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.