Move command plugin interface to libnixcmd

10 files changed, 1583 insertions, 0 deletions

diff --git a/src/libcmd/command.cc b/src/libcmd/command.cc
new file mode 100644
index 000000000..614dee788
--- /dev/null
+++ b/src/libcmd/command.cc
@@ -0,0 +1,237 @@
+#include "command.hh"
+#include "store-api.hh"
+#include "local-fs-store.hh"
+#include "derivations.hh"
+#include "nixexpr.hh"
+#include "profiles.hh"
+
+#include <nlohmann/json.hpp>
+
+extern char * * environ __attribute__((weak));
+
+namespace nix {
+
+RegisterCommand::Commands * RegisterCommand::commands = nullptr;
+
+nix::Commands RegisterCommand::getCommandsFor(const std::vector<std::string> & prefix)
+{
+    nix::Commands res;
+    for (auto & [name, command] : *RegisterCommand::commands)
+        if (name.size() == prefix.size() + 1) {
+            bool equal = true;
+            for (size_t i = 0; i < prefix.size(); ++i)
+                if (name[i] != prefix[i]) equal = false;
+            if (equal)
+                res.insert_or_assign(name[prefix.size()], command);
+        }
+    return res;
+}
+
+nlohmann::json NixMultiCommand::toJSON()
+{
+    // FIXME: use Command::toJSON() as well.
+    return MultiCommand::toJSON();
+}
+
+StoreCommand::StoreCommand()
+{
+}
+
+ref<Store> StoreCommand::getStore()
+{
+    if (!_store)
+        _store = createStore();
+    return ref<Store>(_store);
+}
+
+ref<Store> StoreCommand::createStore()
+{
+    return openStore();
+}
+
+void StoreCommand::run()
+{
+    run(getStore());
+}
+
+StorePathsCommand::StorePathsCommand(bool recursive)
+    : recursive(recursive)
+{
+    if (recursive)
+        addFlag({
+            .longName = "no-recursive",
+            .description = "Apply operation to specified paths only.",
+            .category = installablesCategory,
+            .handler = {&this->recursive, false},
+        });
+    else
+        addFlag({
+            .longName = "recursive",
+            .shortName = 'r',
+            .description = "Apply operation to closure of the specified paths.",
+            .category = installablesCategory,
+            .handler = {&this->recursive, true},
+        });
+
+    addFlag({
+        .longName = "all",
+        .description = "Apply the operation to every store path.",
+        .category = installablesCategory,
+        .handler = {&all, true},
+    });
+}
+
+void StorePathsCommand::run(ref<Store> store)
+{
+    StorePaths storePaths;
+
+    if (all) {
+        if (installables.size())
+            throw UsageError("'--all' does not expect arguments");
+        for (auto & p : store->queryAllValidPaths())
+            storePaths.push_back(p);
+    }
+
+    else {
+        for (auto & p : toStorePaths(store, realiseMode, operateOn, installables))
+            storePaths.push_back(p);
+
+        if (recursive) {
+            StorePathSet closure;
+            store->computeFSClosure(StorePathSet(storePaths.begin(), storePaths.end()), closure, false, false);
+            storePaths.clear();
+            for (auto & p : closure)
+                storePaths.push_back(p);
+        }
+    }
+
+    run(store, std::move(storePaths));
+}
+
+void StorePathCommand::run(ref<Store> store)
+{
+    auto storePaths = toStorePaths(store, Realise::Nothing, operateOn, installables);
+
+    if (storePaths.size() != 1)
+        throw UsageError("this command requires exactly one store path");
+
+    run(store, *storePaths.begin());
+}
+
+Strings editorFor(const Pos & pos)
+{
+    auto editor = getEnv("EDITOR").value_or("cat");
+    auto args = tokenizeString<Strings>(editor);
+    if (pos.line > 0 && (
+        editor.find("emacs") != std::string::npos ||
+        editor.find("nano") != std::string::npos ||
+        editor.find("vim") != std::string::npos))
+        args.push_back(fmt("+%d", pos.line));
+    args.push_back(pos.file);
+    return args;
+}
+
+MixProfile::MixProfile()
+{
+    addFlag({
+        .longName = "profile",
+        .description = "The profile to update.",
+        .labels = {"path"},
+        .handler = {&profile},
+        .completer = completePath
+    });
+}
+
+void MixProfile::updateProfile(const StorePath & storePath)
+{
+    if (!profile) return;
+    auto store = getStore().dynamic_pointer_cast<LocalFSStore>();
+    if (!store) throw Error("'--profile' is not supported for this Nix store");
+    auto profile2 = absPath(*profile);
+    switchLink(profile2,
+        createGeneration(
+            ref<LocalFSStore>(store),
+            profile2, storePath));
+}
+
+void MixProfile::updateProfile(const Buildables & buildables)
+{
+    if (!profile) return;
+
+    std::vector<StorePath> result;
+
+    for (auto & buildable : buildables) {
+        std::visit(overloaded {
+            [&](BuildableOpaque bo) {
+                result.push_back(bo.path);
+            },
+            [&](BuildableFromDrv bfd) {
+                for (auto & output : bfd.outputs) {
+                    /* Output path should be known because we just tried to
+                       build it. */
+                    assert(output.second);
+                    result.push_back(*output.second);
+                }
+            },
+        }, buildable);
+    }
+
+    if (result.size() != 1)
+        throw Error("'--profile' requires that the arguments produce a single store path, but there are %d", result.size());
+
+    updateProfile(result[0]);
+}
+
+MixDefaultProfile::MixDefaultProfile()
+{
+    profile = getDefaultProfile();
+}
+
+MixEnvironment::MixEnvironment() : ignoreEnvironment(false)
+{
+    addFlag({
+        .longName = "ignore-environment",
+        .shortName = 'i',
+        .description = "Clear the entire environment (except those specified with `--keep`).",
+        .handler = {&ignoreEnvironment, true},
+    });
+
+    addFlag({
+        .longName = "keep",
+        .shortName = 'k',
+        .description = "Keep the environment variable *name*.",
+        .labels = {"name"},
+        .handler = {[&](std::string s) { keep.insert(s); }},
+    });
+
+    addFlag({
+        .longName = "unset",
