Tag: Mac

New tool updated: utiluti v1.1

A few weeks back I introduced a new tool: utiluti, which sets default apps for url schemes and file types.

Not standing still, I have updated the tool with new verbs to list url schemes and file types that a given app can handle, as well as inspect and set default apps set for specific files.

You can see the Readme and download the latest release on the GitHub repo.

Installomator v10.8

Further chipping away at the backlog of new and updated with merged 200 PRs merged or closed.

The new PR templates and automations are proving to be a big help! Many thanks Bart for working on these and all the maintainers for staying on top of most things.

This release brings Installomator to 1025 (!) labels!

Many thanks to all the contributors, this tool wouldn’t exist without you!

You can find the detailed release notes and the pkg on the repo!

New tool: utiluti sets default apps

A while back I wrote a post on the Jamf Tech Thoughts blog about managing the default browser on macOS. In that post I introduced a script using JXA to set the default application for a given url scheme. (like http, mailto, ssh etc.) The beauty of using JXA/osascript is that it doesn’t require the installation of an extra tool.

However, there was a follow-up comment asking about default apps for file types, i.e. which app will open PDF files or files with the .sh file extension. Unfortunately, Apple has not bridged those AppKit APIs to AppleScript/JXA, which means it is not possible to use them in a script without dependencies.

Back then, I started working on a command line tool which uses those APIs. I didn’t really plan to publish it, since there were established tools, like duti, cdef and SwiftDefaultApp that provided the functionality. It was a chance to experiment and learn more about Swift Argument Parser. Then life and work happened and other projects required more attention.

A recent discussion on the Mac Admins Slack reminded me of this. Also, none of the above mentioned tools have been updated in the past years. As far as I can tell, none of them have been compiled for the Apple silicon platform. They don’t provide installation pkgs either, which complicates their distribution in a managed deployment.

So, I dusted off the project, cleaned it up a bit, and added a ReadMe file and a signed and notarized installation pkg. The tool is called utiluti (I am a bit proud of that name).

You can use utiluti to set the default app for an url scheme:

$ utiluti url set mailto com.microsoft.Outlook
set com.microsoft.Outlook for mailto

or to set the default app to open a uniform type identifier (UTI):

$ utiluti type set public.plain-text com.barebones.bbedit
set com.barebones.bbedit for public.plain-text

There are bunch of other options, you can read the details in the ReadMe or in the command line with utiluti help.

The functionality is quite basic, but please provide feedback if there are features you’d like to have added.

Installomator v10.6

It is that time again, we have released a new version of Installomator!

Many thanks to all the maintainers and contributors!

It has been quite a while since we published a release and there was quite a backlog. In some ways, Installomator is a victim of its own success. It is quite easy to create a new label and many contributors are doing that. It is also easy to provide an update to a broken label and while many contributors are doing that, there are fewer of them.

This means that we are accumulating a lot of stale and broken labels. Since there is now way for us to gather information on which labels are actually being used and how much, we end up with lot of turnover.

The team of maintainers has some ideas to tackle some of these problems with automation, but human curation will have to remain a step all along the way.

You can get the latest version of Installomator on the repo or download the installer pkg from here.

Swift Command Line Tools and Argument Parser — Part 1

When building tools in Swift, I usually start with a command line tool. This allows me to ignore the complexity of creating a user interface while figuring out the underlying APIs and data models.

Technically, command line tools have a user interface, as well. They print output to pipes, standard out or standard error for data, progress, status or other information. They should provide information using the exit status. They take input from standard in, pipes, environment variables, or from command line arguments.

While there are many subtleties to consider with all of these, these “interfaces” are still less complex to handle than a full user interface built with AppKit, UIKit or SwiftUI.

Swift provides the functionality to deal with files, outputs, pipes with APIs. This post will not cover those. This post will focus on getting arguments from the command line.

Note: this tutorial was written using Swift 5.10, Xcode 15.4, swift-argument-parser 1.5 on macOS 14.6.1. The details and user interfaces may change with different versions, but the fundamentals should remain.

CommandLine.arguments

The built-in way to get the arguments for your process is with CommandLine.arguments which returns an array of String objects. CommandLine.arguments is quite basic, but can be suitable for simple use cases.

Create a project directory named CLArgs and use swift package init to create a swift package to build an executable:

> mkdir CLArgs
> cd CLArgs
> swift package init --type executable

This will create a skeleton project, you will find a basic “Hello, world” code in Sources/main.swift. Replace the print statement there with:

import Foundation
let arguments = CommandLine.arguments.dropFirst()
guard let name = arguments.first
else {
  print("command requires an argument!")
  exit(1)
}
print("Hello, \(name)")

Note: You can use your favorite text editor or IDE to edit SPM projects. You can also use Xcode. When you run xed . in the Swift package directory, Xcode will open the Swift package in a project view. You can edit, build and run the package in Xcode or use Xcode for editing and build and run it from the command line.

In terminal, build and run the project with

> swift run CLArgs Armin

This tells the swift to build the CLArgs target defined in Package.swift and run it with the argument Armin. You should see this output:

> swift run CLArgs Armin
Building for debugging...
[1/1] Write swift-version-39B54973F684ADAB.txt
Build of product 'CLArgs' complete! (0.11s)
Hello, Armin

Let’s look at the code in detail.

let arguments = CommandLine.arguments.dropFirst()

CommandLine.arguments returns an array of strings. By convention, the first argument (arguments[0]) contains the path to the executable. In most situations, you will not be interested in this first argument. One straightforward way to deal with this is to ‘drop’ the first element of the array right away.

guard let name = arguments.first
else {
  print("command requires an argument!")
  exit(1)
}

We get the first element of the arguments array. Additional arguments are simply ignored. When no arguments are provided, this will return nil and guard statement will trigger, where we print an error message and exit the code with a non-zero value, signaling a failure.

print("Hello, \(name)")

The actual point of this sample code: print a greeting with the name.

