Backport 'nix dev-shell' from the flakes branch

This also adds a '--profile' option to 'nix build' (replacing 'nix-env
--set').

9 files changed, 569 insertions, 116 deletions

diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 097ff210a..332c1c43a 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -478,6 +478,17 @@ Path createTempDir(const Path & tmpRoot, const Path & prefix,
 }
 
 
+std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix)
+{
+    Path tmpl(getEnv("TMPDIR").value_or("/tmp") + "/" + prefix + ".XXXXXX");
+    // Strictly speaking, this is UB, but who cares...
+    AutoCloseFD fd(mkstemp((char *) tmpl.c_str()));
+    if (!fd)
+        throw SysError("creating temporary file '%s'", tmpl);
+    return {std::move(fd), tmpl};
+}
+
+
 std::string getUserName()
 {
     auto pw = getpwuid(geteuid());
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 7c3a30242..1f85c7c46 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -122,10 +122,6 @@ void deletePath(const Path & path);
 
 void deletePath(const Path & path, unsigned long long & bytesFreed);
 
-/* Create a temporary directory. */
-Path createTempDir(const Path & tmpRoot = "", const Path & prefix = "nix",
-    bool includePid = true, bool useGlobalCounter = true, mode_t mode = 0755);
-
 std::string getUserName();
 
 /* Return $HOME or the user's home directory from /etc/passwd. */
@@ -205,6 +201,14 @@ public:
 };
 
 
+/* Create a temporary directory. */
+Path createTempDir(const Path & tmpRoot = "", const Path & prefix = "nix",
+    bool includePid = true, bool useGlobalCounter = true, mode_t mode = 0755);
+
+/* Create a temporary file, returning a file handle and its path. */
+std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix = "nix");
+
+
 class Pipe
 {
 public:
diff --git a/src/nix/build.cc b/src/nix/build.cc
index 3c9d2df39..0b0762836 100644
--- a/src/nix/build.cc
+++ b/src/nix/build.cc
@@ -5,7 +5,7 @@
 
 using namespace nix;
 
-struct CmdBuild : MixDryRun, InstallablesCommand
+struct CmdBuild : InstallablesCommand, MixDryRun, MixProfile
 {
     Path outLink = "result";
 
@@ -40,6 +40,10 @@ struct CmdBuild : MixDryRun, InstallablesCommand
                 "To build the build.x86_64-linux attribute from release.nix:",
                 "nix build -f release.nix build.x86_64-linux"
             },
+            Example{
+                "To make a profile point at GNU Hello:",
+                "nix build --profile /tmp/profile nixpkgs.hello"
+            },
         };
     }
 
@@ -49,18 +53,19 @@ struct CmdBuild : MixDryRun, InstallablesCommand
 
         if (dryRun) return;
 
-        for (size_t i = 0; i < buildables.size(); ++i) {
-            auto & b(buildables[i]);
-
-            if (outLink != "")
-                for (auto & output : b.outputs)
+        if (outLink != "") {
+            for (size_t i = 0; i < buildables.size(); ++i) {
+                for (auto & output : buildables[i].outputs)
                     if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>()) {
                         std::string symlink = outLink;
                         if (i) symlink += fmt("-%d", i);
                         if (output.first != "out") symlink += fmt("-%s", output.first);
                         store2->addPermRoot(output.second, absPath(symlink), true);
                     }
+            }
         }
+
+        updateProfile(buildables);
     }
 };
 
diff --git a/src/nix/command.cc b/src/nix/command.cc
index 442bc6c53..99b24d2a2 100644
--- a/src/nix/command.cc
+++ b/src/nix/command.cc
@@ -2,6 +2,9 @@
 #include "store-api.hh"
 #include "derivations.hh"
 #include "nixexpr.hh"
+#include "profiles.hh"
+
+extern char * * environ;
 