+        .shortName = 'u',
+        .description = "Unset the environment variable *name*.",
+        .labels = {"name"},
+        .handler = {[&](std::string s) { unset.insert(s); }},
+    });
+}
+
+void MixEnvironment::setEnviron() {
+    if (ignoreEnvironment) {
+        if (!unset.empty())
+            throw UsageError("--unset does not make sense with --ignore-environment");
+
+        for (const auto & var : keep) {
+            auto val = getenv(var.c_str());
+            if (val) stringsEnv.emplace_back(fmt("%s=%s", var.c_str(), val));
+        }
+
+        vectorEnv = stringsToCharPtrs(stringsEnv);
+        environ = vectorEnv.data();
+    } else {
+        if (!keep.empty())
+            throw UsageError("--keep does not make sense without --ignore-environment");
+
+        for (const auto & var : unset)
+            unsetenv(var.c_str());
+    }
+}
+
+}
diff --git a/src/libcmd/command.hh b/src/libcmd/command.hh
new file mode 100644
index 000000000..ed6980075
--- /dev/null
+++ b/src/libcmd/command.hh
@@ -0,0 +1,274 @@
+#pragma once
+
+#include "installables.hh"
+#include "args.hh"
+#include "common-eval-args.hh"
+#include "path.hh"
+#include "flake/lockfile.hh"
+#include "store-api.hh"
+
+#include <optional>
+
+namespace nix {
+
+extern std::string programPath;
+
+extern char * * savedArgv;
+
+class EvalState;
+struct Pos;
+class Store;
+
+static constexpr Command::Category catSecondary = 100;
+static constexpr Command::Category catUtility = 101;
+static constexpr Command::Category catNixInstallation = 102;
+
+static constexpr auto installablesCategory = "Options that change the interpretation of installables";
+
+struct NixMultiCommand : virtual MultiCommand, virtual Command
+{
+    nlohmann::json toJSON() override;
+};
+
+/* A command that requires a Nix store. */
+struct StoreCommand : virtual Command
+{
+    StoreCommand();
+    void run() override;
+    ref<Store> getStore();
+    virtual ref<Store> createStore();
+    virtual void run(ref<Store>) = 0;
+
+private:
+    std::shared_ptr<Store> _store;
+};
+
+struct EvalCommand : virtual StoreCommand, MixEvalArgs
+{
+    ref<EvalState> getEvalState();
+
+    std::shared_ptr<EvalState> evalState;
+};
+
+struct MixFlakeOptions : virtual Args, EvalCommand
+{
+    flake::LockFlags lockFlags;
+
+    MixFlakeOptions();
+
+    virtual std::optional<FlakeRef> getFlakeRefForCompletion()
+    { return {}; }
+};
+
+/* How to handle derivations in commands that operate on store paths. */
+enum class OperateOn {
+    /* Operate on the output path. */
+    Output,
+    /* Operate on the .drv path. */
+    Derivation
+};
+
+struct SourceExprCommand : virtual Args, MixFlakeOptions
+{
+    std::optional<Path> file;
+    std::optional<std::string> expr;
+
+    // FIXME: move this; not all commands (e.g. 'nix run') use it.
+    OperateOn operateOn = OperateOn::Output;
+
+    SourceExprCommand();
+
+    std::vector<std::shared_ptr<Installable>> parseInstallables(
+        ref<Store> store, std::vector<std::string> ss);
+
+    std::shared_ptr<Installable> parseInstallable(
+        ref<Store> store, const std::string & installable);
+
+    virtual Strings getDefaultFlakeAttrPaths();
+
+    virtual Strings getDefaultFlakeAttrPathPrefixes();
+
+    void completeInstallable(std::string_view prefix);
+};
+
+enum class Realise {
+    /* Build the derivation. Postcondition: the
+       derivation outputs exist. */
+    Outputs,
+    /* Don't build the derivation. Postcondition: the store derivation
+       exists. */
+    Derivation,
+    /* Evaluate in dry-run mode. Postcondition: nothing. */
+    Nothing
+};
+
+/* A command that operates on a list of "installables", which can be
+   store paths, attribute paths, Nix expressions, etc. */
+struct InstallablesCommand : virtual Args, SourceExprCommand
+{
+    std::vector<std::shared_ptr<Installable>> installables;
+
+    InstallablesCommand();
+
+    void prepare() override;
+
+    virtual bool useDefaultInstallables() { return true; }
+
+    std::optional<FlakeRef> getFlakeRefForCompletion() override;
+
+private:
+
+    std::vector<std::string> _installables;
+};
+
+/* A command that operates on exactly one "installable" */
+struct InstallableCommand : virtual Args, SourceExprCommand
+{
+    std::shared_ptr<Installable> installable;
+
+    InstallableCommand();
+
+    void prepare() override;
+
+    std::optional<FlakeRef> getFlakeRefForCompletion() override
+    {
+        return parseFlakeRef(_installable, absPath("."));
+    }
+
+private:
+
+    std::string _installable{"."};
+};
+
+/* A command that operates on zero or more store paths. */
+struct StorePathsCommand : public InstallablesCommand
+{
+private:
+
+    bool recursive = false;
+    bool all = false;
+
+protected:
+
+    Realise realiseMode = Realise::Derivation;
+
+public:
+
+    StorePathsCommand(bool recursive = false);
+
+    using StoreCommand::run;
+
+    virtual void run(ref<Store> store, std::vector<StorePath> storePaths) = 0;
+
+    void run(ref<Store> store) override;
+
+    bool useDefaultInstallables() override { return !all; }
+};
+
+/* A command that operates on exactly one store path. */
+struct StorePathCommand : public InstallablesCommand
+{
+    using StoreCommand::run;
+
+    virtual void run(ref<Store> store, const StorePath & storePath) = 0;
+
+    void run(ref<Store> store) override;
+};
+
+/* A helper class for registering commands globally. */
+struct RegisterCommand
+{
+    typedef std::map<std::vector<std::string>, std::function<ref<Command>()>> Commands;
+    static Commands * commands;
+
+    RegisterCommand(std::vector<std::string> && name,
+        std::function<ref<Command>()> command)
+    {
+        if (!commands) commands = new Commands;
+        commands->emplace(name, command);
+    }
+
+    static nix::Commands getCommandsFor(const std::vector<std::string> & prefix);
+};
+
+template<class T>
+static RegisterCommand registerCommand(const std::string & name)
+{
+    return RegisterCommand({name}, [](){ return make_ref<T>(); });
+}
+
