The document says
Macro: AS_IF (test1, [run-if-true1], ..., [run-if-false])Run shell code test1. If test1 exits with a zero status then run shell code run-if-true1, else examine further tests. If no test exits with a zero status, run shell code run-if-false, with simplifications if either run-if-true1 or run-if-false is empty.
The configure.ac file contains
AS_IF([test "$have_hdf5" != "no"], [AC_DEFINE([HAVE_HDF5], [], [we have HDF5 library + headers])])
But when I type in the run shell code test1, i.e. test "$have_hdf5" != "no", nothing appeared. No matter what I entered (such as test $have_hdf5), I got nothing. But it actually works when I ./configure. So how does the AS_IF and the test1 shell code work? Is it testing some environment variables?
The test utility doesn't have visible output unless you supply an erroneous expression.
test will exit with a 0 status if the expression was true and a nonzero exit status indicates the expression was false (or invalid).
AS_IF tests the exit status of the expression you supply as its first parameter.
If it was 0 (true), the associated block of code is executed. If not, it moves on to the next test expression and performs with the same behavior.
This happens until the macro runs out of tests. Once all tests are found false, it executes the last parameter's contents if the last parameter was supplied.
Edit
Naturally if you don't have "have_hdf5" defined in your current shell, it won't be true when you execute your test expression alone, but if it is true in the configure script, it will do what it is meant to. In any case, output will not happen unless you use an output statement.