The first commit might help to avoid reviewing moved code.

Tests are failing for me.

rpc_deriveaddresses.py --usecli       | ✖ Failed  | 2 s
wallet_createwallet.py --usecli       | ✖ Failed  | 2 s
wallet_multiwallet.py --usecli        | ✖ Failed  | 2 s
wallet_watchonly.py --usecli          | ✖ Failed  | 1 s

Failing with test_framework.authproxy.JSONRPCException: Requested wallet does not exist or is not loaded (-18).

@kristapsk: should be fixed now.
What currently is not working are nested commands in conjunction with descriptors. I'll work on a fix soon.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• build: Add -Werror=implicit-fallthrough compile flag #21430 (build: Add -Werror=implicit-fallthrough compile flag by hebasto)
• refactor: remove Optional & nullopt #21415 (refactor: remove Optional & nullopt by fanquake)
• util: Replace use of locale dependent atoi(…) with locale-independent std::from_chars(…) (C++17) #20452 (util: Replace use of locale dependent atoi(…) with locale-independent std::from_chars(…) (C++17) by practicalswift)
• cli/gui: support "@height" in place of blockhash for getblock on client side #16439 (cli/gui: support "@height" in place of blockhash for getblock on client side by ajtowns)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

The command line already lets you do nested calls natively, eg:

$ bitcoin-cli getblock $(bitcoin-cli getblockheader $(bitcoin-cli getbestblockhash) | jq -r .previousblockhash) | jq -r .size

You could use shell aliases to reduce typing:

$ bc () { bitcoin-cli "$@"; }
$ bcfield () { local F="$1"; shift; bitcoin-cli "$@" | jq -r "$F"; }
$ bcfield size getblock $(bcfield previousblockhash getblockheader $(bc getbestblockhash))

Personally, the only time it makes a difference to me is when I want to quickly query blocks around a particular height (eg for a in {100..200}; do bitcoin-cli getblockheader @$a; done | grep versionHex), so the approach in #16439 is quicker and easier (less typing and don't have to deal with brackets). Otherwise I'm pretty much writing a script, and the extra typing/aliases is no big deal.

@ajtowns: Good point. Yes. You can also do it with shell magic. Though you could also argue that writing a shell alias (or short script) could replace bitcoin-cli entirely (use cURL instead). Also for #16439, a shell alias/script would be trivial.

I guess bitcoin-cli is a utility. Something that should allow one to perform a task quick and fast (to not make you use cURl or other low level http/RPC clients).

This PR is not a replacement for #16439. It goes in a similar direction and may allow solve the same issues.

IMO this PR extends the power-commands (nested options plus subelement access) we have in the GUI to the CLI which I think is desirable (also for further extensions).

@ajtowns: Good point. Yes. You can also do it with shell magic. Though you could also argue that writing a shell alias (or short script) could replace bitcoin-cli entirely (use cURL instead). Also for #16439, a shell alias/script would be trivial.

More or less: the disadvantage of a shell alias (eg bitcoin-cli getblockheader $(bcbest)) is that the alias can't inherit -conf, -datadir, -testnet, -signet, or -regtest from the "parent" bitcoin-cli command, while getblockheader(getbestblockhash) or @best will automatically adapt to the right context.

For me, that matters when I'm doing a quick oneliner, but not when I'm doing anything more complicated. YMMV obviously.

More or less: the disadvantage of a shell alias (eg bitcoin-cli getblockheader $(bcbest)) is that the alias can't inherit -conf, -datadir, -testnet, -signet, or -regtest from the "parent" bitcoin-cli command, while getblockheader(getbestblockhash) or @best will automatically adapt to the right context.

Indeed. That would also speak for this PR over shell aliases.

I think #16439 could be integrated into this PR. There could be special commands like @best or @H(400000) that just translates into getbestblockhash() or getblockhash(400000).

Concept ACK

Rebased

Concept ACK

Compiled successfully on Ubuntu 20.04.2 LTS. Tests passed.

I am not sure about the correct syntax for nesting so got error without quotes.

Error:

satoshi@ubuntu:~$ bitcoin-cli getblock(getblock(getbestblockhash())[previousblockhash])[tx][0]

bash: syntax error near unexpected token `('

With quotes:

satoshi@ubuntu:~$ bitcoin-cli "getblock(getblock(getbestblockhash())[previousblockhash])[tx][0]"

c06f7c267bae5c5994cd4584f930f4c4800170c4ecb96ebf6290b6956dc23303

mixed feelings on this, general concept NACK -- had no idea you could do this from the GUI, and I generally think the cli should be for dumb rpc calls, including a mini programming langauge seems to me to be asking for trouble.

OTOH I do really like the concept of being able to atomically make RPC queries.

including a mini programming langauge seems to me to be asking for trouble

Security? Interesting point. Although GUI console has limited access and usage, can we try few things in it? Or enable this only for testnet, regtest etc. and experiment for few weeks/months?

I am not sure about the correct syntax for nesting so got error without quotes.

@prayank23, that's issue of shell, nothing we can do about here, just echo ( in bash will fail too.

that's issue of shell, nothing we can do about here, just echo ( in bash will fail too.

Okay. I was interested in this PR because nesting helps in creating scripts for specific tasks. I have used cmdlets in PowerShell, the pipe symbol is used when one cmdlet pipes a result to another cmdlet. For example,

Get-MsolUser | Set-MsolUser -PasswordNeverExpires $true
Will perform a task (Set one parameter true) on all the results returned in the first cmdlet.

A similar thing in Bitcoin Core can be:

listunspent 100 9999999 | setlabel "100 Conf UTXO"

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