What does the presence of shbang in a script file mean and how does it affect program execution?

The shebang (#!) is the first two characters in a script file: #!, followed by the path to the interpreter, for example, #!/bin/bash or #!/usr/bin/env python3.

It indicates to the operating system which interpreter to use to execute the script when run as an executable file. If the shebang is present and the file has execute permissions, the OS will invoke the specified interpreter and pass the script to it.

Without a shebang, the script can only be run explicitly by specifying the interpreter, e.g., bash script.sh or python3 script.py. Having a shebang makes launching easier and the script more portable.

Example of a shebang in a bash script:

#!/bin/bash
 echo "Hello, world!"

If saved, given execute permissions (chmod +x script.sh), and run as ./script.sh, the /bin/bash interpreter will be used.