Cookbooks
Bash
Chapter 2. Strings
Converting a string to lower case

Bash:
Converting a string to lower case

How to:

Here’s the simple way to convert a string to lower case in Bash:

str="Make Me Lower Case"
lower_str=$(echo "$str" | tr '[:upper:]' '[:lower:]')

echo $lower_str

Output:

make me lower case

Bash 4.0 and above has a built-in way with parameter expansion:

str="Make Me Lower Case"
lower_str="${str,,}"

echo $lower_str

Output:

make me lower case

Deep Dive

Before Bash 4.0, commonly used methods to convert strings to lower case involved external utilities like tr, awk, or sed. Each of these provide different ways to manipulate strings beyond simply changing case, but may need to spawn a new process, affecting performance.

The introduction of ${parameter,,pattern} syntax in Bash 4.0 provided a native feature to transform strings which is faster and doesn’t rely on external utilities. There are alternatives within Bash itself:

  1. awk: echo $str | awk '{print tolower($0)}'
  2. sed: echo $str | sed 's/[A-Z]/\L&/g'
  3. tr: echo $str | tr '[:upper:]' '[:lower:]' - as shown above.

In terms of implementation, ${parameter,,pattern} don’t just alter ASCII characters; they’re UTF-8 aware and can handle non-English characters, making them versatile for international applications.

See Also

Concatenating stringsDeleting characters matching a pattern