Add basic "nix run" command

src/nix/run.cc

@@ -0,0 +1,62 @@
+#include "command.hh"
+#include "common-args.hh"
+#include "installables.hh"
+#include "shared.hh"
+#include "store-api.hh"
+#include "derivations.hh"
+
+using namespace nix;
+
+struct CmdRun : StoreCommand, MixInstallables
+{
+    CmdRun()
+    {
+    }
+
+    std::string name() override
+    {
+        return "run";
+    }
+
+    std::string description() override
+    {
+        return "run a shell in which the specified packages are available";
+    }
+
+    void run(ref<Store> store) override
+    {
+        auto elems = evalInstallables(store);
+
+        PathSet pathsToBuild;
+
+        for (auto & elem : elems) {
+            if (elem.isDrv)
+                pathsToBuild.insert(elem.drvPath);
+            else
+                pathsToBuild.insert(elem.outPaths.begin(), elem.outPaths.end());
+        }
+
+        printMissing(store, pathsToBuild);
+
+        store->buildPaths(pathsToBuild);
+
+        PathSet outPaths;
+        for (auto & path : pathsToBuild)
+            if (isDerivation(path)) {
+                Derivation drv = store->derivationFromPath(path);
+                for (auto & output : drv.outputs)
+                    outPaths.insert(output.second.path);
+            } else
+                outPaths.insert(path);
+
+        auto unixPath = tokenizeString<Strings>(getEnv("PATH"), ":");
+        for (auto & path : outPaths)
+            if (pathExists(path + "/bin"))
+                unixPath.push_front(path + "/bin");
+        setenv("PATH", concatStringsSep(":", unixPath).c_str(), 1);
+
+        execlp("bash", "bash", nullptr);
+    }
+};
+
+static RegisterCommand r1(make_ref<CmdRun>());