The difference between a/ and a/* and a/** in .gitignore?

Question

Here is my file folder.

a
├─ b
│  ├─ b1.txt
│  └─ b2.txt
├─ a1.txt
└─ a2.txt
.gitignore

Firstly:

I found if I wanna ignore the folder a, a/ and a/* and a/** all can complete.

Secondly:

I wanna ignore all the things in folder a except the folder b, the only way to do this is:

a/*
!a/b/

I don't know why a/** can't do this?

# this way can't except folder b
a/**
!a/b/

Or you can tell me how can I use a/** ?

Answer 1

** matches any amount of directories, * only one.
So if you e. g. ignore /a/**/d/, then /a/b/d/ and /a/b/c/d/ both are ignored.
If you e. g. ignore /a/*/d/, then /a/b/d/ is ignored while /a/b/c/d/ is not ignored.

If a folder is ignored, then Git does not even look at its contents to check for inclusions. That is why you cannot ignore /a/ and then include /a/b/, as Git does not even look into /a/. Because of this you must instead ignore all contents in /a/ with /a/* and then include /a/b/ to ignore everything in /a/, except all contents of /a/b/.