diff --git a/src/nix/command.hh b/src/nix/command.hh
index b159408c3..479e6fd37 100644
--- a/src/nix/command.hh
+++ b/src/nix/command.hh
@@ -51,7 +51,7 @@ struct Installable
 {
     virtual std::string what() = 0;
 
-    virtual Buildables toBuildable()
+    virtual Buildables toBuildable(bool singular = false)
     {
         throw Error("argument '%s' cannot be built", what());
     }
@@ -97,8 +97,6 @@ struct InstallablesCommand : virtual Args, SourceExprCommand
         expectArgs("installables", &_installables);
     }
 
-    std::vector<std::shared_ptr<Installable>> parseInstallables(ref<Store> store, Strings ss);
-
     enum ToStorePathsMode { Build, NoBuild, DryRun };
 
     PathSet toStorePaths(ref<Store> store, ToStorePathsMode mode);
@@ -112,6 +110,22 @@ private:
     Strings _installables;
 };
 
+struct InstallableCommand : virtual Args, SourceExprCommand
+{
+    std::shared_ptr<Installable> installable;
+
+    InstallableCommand()
+    {
+        expectArg("installable", &_installable);
+    }
+
+    void prepare() override;
+
+private:
+
+    std::string _installable;
+};
+
 /* A command that operates on zero or more store paths. */
 struct StorePathsCommand : public InstallablesCommand
 {
diff --git a/src/nix/edit.cc b/src/nix/edit.cc
index fd07913bc..127be321e 100644
--- a/src/nix/edit.cc
+++ b/src/nix/edit.cc
@@ -8,7 +8,7 @@
 
 using namespace nix;
 
-struct CmdEdit : InstallablesCommand
+struct CmdEdit : InstallableCommand
 {
     std::string name() override
     {
@@ -34,44 +34,42 @@ struct CmdEdit : InstallablesCommand
     {
         auto state = getEvalState();
 
-        for (auto & i : installables) {
-            auto v = i->toValue(*state);
+        auto v = installable->toValue(*state);
 
-            Value * v2;
-            try {
-                auto dummyArgs = state->allocBindings(0);
-                v2 = findAlongAttrPath(*state, "meta.position", *dummyArgs, *v);
-            } catch (Error &) {
-                throw Error("package '%s' has no source location information", i->what());
-            }
-
-            auto pos = state->forceString(*v2);
-            debug("position is %s", pos);
-
-            auto colon = pos.rfind(':');
-            if (colon == std::string::npos)
-                throw Error("cannot parse meta.position attribute '%s'", pos);
-
-            std::string filename(pos, 0, colon);
-            int lineno = std::stoi(std::string(pos, colon + 1));
-
-            auto editor = getEnv("EDITOR", "cat");
-
-            Strings args{editor};
-
-            if (editor.find("emacs") != std::string::npos ||
-                editor.find("nano") != std::string::npos ||
-                editor.find("vim") != std::string::npos)
-                args.push_back(fmt("+%d", lineno));
-
-            args.push_back(filename);
-
-            stopProgressBar();
-
-            execvp(editor.c_str(), stringsToCharPtrs(args).data());
-
-            throw SysError("cannot run editor '%s'", editor);
+        Value * v2;
+        try {
+            auto dummyArgs = state->allocBindings(0);
+            v2 = findAlongAttrPath(*state, "meta.position", *dummyArgs, *v);
+        } catch (Error &) {
+            throw Error("package '%s' has no source location information", installable->what());
         }
+
+        auto pos = state->forceString(*v2);
+        debug("position is %s", pos);
+
+        auto colon = pos.rfind(':');
+        if (colon == std::string::npos)
+            throw Error("cannot parse meta.position attribute '%s'", pos);
+
+        std::string filename(pos, 0, colon);
+        int lineno = std::stoi(std::string(pos, colon + 1));
+
+        auto editor = getEnv("EDITOR", "cat");
+
+        Strings args{editor};
+
+        if (editor.find("emacs") != std::string::npos ||
+            editor.find("nano") != std::string::npos ||
+            editor.find("vim") != std::string::npos)
+            args.push_back(fmt("+%d", lineno));
+
+        args.push_back(filename);
+
+        stopProgressBar();
+
+        execvp(editor.c_str(), stringsToCharPtrs(args).data());
+
+        throw SysError("cannot run editor '%s'", editor);
     }
 };
 
diff --git a/src/nix/installables.cc b/src/nix/installables.cc
index 9d812fee4..bdd273d50 100644
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@@ -78,7 +78,7 @@ struct InstallableStoreDrv : Installable
 
     std::string what() override { return storePath; }
 
-    Buildables toBuildable() override
+    Buildables toBuildable(bool singular) override
     {
         return {{storePath, {}}};
     }
@@ -92,25 +92,21 @@ struct InstallableStorePath : Installable
 
     std::string what() override { return storePath; }
 
-    Buildables toBuildable() override
+    Buildables toBuildable(bool singular) override
     {
         return {{storePath, {}}};
     }
 };
 
-struct InstallableExpr : Installable
+struct InstallableValue : Installable
 {
-    InstallablesCommand & installables;
-    std::string text;
+    SourceExprCommand & cmd;
 
-    InstallableExpr(InstallablesCommand & installables, const std::string & text)
-         : installables(installables), text(text) { }
+    InstallableValue(SourceExprCommand & cmd) : cmd(cmd) { }
 
