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moving BTCdeb to the next section
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Shannon Appelcline
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# 7.2: Running a Bitcoin Script
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# 9.2: Running a Bitcoin Script
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> :information_source: **NOTE:** This is a draft in progress, so that I can get some feedback from early reviewers. It is not yet ready for learning.
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Bitcoin Scripts may not initially seem that intuitive, but their execution is quite simple, using reverse Polish notation and a stack.
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## Running Bitcoin Script code
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It is recommended that you run through the examples in a Bitcoin Script Debugger (`btcdeb`) to see the transformations happening
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on the stack. This will require setting up C++ and a few other accessories on your machine, so choose if you want to add this additional material to your machine.
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### Installing btcdeb
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From some appropriate folder (e.g. `~/workspace`), clone the btcdeb project from Github and compile/install it.
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```Bash
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$ sudo apt-get install git
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$ git clone https://github.com/kallewoof/btcdeb.git
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```
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Then, get C++ and other packages installed, so that you can get `btcdeb` running.
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```Bash
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$ sudo apt-get install autoconf libtool g++ pkg-config make
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$ cd btcdeb
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$ ./autogen.sh
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$ ./configure
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$ make
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$ sudo make install
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```
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After all of that, you should have a copy of `btcdeb`:
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```
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$ which btcdeb
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/usr/local/bin/btcdeb
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```
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It is recommended that you all install readline, as this makes the debugger a lot easier to use by supporting history using up/down arrows, left-right movement, autocompletion using tab, etc. The package is usually called `libreadline-dev` (linux) or just `readline` (mac).
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```Bash
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$ sudo apt-get install libreadline-dev
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```
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### Bitcoin Script Debugging Primer
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`btcdeb` takes a script, as well as any number of stack entries, as startup arguments. If you start it up with no arguments, you simply get an interpreter
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where you may issue `exec [opcode]` commands to perform actions directly.
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`btcc` takes script opcodes and data and outputs a Bitcoin Script in hexadecimal form.
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We will make use of both of these in the sections below.
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## Understand the Scripting Language
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A Bitcoin Script has three parts: it has a line of input; it has a stack for storage; and it has specific commands for execution.
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@ -61,7 +20,7 @@ For example, if you were adding together "1" and "2", your Bitcoin Script for th
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It's actually not quite correct to say that an operator applies to the inputs before it. Really, an operator applies to the top inputs in Bitcoin's stack.
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_What is a stack?_ A stack is a LIFO (last-in-first-out) data structure. It has two access functions: push and pop. Push places a new object on top of the stack, pushing down everything below it. Pop removes the top object from the stack.
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> :book: ***What is a stack?*** A stack is a LIFO (last-in-first-out) data structure. It has two access functions: push and pop. Push places a new object on top of the stack, pushing down everything below it. Pop removes the top object from the stack.
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Whenever Bitcoin Script encounters a constant, it pushes it on the stack. So the above example of `1 2 OP_ADD` would actually look like this as it was processed:
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```
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@ -76,49 +35,12 @@ Stack: [ 1 2 ]
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```
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_Note that in this and in following examples the top of the stack is to the right and the bottom is to the left._
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Let's try this out:
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```Bash
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$ btcc OP_1 OP_2 OP_ADD
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515293
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$ btcdeb '[OP_1 OP_2 OP_ADD]' # or: btcdeb 0x515293
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btcdeb -- type `btcdeb -h` for start up options
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valid script
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3 op script loaded. type `help` for usage information
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script | stack
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--------+--------
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1 |
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2 |
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OP_ADD |
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#0001 1
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btcdeb> step
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<> PUSH stack 01
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script | stack
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--------+--------
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2 | 01
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OP_ADD |
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#0002 2
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btcdeb> step
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<> PUSH stack 02
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script | stack
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--------+--------
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OP_ADD | 02
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| 01
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#0003 OP_ADD
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btcdeb> step
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<> POP stack
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<> POP stack
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<> PUSH stack 03
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script | stack
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--------+--------
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| 03
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```
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> `btcdeb` allows you to repeat the previous command by hitting enter. We will be doing this in subsequent examples, so don't be surprised about `btcdeb>` prompts with nothing as input. It is simply repeating the previous (often `step`) command.
