Shebang (#!)

• The #! shebang is used to tell the kernel which interpreter should be used to run the commands present in the file.
• Example: if we have a file with the following content inside

#!/bin/bash  
echo "Hello World!"

• The text after the shebang(#!) which is (/bin/bash) tells the kernel this file should be run with the command /bin/bash in front.

./hello.sh  
# the kernel actually will turn the above command as  
/bin/bash ./hello.sh

• Example 2:

#!/bin/cat  
All the contents of this file will be  
printed in the screen when it's executed 

./hello.sh  
# the kernel will turn it into /bin/cat ./hello.sh  
# the result will be the following output  
#!/bin/cat  
All the contents of this file will be  
printed in the screen when it's executed

For our script header we can first find path of the lang with which program and add it on top of the script !#+result-of(which program) ex: !#+result-of(which python)

When to Use Env?

• When the interpreter is within the Linux system such as (cat, ls, bash…) we can use the #!bin/Linux-command format, however, for user-installed interpreters such as (node, python…) we need to use their environment variables

We can still use #!/path-to-node but that won’t be portable to other machines as the path is dependent on where the user installed node.js.

#!/usr/bin/env node  
console.log('Hello World from NodeJS');  
  
# now we can run the application as  
./hello.js  
# the kernel will turn it into path-to-node ./hello.js