Command-line Swift

Save the following as hello.swift:

#!/usr/bin/env xcrun swift
print("Hello World")

After saving, make the file executable by running chmod a+x hello.swift. The program can then be run by typing ./hello.swift, and the traditional greeting will be seen:

Hello World

Arguments can be passed from the command line and interrogated in the process using the Process class through the arguments constant. As with other Unix commands, the first element (0) is the name of the process executable; the arguments that are passed from the command line start from one (1).

The program can be terminated using the exit function; however, this is defined in the operating system libraries and so it needs to be imported in order to call this function. Modules in Swift correspond to Frameworks in Objective-C and give access to all functions that are defined as public API in the module. The syntax to import all elements from a module is import module although it's also possible to import a single function using import func module.functionName.

A Swift program to print arguments in uppercase can be implemented as a script:

#!/usr/bin/env xcrun swift
import func Darwin.exit
# import func Glibc.exit # for Linux
let args = Process.arguments[1..<Process.arguments.count]
for arg in args {
  print("\(arg.uppercaseString)")
}
exit(0)

Running this with hello world results in the following:

$ ./upper.swift hello world
HELLO
WORLD

Conventionally, the entry point to Swift programs is via a script called main.swift. If starting a Swift-based command-line application project in Xcode, a main.swift file will be created automatically. Scripts do not need to have a .swift extension; for example, the previous example could be called upper and it would still work.

While interpreted Swift scripts are useful for experimenting and writing, each time the script is started, it is interpreted using the Swift compiler and then executed. For simple scripts (such as converting arguments to upper case), this can be a large proportion of the script's execution time.

To compile a Swift script into a native executable, use the swiftc command with the -o output flag to specify a file to write to. This will then generate an executable that does exactly the same as the interpreted script, only much faster. The time command can be used to compare the running time of the interpreted and compiled versions:

$ time ./upper.swift hello world    # Interpreted
HELLO
WORLD
real  0m0.145s
$ xcrun swiftc -o upper upper.swift # Compile step
$ time ./upper hello world          # Compiled
HELLO
WORLD
real  0m0.012s

Of course, the numbers will vary, and the initial step only happens once, but startup is very lightweight in Swift. The numbers are not meant to be taken in magnitude but rather as relative to each other.

The compile step can also be used to link together many individual Swift files into one executable, which helps create a more organized project; Xcode will encourage having multiple Swift files as well.