O traço principal indica um shell de login, de man bash :
A login shell is one whose first character of argument zero is a
-, or one started with the--loginoption.When bash is invoked as an interactive login shell, or as a non-interactive shell with the
--loginoption, it first reads and executes commands from the file/etc/profile, if that file exists. After reading that file, it looks for~/.bash_profile,~/.bash_login, and~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. The--noprofileoption may be used when the shell is started to inhibit this behavior.When a login shell exits, bash reads and executes commands from the file
~/.bash_logout, if it exists.
bash e /bin/bash são os mesmos, eles foram chamados de maneira diferente (o primeiro não foi chamado usando o caminho completo).