 namespace nix {
 
@@ -96,4 +99,95 @@ Strings editorFor(const Pos & pos)
     return args;
 }
 
+MixProfile::MixProfile()
+{
+    mkFlag()
+        .longName("profile")
+        .description("profile to update")
+        .labels({"path"})
+        .dest(&profile);
+}
+
+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, store->printStorePath(storePath)));
+}
+
+void MixProfile::updateProfile(const Buildables & buildables)
+{
+    if (!profile) return;
+
+    std::optional<StorePath> result;
+
+    for (auto & buildable : buildables) {
+        for (auto & output : buildable.outputs) {
+            if (result)
+                throw Error("'--profile' requires that the arguments produce a single store path, but there are multiple");
+            result = output.second.clone();
+        }
+    }
+
+    if (!result)
+        throw Error("'--profile' requires that the arguments produce a single store path, but there are none");
+
+    updateProfile(*result);
+}
+
+MixDefaultProfile::MixDefaultProfile()
+{
+    profile = getDefaultProfile();
+}
+
+MixEnvironment::MixEnvironment() : ignoreEnvironment(false) {
+    mkFlag()
+        .longName("ignore-environment")
+        .shortName('i')
+        .description("clear the entire environment (except those specified with --keep)")
+        .set(&ignoreEnvironment, true);
+
+    mkFlag()
+        .longName("keep")
+        .shortName('k')
+        .description("keep specified environment variable")
+        .arity(1)
+        .labels({"name"})
+        .handler([&](std::vector<std::string> ss) { keep.insert(ss.front()); });
+
+    mkFlag()
+        .longName("unset")
+        .shortName('u')
+        .description("unset specified environment variable")
+        .arity(1)
+        .labels({"name"})
+        .handler([&](std::vector<std::string> ss) { unset.insert(ss.front()); });
+}
+
+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/nix/command.hh b/src/nix/command.hh
index a954a7d04..23f5c9898 100644
--- a/src/nix/command.hh
+++ b/src/nix/command.hh
@@ -1,5 +1,6 @@
 #pragma once
 
+#include "installables.hh"
 #include "args.hh"
 #include "common-eval-args.hh"
 #include "path.hh"
@@ -22,34 +23,7 @@ private:
     std::shared_ptr<Store> _store;
 };
 
-struct Buildable
-{
-    std::optional<StorePath> drvPath;
-    std::map<std::string, StorePath> outputs;
-};
-
-typedef std::vector<Buildable> Buildables;
-
-struct Installable
-{
-    virtual ~Installable() { }
-
-    virtual std::string what() = 0;
-
-    virtual Buildables toBuildables()
-    {
-        throw Error("argument '%s' cannot be built", what());
-    }
-
-    Buildable toBuildable();
-
-    virtual std::pair<Value *, Pos> toValue(EvalState & state)
-    {
-        throw Error("argument '%s' cannot be evaluated", what());
-    }
-};
-
-struct SourceExprCommand : virtual Args, StoreCommand, MixEvalArgs
+struct SourceExprCommand : virtual StoreCommand, MixEvalArgs
 {
     Path file;
 
@@ -184,4 +158,36 @@ std::set<StorePath> toDerivations(ref<Store> store,
    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();
+};
+
 }
diff --git a/src/nix/installables.cc b/src/nix/installables.cc
index 013218cd9..f464d0aa1 100644
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@@ -109,6 +109,11 @@ struct InstallableStorePath : Installable
         bs.push_back(std::move(b));
         return bs;
     }
+
+    std::optional<StorePath> getStorePath() override
+    {
+        return storePath.clone();
+    }
 };
 
 struct InstallableValue : Installable
diff --git a/src/nix/installables.hh b/src/nix/installables.hh
new file mode 100644
index 000000000..503984220
--- /dev/null
+++ b/src/nix/installables.hh
@@ -0,0 +1,45 @@
+#pragma once
+
+#include "util.hh"
+#include "path.hh"
+#include "eval.hh"
+
+#include <optional>
+
+namespace nix {
+
+struct Buildable
+{
+    std::optional<StorePath> drvPath;
+    std::map<std::string, StorePath> outputs;
+};
+
+typedef std::vector<Buildable> Buildables;
+
+struct Installable
+{
+    virtual ~Installable() { }
+
+    virtual std::string what() = 0;
+
+    virtual Buildables toBuildables()
+    {
+        throw Error("argument '%s' cannot be built", what());
+    }
+
+    Buildable toBuildable();
+
+    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 {};
+    }
+};
+
+}
diff --git a/src/nix/run.cc b/src/nix/run.cc
index f885c5e49..8e30264c0 100644
--- a/src/nix/run.cc
+++ b/src/nix/run.cc
@@ -8,6 +8,7 @@
 #include "fs-accessor.hh"
 #include "progress-bar.hh"
 #include "affinity.hh"
+#include "eval.hh"
 