+template<class T>
+static RegisterCommand registerCommand2(std::vector<std::string> && name)
+{
+    return RegisterCommand(std::move(name), [](){ return make_ref<T>(); });
+}
+
+Buildables build(ref<Store> store, Realise mode,
+    std::vector<std::shared_ptr<Installable>> installables, BuildMode bMode = bmNormal);
+
+std::set<StorePath> toStorePaths(ref<Store> store,
+    Realise mode, OperateOn operateOn,
+    std::vector<std::shared_ptr<Installable>> installables);
+
+StorePath toStorePath(ref<Store> store,
+    Realise mode, OperateOn operateOn,
+    std::shared_ptr<Installable> installable);
+
+std::set<StorePath> toDerivations(ref<Store> store,
+    std::vector<std::shared_ptr<Installable>> installables,
+    bool useDeriver = false);
+
+/* Helper function to generate args that invoke $EDITOR on
+   filename:lineno. */
+Strings editorFor(const Pos & pos);
+
+struct MixProfile : virtual StoreCommand
+{
+    std::optional<Path> profile;
+
+    MixProfile();
+
+    /* If 'profile' is set, make it point at 'storePath'. */
+    void updateProfile(const StorePath & storePath);
+
+    /* If 'profile' is set, make it point at the store path produced
+       by 'buildables'. */
+    void updateProfile(const Buildables & buildables);
+};
+
+struct MixDefaultProfile : MixProfile
+{
+    MixDefaultProfile();
+};
+
+struct MixEnvironment : virtual Args {
+
+    StringSet keep, unset;
+    Strings stringsEnv;
+    std::vector<char*> vectorEnv;
+    bool ignoreEnvironment;
+
+    MixEnvironment();
+
+    /* Modify global environ based on ignoreEnvironment, keep, and unset. It's expected that exec will be called before this class goes out of scope, otherwise environ will become invalid. */
+    void setEnviron();
+};
+
+void completeFlakeRef(ref<Store> store, std::string_view prefix);
+
+void completeFlakeRefWithFragment(
+    ref<EvalState> evalState,
+    flake::LockFlags lockFlags,
+    Strings attrPathPrefixes,
+    const Strings & defaultFlakeAttrPaths,
+    std::string_view prefix);
+
+std::string showVersions(const std::set<std::string> & versions);
+
+void printClosureDiff(
+    ref<Store> store,
+    const StorePath & beforePath,
+    const StorePath & afterPath,
+    std::string_view indent);
+
+}
diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
new file mode 100644
index 000000000..4e6bf4a9a
--- /dev/null
+++ b/src/libcmd/installables.cc
@@ -0,0 +1,824 @@
+#include "installables.hh"
+#include "command.hh"
+#include "attr-path.hh"
+#include "common-eval-args.hh"
+#include "derivations.hh"
+#include "eval-inline.hh"
+#include "eval.hh"
+#include "get-drvs.hh"
+#include "store-api.hh"
+#include "shared.hh"
+#include "flake/flake.hh"
+#include "eval-cache.hh"
+#include "url.hh"
+#include "registry.hh"
+
+#include <regex>
+#include <queue>
+
+#include <nlohmann/json.hpp>
+
+namespace nix {
+
+nlohmann::json BuildableOpaque::toJSON(ref<Store> store) const {
+    nlohmann::json res;
+    res["path"] = store->printStorePath(path);
+    return res;
+}
+
+nlohmann::json BuildableFromDrv::toJSON(ref<Store> store) const {
+    nlohmann::json res;
+    res["drvPath"] = store->printStorePath(drvPath);
+    for (const auto& [output, path] : outputs) {
+        res["outputs"][output] = path ? store->printStorePath(*path) : "";
+    }
+    return res;
+}
+
+nlohmann::json buildablesToJSON(const Buildables & buildables, ref<Store> store) {
+    auto res = nlohmann::json::array();
+    for (const Buildable & buildable : buildables) {
+        std::visit([&res, store](const auto & buildable) {
+            res.push_back(buildable.toJSON(store));
+        }, buildable);
+    }
+    return res;
+}
+
+void completeFlakeInputPath(
+    ref<EvalState> evalState,
+    const FlakeRef & flakeRef,
+    std::string_view prefix)
+{
+    auto flake = flake::getFlake(*evalState, flakeRef, true);
+    for (auto & input : flake.inputs)
+        if (hasPrefix(input.first, prefix))
+            completions->add(input.first);
+}
+
+MixFlakeOptions::MixFlakeOptions()
+{
+    auto category = "Common flake-related options";
+
+    addFlag({
+        .longName = "recreate-lock-file",
+        .description = "Recreate the flake's lock file from scratch.",
+        .category = category,
+        .handler = {&lockFlags.recreateLockFile, true}
+    });
+
+    addFlag({
+        .longName = "no-update-lock-file",
+        .description = "Do not allow any updates to the flake's lock file.",
+        .category = category,
+        .handler = {&lockFlags.updateLockFile, false}
+    });
+
+    addFlag({
+        .longName = "no-write-lock-file",
+        .description = "Do not write the flake's newly generated lock file.",
+        .category = category,
+        .handler = {&lockFlags.writeLockFile, false}
+    });
+
+    addFlag({
+        .longName = "no-registries",
+        .description = "Don't allow lookups in the flake registries.",
+        .category = category,
+        .handler = {&lockFlags.useRegistries, false}
+    });
+
+    addFlag({
+        .longName = "commit-lock-file",
+        .description = "Commit changes to the flake's lock file.",
+        .category = category,
+        .handler = {&lockFlags.commitLockFile, true}
+    });
+
+    addFlag({
+        .longName = "update-input",
+        .description = "Update a specific flake input (ignoring its previous entry in the lock file).",
+        .category = category,
+        .labels = {"input-path"},
+        .handler = {[&](std::string s) {
+            lockFlags.inputUpdates.insert(flake::parseInputPath(s));
+        }},
+        .completer = {[&](size_t, std::string_view prefix) {
+            if (auto flakeRef = getFlakeRefForCompletion())
+                completeFlakeInputPath(getEvalState(), *flakeRef, prefix);
+        }}
+    });
+
+    addFlag({
+        .longName = "override-input",
+        .description = "Override a specific flake input (e.g. `dwarffs/nixpkgs`).",
+        .category = category,
+        .labels = {"input-path", "flake-url"},
+        .handler = {[&](std::string inputPath, std::string flakeRef) {
