This command has an empty output.
su user -c "ABC=abc;echo $ABC"
Any idea, how can I define a variable in the input command?
Change your quotes to single quotes. The double quotes allow the variable to be substituted in the current environment where it's not set yet. To see the difference, try your version with $USER
and compare it to this one:
su user -c 'ABC=abc; echo $ABC; echo $USER'