Many scripted macOS workflows require determining the username of the currently logged-in user. Whether you wish to execute a command as that user via su or you just want to log the username during your script's execution, you may need to query macOS for this information.
This is a solved problem, and Armin Briegel's excellent article on Getting the current user in macOS outlines the best method.
In addition to determining the logged-in user's username, you may also need their user ID number, or UID. For example, loading a LaunchAgent as a specific user requires providing that user's UID.
Previous solutions involved grabbing the logged-in user's username, then feeding that to id -u $loggedInUser – but you can get the currently logged-in user's UID in one step.
The code
A slight modification of Armin's recommendation for grabbing the username will give us the UID.
For bash and zsh
loggedInUserID=$( scutil <<< "show State:/Users/ConsoleUser" | awk '/kCGSSessionUserIDKey :/ { print $3 }' )
For shell
Vanilla shell does not support the <<< here-string used in the previous example, so instead we must pipe the show statement to scutil.
loggedInUserID=$( echo "show State:/Users/ConsoleUser" | scutil | awk '/kCGSSessionUserIDKey :/ { print $3 }' )
Both variants will return the UID of the currently logged-in user, or an empty string if the Mac is sitting at the login window.
An example and a note on launchctl syntax
With a UID in hand, you can now load a LaunchAgent as that user:
launchctl asuser <uid> launchctl load /Library/LaunchAgents/com.org.example.plist
However! In macOS Big Sur, I'm finding launchctl is more reliable using the launchctl 2.0 syntax (as documented by Balmes Pavlov). This makes sense, as subcommands like asuser have been marked as "legacy" for some time now.
Switching from the "legacy" subcommand asuser to bootstrap gui/"${loggedInUserID}" consistently loads launchd tasks, and is my current recommendation.
A complete example of loading a LaunchAgent as the currently logged-in user:
loggedInUserID=$( scutil <<< "show State:/Users/ConsoleUser" | awk '/kCGSSessionUserIDKey :/ && ! /loginwindow/ { print $3 }' )
/bin/launchctl bootstrap gui/"${loggedInUserID}" /Library/LaunchAgents/com.org.example.plist