When you do not have any #! (called shebang) line, sh is used. To check thatwhich shell is used, you can run the following script.
ps -p $$ echo -n "The real shell is: " realpath /proc/$$/exe On my computer I get
PID TTY TIME CMD 13718 pts/16 00:00:00 sh The real shell is: /usr/bin/bash even if my default shell is zsh. It uses bash since on my machine, the sh command is implemented by bash and zsh picks sh if no shebang is specified.