Whenever you execute a program, any program, it executes in a new shell, a copy of your log in shell, with a new process identification number (PID). You type the command (and any arguments), a copy of your current shell is created, the program executes and the copy shell dies.
A shell program (a "script" is a shell program) can be executed in your current shell (not a copy) with the dot
command. This is a common way to modify your current shell, for example you might wish to alter the content of a shell environment variable and you can do that by using the dot command.
The dot command does not work with anything but a shell program.
Something that seems to have gone out of favor is writing a shell program source (with the .sh
extension) then making it executable with the make
utility (which copies the file without the .sh
and makes it executable).
prompt: cat hello.sh
print "Hello, world."
prompt: make hello
cat hello.sh >hello
chmod a+x hello
prompt: . hello
The "pound-bang" (#!) is how you specify the shell you wish to use for your shell program. In this case, I specified KornShell; I could have specified BASH or Bourne or whatever that way so that my shell program, written in the grammar and syntax of KornShell will run in that shell environment -- my log in shell might be BASH but my program will be executed by KornShell. The shell(s) are binary executable programs: that's why
does not work (you're already in a shell).
As time goes on, you'll be writing programs in multiple languages; C, C++, shell, AWK, Perl, what have you. You'll learn that you want to use some sort of source code control where you keep the source separate from the executable. That's particularly important when you develop applications that (most likely) will be a mix of sources and the make
utility will be a valuable tool for building applications from a mix of source (such as pretty much everything in Linux is). make
will make an executable from a shell source file (or C or C++ or whatever) and it will not tack an extension on the executable -- Linux doesn't need a .exe
tacked on, that's a Windows thing.
Hope this helps some.