Lets assume you have a Ruby file ./program with the following line of code:
puts 'hello world'
If you run ./program in your terminal, your computer would be agnostic to what type of program you are trying to run. For your computer ./program might be a Bash script, Perl, Python, or an actual binary executable. This is when shebangs become relevant.
For the terminal, the shebang acts as a hint on how to execute a file if you don't explicitly tell your computer what to use to execute such file. For example, if you run ruby ./program, you are explicitly telling your computer to use ruby to run your program, but if you simply run ./program, your computer would know to use ruby as the interpreter if your program includes a shebang.
Tip: Note that having a shebang at the beginning of your file would not affect the program itself.
So what is a shebang?!
This character sequence is called a shebang:
#!
and it is used to tell a Unix-like operating system (such as OS X or Linux) which interpreter to use to parse or execute a file.
A #! (shebang) is followed by an interpreter directive path, which specifies the program to be run.
See below the structure of a shebang interpreter directive:
#![path to your interpreter directive] argument`
Note that your shebang interpreter directive should be the first line of your script.
There are 2 ways to use the shebang directive and set the interpreter:
1) Using the absolute path
2) Using the env directive
Here is an example using an absolute path:
#!usr/bin/python
In this example, your computer would execute the file using python.
Here is an example using the env directive:
#!/usr/bin/env ruby
In this example the env command would locate the path to the ruby interpreter by itself, whether it is located in the /usr/bin/directory, or somewhere else, you would not need to find the absolute path by yourself, instead the env program would figure it out at runtime.
Tip: Using the env command makes your program more portable.
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