Script
Bitcoin uses a scripting system for transactions. Forthlike, Script is simple, stackbased, and processed from left to right. It is purposefully not Turingcomplete, with no loops.
A script is essentially a list of instructions recorded with each transaction that describe how the next person wanting to spend the Bitcoins being transferred can gain access to them. The script for a typical Bitcoin transfer to destination Bitcoin address D simply encumbers future spending of the bitcoins with two things: the spender must provide
 a public key that, when hashed, yields destination address D embedded in the script, and
 a signature to show evidence of the private key corresponding to the public key just provided.
Scripting provides the flexibility to change the parameters of what's needed to spend transferred Bitcoins. For example, the scripting system could be used to require two private keys, or a combination of several, or even no keys at all.
A transaction is valid if nothing in the combined script triggers failure and the top stack item is true (nonzero). The party who originally sent the Bitcoins now being spent, dictates the script operations that will occur last in order to release them for use in another transaction. The party wanting to spend them must provide the input(s) to the previously recorded script that results in those operations occurring last leaving behind true (nonzero).
The stacks hold byte vectors. When used as numbers, byte vectors are interpreted as littleendian variablelength integers with the most significant bit determining the sign of the integer. Thus 0x81 represents 1. 0x80 is another representation of zero (so called negative 0). Positive 0 is represented by a nulllength vector. Byte vectors are interpreted as Booleans where False is represented by any representation of zero, and True is represented by any representation of nonzero.
Contents
Words
This is a list of all Script words, also known as opcodes, commands, or functions.
There are some words which existed in very early versions of Bitcoin but were removed out of a concern that the client might have a bug in their implementation. This fear was motivated by a bug found in OP_LSHIFT which could crash any Bitcoin node if exploited, and by other bugs in how Script worked which allowed anyone to spend anyone's bitcoins. The removed opcodes are sometimes said to be "disabled" opcodes, but this is something of a misnomer because there is absolutely no way for anyone using Bitcoin to use these opcodes (they simply do not exist anymore in the protocol), and there are also no solid plans to ever reenable all of these opcodes. They are listed here only for historical interest.
New opcodes can be added by means of a carefully designed and executed softfork using OP_NOP1OP_NOP10.
False is zero or negative zero and True is anything else.
Constants
When talking about scripts, these valuepushing words are usually omitted.
Word  Opcode  Hex  Input  Output  Description 

OP_0, OP_FALSE  0  0x00  Nothing.  (empty value)  An empty array of bytes is pushed onto the stack. (This is not a noop: an item is added to the stack.) 
N/A  175  0x010x4b  (special)  data  The next opcode bytes is data to be pushed onto the stack 
OP_PUSHDATA1  76  0x4c  (special)  data  The next byte contains the number of bytes to be pushed onto the stack. 
OP_PUSHDATA2  77  0x4d  (special)  data  The next two bytes contain the number of bytes to be pushed onto the stack. 
OP_PUSHDATA4  78  0x4e  (special)  data  The next four bytes contain the number of bytes to be pushed onto the stack. 
OP_1NEGATE  79  0x4f  Nothing.  1  The number 1 is pushed onto the stack. 
OP_1, OP_TRUE  81  0x51  Nothing.  1  The number 1 is pushed onto the stack. 
OP_2OP_16  8296  0x520x60  Nothing.  216  The number in the word name (216) is pushed onto the stack. 
Flow control
Word  Opcode  Hex  Input  Output  Description 

