because abc* matches the longest prefix, so nothing remains -> Option C

4. Quick Check:

   Longest prefix with ## + greedy * = B [OK]

Hint: Wildcard * matches greedily with longest prefix ## [OK]

Common Mistakes:

• Confusing ## (longest) with # (shortest)
• Underestimating greedy nature of * in longest match
• Assuming pattern causes error or variable is empty

5. You have a variable path="folder/home/user/docs/report.txt". Which command removes only the first slash and everything before it, leaving home/user/docs/report.txt?

hard

A. echo ${path#/*}

B. echo ${path##/*}

C. echo ${path##*/}

D. echo ${path#*/}

Solution

1. Step 1: Understand the variable value

   The variable path is "folder/home/user/docs/report.txt".

2. Step 2: Analyze each option's pattern

   ${path#/*} removes shortest prefix matching slash followed by anything, but path doesn't start with /, so no match.
   ${path##/*} removes longest prefix matching slash-anything, no match.
   ${path##*/} removes longest prefix up to last slash, leaving only filename "report.txt".
   ${path#*/} removes shortest prefix up to first slash ("folder/"), leaving "home/user/docs/report.txt".

3. Step 3: Choose correct option

   To remove up to the first slash, use ${path#*/}.

4. Final Answer:

   echo ${path#*/} -> Option D

5. Quick Check:

   Remove shortest prefix up to first slash = D [OK]

Hint: Use #*/ to remove up to first slash only [OK]

Common Mistakes:

• Using ##*/ removes up to last slash (only filename)
• Using #/* or ##/* which don't match without leading slash
• Confusing prefix removal with suffix removal