pattern matching without echo

Question

Why are the outputs different?

touch a aa bb ccc

echo *

a aa bb ccc // I understood, * means matches zero or more

*

aa bb ccc // I'm confused.

echo ?

a //OK

?

a aa bb ccc // confused again.

*?

aa bb ccc // very confused

* *

a aa aa bb bb ccc ccc // very very confused

I wonder what's the difference between using echo and not using it.

Answer 1

Each line you write must have a command, usually the first word.

To get something printed, a common command is echo.

If the pwd (present working directory) has files a, aa, bb, and ccc.
Then, this command will print all files in the directory:

 $ echo *
 a aa bb ccc

And this command will print all files in the pwd that have one character:

$ echo ?
a

---

However, if you do not provide a command, the first "word" (most of the time: up to the first white space) will be taken to be the command.

If you only supply the asterisk *, the first word that results from the expansion of the asterisk will be used as the command to execute.

If the command a does not exist, an error will be printed:

$ *
bash: a: command not found

If there is a command called a, it will be called with the three arguments aa bb ccc.

As you provide no more detail, we can only make a wild guess of which command is being executed with the name a (it is not a default command). Also we don't know which shell are you using.

But all your examples show that the first a is missing:

$ *
aa bb ccc              ### missing first `a`.
$ *?
aa bb ccc              ### missing first `a`.
$ * *
a aa aa bb bb ccc ccc  ### missing first `a`.

Except for:

$ ?
a aa bb ccc

Of which is difficult to make any conclusion as we don't know what command is a.