In this simplest of examples, we spend a majority of the code on preparing the arguments and verifying that they meet our requirements.

CommandLine.arguments will serve you well for simple needs and quick command line tools. However, you will quickly notice that a robust command line tool needs to verify the existence of certain arguments, whether the value matches certain criteria, and print error messages and usage directions when the arguments don’t match the expectations. Many command line tools also have flags and options with short and long forms that need to be processed.

This turns into a lot of code very quickly.

Swift Argument Parser

Enter Swift Argument Parser. A package that provides “straightforward, type-safe argument parsing for Swift.”

You could modify the Package.swift file in our CLArgs project to import Swift Argument Parser but there is an even easier way to start. Back out of the CLArgs project directory and create a new one:

> cd ..
> mkdir SwiftArg
> cd SwiftArg
> swift package init --type tool

When you inspect the Package.swift file in this new project, you will see that it is already linked to the Swift Argument Parser package. Sources/SwiftArgs.swift contains another “Hello, world” template code, but using Swift Argument Parser.

import ArgumentParser
@main
struct SwiftArgs: ParsableCommand {
  mutating func run() throws {
    print("Hello, world!")
  }
}

The struct here implements the ParsableCommand protocol which allows us to use all the nice functionality from the ArgumentParser library. It is also marked with the @main tag, which tells the compiler to run the main() function in this when the binary is launched. The main() function is implemented by ParsableCommand which, well, parses the arguments and then launches the run() function.

Swift Package Manager vs Xcode projects

You can open and edit Swift Package Manager projects in Xcode with the xed . command. Recent Xcode versions know how to work with SPM projects without needing to create an Xcode project file. Xcode will use the configurations in the Package.swift file. This is useful when you like to work in Xcode, but want the project to remain transferable to other editors or IDEs.

There is a weird quirk. When you build and/or run the project from within Xcode it will use the default Xcode build directory (default is ~/Library/Developer/Xcode/DerivedData/). This is different from the location that the swift build or swift run commands in Terminal use (.build in the package directory). This can lead to longer build times and confusion.

You can also use swift-argument-parser and other packages within Xcode projects. This can be necessary if you are building the command line as a target within a larger Xcode project. Maybe you want to use some of Xcode’s advanced features for managing projects, like build phases and Archives. Or maybe you just prefer working in Xcode.

To create a command line with ArgumentParser in Xcode, create a new Project and select the ‘Command Line Tool’ template for macOS. Once the new project is created, select ‘Add Package Dependencies…’ from the File menu. Locate ‘swift-argument-parser’ in the ‘Apple Swift Packages’ collection or just enter the URL in the search field and click ‘Add Package…’ (twice)

Then, you have to delete the main.swift file from the template and create a new SwiftArgs.swift with this code:

import Foundation
import ArgumentParser
@main
struct SwiftArgs: ParsableCommand {
  mutating func run() throws {
    print("Hello, world!")
  }
}

This is the same as the template code created with the swift package init --type tool from above.

When testing and running the command line tool in Xcode will will want to pass arguments into the binary. You can do so by editing the scheme. Choose Product > Scheme > Edit Scheme… from the menu or click on the target icon (the one with the command line icon in the center of the menu bar) and select Edit Scheme… Make sure you are on the ‘Run’ section in that dialog and select the ‘Arguments’ tab. Here you can add, remove, enable or disable the arguments that Xcode passes into your tool when you run it from Xcode.

Continually changing the arguments in the scheme editor can be tedious. You can also use ‘Show Build Folder in Finder’ from the ‘Product’ menu, open the Products/Debug folder in Terminal by dragging that folder on to the Terminal icon in the Dock and run the built command from there with ./SwiftArgs

Whichever way you prefer to create and work with your project, the rest of this tutorial will work the same way.

Using Swift Argument Parser

Right now, we are just running print("Hello, world!"), which is quite underwhelming. Let’s step this up just a little bit:

@main
struct SwiftArgs: ParsableCommand
  @Argument var name: String
  func run() {
    print("Hello, \(name)")
  }
}

First we create a property called name of type Stringwith the @Argument property wrapper. This tells the ArgumentParser library, that we want this variable filled with an argument from the command line. When the run() function is called, we can “just use” the name property, like any other Swift property.

When you run this, something interesting happens: we get an error!

> swift run SwiftArgs
Error: Missing expected argument '<name>'
USAGE: swift-args <name>
ARGUMENTS:
  <name>
OPTIONS:
  -h, --help              Show help information.

When you check the exit code of the command with echo $? you see it return an error code of 64. This means it was missing arguments or got malformed arguments. As should be good practice for command line tools, our tool did print a help message, describing what it was expecting. Here we see that our SwiftArgs command expects a single argument giving a name.

Run the command again, but with an argument:

> swift run SwiftArgs Armin
Hello, Armin

Now everything works as expected. When our tool launches, ArgumentParser grabs the argument, places it in our name property and executes the run() function in our struct that implements ParsableCommand. Since ArgumentParser errors out with the help message, when an argument is missing or too many arguments are present, we can be certain that the name variable is populated when our code runs.

Command Configuration

There is a small detail that is bugging me, though. The help message generated by ArgumentParser deduced that the name of binary should be swift-args instead of SwiftArgs, but the binary name is SwiftArgs, which is the name of the directory we initialized the project in. This is because of different naming standards for Swift types and command line tools. You can change the name of the executable created in the Package.swift file in line 15 under .executableTarget.

We could change the name to something completely different here, say apdemo for ‘Argument Parser Demo. When you apply that change inPackage.swift` it changes the name of the binary, but the auto-generated help message does not pick that up. It still use the auto-generated name.

> swift run apdemo --help
USAGE: swift-args <name>
ARGUMENTS:
  <name>
OPTIONS:
  -h, --help              Show help information.

