Although most shells have certain built-in capabilities, almost everything they do is done by other programs. That is to say, a shell is a program that runs other programs. We’ve used a shell many times to do this, such as when we first executed our Hello, World program back in Chapter 1, The Basics: Input and Output, like this: ./target/scala-2.11/hello-out. We also learned that hello-out is an executable file that contains metadata and machine instructions that the operating system can load and execute.
Binaries and Scripts | |
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In a UNIX OS, there are two different kinds of executable files: binary files that can be executed directly, and scripts that can be interpreted by another program, like a shell. Plain text files that begin with a special “shebang” line like #!/bin/bash, which means: “interpret the rest of this file with the program at /bin/bash.” Scala Native’s output, however, isn’t interpreted; it’s executed directly, just like the output of a C compiler. |
In UNIX operating systems, loading and running a program isn’t magic, and shells are themselves mostly ordinary programs—everything they do, they do using a few standard system calls. Just as with networking and file I/O, we can break down the functionality of the shell into the underlying system calls and put them back together with idiomatic Scala.