There is no simple 'tolower' command on the bash, but you can convert uppercase characters to lowercase with a little shell script. The script uses the tr command internally for converting the chars.
Create a shell script with the name 'tolower' that converts all text that is given as a command-line argument to lower case:
nano /usr/local/bin/tolower
and enter the following content:
#!/bin/sh echo $1 | tr '[:upper:]' '[:lower:]'
Then make the script executable:
chmod +x /usr/local/bin/tolower
An test it by executing this command on the shell:
tolower "Thats a Test"
will convert the string to lowercase and show the result on the shell:
thats a test