To understand the practical use of Grep's option -H in different situations

Question

This question is based on this answer.

Why do you get the same output from the both commands?

Command A

$sudo grep muel *                                                             /tmp
masi:muel

Command B

$sudo grep -H muel *                                                          /tmp
masi:muel

Rob's comment suggests me that Command A should not give me masi:, but only muel.

In short, what is the practical purpose of -H?

Answer 1

Grep will list the filenames by default if more than one filename is given. The -H option makes it do that even if only one filename is given. In both your examples, more than one filename is given.

Here's a better example:

$ grep Richie notes.txt
Richie wears glasses.

$ grep -H Richie notes.txt
notes.txt:Richie wears glasses.

It's more useful when you're giving it a wildcard for an unknown number of files, and you always want the filenames printed even if the wildcard only matches one file.