Using pipes in Swift scripts

I have a few Swift scripts to automate tedious tasks for maintaining my blog. I updated one today to use pipes. It took me a minute to figure out, because it did not feel very intuitive. I’m not sure if I feel that way because the interface is actually that clunky, or if I’m just inexperienced with Swift scripting. In any case, here’s how it works.

First, some background: when I return to a post to update it after publishing, I like to do the proper bookkeeping to show that the post has been updated. I have a post-updated: field in my Jekyll Front Matter for this, and my template clearly displays this date for an updated post to notify the reader. Jekyll expects dates in ISO 8601 format, which are very easy to generate in Swift. My original script looked like this:

#!/usr/bin/swift

import Foundation

let fullDateTime = ISO8601DateFormatter.string(
    from: Date(),
    timeZone: .current,
    formatOptions: [.withFullDate, .withFullTime]
)

print(fullDateTime)

This uses a date formatter to simply print the current datetime via stdout.

$ ./scripts/current-date-time.swift
$ 2021-03-18T11:10:51-07:00

I would run this in the terminal, then manually copy the date. That is rather tedious, so I decided I wanted use pbcopy to automatically copy the date to my clipboard.

The trick is to create a separate Process for each command (echo and pbcopy) then appropriately connect their standardOutput and standardInput. The final script now looks like this:

#!/usr/bin/swift

import Foundation

let fullDateTime = ISO8601DateFormatter.string(
    from: Date(),
    timeZone: .current,
    formatOptions: [.withFullDate, .withFullTime]
)

print(fullDateTime)

let pipe = Pipe()

let echo = Process()
echo.executableURL = URL(fileURLWithPath: "/usr/bin/env")
echo.arguments = ["echo", fullDateTime]
echo.standardOutput = pipe

let pbcopy = Process()
pbcopy.executableURL = URL(fileURLWithPath: "/usr/bin/env")
pbcopy.arguments = ["pbcopy"]
pbcopy.standardInput = pipe

do {
    try echo.run()
    try pbcopy.run()
    pbcopy.waitUntilExit()
} catch {
    print("Error: \(error)")
}

I continue to print the generated datetime because it is nice to have some visual feedback. (If you do not closely follow Swift scripting, like me, note that there have been some API deprecations for Process.)

This is a simple example. But if you need to chain many commands, connecting all these pipes correctly feels error-prone and tedious. It would be worth writing some abstraction on top of this to simplify and ensure correctness, but I’ll leave that as an exercise for the reader.