Ten Bash Parameter Expansions You'll Actually Use

One of the quiet superpowers of Bash is something many shell users overlook: parameter expansion.

In this post, we explored a major benefit of bash’s parameter expansions. In this nugget, we are going to look at ten useful things you can accomplish with them.

If you’ve ever written a command that pipes a variable through sed, cut, or tr just to remove a filename extension or change a few characters, chances are Bash could have done it for you without invoking another program.

Parameter expansion is Bash’s built-in language for manipulating strings. It is fast, expressive, and once you learn a handful of expansions, you’ll find yourself using them almost every day.

Here are ten that every textsmith should know.

1. Remove a Filename Extension

file="report.pdf"
echo "${file%.*}"

Output:

report

The % operator removes the shortest suffix matching the pattern from the end of the string.

Perfect for stripping file extensions before creating new filenames.

2. Remove the Longest Suffix

archive="backup.tar.gz"
echo "${archive%%.*}"

Output:

backup

The double %% removes the longest matching suffix.

This is useful when filenames contain multiple extensions.

3. Remove Everything Before the First Slash

path="/home/ishe/Documents/report.txt"
echo "${path#*/}"

Output:

home/ishe/Documents/report.txt

The # operator removes the shortest matching prefix.

4. Extract Just the Filename

path="/home/ishe/Documents/report.txt"
echo "${path##*/}"

Output:

report.txt

The double ## removes the longest matching prefix.

This is one of the most common Bash tricks and behaves much like the basename command.

5. Replace the First Match

name="chapter_one_draft"
echo "${name/_/-}"

Output:

chapter-one_draft

Only the first occurrence is replaced.

6. Replace Every Match

name="chapter_one_draft"
echo "${name//_/-}"

Output:

chapter-one-draft

The double slash replaces all occurrences.

A quick way to normalize filenames without using sed.

7. Find the Length of a String

title="The Textsmith"
echo "${#title}"

Output:

13

This is invaluable when validating user input or generating reports.

8. Convert to Uppercase

echo "${title^^}"

Output:

THE TEXTSMITH

Useful when generating identifiers, headings, or environment variables.

9. Convert to Lowercase

title="README"
echo "${title,,}"

Output:

readme

This is especially handy when normalizing filenames or user input.

10. Provide a Default Value

echo "${EDITOR:-vim}"

If EDITOR is unset or empty:

vim

Otherwise, Bash prints the value already stored in the variable.

This is a common pattern in portable shell scripts because it allows users to override defaults through environment variables.

Quick Reference

Why This Matters

One lesson emerges from all of these examples: many shell scripts become simpler when you let Bash manipulate strings itself.

Instead of writing:

filename=$(echo "$file" | sed 's/\.txt$//')

you can simply write:

filename="${file%.txt}"

Not only is this easier to read, but it also avoids starting another process. While the performance difference is rarely noticeable for a single command, it becomes significant inside loops that process hundreds or thousands of files.

More importantly, parameter expansion encourages a different way of thinking. Rather than seeing Bash merely as a launcher for external utilities, you begin to appreciate it as a small but capable programming language in its own right. External tools such as sed, awk, and grep remain indispensable, but many everyday text transformations never need to leave the shell.

As a textsmith, that’s a useful habit to cultivate: before reaching for another command, ask yourself whether Bash already knows how to do it.

• Weekly Nugget

• Bash Variables
• Parameter Expansions
• String Manipulation
• Native Text-Processing