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In Bash scripting, parameter expansion lets you transform variable values using patterns inside ${}. Previously, we replaced “file.txt” with “data” in a path: 
#!/usr/bin/env bash
path="/home/user/file.txt"
echo "${path/file.txt/data}"
# Output: /home/user/data
In this lesson, we’ll focus on pattern matching and how to remove prefixes and suffixes from strings using Bash parameter expansion.

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Patterns in Bash

A pattern is a sequence of characters recognized by the shell for matching or substitution. You’ll find patterns in file globbing (e.g., *.txt), text processing, and data validation. In Bash, patterns power parameter expansion, letting you transform variable content without external tools.

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Variable Expansion vs. Parameter Expansion

By default, ${var} simply expands to its value: 
name="John Doe"
echo "Hello, ${name}"
# Hello, John Doe
When you include operators like # or % inside the braces, Bash invokes parameter expansion, applying pattern-based modifications to the variable’s value.
Always enclose the expression in double quotes ("${...}") to preserve spaces and prevent word splitting.

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Removing Prefixes and Suffixes

Bash supports removing the shortest matching prefix or suffix from a string with ${var#pattern} and ${var%pattern}.

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Understanding Prefix and Suffix

Consider these job titles in an IT company:
The image lists different engineering job titles: Mid DevOps Engineer, Jr. Software Engineer, Sr. DevOps Engineer, and Associate DevOps Engineer, alongside an icon of a briefcase with a magnifying glass.
  • Prefixes (associate, senior, junior, mid) indicate tenure or level.
  • The suffix (Engineer) indicates the job field.

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Prefix Removal with

The # operator deletes the shortest matching pattern from the start of the string:
The image illustrates the concept of "Parameter Expansions" with a focus on removing a prefix, represented by a hashtag symbol and scissors icon.
Example 1: Remove the leading H: 
greetings="Hello World"
echo "${greetings#H}"
# Output: ello World
If the pattern isn’t found at the beginning, the value remains unchanged: 
echo "${greetings#e}"
# Output: Hello World
Example 2: Remove the word “Hello ” (including the space): 
echo "${greetings#Hello }"
# Output: World
Patterns are case-sensitive: "h" won’t match "H". 
echo "${greetings#h}"
# Output: Hello World

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Suffix Removal with %

The % operator removes the shortest matching pattern from the end of the string: Example 1: Drop the trailing d: 
echo "${greetings%d}"
# Output: Hello Worl
Example 2: Remove “rld” at the end: 
echo "${greetings%rld}"
# Output: Hello Wo
If the suffix doesn’t match exactly, the string stays the same: 
echo "${greetings%world}"
# Output: Hello World
Pattern matching in Bash is case-sensitive. Ensure your pattern matches the exact case of the prefix or suffix.

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Quick Reference Table

OperatorActionExample
${var#pattern}Remove shortest prefix match${greetings#Hello } → World
${var%pattern}Remove shortest suffix match${greetings%rld} → Hello Wo
The image illustrates parameter expansions, showing a blue circle with a hash symbol for "Prefix" and a red circle with a percent symbol for "Suffix."

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Next Steps

In the next lesson, we’ll explore longest-match removals using ## and %% to strip more complex patterns.

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References

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