Bitcoin Script 101 (1): Stack Model and Core Concepts

Bitcoin Script is the validation system used to spend UTXOs. It’s not a general-purpose language; it’s stack-based, loop-free, and terminating. The first step to understanding Script is the “stack + UTXO” mindset.

1) UTXOs and Scripts

Each UTXO carries a locking condition (locking script / scriptPubKey). To spend it, you must provide an unlocking script (scriptSig or witness) that satisfies that condition.

Put simply:
Locking script says: “To spend me, satisfy these conditions.”
Unlocking script says: “Here is the proof.”

The two scripts are concatenated and executed. If the final result is true, the UTXO can be spent.

2) Stack Machine Model

Script is a LIFO stack machine.
Opcodes push data, pop data, and push results back.

Example (conceptual):

2 3 OP_ADD 5 OP_EQUAL

Execution:

• Push 2, push 3
• OP_ADD pops 3 and 2, pushes 5
• Push 5
• OP_EQUAL compares and pushes true

Top of stack is true → script passes.

3) Three Rules of Script

1. No loops: always terminates
2. Stateless: depends only on inputs and scripts
3. Boolean result: stack top must be true

This makes Script ideal for verification, not general computation.

4) Common Opcodes

Starter opcodes:

• OP_DUP: duplicate top stack item
• OP_HASH160: hash = RIPEMD160(SHA256(x))
• OP_EQUAL / OP_EQUALVERIFY: compare
• OP_CHECKSIG: verify signature
• OP_VERIFY: assert true, otherwise fail

5) A Minimal Example: P2PKH Intuition

Classic P2PKH (details next) is:

1. Unlocking script provides signature and public key
2. Locking script verifies signature vs public key
3. Public key hash matches the address hash in the output

That’s Script’s way of saying “only the key owner can spend.”

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Next: Standard Script Templates (P2PKH/P2SH/SegWit/Taproot)