Say I have this script mm in dir1:
#Contents of mm
echo "Hello Main!"
./hd
My pwd is dir2, and I now execute mm:
$ ../dir1/mm
There is an executable named hd in both dir1 and dir2. I tried it and the hd in dir2 is executed. Why isn't hd in dir1 executed instead?
Because execing a file doesn't change the current working directory of your process into the directory of the file.
The executable may choose do it after it's exec'ed. E.g., I sometimes do:
cd "$(dirname "$0")"
in my script to make sure my current working directory is the directory of the executable, but it's under no obligation to do so.
dir2in your$PATH?