SPM command-line interface

In the preceding articles, we’ve been using Xcode as a tool for creating, developing, and fetching Swift packages. At the end of the day, the program doing the heavy job is SPM. Xcode interfaces with it, providing a nice GUI.

In this article, we’ll take a closer look at SPM itself, an independent program belonging to the Swift project. We’ll explore its main commands by building and releasing a trivial swift executable.

Main commands

If we type man swift in the terminal, it’ll display the manual page for Swift:

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As we can see, the commands for controlling SPM are:

• build
• test
• run
• package and its subcommands

If we type swift package in the terminal, it lists the following sub-commands (notice how many they are, as opposed to the three commands Xcode provides):

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Creating an executable

Besides libraries, a Swift Package can produce executables too. This is how the Swift project creates its command-line programs (e.g: docc, swift-driver). To create an executable package, we need to type the following in the terminal (I’m using ZSH):

mkdir simple-executable
cd simple-executable
swift package init --name "Simple Executable" --type executable

# If you wish to see other options available, type `swift package --help`

This will initialize a package with an executable target:

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Here’s what the manifest looks like:

// swift-tools-version:5.5

import PackageDescription

let package = Package(
    name: "Simple Executable",
    dependencies: [],
    targets: [
        .executableTarget(
            name: "Simple Executable",
            dependencies: []),
        .testTarget(
            name: "Simple ExecutableTests",
            dependencies: ["Simple Executable"]),
    ]
)

Building

To build our package, we use the swift build command:

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Use the swift package clean command to clear the build caches.

Running

Running the executable is simple, simply type swift run:

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Resolving & updating dependencies

Let’s first include a dependency in our manifest:

let package = Package(
    // ...
    dependencies: [
        .package(url: "https://github.com/apple/swift-argument-parser", from: "1.0.0"),
    ],
    targets: [
        .executableTarget(
            name: "Simple Executable",
            dependencies: [
                .product(name: "ArgumentParser", package: "swift-argument-parser")
            ]),
    // ...
    ]
)

With this dependency now declared, we can use the following commands:

• swift package resolve to resolve the dependencies
• swift package show-dependencies to show the currently resolved dependencies
• swift package update to update the dependencies
• swift package reset to reset the cache and build directories

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Releasing

Unfortunately, the swift package command doesn’t explain how we can build an executable for release. You can find this information in the GitHub repository. Here’s how it works:

1. Use swift build -c release to build the executable using the release configuration
2. The built executable lives inside the .build/release folder. We can invoke it directly from there: type .build/release/Simple\ Package to see what happens.

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Invoking it from ZSH

We can include the executable in one of the folders used in the $PATH variable. This allows ZSH to find and run the executable we created. Type the following in your terminal:

1. echo $PATH, this will display the folders separated by colon characters (:)
2. Rename the executable to an invokable name:
   1. mv .build/release/Simple\ Executable ./say-hello
   2. If /usr/local/bin doesn’t exist, create it with this command: sudo mkdir /usr/local/bin
   3. sudo mv ./say-hello /usr/local/bin
3. Now we can simply invoke our command by typing say-hello

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Conclusion

SPM is a powerful program capable of managing dependencies and building libraries and executables. It is a key component of the Swift project, supporting the usage of the language in different operating systems.

In this article, we saw how we can create a trivial package that builds an executable. We also explored the main SPM commands, and how we can invoke our executables directly from the terminal’s $PATH variable.