 #if __linux__
 #include <sys/mount.h>
@@ -19,11 +20,46 @@ using namespace nix;
 
 std::string chrootHelperName = "__run_in_chroot";
 
-struct CmdRun : InstallablesCommand
+struct RunCommon : virtual Command
+{
+    void runProgram(ref<Store> store,
+        const std::string & program,
+        const Strings & args)
+    {
+        stopProgressBar();
+
+        restoreSignals();
+
+        restoreAffinity();
+
+        /* If this is a diverted store (i.e. its "logical" location
+           (typically /nix/store) differs from its "physical" location
+           (e.g. /home/eelco/nix/store), then run the command in a
+           chroot. For non-root users, this requires running it in new
+           mount and user namespaces. Unfortunately,
+           unshare(CLONE_NEWUSER) doesn't work in a multithreaded
+           program (which "nix" is), so we exec() a single-threaded
+           helper program (chrootHelper() below) to do the work. */
+        auto store2 = store.dynamic_pointer_cast<LocalStore>();
+
+        if (store2 && store->storeDir != store2->realStoreDir) {
+            Strings helperArgs = { chrootHelperName, store->storeDir, store2->realStoreDir, program };
+            for (auto & arg : args) helperArgs.push_back(arg);
+
+            execv(readLink("/proc/self/exe").c_str(), stringsToCharPtrs(helperArgs).data());
+
+            throw SysError("could not execute chroot helper");
+        }
+
+        execvp(program.c_str(), stringsToCharPtrs(args).data());
+
+        throw SysError("unable to execute '%s'", program);
+    }
+};
+
+struct CmdRun : InstallablesCommand, RunCommon, MixEnvironment
 {
     std::vector<std::string> command = { "bash" };
-    StringSet keep, unset;
-    bool ignoreEnvironment = false;
 
     CmdRun()
     {
@@ -37,28 +73,6 @@ struct CmdRun : InstallablesCommand
                 if (ss.empty()) throw UsageError("--command requires at least one argument");
                 command = ss;
             });
-
-        mkFlag()
-            .longName("ignore-environment")
-            .shortName('i')
-            .description("clear the entire environment (except those specified with --keep)")
-            .set(&ignoreEnvironment, true);
-
-        mkFlag()
-            .longName("keep")
-            .shortName('k')
-            .description("keep specified environment variable")
-            .arity(1)
-            .labels({"name"})
-            .handler([&](std::vector<std::string> ss) { keep.insert(ss.front()); });
-
-        mkFlag()
-            .longName("unset")
-            .shortName('u')
-            .description("unset specified environment variable")
-            .arity(1)
-            .labels({"name"})
-            .handler([&](std::vector<std::string> ss) { unset.insert(ss.front()); });
     }
 
     std::string description() override
@@ -94,35 +108,13 @@ struct CmdRun : InstallablesCommand
 
         auto accessor = store->getFSAccessor();
 
-        if (ignoreEnvironment) {
-
-            if (!unset.empty())
-                throw UsageError("--unset does not make sense with --ignore-environment");
-
-            std::map<std::string, std::string> kept;
-            for (auto & var : keep) {
-                auto s = getenv(var.c_str());
-                if (s) kept[var] = s;
-            }
-
-            clearEnv();
-
-            for (auto & var : kept)
-                setenv(var.first.c_str(), var.second.c_str(), 1);
-
-        } else {
-
-            if (!keep.empty())
-                throw UsageError("--keep does not make sense without --ignore-environment");
-
-            for (auto & var : unset)
-                unsetenv(var.c_str());
-        }
 
         std::unordered_set<StorePath> done;
         std::queue<StorePath> todo;
         for (auto & path : outPaths) todo.push(path.clone());
 
