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>> The '~' usually means that what follows is a User ID on a UNIX
>> system.
Actually, that's an alias for your home directory (an user's own home dir).
What follows isn't an user ID but instead, it would be whatever file and/or
subdirectories under your home directory (user's home dir). If I'm Bob and my
account name is bob, my physical home dir would be something like /home/bob and
a file named bob.txt in my home dir would have a full path of
/home/bob/bob.txt in which case I could also reach it anywhere on the system
using simply ~/bob.txt.