(Isn’t it neat that ArgumentParser automatically implements --help and -h flags?)

We could change the name of our struct, which will work in simple situations. But you will have a situation where the struct name will not match what you want for the executable name. There is a way to tell ArgumentParser exactly what we want, though.

Insert this code below the struct SwiftArgs line and above the @Argument:

  static let configuration = CommandConfiguration(
    commandName: "apdemo"
  )

When you now look at the generated help again, the command name matches:

OVERVIEW: apdemo - swift-argument-parser tutorial tool
USAGE: apdemo <name>
ARGUMENTS:
  <name>
OPTIONS:
  -h, --help              Show help information.

There is more information we can provide in the command configuration. Extend the CommandConfiguration initializer like this:

  static let configuration = CommandConfiguration(
    commandName: "apdemo",
    abstract: "apdemo - swift-argument-parser tutorial tool",
    version: "0.1"
  )

and run the command to get the help message again.

OVERVIEW: apdemo - swift-argument-parser tutorial tool
USAGE: apdemo <name>
ARGUMENTS:
  <name>
OPTIONS:
  --version               Show the version.
  -h, --help              Show help information.

The abstract appears as an ‘overview’ above the help message and we now see a new option --version. When you run the tool with that option, you will not be surprised to see the 0.1 provided in the configuration, but it is useful nonetheless.

There are more fields you can provide in the CommandConfiguration: discussion allows you to provide a long form description of the command. usage allows you to override the auto-generated usage text. There are some more that we will explore later.

More Arguments

You can add more @Arguments and they will be filled in order from the arguments provided at the command line. Add another property with the @Argument wrapper:

  @Argument var name: String
  @Argument var age: Int
  func run() {
    print("Hello, \(name)!")
    print("You are \(age) years old.")
  }

When you run the tool without any arguments, you can inspect the updated help message. The usage and arguments area now shows both expected arguments. When you run the tool with a single argument, you get an abbreviated help, showing only the missing argument. When you provide a name and a number as command line arguments, everything works as expected.

But what if you provide two strings?

> swift run apdemo Armin lalala            
Error: The value 'lalala' is invalid for '<age>'
Help:  <age>  
Usage: apdemo <name> <age>
  See 'apdemo --help' for more information.

We declared the age property as an Int, so ArgumentParser expects an integer number for the second argument. When the second argument cannot be parsed into an integer, it shows the error.

Change the type of the age property to a double and run it again with a decimal for the age.

Some Help, please?

name and age might be enough to tell a user of your command line tool what to enter. But I think we should provide a bit more explanation. You can attach a help message to the argument:

  @Argument(help: "a name")
  var name: String
  @Argument(help: "age (integer number)")
  var age: Int

I have broken the property declarations into two lines each for clarity and changed the age back to an Int for simplicity. The help messages will appear next to the argument names in the long and short help messages.

Off to a good start

We have just started to scratch the surface of what swift-argument-parser can do for us. In the next part, we will cover options and flags.

Building a LaunchD Installer pkg for desktoppr (and other tools)

In my last post introducing desktoppr 0.5, I said that a Mac admin could build a customized pkg with a customized LaunchAgent property list, that would run desktoppr manage at login and every few hours in the background to re-set the wallpaper when the configuration profile is updated.

I admit I glossed over the process of building a package that installs and launches a LaunchAgent (or LaunchDaemon). Since that is not a trivial configuration, I will describe the details in this post.

LaunchAgent? LaunchDaemon? LaunchD?

On macOS, launchd is the system-wide process that controls and launches every other process running on the system. When you look at the hierarchical process list in Activity Monitor, you will always see launchd running with the process ID 1 as the single child process of kernel_task and all other processes are a child of launchd.

More practically, users and admins can use launchd to control processes that need to be available at all times or be launched in certain intervals or at certain events.

launchd has two types of processes: LaunchDaemons and LaunchAgents. Since the terms daemons (or demons) and agents do not have clear definitions outside of launchd, a bit of explanation is required.

LaunchDaemons

LaunchDaemons are processes that run with system privileges (“as root”). Processes that belong to your management system and security suites may need to run periodically or on certain events in the background and require root level access to the system to do their jobs. Other processes or services need to listen for incoming network traffic on privileged ports.

Note that background process are often called ‘agents’ regardless of whether they run with root privileges or as a user. The launchd terminology is more precise here.

Apple has done a lot of work over the past few year to provide Endpoint Security or Network Extension frameworks that mitigate the need for third party root daemons, but there are still relevant use cases, especially when you are a managing Macs.

A LaunchDaemon is configured by providing a property list file in /Library/LaunchDaemons. This configuration plist will be automatically loaded at reboot, or can be loaded using the launchctl command.

LaunchAgents

LaunchAgents are processes that run with user privileges. Generally, if something needs to read or write to anything in the user home folder, it should be a LaunchAgent. Over the recent years, Apple has put restrictions on what data process can access in user space, which has made LaunchAgents more of a pain to manage, but they still have their use cases.

A LaunchAgent is configured by providing a property list file in /Library/LaunchAgents on the system level or in ~/Library/LaunchAgents in a user’s home folder. Configuration property lists that are in a user home folder, will only run for that user, while LaunchAgents that are configured in the system wide /Library/LaunchAgents will be loaded for all users on the system.

A LaunchAgent configuration plist will be automatically loaded at login, or can be loaded using the launchctl command.

Since desktoppr affects a user setting (the desktop/wallpaper), it has to run as the user, so we need to build and install a LaunchAgent. Creating and installing a LaunchDaemon is pretty much the same process, though.

Ingredients

To build an installation package to install and run a LaunchAgent or LaunchDaemon, you need three ingredients:

  • the binary or script which will be launched
  • a launchd configuration property list
  • installation scripts to unload and load the configuration

For desktoppr, we already have the binary (downloadable as a zip from the project release page) and a sample configuration property list file.

