Thank you for taking time to reply to my question.
I am talking about interface id. When used in terminal command find works well no matter if the interface id starts with capital or small letter. When used in a script it requires only capital letters. This is my problem.
Do you have any ppp interfaces? Can you add manually via winbox an interface to a test list. Then disconnect the interface and show a /interface list member print.
I feel so lame. I spent hours on debugging a script to add/remove interfaces from a list your last post made me flash for a moment. It could have been so simple. Thank you.
Another way to do it was given to my by the support:
On-up:
:local interfaceName [/interface get $interface name]
/interface list member add interface=$interfaceName list=OPERATORS_IFs
On-down:
delay 1
:local interfaceName [:toid $interface]
/interface list member remove [find interface=$interfaceName]