Effects of comment (#) lines before and/or after the comment-like #!/bin/sh line

Question

Example one

#!/bin/sh
# purpose: print out current directory name and contents
pwd
ls

Example two

# purpose: print out current directory name and contents
#!/bin/sh
pwd
ls

What is the difference – if I make the first line a comment(#), with #!/bin/sh as the second line, what will happen?

What is meaning of #!/bin/sh ?

Answer 1

Normally a shell script is run by your default shell defined in the /etc/passwd file. But you can define explicitly a program which can run your script.

Unices uses a common method to determine what program needed to run a specific script (man execve(2)). If the script has the proper execute rights set and in a script the first line starts with a #! characters, it will run by the program defined afterwards.

For example if the first line is #!/usr/bin/awk -f then the rest of the file will be passed to the awk program (so it has to use awk syntax). Or if a Makefile starts with #!/usr/bin/make -f then the rest of the file will be passed to make. You can start the script as a normal program and the script can be written in awk or make (or whatever defined) syntax.

If execve does not find #! as the first two character of the file, it will consider as a normal script file and it will run as it is.

So using #! You can determine the script language and You do not need to know what shell is used by the other user using your script. In any other line #! will be interpretered your default shell, which is usually just a comment line.