Bitcoin Script Language Basics

Bitcoin is often described as "digital gold" — a simple store of value. But beneath the surface, every Bitcoin transaction is powered by a small but powerful programming language called Bitcoin Script. This language determines the conditions under which Bitcoin can be spent, and understanding it unlocks a deeper appreciation of what Bitcoin can actually do.

What Is Bitcoin Script?

Bitcoin Script is a stack-based, Forth-like programming language that is intentionally simple and limited. Every Bitcoin transaction contains two scripts:

• ScriptPubKey (Locking Script): Attached to the transaction output — it defines the conditions that must be met to spend the Bitcoin. Think of it as a lock on a safe.
• ScriptSig (Unlocking Script): Provided by the spender in a new transaction — it provides the data (like a digital signature) that satisfies the locking conditions. Think of it as the key.

When a node validates a transaction, it combines the unlocking script with the locking script and executes them together. If the combined script evaluates to TRUE, the transaction is valid and the Bitcoin can be spent.

Stack-Based Execution

Unlike languages like JavaScript or Python, Bitcoin Script does not use variables or loops. Instead, it uses a stack — a last-in, first-out (LIFO) data structure. Operations (called opcodes) push data onto the stack, manipulate it, or pop data off. For example:

• OP_DUP: Duplicates the top item on the stack.
• OP_HASH160: Pops the top item, hashes it with SHA-256 and then RIPEMD-160, and pushes the result.
• OP_EQUALVERIFY: Checks that the top two stack items are equal; fails the script if they are not.
• OP_CHECKSIG: Verifies a digital signature against a public key.

The Standard P2PKH Transaction

The most common Bitcoin script pattern is Pay-to-Public-Key-Hash (P2PKH). Here is how it works step by step:

1. The locking script says: "Provide a public key that hashes to this address, and a valid signature for that key."
2. The unlocking script provides: the signature and the public key.
3. The node executes: duplicate the public key, hash it, compare it to the expected hash, then verify the signature.
4. If everything checks out, the script returns TRUE and the spend is valid.

Why Script Is Intentionally Limited

Satoshi Nakamoto deliberately made Bitcoin Script non-Turing-complete. It has no loops, no recursion, and limited opcodes. This was a critical design decision:

• Security: A limited language has a smaller attack surface. No infinite loops means no denial-of-service attacks via script execution.
• Predictability: Every script execution is guaranteed to terminate in a bounded number of steps.
• Auditability: Simple scripts are easier to verify and reason about — critical when billions of dollars are at stake.

For Sri Lankan developers interested in Bitcoin development, understanding Script is the foundation. It reveals that Bitcoin is not just money — it is a programmable settlement layer with carefully designed constraints.