The sample launchd configuration looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>com.scriptingosx.desktopprmanage</string>
    <key>ProgramArguments</key>
    <array>
        <string>/usr/local/bin/desktoppr</string>
        <string>manage</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
    <key>StartInterval</key>
    <integer>10800</integer>
</dict>
</plist>

The label is required and is used to uniquely identify the LaunchAgent or Daemon with launchd and the launchctl command. Reverse domain notification is common. The filename of the configuration property list should match the label.

The ProgramArguments key provides an array with the command and arguments that will be launched. The first item in the array should provide the full path to the binary or script. Then you provide arguments, one string item per argument. Since the individual arguments are separated by the string tags, you do not need to escape spaces or other special characters in the arguments.

It is worth noting that launchd will launch the binary or script directly, so syntax and substitutions that are done in shells will not work. This includes variable and command substitution like $HOME, $USER, or $(date) as well as pipes and output redirection. You get a single command with a list of static arguments. (You can use the StandardOutPath and StandardErrorPath keys in the launchd config plist to redirect output.)

For the desktoppr LaunchAgent, that suits us just fine, as we are only passing the manage argument which tells desktoppr to get the details from a configuration profile (sample here).

The next key, RunAtLoad tells launchd, to run the binary with the arguments immediately when the configuration file is loaded. Since configuration files in /Library/LaunchAgents and ~/Library/LaunchAgents are automatically loaded when a user logs in, this generally means desktoppr manage will be run when a user logs in, which suits us well.

The last key StartInterval tells launchd to re-launch the process after a certain time (given in seconds, the 10800 seconds in our sample file translate to three hours). Should the system be sleeping at the time, it will not run the LaunchAgent at that time, or when the system wakes up, but wait until the next interval period comes around.

There are other keys that control when the process gets launched, such as StartCalendarInterval or WatchPath. You can read details in the man page for launchd.plist or on the excellent launchd.info page. There are also apps, like Peter Borg’s Lingon that provide a user interface for creating these plist files.

Move or copy the plist file into the correct folder. The plist files in /Library/LaunchDaemons and /Library/LaunchAgents must be owned by root. They must not be writable by group or other. The file mode 644 (rw-r--r--) is recommended. Similarly, the binary or script needs to be owned by root and cannot be writable by group or other. The file mode 755 (rwxr-xr-x) is recommended. Wrong file privileges will result in a Load/Bootstrap failed: 5: Input/output error.

Make sure that the binary or script does not have a quarantine flag attached. You can check with ls -al@ or xattr -p com.apple.quarantine /path/to/file and remove a quarantine flag with xattr -d com.apple.quarantine /path/to/file. When the binary or script has the quarantine flag, the configuration file will load fine but the actual execution will fail quietly.

Loading the LaunchAgent or LaunchDaemon

The easiest way to load a LaunchDaemon is to restart the Mac. The easiest way to load a LaunchAgent is to log out and login. This is usually not practical.

The launchctl command manages LaunchDaemons and LaunchAgents and we can use this to load LaunchAgents and LaunchDaemons on demand.

To load our desktoppr agent, use

> launchctl load /Library/LaunchAgents/com.scriptingosx.desktopprmanage.plist

People are already readying their pitchforks here. “Hold on,” they say. “Apple has marked load and unload as ‘legacy.’ You shouldn’t use them!”

While it is true that load and unload (and a bunch of other commands) are labeled as ‘legacy’ in the launchctl man page, this does not mean they are deprecated and should be avoided. The difference between the legacy and the ‘new’ commands is that the legacy commands pick up whether a process should be run in user or root context (the ‘target domain’) from the context that launchctl runs in, whereas the new bootstrap and bootout commands need the target domain to be stated explicitly.

This makes the modern commands more precise, but often more wordy to use. You will see that the legacy commands are simpler to use in the interactive terminal, while the ‘modern’ commands are more useful in scripts. The equivalent ‘modern’ command is:

> launchctl bootstrap gui/501 /Library/LaunchAgents/com.scriptingosx.desktopprmanage.plist

where 501 is the user ID for your user account. This is 501 by default for the first user on on a macOS system, but you should verify what your account’s user ID actually is with id -u.

See what I mean with “more wordy to use”? That doesn’t mean they are bad or should be avoided, but that we can be picky about when to use which format. You could argue that using the modern commands throughout would be more consistent, but you still find the ‘legacy’ command is lots of documentation, so I feel a Mac admin needs to be aware of both.

Note: the ‘modern’ commands were introduced in OS X Yosemite 10.10 in 2014.

To load a LaunchDaemon, you need to run launchctl with root privileges. In the interactive shell, that means with sudo:

> sudo launchctl load /Library/LaunchDaemons/com.example.daemon.plist

or

> sudo launchctl bootstrap system/ /Library/LaunchDaemons/com.example.daemon.plist

To stop a LaunchAgent from being launched going forward, unload the configuration:

> launchctl unload /Library/LaunchAgents/com.scriptingosx.desktopprmanage.plist

or

> launchctl bootout gui/501/com.scriptingosx.desktopprmanage

The bootout command uses the label instead of the file path.

Putting it all together in an installer

Note: I will be showing how to build and installer package for our LaunchAgent using command line tools. These instructions should have all the information you need, even if you prefer using apps such as Whitebox Packages or Composer.

If you are not comfortable with using Terminal on macOS yet, please consider my book “macOS Terminal and Shell.” If you want to learn more about building installation packages, consider my book “Packaging for Apple Administrators.”

Download the zip file for desktoppr from the releases page. Download the sample launchd property list file and modify the launch criteria (StartInterval or StartCalendarInterval) to your requirements.