+        setEnviron();
+
         auto unixPath = tokenizeString<Strings>(getEnv("PATH").value_or(""), ":");
 
         while (!todo.empty()) {
@@ -142,38 +134,10 @@ struct CmdRun : InstallablesCommand
 
         setenv("PATH", concatStringsSep(":", unixPath).c_str(), 1);
 
-        std::string cmd = *command.begin();
         Strings args;
         for (auto & arg : command) args.push_back(arg);
 
-        stopProgressBar();
-
-        restoreSignals();
-
-        restoreAffinity();
-
-        /* If this is a diverted store (i.e. its "logical" location
-           (typically /nix/store) differs from its "physical" location
-           (e.g. /home/eelco/nix/store), then run the command in a
-           chroot. For non-root users, this requires running it in new
-           mount and user namespaces. Unfortunately,
-           unshare(CLONE_NEWUSER) doesn't work in a multithreaded
-           program (which "nix" is), so we exec() a single-threaded
-           helper program (chrootHelper() below) to do the work. */
-        auto store2 = store.dynamic_pointer_cast<LocalStore>();
-
-        if (store2 && store->storeDir != store2->realStoreDir) {
-            Strings helperArgs = { chrootHelperName, store->storeDir, store2->realStoreDir, cmd };
-            for (auto & arg : args) helperArgs.push_back(arg);
-
-            execv(readLink("/proc/self/exe").c_str(), stringsToCharPtrs(helperArgs).data());
-
-            throw SysError("could not execute chroot helper");
-        }
-
-        execvp(cmd.c_str(), stringsToCharPtrs(args).data());
-
-        throw SysError("unable to exec '%s'", cmd);
+        runProgram(store, *command.begin(), args);
     }
 };
 