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### Understand the Opcodes
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When a Bitcoin Script encounters an operator, it evaluates it. Each operator pops zero or more elements off the stack as inputs, usually one or two. It then processes them in a specific way before pushing zero or more elements back on the stack, usually one or two.
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> :book: ***What is an Opcode?*** Opcode stands for "operation code". It's typically associated with machine-language code, and is a simple function (or "operator").
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OP_ADD pops two items off the stack (here: 2 then 1), adds then together, and pushes the result back on the stack (here: 3).
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```
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Script:
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@ -151,62 +73,6 @@ Running: 5 4 OP_SUB
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Stack: [ 1 ]
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```
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Let's try this one too:
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```Bash
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$ btcdeb '[OP_3 OP_2 OP_ADD OP_4 OP_SUB]'
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btcdeb -- type `btcdeb -h` for start up options
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valid script
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5 op script loaded. type `help` for usage information
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script | stack
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--------+--------
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3 |
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2 |
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OP_ADD |
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4 |
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OP_SUB |
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#0001 3
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btcdeb> step
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<> PUSH stack 03
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script | stack
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--------+--------
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2 | 03
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OP_ADD |
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4 |
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OP_SUB |
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#0002 2
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btcdeb>
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<> PUSH stack 02
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script | stack
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--------+--------
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OP_ADD | 02
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4 | 03
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OP_SUB |
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#0003 OP_ADD
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btcdeb>
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<> POP stack
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<> POP stack
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<> PUSH stack 05
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script | stack
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--------+--------
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4 | 05
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OP_SUB |
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#0004 4
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btcdeb>
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<> PUSH stack 04
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script | stack
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--------+--------
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OP_SUB | 04
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| 05
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#0005 OP_SUB
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btcdeb>
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<> POP stack
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<> POP stack
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<> PUSH stack 01
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script | stack
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--------+--------
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| 01
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```
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## Understand the Usage of Bitcoin Script
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That's pretty much Bitcoin Scripting ... other than a few intricacies for how this scripting language interacts with Bitcoin itself.
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@ -241,8 +107,7 @@ Stack: [ True ]
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```
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This abstraction isn't quite accurate: for security reasons, the `scriptSig` is run, then the contents of the stack are transferred for the `scriptPubKey` to run, but it's accurate enough for understanding how the key of `scriptSig` fits into the lock of `scriptPubKey`.
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> :warning: **WARNING** The above is a non-standard transaction type. It would not actually be accepted by nodes running Bitcoin Core with the standard settings. [§8.1: Building a Bitcoin Script with P2SH](08_1_Understanding_the_Foundation_of_P2SH.md
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) discusses how you actually _could_ run a Bitcoin Script like this, using the power of P2SH.
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> :warning: **WARNING** The above is a non-standard transaction type. It would not actually be accepted by nodes running Bitcoin Core with the standard settings. [§10.1: Building a Bitcoin Script with P2SH](10_1_Understanding_the_Foundation_of_P2SH.md) discusses how you actually _could_ run a Bitcoin Script like this, using the power of P2SH.
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### Get the Results
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@ -259,5 +124,4 @@ To process a Bitcoin Script, a `scriptSig` is run followed by the `scriptPubKey`
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## What's Next?
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Continue "Introducing Bitcoin Scripts" with [§7.3: Scripting a P2PKH](07_3_Scripting_a_P2PKH.md).
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Continue "Introducing Bitcoin Scripts" with [§9.3: Testing a Bitcoin Script](09_3_Testing_a_Bitcoin_Script.md).
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