+            lockFlags.inputOverrides.insert_or_assign(
+                flake::parseInputPath(inputPath),
+                parseFlakeRef(flakeRef, absPath(".")));
+        }}
+    });
+
+    addFlag({
+        .longName = "inputs-from",
+        .description = "Use the inputs of the specified flake as registry entries.",
+        .category = category,
+        .labels = {"flake-url"},
+        .handler = {[&](std::string flakeRef) {
+            auto evalState = getEvalState();
+            auto flake = flake::lockFlake(
+                *evalState,
+                parseFlakeRef(flakeRef, absPath(".")),
+                { .writeLockFile = false });
+            for (auto & [inputName, input] : flake.lockFile.root->inputs) {
+                auto input2 = flake.lockFile.findInput({inputName}); // resolve 'follows' nodes
+                if (auto input3 = std::dynamic_pointer_cast<const flake::LockedNode>(input2)) {
+                    overrideRegistry(
+                        fetchers::Input::fromAttrs({{"type","indirect"}, {"id", inputName}}),
+                        input3->lockedRef.input,
+                        {});
+                }
+            }
+        }},
+        .completer = {[&](size_t, std::string_view prefix) {
+            completeFlakeRef(getEvalState()->store, prefix);
+        }}
+    });
+}
+
+SourceExprCommand::SourceExprCommand()
+{
+    addFlag({
+        .longName = "file",
+        .shortName = 'f',
+        .description = "Interpret installables as attribute paths relative to the Nix expression stored in *file*.",
+        .category = installablesCategory,
+        .labels = {"file"},
+        .handler = {&file},
+        .completer = completePath
+    });
+
+    addFlag({
+        .longName = "expr",
+        .description = "Interpret installables as attribute paths relative to the Nix expression *expr*.",
+        .category = installablesCategory,
+        .labels = {"expr"},
+        .handler = {&expr}
+    });
+
+    addFlag({
+        .longName = "derivation",
+        .description = "Operate on the store derivation rather than its outputs.",
+        .category = installablesCategory,
+        .handler = {&operateOn, OperateOn::Derivation},
+    });
+}
+
+Strings SourceExprCommand::getDefaultFlakeAttrPaths()
+{
+    return {"defaultPackage." + settings.thisSystem.get()};
+}
+
+Strings SourceExprCommand::getDefaultFlakeAttrPathPrefixes()
+{
+    return {
+        // As a convenience, look for the attribute in
+        // 'outputs.packages'.
+        "packages." + settings.thisSystem.get() + ".",
+        // As a temporary hack until Nixpkgs is properly converted
+        // to provide a clean 'packages' set, look in 'legacyPackages'.
+        "legacyPackages." + settings.thisSystem.get() + "."
+    };
+}
+
+void SourceExprCommand::completeInstallable(std::string_view prefix)
+{
+    if (file) return; // FIXME
+
+    completeFlakeRefWithFragment(
+        getEvalState(),
+        lockFlags,
+        getDefaultFlakeAttrPathPrefixes(),
+        getDefaultFlakeAttrPaths(),
+        prefix);
+}
+
+void completeFlakeRefWithFragment(
+    ref<EvalState> evalState,
+    flake::LockFlags lockFlags,
+    Strings attrPathPrefixes,
+    const Strings & defaultFlakeAttrPaths,
+    std::string_view prefix)
+{
+    /* Look for flake output attributes that match the
+       prefix. */
+    try {
+        auto hash = prefix.find('#');
+        if (hash != std::string::npos) {
+            auto fragment = prefix.substr(hash + 1);
+            auto flakeRefS = std::string(prefix.substr(0, hash));
+            // FIXME: do tilde expansion.
+            auto flakeRef = parseFlakeRef(flakeRefS, absPath("."));
+
+            auto evalCache = openEvalCache(*evalState,
+                std::make_shared<flake::LockedFlake>(lockFlake(*evalState, flakeRef, lockFlags)));
+
+            auto root = evalCache->getRoot();
+
+            /* Complete 'fragment' relative to all the
+               attrpath prefixes as well as the root of the
+               flake. */
+            attrPathPrefixes.push_back("");
+
+            for (auto & attrPathPrefixS : attrPathPrefixes) {
+                auto attrPathPrefix = parseAttrPath(*evalState, attrPathPrefixS);
+                auto attrPathS = attrPathPrefixS + std::string(fragment);
+                auto attrPath = parseAttrPath(*evalState, attrPathS);
+
+                std::string lastAttr;
+                if (!attrPath.empty() && !hasSuffix(attrPathS, ".")) {
+                    lastAttr = attrPath.back();
+                    attrPath.pop_back();
+                }
+
+                auto attr = root->findAlongAttrPath(attrPath);
+                if (!attr) continue;
+
+                for (auto & attr2 : attr->getAttrs()) {
+                    if (hasPrefix(attr2, lastAttr)) {
+                        auto attrPath2 = attr->getAttrPath(attr2);
+                        /* Strip the attrpath prefix. */
+                        attrPath2.erase(attrPath2.begin(), attrPath2.begin() + attrPathPrefix.size());
+                        completions->add(flakeRefS + "#" + concatStringsSep(".", attrPath2));
+                    }
+                }
+            }
+
+            /* And add an empty completion for the default
+               attrpaths. */
+            if (fragment.empty()) {
+                for (auto & attrPath : defaultFlakeAttrPaths) {
+                    auto attr = root->findAlongAttrPath(parseAttrPath(*evalState, attrPath));
+                    if (!attr) continue;
+                    completions->add(flakeRefS + "#");
+                }
+            }
+        }
+    } catch (Error & e) {
+        warn(e.msg());
+    }
+
+    completeFlakeRef(evalState->store, prefix);
+}
+
+ref<EvalState> EvalCommand::getEvalState()
+{
+    if (!evalState)
+        evalState = std::make_shared<EvalState>(searchPath, getStore());
+    return ref<EvalState>(evalState);
+}
+
+void completeFlakeRef(ref<Store> store, std::string_view prefix)
+{
+    if (prefix == "")
