I am coming back with my stupid questions.
I tried the '$/' in the .args but it prints ...'$/'. Also when the field IPTC:caption-abstract doesn't exist in the file there is an error Warning: [minor] Tag 'caption-abstract' not defined - -, I read the manual (If a tag does not exist, the output value is set to '-' if the -f option is used.) but I tried to put -f but without success and my file is corrupted (4K).
I tried the -p option but it doesn't seem to work either in case of .args file. Or simply I don't get the syntax.