When reading some rpmbuild spec files, I come across some of the conditional macros which puzzle me.
example1
%if 0%{?rhel} > 7
blah blah
%endif
# I understand the above block tries to check if the
# red hat enterprise linux version is above 7, then blah blah
# But what is the usage of the '0'?
example 2
%if 0%{!?pkg_name:1}
%define pkg_name foo
%endif
# I understand the above block tries to check if the pkg_name
# is not defined, then define it with the value foo.
# But what is the usage of the '0'?
My guess is that '0' indicates the next expression to be either 'nil' or a number so that rpm would consider them as a number (such as 06, 0, or 01 in above examples) instead of a string or empty string. But I am not sure about it.
Unfortunatly, most of the online tutorial materials did not cover this topic.
You got it right; it's a safeguard. The %{?rhel} says "replace with the rhel macro if it exists and it is OK if it does not (the ?)."
So, if rpmbuild replaced it with nothing, the resulting if > 7 would barf.