+        completions->add(".");
+
+    completeDir(0, prefix);
+
+    /* Look for registry entries that match the prefix. */
+    for (auto & registry : fetchers::getRegistries(store)) {
+        for (auto & entry : registry->entries) {
+            auto from = entry.from.to_string();
+            if (!hasPrefix(prefix, "flake:") && hasPrefix(from, "flake:")) {
+                std::string from2(from, 6);
+                if (hasPrefix(from2, prefix))
+                    completions->add(from2);
+            } else {
+                if (hasPrefix(from, prefix))
+                    completions->add(from);
+            }
+        }
+    }
+}
+
+Buildable Installable::toBuildable()
+{
+    auto buildables = toBuildables();
+    if (buildables.size() != 1)
+        throw Error("installable '%s' evaluates to %d derivations, where only one is expected", what(), buildables.size());
+    return std::move(buildables[0]);
+}
+
+std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>>
+Installable::getCursors(EvalState & state)
+{
+    auto evalCache =
+        std::make_shared<nix::eval_cache::EvalCache>(std::nullopt, state,
+            [&]() { return toValue(state).first; });
+    return {{evalCache->getRoot(), ""}};
+}
+
+std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>
+Installable::getCursor(EvalState & state)
+{
+    auto cursors = getCursors(state);
+    if (cursors.empty())
+        throw Error("cannot find flake attribute '%s'", what());
+    return cursors[0];
+}
+
+struct InstallableStorePath : Installable
+{
+    ref<Store> store;
+    StorePath storePath;
+
+    InstallableStorePath(ref<Store> store, StorePath && storePath)
+        : store(store), storePath(std::move(storePath)) { }
+
+    std::string what() override { return store->printStorePath(storePath); }
+
+    Buildables toBuildables() override
+    {
+        if (storePath.isDerivation()) {
+            std::map<std::string, std::optional<StorePath>> outputs;
+            auto drv = store->readDerivation(storePath);
+            for (auto & [name, output] : drv.outputsAndOptPaths(*store))
+                outputs.emplace(name, output.second);
+            return {
+                BuildableFromDrv {
+                    .drvPath = storePath,
+                    .outputs = std::move(outputs)
+                }
+            };
+        } else {
+            return {
+                BuildableOpaque {
+                    .path = storePath,
+                }
+            };
+        }
+    }
+
+    std::optional<StorePath> getStorePath() override
+    {
+        return storePath;
+    }
+};
+
+Buildables InstallableValue::toBuildables()
+{
+    Buildables res;
+
+    std::map<StorePath, std::map<std::string, std::optional<StorePath>>> drvsToOutputs;
+
+    // Group by derivation, helps with .all in particular
+    for (auto & drv : toDerivations()) {
+        auto outputName = drv.outputName;
+        if (outputName == "")
+            throw Error("derivation '%s' lacks an 'outputName' attribute", state->store->printStorePath(drv.drvPath));
+        drvsToOutputs[drv.drvPath].insert_or_assign(outputName, drv.outPath);
+    }
+
+    for (auto & i : drvsToOutputs)
+        res.push_back(BuildableFromDrv { i.first, i.second });
+
+    return res;
+}
+
+struct InstallableAttrPath : InstallableValue
+{
+    SourceExprCommand & cmd;
+    RootValue v;
+    std::string attrPath;
+
+    InstallableAttrPath(ref<EvalState> state, SourceExprCommand & cmd, Value * v, const std::string & attrPath)
+        : InstallableValue(state), cmd(cmd), v(allocRootValue(v)), attrPath(attrPath)
+    { }
+
+    std::string what() override { return attrPath; }
+
+    std::pair<Value *, Pos> toValue(EvalState & state) override
+    {
+        auto [vRes, pos] = findAlongAttrPath(state, attrPath, *cmd.getAutoArgs(state), **v);
+        state.forceValue(*vRes);
+        return {vRes, pos};
+    }
+
+    virtual std::vector<InstallableValue::DerivationInfo> toDerivations() override;
+};
+
+std::vector<InstallableValue::DerivationInfo> InstallableAttrPath::toDerivations()
+{
+    auto v = toValue(*state).first;
+
+    Bindings & autoArgs = *cmd.getAutoArgs(*state);
+
+    DrvInfos drvInfos;
+    getDerivations(*state, *v, "", autoArgs, drvInfos, false);
+
+    std::vector<DerivationInfo> res;
+    for (auto & drvInfo : drvInfos) {
+        res.push_back({
+            state->store->parseStorePath(drvInfo.queryDrvPath()),
+            state->store->maybeParseStorePath(drvInfo.queryOutPath()),
+            drvInfo.queryOutputName()
+        });
+    }
+
+    return res;
+}
+
+std::vector<std::string> InstallableFlake::getActualAttrPaths()
+{
+    std::vector<std::string> res;
+
+    for (auto & prefix : prefixes)
+        res.push_back(prefix + *attrPaths.begin());
+
+    for (auto & s : attrPaths)
+        res.push_back(s);
+
+    return res;
+}
+
+Value * InstallableFlake::getFlakeOutputs(EvalState & state, const flake::LockedFlake & lockedFlake)
+{
+    auto vFlake = state.allocValue();
+
+    callFlake(state, lockedFlake, *vFlake);
+
+    auto aOutputs = vFlake->attrs->get(state.symbols.create("outputs"));
+    assert(aOutputs);
+
+    state.forceValue(*aOutputs->value);
+
+    return aOutputs->value;
+}
+
+ref<eval_cache::EvalCache> openEvalCache(
+    EvalState & state,
+    std::shared_ptr<flake::LockedFlake> lockedFlake)
+{
+    auto fingerprint = lockedFlake->getFingerprint();
+    return make_ref<nix::eval_cache::EvalCache>(
+        evalSettings.useEvalCache && evalSettings.pureEval
+            ? std::optional { std::cref(fingerprint) }
+            : std::nullopt,
+        state,
+        [&state, lockedFlake]()
+        {
+            /* For testing whether the evaluation cache is
+               complete. */
+            if (getEnv("NIX_ALLOW_EVAL").value_or("1") == "0")
+                throw Error("not everything is cached, but evaluation is not allowed");
+
+            auto vFlake = state.allocValue();
+            flake::callFlake(state, *lockedFlake, *vFlake);
+