OP_NOP  97  0x61  Nothing  Nothing  Does nothing. 
OP_IF  99  0x63  <expression> if [statements] [else [statements]]* endif  If the top stack value is not 0, the statements are executed. The top stack value is removed.  
OP_NOTIF  100  0x64  <expression> if [statements] [else [statements]]* endif  If the top stack value is 0, the statements are executed. The top stack value is removed.  
OP_ELSE  103  0x67  <expression> if [statements] [else [statements]]* endif  If the preceding OP_IF or OP_NOTIF or OP_ELSE was not executed then these statements are and if the preceding OP_IF or OP_NOTIF or OP_ELSE was executed then these statements are not.  
OP_ENDIF  104  0x68  <expression> if [statements] [else [statements]]* endif  Ends an if/else block. All blocks must end, or the transaction is invalid. An OP_ENDIF without OP_IF earlier is also invalid.  
OP_VERIFY  105  0x69  True / false  Nothing / False  Marks transaction as invalid if top stack value is not true. 
OP_RETURN  106  0x6a  Nothing  Nothing  Marks transaction as invalid. A standard way of attaching extra data to transactions is to add a zerovalue output with a scriptPubKey consisting of OP_RETURN followed by exactly one pushdata op. Such outputs are provably unspendable, reducing their cost to the network. Currently it is usually considered nonstandard (though valid) for a transaction to have more than one OP_RETURN output or an OP_RETURN output with more than one pushdata op. 
Stack
Word  Opcode  Hex  Input  Output  Description 

OP_TOALTSTACK  107  0x6b  x1  (alt)x1  Puts the input onto the top of the alt stack. Removes it from the main stack. 
OP_FROMALTSTACK  108  0x6c  (alt)x1  x1  Puts the input onto the top of the main stack. Removes it from the alt stack. 
OP_IFDUP  115  0x73  x  x / x x  If the top stack value is not 0, duplicate it. 
OP_DEPTH  116  0x74  Nothing  <Stack size>  Puts the number of stack items onto the stack. 
OP_DROP  117  0x75  x  Nothing  Removes the top stack item. 
OP_DUP  118  0x76  x  x x  Duplicates the top stack item. 
OP_NIP  119  0x77  x1 x2  x2  Removes the secondtotop stack item. 
OP_OVER  120  0x78  x1 x2  x1 x2 x1  Copies the secondtotop stack item to the top. 
OP_PICK  121  0x79  xn ... x2 x1 x0 <n>  xn ... x2 x1 x0 xn  The item n back in the stack is copied to the top. 
OP_ROLL  122  0x7a  xn ... x2 x1 x0 <n>  ... x2 x1 x0 xn  The item n back in the stack is moved to the top. 
OP_ROT  123  0x7b  x1 x2 x3  x2 x3 x1  The top three items on the stack are rotated to the left. 
OP_SWAP  124  0x7c  x1 x2  x2 x1  The top two items on the stack are swapped. 
OP_TUCK  125  0x7d  x1 x2  x2 x1 x2  The item at the top of the stack is copied and inserted before the secondtotop item. 
OP_2DROP  109  0x6d  x1 x2  Nothing  Removes the top two stack items. 
OP_2DUP  110  0x6e  x1 x2  x1 x2 x1 x2  Duplicates the top two stack items. 
OP_3DUP  111  0x6f  x1 x2 x3  x1 x2 x3 x1 x2 x3  Duplicates the top three stack items. 
OP_2OVER  112  0x70  x1 x2 x3 x4  x1 x2 x3 x4 x1 x2  Copies the pair of items two spaces back in the stack to the front. 
OP_2ROT  113  0x71  x1 x2 x3 x4 x5 x6  x3 x4 x5 x6 x1 x2  The fifth and sixth items back are moved to the top of the stack. 
OP_2SWAP  114  0x72  x1 x2 x3 x4  x3 x4 x1 x2  Swaps the top two pairs of items. 
Splice
If any opcode marked as disabled is present in a script, it must abort and fail.
Word  Opcode  Hex  Input  Output  Description 

OP_CAT  126  0x7e  x1 x2  out  Concatenates two strings. disabled. 
OP_SUBSTR  127  0x7f  in begin size  out  Returns a section of a string. disabled. 
OP_LEFT  128  0x80  in size  out  Keeps only characters left of the specified point in a string. disabled. 
OP_RIGHT  129  0x81  in size  out  Keeps only characters right of the specified point in a string. disabled. 
OP_SIZE  130  0x82  in  in size  Pushes the string length of the top element of the stack (without popping it). 
Bitwise logic
If any opcode marked as disabled is present in a script, it must abort and fail.
Word  Opcode  Hex  Input  Output  Description 