Open Terminal, change directory to a location where you want to create the project directory and create a project directory:

> mkdir DesktopprManagePkg
> cd DesktopprManagePkg

Then create a payload and a scripts directory in this project folder:

> mkdir payload
> mkdir scripts

Unzip the desktoppr binary from the downloaded zip archive into the right folder hierarchy in the payload folder:

> mkdir -p payload/usr/local/bin/
> ditto -x -k ~/Downloads/desktoppr-0.5-218.zip payload/usr/local/bin/

Remove the quarantine flag from the expanded desktoppr binary and test it by getting the version and the current wallpaper/desktop picture:

> xattr -d com.apple.quarantine payload/usr/local/bin/desktoppr
> payload/usr/local/bin/desktoppr version
0.5
> payload/usr/local/bin/desktoppr
/System/Library/CoreServices/DefaultDesktop.heic

Create the /Library/LaunchAgents directory in the payload and copy or move the LaunchAgent configuration plist there:

> mkdir -p payload/Library/LaunchAgents/
> cp ~/Downloads/com.scriptingosx.desktopprmanage.plist payload/Library/LaunchAgents/

When the installation package runs, the files in the payload will be moved to the respective locations on the target drive. The pkgbuild tool will set the owner of the files to root when building the package, but we should verify that the file mode is correct:

> stat -f %Sp payload/usr/local/bin/desktoppr
-rwxr-xr-x
> stat -f %Sp payload/Library/LaunchAgents/com.scriptingosx.desktopprmanage.plist
-rw-r--r--

If the privileges don’t match the desired mode, set them with chmod

> chmod 755 payload/usr/local/bin/desktoppr
> chmod 644 payload/Library/LaunchAgents/com.scriptingosx.desktopprmanage.plist

and run the stat commands from above again to verify.

If you built the installer package now, it will install the files in the right locations and the LaunchAgent will load and run desktoppr manage on next login. This will set the desktop/wallpaper according to the information in the configuration profile. You should install and update the configuration profile using your management system.

Loading the LaunchAgent or LaunchDaemon

However, as I said earlier, requiring or waiting for a logout or reboot is not ideal, we would like to have the LaunchAgent load immediately when the package is installed. We can achieve this by adding a postinstall script to the installation package. This script will be executed after the payload files have been installed.

With your favored text editor, create a file named postinstall (no file extension) with this content in the scripts sub directory.

#!/bin/sh

export PATH=/usr/bin:/bin:/usr/sbin:/sbin

label="com.scriptingosx.desktopprmanage"
launchdPlist="/Library/LaunchAgents/${label}.plist"

# do not load when target volume ($3) is not current startup volume
if [ "$3" != "/" ]; then
    echo "not installing on startup volume, exiting"
    exit 0
fi

# for LaunchAgent, check if user is logged in
# see https://scriptingosx.com/2019/09/get-current-user-in-shell-scripts-on-macos/

currentUser=$(echo "show State:/Users/ConsoleUser" | scutil | awk '/Name :/ { print $3 }')

# don't load at loginwindow 
if [ "$currentUser" = "loginwindow" ] || [ "$currentUser" = "_mbsetupuser" ]; then
    echo "no user logged in, exiting"
    exit 0
fi

uid=$(id -u "$currentUser")

echo "loading $label"
launchctl bootstrap "gui/$uid" "$launchdPlist"

Set the proper file privileges on the postinstall file with

> chmod 755 scripts/postinstall

First the script sets the PATH. Then some variables for the label and bath to the label. You can change these to adapt the script to other LaunchAgents or LaunchDaemons.

Then the scripts checks the $3 argument. The installer system passes the target volume to the postinstall script in this argument. While it is a not common in deployment workflows any more, you can still install a pkg on a target volume that is not the current boot volume “/“. We remain safe with our script and check for that possibility, and exit without loading the agent.

LaunchAgents have to be loaded or bootstrapped into the current user’s context. The script gets the current user and checks whether the system might be sitting at the login window. We exit without loading if the system is at the login window or if the current user is _mbsetupuser which means the package is being installed while the system is sitting at Setup Assistant during first setup. Now that the LaunchAgent plist is in place, it will be loaded the first time the user logs in.

When there is a user logged in, we bootstrap the LaunchAgent right away. This is where the modern launchctl syntax shows its advantages. With the legacy commands, this would be:

launchctl asuser "$uid" launchctl load "$launchdPlist"

When you load a LaunchDaemon in a postinstall, the script is much simpler, as you don’t need to check for a current user. You can directly load the daemon into the system target domain:

launchctl bootstrap system/ "$launchdPlist"

Unloading before installation

When you are updating existing software with a package, you have to consider the situation where a LaunchAgent or LaunchDaemon is already running. In this case, we should unload before the installation. We can add a preinstall script (also no file extension) to the scripts folder that will be run before the payload is installed. This closely parallels the postinstall script.

#!/bin/sh

export PATH=/usr/bin:/bin:/usr/sbin:/sbin

label="com.scriptingosx.desktopprmanage"
launchdPlist="/Library/LaunchAgents/${label}.plist"

# do not unload when target volume ($3) is not current startup volume
if [ "$3" != "/" ]; then
    echo "not installing on startup volume, exiting"
    exit 0
fi

if ! launchctl list | grep -q "$label"; then
    echo "$label not loaded, exiting"
    exit 0
fi

# for LaunchAgent, check if user is logged in
# see https://scriptingosx.com/2019/09/get-current-user-in-shell-scripts-on-macos/

currentUser=$(echo "show State:/Users/ConsoleUser" | scutil | awk '/Name :/ { print $3 }')

# don't unload at loginwindow or Setup Assistant
if [ "$currentUser" = "loginwindow" ] || [ "$currentUser" = "_mbsetupuser" ]; then
    echo "no user logged in, exiting"
    exit 0
fi

uid=$(id -u "$currentUser")

echo "loading $label"
launchctl bootout "gui/$uid" "$launchdPlist"

Again, set the proper file privileges on the preinstall file with

> chmod 755 scripts/preinstall

There is one extra step compared to the postinstall script. The preinstall uses launchctl list to check if the launchd configuration is loaded, before attempting to unload it.