+            state.forceAttrs(*vFlake);
+
+            auto aOutputs = vFlake->attrs->get(state.symbols.create("outputs"));
+            assert(aOutputs);
+
+            return aOutputs->value;
+        });
+}
+
+static std::string showAttrPaths(const std::vector<std::string> & paths)
+{
+    std::string s;
+    for (const auto & [n, i] : enumerate(paths)) {
+        if (n > 0) s += n + 1 == paths.size() ? " or " : ", ";
+        s += '\''; s += i; s += '\'';
+    }
+    return s;
+}
+
+std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableFlake::toDerivation()
+{
+    auto lockedFlake = getLockedFlake();
+
+    auto cache = openEvalCache(*state, lockedFlake);
+    auto root = cache->getRoot();
+
+    for (auto & attrPath : getActualAttrPaths()) {
+        auto attr = root->findAlongAttrPath(parseAttrPath(*state, attrPath));
+        if (!attr) continue;
+
+        if (!attr->isDerivation())
+            throw Error("flake output attribute '%s' is not a derivation", attrPath);
+
+        auto drvPath = attr->forceDerivation();
+
+        auto drvInfo = DerivationInfo{
+            std::move(drvPath),
+            state->store->maybeParseStorePath(attr->getAttr(state->sOutPath)->getString()),
+            attr->getAttr(state->sOutputName)->getString()
+        };
+
+        return {attrPath, lockedFlake->flake.lockedRef, std::move(drvInfo)};
+    }
+
+    throw Error("flake '%s' does not provide attribute %s",
+        flakeRef, showAttrPaths(getActualAttrPaths()));
+}
+
+std::vector<InstallableValue::DerivationInfo> InstallableFlake::toDerivations()
+{
+    std::vector<DerivationInfo> res;
+    res.push_back(std::get<2>(toDerivation()));
+    return res;
+}
+
+std::pair<Value *, Pos> InstallableFlake::toValue(EvalState & state)
+{
+    auto lockedFlake = getLockedFlake();
+
+    auto vOutputs = getFlakeOutputs(state, *lockedFlake);
+
+    auto emptyArgs = state.allocBindings(0);
+
+    for (auto & attrPath : getActualAttrPaths()) {
+        try {
+            auto [v, pos] = findAlongAttrPath(state, attrPath, *emptyArgs, *vOutputs);
+            state.forceValue(*v);
+            return {v, pos};
+        } catch (AttrPathNotFound & e) {
+        }
+    }
+
+    throw Error("flake '%s' does not provide attribute %s",
+        flakeRef, showAttrPaths(getActualAttrPaths()));
+}
+
+std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>>
+InstallableFlake::getCursors(EvalState & state)
+{
+    auto evalCache = openEvalCache(state,
+        std::make_shared<flake::LockedFlake>(lockFlake(state, flakeRef, lockFlags)));
+
+    auto root = evalCache->getRoot();
+
+    std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>> res;
+
+    for (auto & attrPath : getActualAttrPaths()) {
+        auto attr = root->findAlongAttrPath(parseAttrPath(state, attrPath));
+        if (attr) res.push_back({attr, attrPath});
+    }
+
+    return res;
+}
+
+std::shared_ptr<flake::LockedFlake> InstallableFlake::getLockedFlake() const
+{
+    if (!_lockedFlake) {
+        _lockedFlake = std::make_shared<flake::LockedFlake>(lockFlake(*state, flakeRef, lockFlags));
+        _lockedFlake->flake.config.apply();
+        // FIXME: send new config to the daemon.
+    }
+    return _lockedFlake;
+}
+
+FlakeRef InstallableFlake::nixpkgsFlakeRef() const
+{
+    auto lockedFlake = getLockedFlake();
+
+    if (auto nixpkgsInput = lockedFlake->lockFile.findInput({"nixpkgs"})) {
+        if (auto lockedNode = std::dynamic_pointer_cast<const flake::LockedNode>(nixpkgsInput)) {
+            debug("using nixpkgs flake '%s'", lockedNode->lockedRef);
+            return std::move(lockedNode->lockedRef);
+        }
+    }
+
+    return Installable::nixpkgsFlakeRef();
+}
+
+std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
+    ref<Store> store, std::vector<std::string> ss)
+{
+    std::vector<std::shared_ptr<Installable>> result;
+
+    if (file || expr) {
+        if (file && expr)
+            throw UsageError("'--file' and '--expr' are exclusive");
+
+        // FIXME: backward compatibility hack
+        if (file) evalSettings.pureEval = false;
+
+        auto state = getEvalState();
+        auto vFile = state->allocValue();
+
+        if (file)
+            state->evalFile(lookupFileArg(*state, *file), *vFile);
+        else {
+            auto e = state->parseExprFromString(*expr, absPath("."));
+            state->eval(e, *vFile);
+        }
+
+        for (auto & s : ss)
+            result.push_back(std::make_shared<InstallableAttrPath>(state, *this, vFile, s == "." ? "" : s));
+
+    } else {
+
+        for (auto & s : ss) {
+            std::exception_ptr ex;
+
+            try {
+                auto [flakeRef, fragment] = parseFlakeRefWithFragment(s, absPath("."));
+                result.push_back(std::make_shared<InstallableFlake>(
+                        getEvalState(), std::move(flakeRef),
+                        fragment == "" ? getDefaultFlakeAttrPaths() : Strings{fragment},
+                        getDefaultFlakeAttrPathPrefixes(), lockFlags));
+                continue;
+            } catch (...) {
+                ex = std::current_exception();
+            }
+
+            if (s.find('/') != std::string::npos) {
+                try {
+                    result.push_back(std::make_shared<InstallableStorePath>(store, store->followLinksToStorePath(s)));
+                    continue;
+                } catch (BadStorePath &) {
+                } catch (...) {
+                    if (!ex)
+                        ex = std::current_exception();
+                }
+            }
+
+            std::rethrow_exception(ex);
+
+            /*
+            throw Error(
+                pathExists(s)
+                ? "path '%s' is not a flake or a store path"
+                : "don't know how to handle argument '%s'", s);
+            */
+        }
+    }
+
+    return result;
+}
+
+std::shared_ptr<Installable> SourceExprCommand::parseInstallable(
+    ref<Store> store, const std::string & installable)
+{