diff --git a/src/nix/shell.cc b/src/nix/shell.cc
new file mode 100644
index 000000000..82b1da552
--- /dev/null
+++ b/src/nix/shell.cc
@@ -0,0 +1,319 @@
+#include "eval.hh"
+#include "command.hh"
+#include "common-args.hh"
+#include "shared.hh"
+#include "store-api.hh"
+#include "derivations.hh"
+#include "affinity.hh"
+#include "progress-bar.hh"
+
+#include <regex>
+
+using namespace nix;
+
+struct Var
+{
+    bool exported;
+    std::string value; // quoted string or array
+};
+
+struct BuildEnvironment
+{
+    std::map<std::string, Var> env;
+    std::string bashFunctions;
+};
+
+BuildEnvironment readEnvironment(const Path & path)
+{
+    BuildEnvironment res;
+
+    std::set<std::string> exported;
+
+    debug("reading environment file '%s'", path);
+
+    auto file = readFile(path);
+
+    auto pos = file.cbegin();
+
+    static std::string varNameRegex =
+        R"re((?:[a-zA-Z_][a-zA-Z0-9_]*))re";
+
+    static std::regex declareRegex(
+        "^declare -x (" + varNameRegex + ")" +
+        R"re((?:="((?:[^"\\]|\\.)*)")?\n)re");
+
+    static std::string simpleStringRegex =
+        R"re((?:[a-zA-Z0-9_/:\.\-\+=]*))re";
+
+    static std::string quotedStringRegex =
+        R"re((?:\$?'(?:[^'\\]|\\[abeEfnrtv\\'"?])*'))re";
+
+    static std::string arrayRegex =
+        R"re((?:\(( *\[[^\]]+\]="(?:[^"\\]|\\.)*")*\)))re";
+
+    static std::regex varRegex(
+        "^(" + varNameRegex + ")=(" + simpleStringRegex + "|" + quotedStringRegex + "|" + arrayRegex + ")\n");
+
+    static std::regex functionRegex(
+        "^" + varNameRegex + " \\(\\) *\n");
+
+    while (pos != file.end()) {
+
+        std::smatch match;
+
+        if (std::regex_search(pos, file.cend(), match, declareRegex)) {
+            pos = match[0].second;
+            exported.insert(match[1]);
+        }
+
+        else if (std::regex_search(pos, file.cend(), match, varRegex)) {
+            pos = match[0].second;
+            res.env.insert({match[1], Var { (bool) exported.count(match[1]), match[2] }});
+        }
+
+        else if (std::regex_search(pos, file.cend(), match, functionRegex)) {
+            res.bashFunctions = std::string(pos, file.cend());
+            break;
+        }
+
+        else throw Error("shell environment '%s' has unexpected line '%s'",
+            path, file.substr(pos - file.cbegin(), 60));
+    }
+
+    return res;
+}
+
+/* Given an existing derivation, return the shell environment as
+   initialised by stdenv's setup script. We do this by building a
+   modified derivation with the same dependencies and nearly the same
+   initial environment variables, that just writes the resulting
+   environment to a file and exits. */
+StorePath getDerivationEnvironment(ref<Store> store, Derivation drv)
+{
+    auto builder = baseNameOf(drv.builder);
+    if (builder != "bash")
+        throw Error("'nix shell' only works on derivations that use 'bash' as their builder");
+
+    drv.args = {
+        "-c",
+        "set -e; "
+        "export IN_NIX_SHELL=impure; "
+        "export dontAddDisableDepTrack=1; "
+        "if [[ -n $stdenv ]]; then "
+        "  source $stdenv/setup; "
+        "fi; "
+        "export > $out; "
+        "set >> $out "};
+
+    /* Remove derivation checks. */
+    drv.env.erase("allowedReferences");
+    drv.env.erase("allowedRequisites");
+    drv.env.erase("disallowedReferences");
+    drv.env.erase("disallowedRequisites");
+
+    // FIXME: handle structured attrs
+
+    /* Rehash and write the derivation. FIXME: would be nice to use
+       'buildDerivation', but that's privileged. */
+    auto drvName = drv.env["name"] + "-env";
+    for (auto & output : drv.outputs)
+        drv.env.erase(output.first);
+    drv.env["out"] = "";
+    drv.env["outputs"] = "out";
+    Hash h = hashDerivationModulo(*store, drv, true);
+    auto shellOutPath = store->makeOutputPath("out", h, drvName);
+    drv.outputs.insert_or_assign("out", DerivationOutput(shellOutPath.clone(), "", ""));
+    drv.env["out"] = store->printStorePath(shellOutPath);
+    auto shellDrvPath2 = writeDerivation(store, drv, drvName);
+
+    /* Build the derivation. */
+    store->buildPaths({shellDrvPath2});
+
+    assert(store->isValidPath(shellOutPath));
+
+    return shellOutPath;
+}
+
+struct Common : InstallableCommand, MixProfile
+{
+    std::set<string> ignoreVars{
+        "BASHOPTS",
+        "EUID",
+        "HOME", // FIXME: don't ignore in pure mode?
+        "NIX_BUILD_TOP",
+        "NIX_ENFORCE_PURITY",
+        "NIX_LOG_FD",
+        "PPID",
+        "PWD",
+        "SHELLOPTS",
+        "SHLVL",
+        "SSL_CERT_FILE", // FIXME: only want to ignore /no-cert-file.crt
+        "TEMP",
+        "TEMPDIR",
+        "TERM",
+        "TMP",
+        "TMPDIR",
+        "TZ",
+        "UID",
+    };
+
