Why does running "echo $-" output "himBH" on the bash shell?

Question

I'm using macOS. When I open a bash shell and execute echo $-, it outputs himBH. What does that mean?

Answer 1

It shows your Builtin Set Flags. man bash then look for SHELL BUILTIN COMMANDS and then look for the set subsection. You will find the meanings of all those flags:

h: Remember the location of commands as they are looked up for execution.  This is enabled by default.
i: interactive
m: Monitor mode.  Job control is enabled
B: The shell performs brace expansion (see Brace Expansion above).  This is on by default
H: Enable !  style history substitution.  This option is on by default when the shell is interactive.