Building the Package Installer

Now we get to put everything together. You can use pkgbuild to build an installer package:

> pkgbuild --root payload/ --scripts scripts/ --version 1 --identifier com.scriptingosx.desktopprmanage --install-location / DesktopprManage-1.pkg   
pkgbuild: Inferring bundle components from contents of payload/
pkgbuild: Adding top-level preinstall script
pkgbuild: Adding top-level postinstall script
pkgbuild: Wrote package to DesktopprManage-1.pkg

This works well, but it is difficult to memorize all the proper arguments for the pkgbuild command. If you forget or misconfigure one of them it will either fail outright or lead to a pkg installer file with unwanted behavior. I prefer to put the command in a script. Create a script file buildDesktopprManages.sh at the top of your project directory with these contents:

#!/bin/sh

export PATH=/usr/bin:/bin:/usr/sbin:/sbin

pkgname="DesktopprManage"
version="1.1"
identifier="com.scriptingosx.${pkgname}"
install_location="/"
minOSVersion="10.13"

# determine enclosing folder
projectfolder=$(dirname "$0")

# build the component package
pkgbuild --root "${projectfolder}/payload" \
         --identifier "${identifier}" \
         --version "${version}" \
         --ownership recommended \
         --install-location "${install_location}" \
         --scripts "${projectfolder}/scripts" \
         --min-os-version "${minOSVersion}" \
         --compression latest \
         "${pkgname}-${version}.pkg"

This way, you do not have to memorize the pkgbuild command and its arguments and it is easier to modify parameters such as the version. Remember to change the version as you build newer versions of this package going forward!

Some management systems deploy a pkg file using certain MDM commands. Then you need a signed distribution package instead of a simple component pkg file. (You can learn more about the different types of packages in this blog post or this MacDevOps YVR presentation. Building a sign distribution pkg is somewhat more involved. I have a template script for that in the repo.

Conclusion

As I said, building a LaunchAgent for desktoppr manage is not trivial. However, building LaunchAgents and LaunchDaemons and proper installer pkgs to deploy them is an important skill for Mac admins.

There are some other tools available, such as outset, or your management system might have options to run scripts at certain triggers. But in general, I recommend to understand the underlying processes and technologies before using tools, even when the tools are great and a good fit for the task at hand.

You can use this process of building a LaunchAgent and installer pkg for desktoppr as a template for your own scripts and projects. You can find all the files in the desktoppr repo example directory.

desktoppr 0.5 — Managed by profile

Earlier this year, I released a beta version of desktoppr 0.5 which added an option to control the tool using a configuration profile. You can read all the details of how I built out this workflow in this blog post.

I have not received any feedback on it, which can mean two things: either it is working just fine or people simply aren’t aware of the beta. I guess both could be true at the same time here?

The new release is now available in the desktoppr repo. Nothing has changed compared to the 0.5beta except the version number and updated documentation.

Using desktoppr manage

The new manage verb allows a Mac Admin to set the wallpaper using data in a configuration profile. Together with the new option to download an image file for the wallpaper from an URL, this removes the requirement of updating two or more packages when you want to update the wallpaper on managed devices.

To use the new manage option, you need three pieces

desktoppr LaunchAgent

The LaunchAgent plist file controls when desktoppr will run.

The sample LaunchAgent file has both the RunAtLoad key and a StartInterval key. This means that desktoppr will run when the LaunchAgent loads and repeatedly after the time set in the StartInterval (in seconds, the sample file has 10800, so every three hours, as long as the system is awake).

The desktoppr profile provides a setOnlyOnce key which stops desktoppr from re-setting the same wallpaper over and over again. This way we can run desktoppr frequently, but it will only reset the wallpaper when the picture key in the profile has changed.

Build a pkg to install the LaunchAgent with your preferred frequency (or none) in /Library/LaunchAgents and deploy that together with the desktoppr pkg.

Configuration Profile

Since macOS Ventura, managed binaries and scripts launched by a LaunchAgent or Daemon should be pre-approved with a configuration profile from the management system, so that the user gets less dialogs about them and cannot disable them. Since desktoppr is a signed binary, this is quite straightforward and the sample configuration profile already contains the com.apple.servicemanagement section.

The second payload in the sample profile contains the settings. The only one that is required is the picture key which contains either a path to a local image file for the wallpaper, or a URL to an image file. Using a hosted file allows you to provide a custom branded wallpaper and change it without needing to deploy files to the client using more custom pkgs.

When you are not using a local file, you can (and probably should) provide a sha256 key with a checksum of the image file. This should protect from attempt to inject something malicious by hi-jacking the download URL. You can generate a sha256 sum for the image file with shasum -a 256 <path/to/file>.

The setOnlyOnce key controls whether desktoppr manage will re-set the wallpaper every time it runs (when setOnlyOnce is false) or only when the picture key in the profile changed (when setOnlyOnce is true). Configuring setOnlyOnce to true allows a user to change the wallpaper and it will not be overridden until you change the configuration profile. Then the user can change it again. This seemed like a useful compromise between management and user choice to me.

The scale and color keys work just like the options of the command line tool. Note that setting the color option is broken in macOS 14 as of this writing.

You can of course use different combinations of LaunchAgent configuration and other triggers for desktoppr, such as a self service portal to get all kinds of different workflows and levels of ‘lockdown.’ Remember that Apple provides a config profile setting to fully set and lock the wallpaper.

Conclusion

I have found this new managed desktoppr option useful in my deployments. I hope you do, too. Let me know!

Prefs CLI Tools for Mac Admins

Recently I have been working on some… well… “stuff” that uses custom configuration profiles. Very custom, and since I am testing things, they need to be updated a lot.

