With all credit to developer Jake James, we have the very first jailbreak tool release to the user for Jailbreak iOS 12. But this is targeting developers and totally

About SSH based RootlessJB for Jailbreak iOS 12
This is totally a developer targeted tool update that comes in a SSH based framework. In fact, it is rootless where you will not find anything straightforward. And most notably, this is not for Cydia purpose as it gives no support to Download Cydia and Cydia substrate.
The tool targets iOS 12.0- iOS 12.1.2 firmware. And it allows to install and run on A9-A11 Apple devices that run the given firmware versions. But the latest iPhone XS, XS Max, iPhone XR and new iPad Pro are not yet compatible with the jailbreak. So make sure you are getting ready with the right device to process with no errors. The devices compatible for the tool are as,
iPhone 6s, iPhone 6s Plus, iPhone 7, iPhone 7 Plus, iPhone 8, iPhone 8 Plus, iPhone SE and iPhone X
iPad device models do not have A12 or upper versions of chipsets
The complete step guide to RootlessJB
Step 1: Launch through “jailbreaks.fun” from the iPhone or iPad and get into the processing. From the interface, search rootlessJB through the bar
Step 2: From the search results, continue to “Get button” and download the correct tool file into the iPhone or iPad
Step 3: Give several minutes to download the file to the device and complete installation. Then to trust the program from the device, go to General > device management and select the profile installed. Trust the profile and continue
Step 4: Now enable Airplane mode
Step 5: Launch rootlessJB from Home and make sure “Tweaks” toggle is successfully turned on in the interface. Now give command through a tap on “Jailbreak” button and wait for jailbreak to process
Step 6: It is all that you are required to do and now wait for the device to take time for complete jailbreaking. In several minutes it should respring if the iOS 12 jailbreak is successfully done. In case, if it is restarted, you are required to run the tool once again or more times until it shows successfully done

Final words
With correct guidelines, you can successfully follow RootlessJB Jailbreak iOS 12. But it gives no support to Cydia Download or substrate as this is SSH based. But for the happy fact, it is now with tweak injection. As to James follow up tweets rootlessJB can tweak different parts of SpringBoard and unsandboxed applications. But sandboxed apps like Notes, Home, Messages etc are still not supported for tweaking. So stay tuned and catch more updates to rootlessJB. Until something turns out with complete Cydia install, you can enjoy this limited jailbreak update.