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WARNING: The lack of any code Script interpretors is currently halting this section.
8.3: Testing a Bitcoin Script
NOTE: This is a draft in progress, so that I can get some feedback from early reviewers. It is not yet ready for learning.
Bitcoin Scripting allows for considerable new control over Bitcoin transactions, but it's all somewhat abstract. How do you actually turn that theoretical Bitcoin Script into functional Bitcoin code? The first answer is to test it ... but unfortunately testing is pretty is a category that's pretty underserved right now, which means that this section mainly focuses on hopefuly futures.
Get the Code Right
The object of testing your Bitcoin Scripts is to get the code right. Don't worry about the whole rigamarole of P2SH scripting (yet). Just worry about producing the Bitcoin Script code itself and making sure it runs and does what you expect.
This is a critically important aspect of any Bitcoin Scripting, because there's no other check on your Script. Bitcoin will often protect you from trying to send bad transactions, but it won't protect you from writing bad locking scripts. That's because the P2SH methodology actually keeps Bitcoin from knowing about the actual Script until it tries to unlock it ... and if your script doesn't run at that point, your funds are gone.
So the onus is on you to get that code right.
Test a Script Online
Web simulators seem like they should be a great option for Bitcoin Script testing. Unfortunately, as of the time of this writing, online simulators are few, and the ones that exist are almost fatally buggy. A few are included here mainly for completeness sake: because they used to work pretty well and perhaps might again. For now, they're not particularly recommended though.
The Script Playground
Charlie Marsh has built an excellent Script Playground. Just put together your unlocking script and your locking script and run them. The Playground was run an OP_VERIFY at the end, and tell you with a green checkmark or a red X whether the transaction was unlocked or not.
Type in the test from §7.2: Running a Bitcoin Script.md of 1 98 OP_ADD 99 OP_EQUAL and watch it verify; change it to 1 97 OP_ADD 99 OP_EQUAL and see it fail.
If you prefer you can grab the JavaScript code and run it on your own machine.
Bugs & Challenges
Signature Problems. To really test Scripts you need to be able to verify hashes and signatures. The Playground tried to make that easy with buttons that can be used to cut-and-paste public keys and signatures, but they haven't kept up with Flash security ugprades and no longer work reliably. This makes any testing extremely cumbersome, as you have to dump addresses, public keys, and private keys from bitcoin-cli. Signing is simplified by just checking the signature of a nonce of "Secure", but given that bitcoin-cli produces a base64 signature instead of a hex signature, you're deep in the weeds. As a result of all of this, unless you're deeply comfortable with Bitcoin signatures, you're unlikely to be able to use Script Playground for anything but tests of flow control.
Number Problems. The Script Playground doesn't recognize numbers of 100 or more.
If the signature buttons worked right, the Playground would be great, but until that happens, it's not very useful.
Web BTC Script Debugger
WebBTC has a Script Debugger that not only shows you the execution of a script, but also explains its parts and shows you the stack. The following example is similar to the add-to-99 script of previous chapters: Add to 15 Script.
Bugs & Challenges
Web Problems. The web submit form no longer works, requiring you to type all script into the URL in the form https://webbtc.com/script/[unlocking-script]/[locking-script]. Each script is a series of operators separated by %20s. For example 1%2014 is 1 14 and OP_ADD%2015%20OP_EQUAL is OP_ADD 15 OP_EQUAL.
Signature Problems. Allegedly, all signatures are assumed valid if a scripthash is not provided. This does not seem to be the case. Examining existing transactions demonstrates how their signatures were processed, but checking examples does not work, again majorly impacting the utility of this site.
Number Problems. The BTC debugger doesn't recognize numbers of 17 or more. (This is not unusual: 1 to 16 become OP_1 to OP_16, but beyond that, extra work is required for numbers, work that you're usually insulated from.)
_If signatures were indeed considered valid, this would be a great resource, but between that not working and having to annoyingly type in code as a URL, this is another online script tester that's barely usable currently. The one saving grace is that transactions can be examined down to the Bitcoin script ... but the site hasn't been recording new transactions since December 2016.
Test a Script with Software Packages
Software packages may be another future for the testing of Bitcoin Scripts. A Python package called Hashmal is very promising, but it was still in Alpha at the time of this writing. The Python Transaction Script Compiler is also interesting, though it just converts a script between different representations, rather than actually running it.
Test a Script with Bitcoin
Someday web sites and software packages might offer great opportunities for testing Bitcoin Scripts. However, they'll never be the real thing, because you can't guarantee that they follow the consensus rules. For example, the Script Playground explicitly says that it ignores the extra-pop multisig bug. This means that any multisig code that you successfully test on the Script Playground will break in the real-world.
So the only way to really test Bitcoin Scripts is to try them out on Testnet, and verify that you can redeem coins using the script before you put it on Mainnet. Don't trust your code is write; don't just eyeball it. Doing so it another great way to lose funds on Bitcoin.
Summary: Testing a Bitcoin Script
Testing turns out to not be a great option right now for Bitcoin Scripts, except perhaps for some very simple flow control testing. We hope that some of the options in this section will mature in the future, but for now you're going to need to move straight on to sending and spending transactions with Bitcoin Scripts.