updated todo's

yeah, it'd be nice to not have to output json into jq 👍

From the CLI guidelines:

When needed, commands should offer a --json flag to allow users to easily parse and script the CLI.

It was a conscious decision not to have ad-hoc, non-extensible, unstructured output on stdout in the new CLI. But I realize it's annoying to have to rely on a tool like jq...

Yeah we probably shouldn't add flags for every output option under the sun, the json output is very useful. But since this was the previous behavior of nix-build and came in handy I'd say it's worth it.

Or maybe we could consider having jq style templating build in. Then you can do about anything without a 2nd tool:
--template '.[].outputs.out'
like kubectl has a json-template or go-template output format
kubectl get pods --selector=app=cassandra -o jsonpath='{.items[*].metadata.labels.version}'
kubectl get secret my-secret -o go-template='{{range $k,$v := .data}}{{"### "}}{{$k}}{{"\n"}}{{$v|base64decode}}{{"\n\n"}}{{end}}'

Github cli has a --jq option which doesn't seem to depend on jq being installed because jq isn't in the deps https://archlinux.org/packages/community/x86_64/github-cli/

https://github.com/cli/cli/blob/d794a929da09741f593572c11822b21289cc2f74/pkg/cmdutil/json_flags.go#L90

https://github.com/cli/cli/blob/d794a929da09741f593572c11822b21289cc2f74/pkg/cmdutil/json_flags.go#L140

I don't think adding JSON filtering to nix is necessary. --print-store-paths (or --print-out-paths) and --json should be enough for most purposes.

I agree

Github cli has a --jq option which doesn't seem to depend on jq being installed because jq isn't in the deps

It uses a go library that re-implements jq

github.com/itchyny/gojq

added a release note

any code reviews? 🙂

Not a code review (well, I looked at it but don't have anything to add), but the obligatory bikeshedding over naming: I'd prefer --print-out-paths. I think it's more precise, and fits better with --out-link and drv.outPath. --print-store-paths sounds like it could mean "print all evaluated store paths" or "all fetched store paths" or something else.

“ flag to print the resulting store paths”.

I’m not sure what this means. What is the expectation people can rely on? (This question may also apply to the JSON output.) What will the output contain if combined with remote builders/ eval stores/substitution/symlink generated. What invariant is provided? Eg: Given identical starting stores where one does the build and the other only gets this stdout - what steps can be taken to reach the same Store state, or similar? Does (build + GC ~= substitute with stdout paths)?

The common use case I presume is as a CLI ad-hoc variant of a post-build-hook. What are the semantics compared to this pseudo code:

nix build —option post-build-hook ‘printf “%s\n” $OUT_PATHS’

My intention is not to support or oppose, but to ensure there is consistency so we can apply it in more situations or to understand the distinctions.

@tomberek

> $HOME/nixgits/nix/result/bin/nix build --option builders 'ssh://eu.nixbuild.net' -j0 nixpkgsMaster#bash --print-output-paths
/nix/store/dndi916j6yxzfzzj2sma2llhrlwahq06-bash-5.1-p16

> $HOME/nixgits/nix/result/bin/nix build --option builders 'ssh://eu.nixbuild.net' -j0 /nix/store/dndi916j6yxzfzzj2sma2llhrlwahq06-bash-5.1-p16 --print-output-paths
/nix/store/dndi916j6yxzfzzj2sma2llhrlwahq06-bash-5.1-p16

> $HOME/nixgits/nix/result/bin/nix build --option builders 'ssh://eu.nixbuild.net' -j0 nixpkgsMaster#bash.dev --print-output-paths
/nix/store/lrrv3fzii19il2lsqdcb5d9mia2wxy5r-bash-5.1-p16-dev



> $HOME/nixgits/nix/result/bin/nix build nixpkgsMaster#bash --print-output-paths
/nix/store/dndi916j6yxzfzzj2sma2llhrlwahq06-bash-5.1-p16

> $HOME/nixgits/nix/result/bin/nix build /nix/store/dndi916j6yxzfzzj2sma2llhrlwahq06-bash-5.1-p16 --print-output-paths
/nix/store/dndi916j6yxzfzzj2sma2llhrlwahq06-bash-5.1-p16

> $HOME/nixgits/nix/result/bin/nix build nixpkgsMaster#bash.dev --print-output-paths
/nix/store/lrrv3fzii19il2lsqdcb5d9mia2wxy5r-bash-5.1-p16-dev

there's a difference when the derivation is built for the first time / rebuilt

nix-build -A bash --check
/nix/store/dndi916j6yxzfzzj2sma2llhrlwahq06-bash-5.1-p16

$HOME/nixgits/nix/result/bin/nix build ".#bash" --print-output-paths --rebuild
/nix/store/lrrv3fzii19il2lsqdcb5d9mia2wxy5r-bash-5.1-p16-dev
/nix/store/s6bspl3x0s8h626vc4ikgv1h8sphlm17-bash-5.1-p16-doc
/nix/store/g0n1a3vdx64zkkprfl0y8643320bsb0a-bash-5.1-p16-info
/nix/store/ddgnym0a3nl1ggz50gg8q1cs0arzic87-bash-5.1-p16-man
/nix/store/dndi916j6yxzfzzj2sma2llhrlwahq06-bash-5.1-p16

nix build --json has the same behaviour currently

with nix build when a derivation is built for the first time / rebuilt then all the outputs of it will be symlinked to the current dir

while with nix-build only the main output is symlinked to the current dir

i rebuilt my system with

(self: super: {
  nixUnstable = super.nixUnstable.overrideAttrs (old: {
    src = super.fetchFromGitHub {
      owner = "nixos";
      repo = "nix";
      rev = "abab65f774169c4ebf58823af2a3c327a560deee";
      sha256 = "sha256-kFsJ/WsopJIYKHBpnVGuyXlqdwcrezvvmbT2ycN89r0=";
    };
  });
})

i then ran

nix build "nixpkgs#bash.dev" --print-output-paths --rebuild
/nix/store/c49i1ggnr5cc8gxmk9xm0cn961z104dn-bash-5.1-p12-dev

and

nix build "nixpkgs#bash" --print-output-paths --rebuild
/nix/store/4nmqxajzaf60yjribkgvj5j54x9yvr1r-bash-5.1-p12

both work

Thank you @edolstra !!

I agree with @ncfavier that this should be renamed to --print-out-paths, analogous to --out-link.

Alternatively, maybe --out-link should be renamed to --output-link, since there is no good reason why we're abbreviating "output"...

Alternatively, maybe --out-link should be renamed to --output-link, since there is no good reason why we're abbreviating "output"...

The variable inside builders is called $out not $output though. (Not arguing against rename, just saying.)

print-out-paths sounds good to me

resolved conflict

conflict resolved.

Sorry, but can it be explained why is --print-out-paths not the default behavior? Are there any drawbacks for printing output paths by default for anybody?

Because we don't want the new CLI to be spammy by default. Also, the CLI guideline is to use --json for machine-parseable output rather than ad hoc formats like --print-out-paths.

After years of quickly digging into just-built out paths, using in scripts, etc and seeing no problem with it. And the number of the built out paths is rarely > 5.
It can be really hard to comprehend it, honestly. The only hope is that this will make nix look less "geeky" for newcomers. But the rhetoric question - is nix generally ready to become so user-friendly, that the built out paths information is that much irrelevant to be suppressed? Or a new CLI is going to take credits for a fantastic nix breakthrough 🚀. Thank you for your work.

Is there a way to do this via nix copy as well?