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.