OP_INVERT  131  0x83  in  out  Flips all of the bits in the input. disabled. 
OP_AND  132  0x84  x1 x2  out  Boolean and between each bit in the inputs. disabled. 
OP_OR  133  0x85  x1 x2  out  Boolean or between each bit in the inputs. disabled. 
OP_XOR  134  0x86  x1 x2  out  Boolean exclusive or between each bit in the inputs. disabled. 
OP_EQUAL  135  0x87  x1 x2  True / false  Returns 1 if the inputs are exactly equal, 0 otherwise. 
OP_EQUALVERIFY  136  0x88  x1 x2  True / false  Same as OP_EQUAL, but runs OP_VERIFY afterward. 
Arithmetic
Note: Arithmetic inputs are limited to signed 32bit integers, but may overflow their output.
If any input value for any of these commands is longer than 4 bytes, the script must abort and fail. If any opcode marked as disabled is present in a script  it must also abort and fail.
Word  Opcode  Hex  Input  Output  Description 

OP_1ADD  139  0x8b  in  out  1 is added to the input. 
OP_1SUB  140  0x8c  in  out  1 is subtracted from the input. 
OP_2MUL  141  0x8d  in  out  The input is multiplied by 2. disabled. 
OP_2DIV  142  0x8e  in  out  The input is divided by 2. disabled. 
OP_NEGATE  143  0x8f  in  out  The sign of the input is flipped. 
OP_ABS  144  0x90  in  out  The input is made positive. 
OP_NOT  145  0x91  in  out  If the input is 0 or 1, it is flipped. Otherwise the output will be 0. 
OP_0NOTEQUAL  146  0x92  in  out  Returns 0 if the input is 0. 1 otherwise. 
OP_ADD  147  0x93  a b  out  a is added to b. 
OP_SUB  148  0x94  a b  out  b is subtracted from a. 
OP_MUL  149  0x95  a b  out  a is multiplied by b. disabled. 
OP_DIV  150  0x96  a b  out  a is divided by b. disabled. 
OP_MOD  151  0x97  a b  out  Returns the remainder after dividing a by b. disabled. 
OP_LSHIFT  152  0x98  a b  out  Shifts a left b bits, preserving sign. disabled. 
OP_RSHIFT  153  0x99  a b  out  Shifts a right b bits, preserving sign. disabled. 
OP_BOOLAND  154  0x9a  a b  out  If both a and b are not 0, the output is 1. Otherwise 0. 
OP_BOOLOR  155  0x9b  a b  out  If a or b is not 0, the output is 1. Otherwise 0. 
OP_NUMEQUAL  156  0x9c  a b  out  Returns 1 if the numbers are equal, 0 otherwise. 
OP_NUMEQUALVERIFY  157  0x9d  a b  out  Same as OP_NUMEQUAL, but runs OP_VERIFY afterward. 
OP_NUMNOTEQUAL  158  0x9e  a b  out  Returns 1 if the numbers are not equal, 0 otherwise. 
OP_LESSTHAN  159  0x9f  a b  out  Returns 1 if a is less than b, 0 otherwise. 
OP_GREATERTHAN  160  0xa0  a b  out  Returns 1 if a is greater than b, 0 otherwise. 
OP_LESSTHANOREQUAL  161  0xa1  a b  out  Returns 1 if a is less than or equal to b, 0 otherwise. 
OP_GREATERTHANOREQUAL  162  0xa2  a b  out  Returns 1 if a is greater than or equal to b, 0 otherwise. 
OP_MIN  163  0xa3  a b  out  Returns the smaller of a and b. 
OP_MAX  164  0xa4  a b  out  Returns the larger of a and b. 
OP_WITHIN  165  0xa5  x min max  out  Returns 1 if x is within the specified range (leftinclusive), 0 otherwise. 
Crypto
Word  Opcode  Hex  Input  Output  Description 