+    auto installables = parseInstallables(store, {installable});
+    assert(installables.size() == 1);
+    return installables.front();
+}
+
+Buildables build(ref<Store> store, Realise mode,
+    std::vector<std::shared_ptr<Installable>> installables, BuildMode bMode)
+{
+    if (mode == Realise::Nothing)
+        settings.readOnlyMode = true;
+
+    Buildables buildables;
+
+    std::vector<StorePathWithOutputs> pathsToBuild;
+
+    for (auto & i : installables) {
+        for (auto & b : i->toBuildables()) {
+            std::visit(overloaded {
+                [&](BuildableOpaque bo) {
+                    pathsToBuild.push_back({bo.path});
+                },
+                [&](BuildableFromDrv bfd) {
+                    StringSet outputNames;
+                    for (auto & output : bfd.outputs)
+                        outputNames.insert(output.first);
+                    pathsToBuild.push_back({bfd.drvPath, outputNames});
+                },
+            }, b);
+            buildables.push_back(std::move(b));
+        }
+    }
+
+    if (mode == Realise::Nothing)
+        printMissing(store, pathsToBuild, lvlError);
+    else if (mode == Realise::Outputs)
+        store->buildPaths(pathsToBuild, bMode);
+
+    return buildables;
+}
+
+StorePathSet toStorePaths(ref<Store> store,
+    Realise mode, OperateOn operateOn,
+    std::vector<std::shared_ptr<Installable>> installables)
+{
+    StorePathSet outPaths;
+
+    if (operateOn == OperateOn::Output) {
+        for (auto & b : build(store, mode, installables))
+            std::visit(overloaded {
+                [&](BuildableOpaque bo) {
+                    outPaths.insert(bo.path);
+                },
+                [&](BuildableFromDrv bfd) {
+                    for (auto & output : bfd.outputs) {
+                        if (!output.second)
+                            throw Error("Cannot operate on output of unbuilt CA drv");
+                        outPaths.insert(*output.second);
+                    }
+                },
+            }, b);
+    } else {
+        if (mode == Realise::Nothing)
+            settings.readOnlyMode = true;
+
+        for (auto & i : installables)
+            for (auto & b : i->toBuildables())
+                if (auto bfd = std::get_if<BuildableFromDrv>(&b))
+                    outPaths.insert(bfd->drvPath);
+    }
+
+    return outPaths;
+}
+
+StorePath toStorePath(ref<Store> store,
+    Realise mode, OperateOn operateOn,
+    std::shared_ptr<Installable> installable)
+{
+    auto paths = toStorePaths(store, mode, operateOn, {installable});
+
+    if (paths.size() != 1)
+        throw Error("argument '%s' should evaluate to one store path", installable->what());
+
+    return *paths.begin();
+}
+
+StorePathSet toDerivations(ref<Store> store,
+    std::vector<std::shared_ptr<Installable>> installables, bool useDeriver)
+{
+    StorePathSet drvPaths;
+
+    for (auto & i : installables)
+        for (auto & b : i->toBuildables())
+            std::visit(overloaded {
+                [&](BuildableOpaque bo) {
+                    if (!useDeriver)
+                        throw Error("argument '%s' did not evaluate to a derivation", i->what());
+                    auto derivers = store->queryValidDerivers(bo.path);
+                    if (derivers.empty())
+                        throw Error("'%s' does not have a known deriver", i->what());
+                    // FIXME: use all derivers?
+                    drvPaths.insert(*derivers.begin());
+                },
+                [&](BuildableFromDrv bfd) {
+                    drvPaths.insert(bfd.drvPath);
+                },
+            }, b);
+
+    return drvPaths;
+}
+
+InstallablesCommand::InstallablesCommand()
+{
+    expectArgs({
+        .label = "installables",
+        .handler = {&_installables},
+        .completer = {[&](size_t, std::string_view prefix) {
+            completeInstallable(prefix);
+        }}
+    });
+}
+
+void InstallablesCommand::prepare()
+{
+    if (_installables.empty() && useDefaultInstallables())
+        // FIXME: commands like "nix install" should not have a
+        // default, probably.
+        _installables.push_back(".");
+    installables = parseInstallables(getStore(), _installables);
+}
+
+std::optional<FlakeRef> InstallablesCommand::getFlakeRefForCompletion()
+{
+    if (_installables.empty()) {
+        if (useDefaultInstallables())
+            return parseFlakeRef(".", absPath("."));
+        return {};
+    }
+    return parseFlakeRef(_installables.front(), absPath("."));
+}
+
+InstallableCommand::InstallableCommand()
+{
+    expectArgs({
+        .label = "installable",
+        .optional = true,
+        .handler = {&_installable},
+        .completer = {[&](size_t, std::string_view prefix) {
+            completeInstallable(prefix);
+        }}
+    });
+}
+
+void InstallableCommand::prepare()
+{
+    installable = parseInstallable(getStore(), _installable);
+}
+
+}
diff --git a/src/libcmd/installables.hh b/src/libcmd/installables.hh
new file mode 100644
index 000000000..f37b3f829
--- /dev/null
+++ b/src/libcmd/installables.hh
@@ -0,0 +1,137 @@
+#pragma once
+
+#include "util.hh"
+#include "path.hh"
+#include "eval.hh"
+#include "flake/flake.hh"
+
+#include <optional>
+
+#include <nlohmann/json_fwd.hpp>
+
+namespace nix {
+
+struct DrvInfo;
+struct SourceExprCommand;
+
+namespace eval_cache { class EvalCache; class AttrCursor; }
+
+struct BuildableOpaque {
+    StorePath path;
+    nlohmann::json toJSON(ref<Store> store) const;
+};
+
+struct BuildableFromDrv {
+    StorePath drvPath;
+    std::map<std::string, std::optional<StorePath>> outputs;
+    nlohmann::json toJSON(ref<Store> store) const;
+};
+
+typedef std::variant<
+    BuildableOpaque,
+    BuildableFromDrv
+> Buildable;
+
+typedef std::vector<Buildable> Buildables;
+nlohmann::json buildablesToJSON(const Buildables & buildables, ref<Store> store);
+
+struct App
+{
+    std::vector<StorePathWithOutputs> context;
+    Path program;
+    // FIXME: add args, sandbox settings, metadata, ...