+    void makeRcScript(const BuildEnvironment & buildEnvironment, std::ostream & out)
+    {
+        out << "nix_saved_PATH=\"$PATH\"\n";
+
+        for (auto & i : buildEnvironment.env) {
+            if (!ignoreVars.count(i.first) && !hasPrefix(i.first, "BASH_")) {
+                out << fmt("%s=%s\n", i.first, i.second.value);
+                if (i.second.exported)
+                    out << fmt("export %s\n", i.first);
+            }
+        }
+
+        out << "PATH=\"$PATH:$nix_saved_PATH\"\n";
+
+        out << buildEnvironment.bashFunctions << "\n";
+
+        // FIXME: set outputs
+
+        out << "export NIX_BUILD_TOP=\"$(mktemp -d --tmpdir nix-shell.XXXXXX)\"\n";
+        for (auto & i : {"TMP", "TMPDIR", "TEMP", "TEMPDIR"})
+            out << fmt("export %s=\"$NIX_BUILD_TOP\"\n", i);
+
+        out << "eval \"$shellHook\"\n";
+    }
+
+    StorePath getShellOutPath(ref<Store> store)
+    {
+        auto path = installable->getStorePath();
+        if (path && hasSuffix(path->to_string(), "-env"))
+            return path->clone();
+        else {
+            auto drvs = toDerivations(store, {installable});
+
+            if (drvs.size() != 1)
+                throw Error("'%s' needs to evaluate to a single derivation, but it evaluated to %d derivations",
+                    installable->what(), drvs.size());
+
+            auto & drvPath = *drvs.begin();
+
+            return getDerivationEnvironment(store, store->derivationFromPath(drvPath));
+        }
+    }
+
+    BuildEnvironment getBuildEnvironment(ref<Store> store)
+    {
+        auto shellOutPath = getShellOutPath(store);
+
+        updateProfile(shellOutPath);
+
+        return readEnvironment(store->printStorePath(shellOutPath));
+    }
+};
+
+struct CmdDevShell : Common, MixEnvironment
+{
+    std::vector<std::string> command;
+
+    CmdDevShell()
+    {
+        mkFlag()
+            .longName("command")
+            .shortName('c')
+            .description("command and arguments to be executed insted of an interactive shell")
+            .labels({"command", "args"})
+            .arity(ArityAny)
+            .handler([&](std::vector<std::string> ss) {
+                if (ss.empty()) throw UsageError("--command requires at least one argument");
+                command = ss;
+            });
+    }
+
+    std::string description() override
+    {
+        return "run a bash shell that provides the build environment of a derivation";
+    }
+
+    Examples examples() override
+    {
+        return {
+            Example{
+                "To get the build environment of GNU hello:",
+                "nix dev-shell nixpkgs.hello"
+            },
+            Example{
+                "To store the build environment in a profile:",
+                "nix dev-shell --profile /tmp/my-shell nixpkgs.hello"
+            },
+            Example{
+                "To use a build environment previously recorded in a profile:",
+                "nix dev-shell /tmp/my-shell"
+            },
+        };
+    }
+
+    void run(ref<Store> store) override
+    {
+        auto buildEnvironment = getBuildEnvironment(store);
+
+        auto [rcFileFd, rcFilePath] = createTempFile("nix-shell");
+
+        std::ostringstream ss;
+        makeRcScript(buildEnvironment, ss);
+
+        ss << fmt("rm -f '%s'\n", rcFilePath);
+
+        if (!command.empty()) {
+            std::vector<std::string> args;
+            for (auto s : command)
+                args.push_back(shellEscape(s));
+            ss << fmt("exec %s\n", concatStringsSep(" ", args));
+        }
+
+        writeFull(rcFileFd.get(), ss.str());
+
+        stopProgressBar();
+
+        auto shell = getEnv("SHELL").value_or("bash");
+
+        setEnviron();
+
+        auto args = Strings{std::string(baseNameOf(shell)), "--rcfile", rcFilePath};
+
+        restoreAffinity();
+        restoreSignals();
+
+        execvp(shell.c_str(), stringsToCharPtrs(args).data());
+
+        throw SysError("executing shell '%s'", shell);
+    }
+};
+
+struct CmdPrintDevEnv : Common
+{
+    std::string description() override
+    {
+        return "print shell code that can be sourced by bash to reproduce the build environment of a derivation";
+    }
+
+    Examples examples() override
+    {
+        return {
+            Example{
+                "To apply the build environment of GNU hello to the current shell:",
+                ". <(nix print-dev-env nixpkgs.hello)"
+            },
+        };
+    }
+
+    void run(ref<Store> store) override
+    {
+        auto buildEnvironment = getBuildEnvironment(store);
+
+        stopProgressBar();
+
+        makeRcScript(buildEnvironment, std::cout);
+    }
+};
+
+static auto r1 = registerCommand<CmdPrintDevEnv>("print-dev-env");
+static auto r2 = registerCommand<CmdDevShell>("dev-shell");