OP_RIPEMD160  166  0xa6  in  hash  The input is hashed using RIPEMD160. 
OP_SHA1  167  0xa7  in  hash  The input is hashed using SHA1. 
OP_SHA256  168  0xa8  in  hash  The input is hashed using SHA256. 
OP_HASH160  169  0xa9  in  hash  The input is hashed twice: first with SHA256 and then with RIPEMD160. 
OP_HASH256  170  0xaa  in  hash  The input is hashed two times with SHA256. 
OP_CODESEPARATOR  171  0xab  Nothing  Nothing  All of the signature checking words will only match signatures to the data after the most recentlyexecuted OP_CODESEPARATOR. 
OP_CHECKSIG  172  0xac  sig pubkey  True / false  The entire transaction's outputs, inputs, and script (from the most recentlyexecuted OP_CODESEPARATOR to the end) are hashed. The signature used by OP_CHECKSIG must be a valid signature for this hash and public key. If it is, 1 is returned, 0 otherwise. 
OP_CHECKSIGVERIFY  173  0xad  sig pubkey  True / false  Same as OP_CHECKSIG, but OP_VERIFY is executed afterward. 
OP_CHECKMULTISIG  174  0xae  x sig1 sig2 ... <number of signatures> pub1 pub2 <number of public keys>  True / False  Compares the first signature against each public key until it finds an ECDSA match. Starting with the subsequent public key, it compares the second signature against each remaining public key until it finds an ECDSA match. The process is repeated until all signatures have been checked or not enough public keys remain to produce a successful result. All signatures need to match a public key. Because public keys are not checked again if they fail any signature comparison, signatures must be placed in the scriptSig using the same order as their corresponding public keys were placed in the scriptPubKey or redeemScript. If all signatures are valid, 1 is returned, 0 otherwise. Due to a bug, one extra unused value is removed from the stack. 
OP_CHECKMULTISIGVERIFY  175  0xaf  x sig1 sig2 ... <number of signatures> pub1 pub2 ... <number of public keys>  True / False  Same as OP_CHECKMULTISIG, but OP_VERIFY is executed afterward. 
Pseudowords
These words are used internally for assisting with transaction matching. They are invalid if used in actual scripts.
Word  Opcode  Hex  Description 

OP_PUBKEYHASH  253  0xfd  Represents a public key hashed with OP_HASH160. 
OP_PUBKEY  254  0xfe  Represents a public key compatible with OP_CHECKSIG. 
OP_INVALIDOPCODE  255  0xff  Matches any opcode that is not yet assigned. 
Reserved words
Any opcode not assigned is also reserved. Using an unassigned opcode makes the transaction invalid.
Word  Opcode  Hex  When used... 

OP_RESERVED  80  0x50  Transaction is invalid unless occuring in an unexecuted OP_IF branch 
OP_VER  98  0x62  Transaction is invalid unless occuring in an unexecuted OP_IF branch 
OP_VERIF  101  0x65  Transaction is invalid even when occuring in an unexecuted OP_IF branch 
OP_VERNOTIF  102  0x66  Transaction is invalid even when occuring in an unexecuted OP_IF branch 
OP_RESERVED1  137  0x89  Transaction is invalid unless occuring in an unexecuted OP_IF branch 
OP_RESERVED2  138  0x8a  Transaction is invalid unless occuring in an unexecuted OP_IF branch 
OP_NOP1OP_NOP10  176185  0xb00xb9  The word is ignored. Does not mark transaction as invalid. 
Scripts
This is a list of interesting scripts. Keep in mind that all constants actually use the datapushing commands above. Note that there is a small number of standard script forms that are relayed from node to node; nonstandard scripts are accepted if they are in a block, but nodes will not relay them.
Standard Transaction to Bitcoin address (paytopubkeyhash)
scriptPubKey: OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG scriptSig: <sig> <pubKey>
To demonstrate how scripts look on the wire, here is a raw scriptPubKey:
76 A9 14 OP_DUP OP_HASH160 Bytes to push 89 AB CD EF AB BA AB BA AB BA AB BA AB BA AB BA AB BA AB BA 88 AC Data to push OP_EQUALVERIFY OP_CHECKSIG
Note: scriptSig is in the input of the spending transaction and scriptPubKey is in the output of the previously unspent i.e. "available" transaction.
Here is how each word is processed:
Stack  Script  Description 