+};
+
+struct Installable
+{
+    virtual ~Installable() { }
+
+    virtual std::string what() = 0;
+
+    virtual Buildables toBuildables() = 0;
+
+    Buildable toBuildable();
+
+    App toApp(EvalState & state);
+
+    virtual std::pair<Value *, Pos> toValue(EvalState & state)
+    {
+        throw Error("argument '%s' cannot be evaluated", what());
+    }
+
+    /* Return a value only if this installable is a store path or a
+       symlink to it. */
+    virtual std::optional<StorePath> getStorePath()
+    {
+        return {};
+    }
+
+    virtual std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>>
+    getCursors(EvalState & state);
+
+    std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>
+    getCursor(EvalState & state);
+
+    virtual FlakeRef nixpkgsFlakeRef() const
+    {
+        return FlakeRef::fromAttrs({{"type","indirect"}, {"id", "nixpkgs"}});
+    }
+};
+
+struct InstallableValue : Installable
+{
+    ref<EvalState> state;
+
+    InstallableValue(ref<EvalState> state) : state(state) {}
+
+    struct DerivationInfo
+    {
+        StorePath drvPath;
+        std::optional<StorePath> outPath;
+        std::string outputName;
+    };
+
+    virtual std::vector<DerivationInfo> toDerivations() = 0;
+
+    Buildables toBuildables() override;
+};
+
+struct InstallableFlake : InstallableValue
+{
+    FlakeRef flakeRef;
+    Strings attrPaths;
+    Strings prefixes;
+    const flake::LockFlags & lockFlags;
+    mutable std::shared_ptr<flake::LockedFlake> _lockedFlake;
+
+    InstallableFlake(ref<EvalState> state, FlakeRef && flakeRef,
+        Strings && attrPaths, Strings && prefixes, const flake::LockFlags & lockFlags)
+        : InstallableValue(state), flakeRef(flakeRef), attrPaths(attrPaths),
+          prefixes(prefixes), lockFlags(lockFlags)
+    { }
+
+    std::string what() override { return flakeRef.to_string() + "#" + *attrPaths.begin(); }
+
+    std::vector<std::string> getActualAttrPaths();
+
+    Value * getFlakeOutputs(EvalState & state, const flake::LockedFlake & lockedFlake);
+
+    std::tuple<std::string, FlakeRef, DerivationInfo> toDerivation();
+
+    std::vector<DerivationInfo> toDerivations() override;
+
+    std::pair<Value *, Pos> toValue(EvalState & state) override;
+
+    std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>>
+    getCursors(EvalState & state) override;
+
+    std::shared_ptr<flake::LockedFlake> getLockedFlake() const;
+
+    FlakeRef nixpkgsFlakeRef() const override;
+};
+
+ref<eval_cache::EvalCache> openEvalCache(
+    EvalState & state,
+    std::shared_ptr<flake::LockedFlake> lockedFlake);
+
+}
diff --git a/src/libcmd/legacy.cc b/src/libcmd/legacy.cc
new file mode 100644
index 000000000..6df09ee37
--- /dev/null
+++ b/src/libcmd/legacy.cc
@@ -0,0 +1,7 @@
+#include "legacy.hh"
+
+namespace nix {
+
+RegisterLegacyCommand::Commands * RegisterLegacyCommand::commands = 0;
+
+}
diff --git a/src/libcmd/legacy.hh b/src/libcmd/legacy.hh
new file mode 100644
index 000000000..f503b0da3
--- /dev/null
+++ b/src/libcmd/legacy.hh
@@ -0,0 +1,23 @@
+#pragma once
+
+#include <functional>
+#include <map>
+#include <string>
+
+namespace nix {
+
+typedef std::function<void(int, char * *)> MainFunction;
+
+struct RegisterLegacyCommand
+{
+    typedef std::map<std::string, MainFunction> Commands;
+    static Commands * commands;
+
+    RegisterLegacyCommand(const std::string & name, MainFunction fun)
+    {
+        if (!commands) commands = new Commands;
+        (*commands)[name] = fun;
+    }
+};
+
+}
diff --git a/src/libcmd/local.mk b/src/libcmd/local.mk
new file mode 100644
index 000000000..ab0e0e43d
--- /dev/null
+++ b/src/libcmd/local.mk
@@ -0,0 +1,15 @@
+libraries += libcmd
+
+libcmd_NAME = libnixcmd
+
+libcmd_DIR := $(d)
+
+libcmd_SOURCES := $(wildcard $(d)/*.cc)
+
+libcmd_CXXFLAGS += -I src/libutil -I src/libstore -I src/libexpr -I src/libmain -I src/libfetchers
+
+libcmd_LDFLAGS = -llowdown
+
+libcmd_LIBS = libstore libutil libexpr libmain libfetchers
+
+$(eval $(call install-file-in, $(d)/nix-cmd.pc, $(prefix)/lib/pkgconfig, 0644))
diff --git a/src/libcmd/markdown.cc b/src/libcmd/markdown.cc
new file mode 100644
index 000000000..40788a42f
--- /dev/null
+++ b/src/libcmd/markdown.cc
@@ -0,0 +1,50 @@
+#include "markdown.hh"
+#include "util.hh"
+#include "finally.hh"
+
+#include <sys/queue.h>
+extern "C" {
+#include <lowdown.h>
+}
+
+namespace nix {
+
+std::string renderMarkdownToTerminal(std::string_view markdown)
+{
+    struct lowdown_opts opts {
+        .type = LOWDOWN_TERM,
+        .maxdepth = 20,
+        .cols = std::min(getWindowSize().second, (unsigned short) 80),
+        .hmargin = 0,
+        .vmargin = 0,
+        .feat = LOWDOWN_COMMONMARK | LOWDOWN_FENCED | LOWDOWN_DEFLIST | LOWDOWN_TABLES,
+        .oflags = 0,
+    };
+
+    auto doc = lowdown_doc_new(&opts);
+    if (!doc)
+        throw Error("cannot allocate Markdown document");
+    Finally freeDoc([&]() { lowdown_doc_free(doc); });
+
+    size_t maxn = 0;
+    auto node = lowdown_doc_parse(doc, &maxn, markdown.data(), markdown.size());
+    if (!node)
+        throw Error("cannot parse Markdown document");
+    Finally freeNode([&]() { lowdown_node_free(node); });
+
+    auto renderer = lowdown_term_new(&opts);
+    if (!renderer)
+        throw Error("cannot allocate Markdown renderer");
+    Finally freeRenderer([&]() { lowdown_term_free(renderer); });
+
+    auto buf = lowdown_buf_new(16384);
+    if (!buf)
+        throw Error("cannot allocate Markdown output buffer");
+    Finally freeBuffer([&]() { lowdown_buf_free(buf); });
+
+    lowdown_term_rndr(buf, nullptr, renderer, node);
+
+    return std::string(buf->data, buf->size);
+}
+
+}
diff --git a/src/libcmd/markdown.hh b/src/libcmd/markdown.hh
new file mode 100644
index 000000000..78320fcf5
--- /dev/null
+++ b/src/libcmd/markdown.hh
@@ -0,0 +1,7 @@
+#include "types.hh"
+
+namespace nix {
+
+std::string renderMarkdownToTerminal(std::string_view markdown);
+
+}
diff --git a/src/libcmd/nix-cmd.pc.in b/src/libcmd/nix-cmd.pc.in
new file mode 100644
index 000000000..1761a9f41
--- /dev/null
+++ b/src/libcmd/nix-cmd.pc.in
@@ -0,0 +1,9 @@
+prefix=@prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: Nix
+Description: Nix Package Manager
+Version: @PACKAGE_VERSION@
+Libs: -L${libdir} -lnixcmd
+Cflags: -I${includedir}/nix -std=c++17