String prefix removal (${var#pattern}) in Bash Scripting

Introduction

This helps you quickly remove a part from the start of a text stored in a variable. It is useful when you want to clean or shorten text without changing the original.

You have a file path and want to remove the folder part to get just the file name.

You receive a string with a fixed prefix and want to remove that prefix.

You want to trim a known pattern from the start of a string before processing it.

You are parsing logs and want to remove timestamps or tags at the beginning.

Syntax

Bash Scripting

${variable#pattern}

The # removes the shortest match of the pattern from the start of the variable's value.

Use ## to remove the longest match instead.

Examples

Removes the '/home/user/' prefix from the path, leaving 'file.txt'.

Bash Scripting

filename="/home/user/file.txt"
basename=${filename#"/home/user/"}
echo "$basename"

Removes 'https://' from the start of the URL.

Bash Scripting

url="https://example.com/page"
clean_url=${url#"https://"}
echo "$clean_url"

Removes the shortest 'abc' from the start, resulting in '123abc'.

Bash Scripting

text="abc123abc"
result=${text#abc}
echo "$result"

Sample Program

This script removes the '/var/log/' prefix from the full path to get just the file name.

Bash Scripting

#!/bin/bash

full_path="/var/log/syslog"

# Remove the directory path, keep only the file name
file_name=${full_path#"/var/log/"}

echo "Full path: $full_path"
echo "File name only: $file_name"

Important Notes

If the pattern does not match the start, the variable remains unchanged.

Patterns support wildcards like * and ? for flexible matching.

Use double quotes around variables to avoid word splitting or globbing issues.

Summary

Use ${var#pattern} to remove the shortest matching prefix from a variable's value.

It is useful for cleaning or extracting parts of strings in scripts.

Remember to use quotes to keep your script safe and predictable.

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