Empty.  <sig> <pubKey> OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG  scriptSig and scriptPubKey are combined. 
<sig> <pubKey>  OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG  Constants are added to the stack. 
<sig> <pubKey> <pubKey>  OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG  Top stack item is duplicated. 
<sig> <pubKey> <pubHashA>  <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG  Top stack item is hashed. 
<sig> <pubKey> <pubHashA> <pubKeyHash>  OP_EQUALVERIFY OP_CHECKSIG  Constant added. 
<sig> <pubKey>  OP_CHECKSIG  Equality is checked between the top two stack items. 
true  Empty.  Signature is checked for top two stack items. 
Obsolete paytopubkey transaction
OP_CHECKSIG is used directly without first hashing the public key. This was used by early versions of Bitcoin where people paid directly to IP addresses, before Bitcoin addresses were introduced. scriptPubKeys of this transaction form are still recognized as payments to user by Bitcoin Core. The disadvantage of this transaction form is that the whole public key needs to be known in advance, implying longer payment addresses, and that it provides less protection in the event of a break in the ECDSA signature algorithm.
scriptPubKey: <pubKey> OP_CHECKSIG scriptSig: <sig>
Checking process:
Stack  Script  Description 

Empty.  <sig> <pubKey> OP_CHECKSIG  scriptSig and scriptPubKey are combined. 
<sig> <pubKey>  OP_CHECKSIG  Constants are added to the stack. 
true  Empty.  Signature is checked for top two stack items. 
Provably Unspendable/Prunable Outputs
The standard way to mark a transaction as provably unspendable is with a scriptPubKey of the following form:
scriptPubKey: OP_RETURN {zero or more ops}
OP_RETURN immediately marks the script as invalid, guaranteeing that no scriptSig exists that could possibly spend that output. Thus the output can be immediately pruned from the UTXO set even if it has not been spent. eb31ca1a4cbd97c2770983164d7560d2d03276ae1aee26f12d7c2c6424252f29 is an example: it has a single output of zero value, thus giving the full 0.125BTC fee to the miner who mined the transaction without adding an entry to the UTXO set. You can also use OP_RETURN to add data to a transaction without the data ever appearing in the UTXO set, as seen in 1a2e22a717d626fc5db363582007c46924ae6b28319f07cb1b907776bd8293fc; P2Pool does this with the share chain hash txout in the coinbase of blocks it creates.
AnyoneCanSpend Outputs
Conversely a transaction can be made spendable by anyone at all:
scriptPubKey: (empty) scriptSig: OP_TRUE
With some software changes such transactions can be used as a way to donate funds to miners in addition to transaction fees: any miner who mines such a transaction can also include an additional one after it sending the funds to an address they control. This mechanism may be used in the future for fidelity bonds to sacrifice funds in a provable way.
AnyoneCanSpend outputs are currently considered nonstandard, and are not relayed on the P2P network.
Transaction puzzle
Transaction a4bfa8ab6435ae5f25dae9d89e4eb67dfa94283ca751f393c1ddc5a837bbc31b is an interesting puzzle.
scriptPubKey: OP_HASH256 6fe28c0ab6f1b372c1a6a246ae63f74f931e8365e15a089c68d6190000000000 OP_EQUAL scriptSig:
To spend the transaction you need to come up with some data such that hashing the data twice results in the given hash.
Stack  Script  Description 

Empty.  <data> OP_HASH256 <given_hash> OP_EQUAL  
<data>  OP_HASH256 <given_hash> OP_EQUAL  scriptSig added to the stack. 
<data_hash>  <given_hash> OP_EQUAL  The data is hashed. 
<data_hash> <given_hash>  OP_EQUAL  The given hash is pushed to the stack. 
true  Empty.  The hashes are compared, leaving true on the stack. 
This transaction was successfully spent by 09f691b2263260e71f363d1db51ff3100d285956a40cc0e4f8c8c2c4a80559b1. The required data happened to be the Genesis block, and the given hash was the genesis block hash. Note that while transactions like this are fun, they are not secure, because they do not contain any signatures and thus any transaction attempting to spend them can be replaced with a different transaction sending the funds somewhere else.
See Also
