How to convert string to lower case

There are several ways to convert a string from uppercase to lowercase in Bash:

1. Use tr:

   [tum@ezylinux ~]$ echo 'Hello World!' | tr '[:upper:]' '[:lower:]'
   
   hello world!

2. Use awk:

   [tum@ezylinux ~]$ echo 'Hello World!' | awk '{print tolower($0)}'
   
   hello world!

3. In Bash 4.0 or later, use the following:

   [tum@ezylinux ~]$ A='Hello World!'
   [tum@ezylinux ~]$ echo ${A,,}
   
   hello world!

4. Use Perl:

   [tum@ezylinux ~]$ echo 'Hello World!' | perl -ne 'print lc'
   
   hello world!