This two-characters sequence
#! is called a shebang. It is used in shell scripts to determine which interpreter should be used to run the script.
It’s a good idea to include a shebang
#! at the top of a Bash file. This is so that the sript runs correctly regardless of what shell the user is running.
For example, if a user is using the zsh shell, the script would default to zsh syntax instead of the intended bash syntax. Adding a shebang prevents this issue.
This also means you don’t have to include the
.sh file extension.
Reference: What is the preferred bash shebang