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I am trying to understand how btcdeb (Bitcoin script debugger from Bitcoin core) works. In the usecase of btcdeb --tx=<tx hex> --txin=<txin hex>, how does one obtain the hex of the input of interest? In the example there:

$ btcdeb --tx=010000000001019086ce64fce1bb086395faf6fac37c73f32ba4ea89330432bf8ee8035e9315aa0100000000ffffffff021353b9030000000017a914c3f413d0918853a8e23766678d2e3c2e5c8138bb8725e4973100000000220020701a8d401c84fb13e6baf169d59684e17abd9fa216c8cc5b9fc63d622ff8c58d040047304402207f874ef00f11dcc9a621acad9354f3fca1bf90c43878f607b7e2d358088487e7022052a01b47b8eef5e1c96a6affdc3dac46fdc11b60612464dc8c5921a852090d2701483045022100c56ab2abb17fdf565417228763bc9f2940a6465042fd62fbd9f4c7406345d7f702201cb1a56b45181f8347713627b325ec5df48fc1aee6bdaf937cbb804d7409b10c016952210375e00eb72e29da82b89367947f29ef34afb75e8654f6ea368e0acdfd92976b7c2103a1b26313f430c4b15bb1fdce663207659d8cac749a0e53d70eff01874496feff2103c96d495bfdd5ba4145e3e046fee45e84a8a48ad05bd8dbb395c011a32cf9f88053ae00000000 \
txin=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 

the hex of the txin (0100000000010170a..., 383Bytes) is actually longer than the hex for the whole tx itself (010000000001019..., 381Bytes). How was the hex for txin obtained?

When I run bitcoin-cli decoderawtransaction <txin hex> to see what txin hex is, it gives full JSON of the whole transaction, same as running bitcoin-cli decoderawtransaction <tx hex>

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The txin hex is just the normal hex for a transaction whose output(s) you are spending. It's not something you can compute from the tx hex itself (if you could, you would't need to provide it). It's there because to validate a transaction input you need to know what output(s) it is spending, and those are part of the earlier transaction(s).

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  • Thank you. I still do not understand, if the --tx=<hex> has more than one input that reference the same --txin=<reftx hex>, which tx's input script btcdeb is going to pick to debug? Jun 26, 2023 at 17:07
  • I do not know the answer to that. Jun 26, 2023 at 17:22

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