OP_IF ​

OP_IF is used to perform conditional execution. Scripts in Bitcoin can contain branches—parts of the script that are executed only if certain conditions are met. The OP_IF opcode allows the script to evaluate a value and conditionally execute the following script commands based on that value.

Operation ​

1. Pop the top stack item.
2. If the item is not 0, execute the following opcodes until an OP_ELSE or OP_ENDIF is encountered.
3. If the item is 0, skip the following opcodes until an OP_ELSE or OP_ENDIF is encountered.

You'll see it used like so:

<condition> OP_IF
    <script if condition is true>
OP_ELSE
    <script if condition is false>
OP_ENDIF

Where <condition> is some operation or value that will leave a 0 or non-0 value on top of the stack.

Notes ​

• Bitcoin script does not support loop structures, so all conditional logic is based on branches.
• The script will fail and not complete if there is no matching OP_ENDIF for an OP_IF.

Examples ​

Example 1 ​

The following script would leave 2 on the stack as the condition (1) is true.

# ASM script
OP_1 OP_IF OP_2 OP_ENDIF

# Raw script
51635268

# Final stack
2

Example 2 ​

The following script would leave 3 on the stack as the condition (0) is false.

# ASM script
OP_0 OP_IF OP_2 OP_ELSE OP_3 OP_ENDIF

# Raw script
006352675368

# Final stack
3