String prefix removal (${var#pattern}) in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this prefix removal?

Given the variable file="backup_2024_report.txt", what is the output of echo ${file#backup_}?

Bash Scripting

file="backup_2024_report.txt"
echo ${file#backup_}

A report.txt

B backup_2024_report.txt

C _2024_report.txt

D 2024_report.txt

💡 Hint

The # operator removes the shortest match of the pattern from the start of the string.

The pattern 'backup_' matches the prefix 'backup_' exactly, so it is removed, leaving '2024_report.txt'.

💻 Command Output

intermediate

Removing shortest vs longest prefix

Given path="/home/user/docs/file.txt", what is the output of echo ${path##*/}?

Bash Scripting

path="/home/user/docs/file.txt"
echo ${path##*/}

A file.txt

B /file.txt

C docs/file.txt

D /home/user/docs/file.txt

💡 Hint

The ## operator removes the longest match of the pattern from the start.

The pattern '*/' matches everything up to the last slash, so it removes '/home/user/docs/' leaving 'file.txt'.

🔧 Debug

advanced

Why does this prefix removal fail?

Consider var="version1.2.3". What error or output results from echo ${var#version.}?

Bash Scripting

var="version1.2.3"
echo ${var#version.}

A 1.2.3

B version1.2.3

C Syntax error

D ersion1.2.3

💡 Hint

The pattern must match exactly at the start to be removed.

The pattern 'version.' does not match the start because the string has 'version1' without a dot after 'version'. So no removal happens.

🧠 Conceptual

advanced

What does this prefix removal pattern do?

Given text="error_404_not_found", what is the output of echo ${text#*_}?

Bash Scripting

text="error_404_not_found"
echo ${text#*_}

A 404_not_found

B error_404_not_found

C not_found

D _404_not_found

💡 Hint

The pattern '*_' removes the shortest prefix ending with underscore.

The pattern '*_' matches everything up to the first underscore, so it removes 'error_' leaving '404_not_found'.

🚀 Application

expert

Extract filename without extension using prefix removal

Given filename="archive.tar.gz", which command outputs tar.gz by removing the shortest prefix ending with a dot?

Bash Scripting

filename="archive.tar.gz"
echo ???

A echo ${filename#*.tar}

B echo ${filename##*.}

C echo ${filename#*.}

D echo ${filename#archive.}

💡 Hint

Use # to remove shortest prefix matching pattern including a dot.

The pattern '*.' removes everything up to the first dot, so ${filename#*.} removes 'archive.' leaving 'tar.gz'.