There is no simple tolower command on the bash, but with a little shell script you can convert uppercase characters to lowercase. The script uses the tr command internally for converting the chars.
Create a shell script with the name tolower:
vi /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