Getting started

You can use Swift as scripting language as well as compiled language. For example:

To run this file, execute the command swift <file-name> in Terminal (you need to be in the same directory where the swift file that you want to run exists).

$ swift HelloWorld.swift
Hello, world!

Access to arguments

When you use a command line program, you often pass in arguments to the program. You can pass in arguments to a Swift program too. But, the program-side, how can you use the passed arguments? The answer is to use CommandLine enum.

CommandLine enum is a simple small enum that has only type properties. You can access to arguments with them.

  • argc is a type property that returns the count of the passed arguments. The first argument is always the name of the program, that is the file name here, that is argc always return at least 1.
  • arguments is a type property that returns an array of String type that contains arguments as String type.

Let’s run HelloWorld2.swift file with no argument, and with some arguments.

$ swift HelloWorld.swift
Hello, world!
No arguments are passed.
HelloWorld.swift

$ swift HelloWorld.swift foo bar baz
Hello, world!
Arguments are passed.
HelloWorld.swift
foo
bar
baz

Using Foundation

Awesome! Do you want to use Foundation framework with scripting? Of course you can use it. Just add a line import Foundation as same as a Xcode project.

$ swift HelloWorld.swift foo bar baz
Hello, world!
Arguments are passed.
HelloWorld.swift
foo
bar
baz
2017-01-16 07:31:41 +0000

You can now print out the current date!

More Foundation

Let’s do something more interesting. How about using FileManager class to explore your computer? Let’s check your home directory!

  • homeDirectoryForCurrentUser is an instance property returns the current user’s home directory URL.
  • contentsOfDirectory(atPath:) is an instance method returns an array of String type that contains file names under the passed directory path.
  • contentsOfDirectory(atPath:) is a throwing method. You need to add try keyword to call it, and you can use do-catch to call try.
  • You can get a path string from a URL object’s path instance property.

Cool, you would get the contents in Terminal when you run it.

$ swift HelloWorld.swift foo bar baz
HelloWorld.swift:26:30: error: 'homeDirectoryForCurrentUser' is only available on OS X 10.12 or newer
let homeDirURL = fileManager.homeDirectoryForCurrentUser
                             ^
HelloWorld.swift:26:30: note: add 'if #available' version check
let homeDirURL = fileManager.homeDirectoryForCurrentUser
                             ^

Whaaaaat. When I got the above error when running the Swift file, and you? You too right? After a while debugging, I got why the error is thrown. When you get an error, always read the returned error description, which I didn’t 😭

error: 'homeDirectoryForCurrentUser' is only available on OS X 10.12 or newer

This means that homeDirectoryForCurrentUser method is a machine environment dependent method. It is only available on macOS (OS X) 10.12. It means this method is NOT available on Linux environment, for example.

note: add 'if #available' version check

Availability condition

A solution is, as the above note says, add an available condition before calling the method. The condition checks the current environment. The method will be invoked since I run this Swift file on my Mac.

Let’s run the Swift file.

$ swift HelloWorld.swift foo bar baz
Hello, world!
Arguments are passed.
HelloWorld.swift
foo
bar
baz
2017-01-16 08:13:20 +0000
.atom
.bash_history
.bash_profile
# Many dot files.
.subversion
.Trash
.viminfo
Desktop
dev
Documents
Downloads
Library
Movies
Music
Pictures
Public

You are now able to print out the home directory contents here!

Using -target option

Another solution is using -target option with swift command. When you check swift --help:

$ swift --help
  -target <value>        Generate code for the given target

With this option, you can tell the Swift (compiler maybe?) which the environment is used. To try this option out, remove or comment out the if #available condition check, then run the following command.

$ swift -target x86_64-macosx10.12 HelloWorld.swift foo bar baz
# You will get the same result above.

It works, but it would not be a good idea probably, since it just skipped the availability check, then force the Swift compiler to believe the current environment is the passed one. If it’s not true, the program won’t be run, I guess.

Conclusion

  • Use CommandLine enum to access to arguments.
  • Use import Foundation to access to awesome Foundation APIs.
  • Use if #available condition check.
  • Use Swift Scripting, because it’s super fun!

Reference

The links to the awesome reference resources.


I will check out the following later.

The * argument is required and specifies that on any other platform, the body of the code block guarded by the availability condition executes on the minimum deployment target specified by your target.

SWIFT_RUNTIME_STDLIB_INTERFACE
char * _Nullable * _Nonnull
_swift_stdlib_getUnsafeArgvArgc(int * _Nonnull outArgLen);

https://github.com/apple/swift/blob/master/stdlib/public/SwiftShims/RuntimeStubs.h#L34-L36