The issue with defaults

When you are working with defaults/preferences/settings/property lists on macOS, you will be familiar with the defaults command line tool. But, as useful as defaults can be, it has some downsides.

One of the great advantages of macOS’ preference system is that settings can be provided on multiple levels or domains. In my book “Property Lists, Preferences and Profiles for Apple Administrators, I have identified 19 different levels where settings for a single application can originate.

You will be most familiar with plist files in /Library/Preferences (system), ~/Library/Preferences (user), and managed configuration profiles (managed). When an app or tool requests a setting, the preferences system will merge all those levels together and present only the most relevant value. When the developer uses the system APIs (correctly), they do not have to worry about all the underlying levels, domains and mechanisms very much, but automatically gain support for things like separated system and user level settings files and support for management through configuration profiles.

The macOS defaults command line tool can work with settings on different levels or domains, but will only show the settings from one at a time. By default it only works with the user domain settings stored in ~/Library/Preferences/. When you have settings in multiple levels or from configuration profiles, you may be able to point defaults directly at the files. Or in the case of managed settings from profiles, you have to use a different tool. Either way, you have to determine which setting might override another and which final value might be visible to the app or process.

A new prefs tool

Years back, I had built a python script, called prefs.py, which would not only show the coalesced set of settings but their origin level. When macOS removed Python 2 in macOS 12.3, this tool obviously broke.

While working with preferences and profiles recently, this feature would have been quite useful to debug and verify preferences. I could have adapted the existing tool to work with MacAdmins Python 3, but felt I would learn something from recreating it in Swift. I had already started down that road just a bit for my sample project in this post.

So, you can find the new Swift-based prefs command line tool on GitHub. You can also download a signed and notarized pkg which will install the binary in /usr/local/bin/.

If its most basic form, you run it with a domain or application identifier. It will then list the merged settings for that preference domain, showing the level where the final value came from.

% prefs com.apple.screensaver
moduleDict [host]: {
    moduleName = "Computer Name";
    path = "/System/Library/Frameworks/ScreenSaver.framework/PlugIns/Computer Name.appex";
    type = 0;
}
PrefsVersion [host]: 100
idleTime [host]: 0
lastDelayTime [host]: 1200
tokenRemovalAction [host]: 0
showClock [host]: 0
CleanExit [host]: 1

I find this useful when researching where services and applications store their settings and also to see if a custom configuration profile is set up and applying correctly. There is a bit of documentation in the repo’s ReadMe and you can get a description of the options with prefs --help.

plist2profile

Another tool that would have been useful to my work, but that was also written in python 2 is Tim Sutton’s mcxToProfile. Back in the day, this tool was very useful when transitioning from Workgroup Manager and mcx based management to the new MDM and configuration profile based methods. If you have a long-lived management service, you will probably find some references to mcxToProfile in the custom profiles.

Even after Workgroup Manager and mcx based settings management was retired, Tim’s tool allowed to create a custom configuration profile from a simple property list file. Configuration Profiles require a lot of metadata around the actual settings keys and values, and mcxToProfile was useful in automating that step.

Some management systems, like Jamf Pro, have this feature built in. Many other management systems, however, do not. (Looking at you Jamf School.) But even then creating a custom profile on your admin Mac or as part of an automation, can be useful.

So, you probably guessed it, I also recreated mcxToProfile in Swift. The new tool is called plist2profile and available in the same repo and pkg. I have focused on the features I need right now, so plist2profile is missing several options compared to mcxToProfile. Let me know if this is useful and I might put some more work into it.

That said, I added a new feature. There are two different formats or layouts that configuration profiles can use to provide custom setting. The ‘traditional’ layout goes back all the way to the mcx data format in Workgroup Manager. This is what mcxToProfile would create as well. There is another, flatter format which has less metadata around it. Bob Gendler has a great post about the differences.

From what I can tell, the end effect is the same between the two approaches. plist2profile uses the ‘flatter’, simpler layout by default, but you can make it create the traditional mcx format by adding the --mcx option.

Using it is simple. You just need to give it an identifier and one or more plist files from which it will build a custom configuration profile:

% plist2profile --identifier example.settings com.example.settings.plist

You can find more instructions in the ReadMe and in the commands help with plist2profile --help

Conclusion

As I had anticipated, I learned a lot putting these tools together. Not just about the preferences system, but some new (and old) Swift strategies that will be useful for the actual problems I am trying to solve.

I also learnt more about the ArgumentParser package to parse command line arguments. This is such a useful and powerful package, but their documentation fails in the common way. It describes what you can do, but not why or how. There might be posts about that coming up.

Most of all, these two tools turned out to be useful to my work right now. Hope they will be useful to you!

zsh scripts and Root Escalations

There was an update for the Support.app this week which fixed a CVE. There is an argument to be had about whether this CVE deserves its high rating, but it is worth discussing the underlying issue and presenting some solutions. (Erik Gomez has some great comments on Mac Admins Slack.)

zsh is far more configurable than most other shells, most certainly more configurable than the aging bash 3.2 that comes with macOS. One aspect of being more configurable is that it has multiple different configuration files that are loaded at different times when the shell starts. The most common configuration file you will have encountered is ~/.zshrc which is loaded for all interactive shells.

Another, less commonly used, configuration file, ~/.zshenv (and its global sibling /etc/zshenv) is loaded every time a zsh process launches, including script launches. That means, when you (or something else in the system) launches a script with a #!/bin/zsh shebang, the zsh process will read the files /etc/zshenv and ~/.zshenv when they exist, and execute their code.

Update 2024-03-22: some links for further reading:

The Setup

We can see this in action.

First, create a simple zsh “hello.sh” type script with this content:

#!/bin/zsh
echo "Hello, $(whoami)"

Make the script file executable (chmod +x hello.sh) and run it. You should see output like

% ./hello.sh
Hello, armin

where armin is replaced with your current username.

