Below is the abstract of the script which is working fine.

if [[...
${l_file_mm} -gt 12 || ${l_file_mm} -eq 00 || \
]]
then
error_process "Invalid month format."
return 1
fi

I am doing validation for month and it errors if the value is > 12 or < 0. In addition, I want to add another condition to error if it is not a number.
Say, if it is 'ab', I want to error. What's the simpler way ?
Thanks.