Next, create a new file ~/.zshenv (~/ means at the root of your home directory) with your favored text editor and add the following line:

echo "zshenv: as $(whoami) called from $0"

If you already have a ~/.zshenv you will want to rename it for now so we don’t modify that. (mv ~/.zshenv ~/.zshenv_old)

Note that the configuration file neither has a shebang, nor does it need to be executable.

When you open a new Terminal window, you should see the line

zshenv: as armin called from -zsh

among the other output at the top of the Terminal window, since ~/.zshenv is read and evaluated every time a zsh process starts. The shebang at the beginning of the script ensures a new zsh environment is created for it, even when your interactive shell is not zsh.

When you run hello.sh you will see that ~/.zshenv is executed as well:

% ./hello.sh
zshenv: as armin called from /bin/zsh
Hello, armin

So far, so good. This is how zshenv is supposed to work. It’s purpose is to contain environment variables and other settings that apply to all processes on the system. On macOS this is undermined by the fact that most apps and process are not started from a shell, so they don’t see environment variables set anywhere in the zsh (or another shell’s) configuration files, so .zshenv is rarely used. .zshrc is far more useful.

But now we come to escalation aspect: run hello.sh with root privileges using sudo:

% sudo ./hello.sh
zshenv: as root called from /bin/zsh
Hello, root

We see that both our script and ~/.zshenv are run with root privileges. Again, that is how sudo and .zshenv are supposed to work.

Since the zshenv configuration files are evaluated every time a zsh is launched, it is far more likely that they will eventually run with elevated privileges, than the other configuration files. Other shells, like bash and sh have no equivalent configuration file that gets run on every launch.

The Escalation

The potential danger is the following: a process only needs user privileges to create or modify ~/.zshenv. A malicious attacker can inject code into your .zshenv, wait patiently for a script with a zsh shebang to be run with root privileges, and then execute some malicious code with root privileges on your system.

This can also occur with preinstall or postinstall scripts with a zsh shebang in installation package files (pkg files) when the installation is initiated by the user. A malicious attacker could scan your system for apps where they know the installer package contains zsh scripts, which makes the chance that eventually a zsh script is run with root permissions quite certain. Then all they have to do is be patient and wait…

As root escalations go, this is not a very efficient one. The attacker is at the mercy of the user to wait when they execute a zsh script with escalated root privileges. On some Macs, that may very well be never.

This also only works when the user can gain administrative privileges. Standard users cannot use sudo to gain root privileges, or run installation packages.

There are more effective means of escalating from user to root privileges on macOS. Most easily by asking the user directly for the password with a dialog that appears benign. However, if you are a frequent user or author of zsh scripts, it is important to be aware.

From what I can tell so far, this does not affect scripts launched from non-user contexts, such as scripts from Munki, Jamf Pro or other management solutions, though there may be odd, unexpected edge cases.

As an organization, you can monitor changes to all shell configuration files with a security tool (such as Jamf Protect). There are more shenanigans an attacker could achieve by modifying these files. There are, however, many legit reasons to change these files, so most changes will not indicate an attack or intrusion. Nevertheless, the information could be an important puzzle piece in combination with other behaviors, when putting together the progress or pattern of an intrusion.

Update 2024-03-22: Matteo Bolognini has added a custom analytic for Jamf Protect to detect changes to the zshenv file.

Configuration file protection

This weakness is fairly straightforward to protect against. First, since the attack requires modification of ~/.zshenv, you can protect that file.

If you did not have a ~/.zshenv, create an empty file in its place or remove the line of code from our sample .zshenv before. Then apply the following flag:

% sudo chflags schg ~/.zshenv

This command sets the system immutable flag (schg, ‘system change’), which makes it require root privileges to modify, rename or delete the file.

If you want to later modify this file, you can unlock it with

% sudo chflags noschg ~/.zshenv

Just remember to protect it again when you are done.

Script protection

Since most Mac users will never open terminal and never touch any shell configuration file, relying on protecting the configuration file is not sufficient. However, it is quite easy to protect the zsh scripts you create from this kind of abuse.

One solution is to use a $!/bin/sh shebang instead of #!/bin/zsh. Posix sh does not have a configuration file which gets loaded on every launch, so this problem does not exist for sh.

If you are using zsh features that are not part of the POSIX sh standard you will need to change the script. In my opinion, installation scripts that are too complex to use POSIX sh, are attempting to do too much and should be simplified, anyway. So, this can also work as an indicator for “good” installation scripts.

In addition, installer packages might be used in situations where zsh is not available. Installation tools that install package files when the Mac is booted to Recovery (where there is no zsh) are used far less than they used to, but it is still possible and making your installer pkgs resilient to all use cases is generally a good choice.

I believe for installation package scripts, using an sh shebang is a good recommendation.

Some scripts popular with Mac Admins, like Installomator, do not usually run from installer packages, but are regularly run with root privileges. For these cases, (or for installation scripts, where cannot or do not want to use sh) you can protect from the above escalation, by adding the --no-rcs option to the shebang:

#!/bin/zsh --no-rcs
echo "Hello, $(whoami)"

zsh’s --no-rcs option suppresses the launch of user level configuration files. The rc stands for ‘Run Command’ files and is the same rc that appears in zshrc or bashrc or several other unix configuration files.

That allows you to keep using zsh and zsh features, but still have safe scripts. This is the solution that Support.app and the latest version of Nudge have implemented. I have also created a PR for Installomator, which should be merged in the next release.

Conclusion

Modifying ~/.zshenv is not a very effective means of gaining root privileges, but it is something developers and Mac admins that create zsh script that may be run with root privileges should be aware of. Switch to an sh shebang or add --no-rcs to the zsh shebang of scripts that might be run with root privileges to protect.