-    std::string what() override { return text; }
-
-    Buildables toBuildable() override
+    Buildables toBuildable(bool singular) override
     {
-        auto state = installables.getEvalState();
+        auto state = cmd.getEvalState();
 
         auto v = toValue(*state);
 
@@ -121,6 +117,9 @@ struct InstallableExpr : Installable
         DrvInfos drvs;
         getDerivations(*state, *v, "", autoArgs, drvs, false);
 
+        if (singular && drvs.size() != 1)
+            throw Error("installable '%s' evaluates to %d derivations, where only one is expected", what(), drvs.size());
+
         Buildables res;
 
         for (auto & drv : drvs)
@@ -129,6 +128,16 @@ struct InstallableExpr : Installable
 
         return res;
     }
+};
+
+struct InstallableExpr : InstallableValue
+{
+    std::string text;
+
+    InstallableExpr(SourceExprCommand & cmd, const std::string & text)
+         : InstallableValue(cmd), text(text) { }
+
+    std::string what() override { return text; }
 
     Value * toValue(EvalState & state) override
     {
@@ -138,42 +147,19 @@ struct InstallableExpr : Installable
     }
 };
 
-struct InstallableAttrPath : Installable
+struct InstallableAttrPath : InstallableValue
 {
-    InstallablesCommand & installables;
     std::string attrPath;
 
-    InstallableAttrPath(InstallablesCommand & installables, const std::string & attrPath)
-        : installables(installables), attrPath(attrPath)
+    InstallableAttrPath(SourceExprCommand & cmd, const std::string & attrPath)
+        : InstallableValue(cmd), attrPath(attrPath)
     { }
 
     std::string what() override { return attrPath; }
 
-    Buildables toBuildable() override
-    {
-        auto state = installables.getEvalState();
-
-        auto v = toValue(*state);
-
-        // FIXME
-        std::map<string, string> autoArgs_;
-        Bindings & autoArgs(*evalAutoArgs(*state, autoArgs_));
-
-        DrvInfos drvs;
-        getDerivations(*state, *v, "", autoArgs, drvs, false);
-
-        Buildables res;
-
-        for (auto & drv : drvs)
-            for (auto & output : drv.queryOutputs())
-                res.emplace(output.second, Whence{output.first, drv.queryDrvPath()});
-
-        return res;
-    }
-
     Value * toValue(EvalState & state) override
     {
-        auto source = installables.getSourceExpr(state);
+        auto source = cmd.getSourceExpr(state);
 
         // FIXME
         std::map<string, string> autoArgs_;
@@ -190,20 +176,21 @@ struct InstallableAttrPath : Installable
 std::string attrRegex = R"([A-Za-z_][A-Za-z0-9-_+]*)";
 static std::regex attrPathRegex(fmt(R"(%1%(\.%1%)*)", attrRegex));
 
-std::vector<std::shared_ptr<Installable>> InstallablesCommand::parseInstallables(ref<Store> store, Strings ss)
+static std::vector<std::shared_ptr<Installable>> parseInstallables(
+    SourceExprCommand & cmd, ref<Store> store, Strings ss, bool useDefaultInstallables)
 {
     std::vector<std::shared_ptr<Installable>> result;
 
-    if (ss.empty() && useDefaultInstallables()) {
-        if (file == "")
-            file = ".";
+    if (ss.empty() && useDefaultInstallables) {
+        if (cmd.file == "")
+            cmd.file = ".";
         ss = Strings{""};
     }
 
     for (auto & s : ss) {
 
         if (s.compare(0, 1, "(") == 0)
-            result.push_back(std::make_shared<InstallableExpr>(*this, s));
+            result.push_back(std::make_shared<InstallableExpr>(cmd, s));
 
         else if (s.find("/") != std::string::npos) {
 
@@ -218,7 +205,7 @@ std::vector<std::shared_ptr<Installable>> InstallablesCommand::parseInstallables
         }
 
         else if (s == "" || std::regex_match(s, attrPathRegex))
-            result.push_back(std::make_shared<InstallableAttrPath>(*this, s));
+            result.push_back(std::make_shared<InstallableAttrPath>(cmd, s));
 
         else
             throw UsageError("don't know what to do with argument '%s'", s);
@@ -250,7 +237,14 @@ PathSet InstallablesCommand::toStorePaths(ref<Store> store, ToStorePathsMode mod
 
 void InstallablesCommand::prepare()
 {
-    installables = parseInstallables(getStore(), _installables);
+    installables = parseInstallables(*this, getStore(), _installables, useDefaultInstallables());
+}
+
+void InstallableCommand::prepare()
+{
+    auto installables = parseInstallables(*this, getStore(), {_installable}, false);
+    assert(installables.size() == 1);
+    installable = installables.front();
 }
 
 }
diff --git a/src/nix/log.cc b/src/nix/log.cc
index 0fb45c145..ba5e71c10 100644
--- a/src/nix/log.cc
+++ b/src/nix/log.cc
@@ -2,10 +2,11 @@
 #include "common-args.hh"
 #include "shared.hh"
 #include "store-api.hh"
+#include "progress-bar.hh"
 
 using namespace nix;
 
-struct CmdLog : InstallablesCommand
+struct CmdLog : InstallableCommand
 {
     CmdLog()
     {
@@ -27,21 +28,22 @@ struct CmdLog : InstallablesCommand
 
         subs.push_front(store);
 
-        for (auto & inst : installables) {
-            for (auto & b : inst->toBuildable()) {
-                auto path = b.second.drvPath != "" ? b.second.drvPath : b.first;
-                bool found = false;
-                for (auto & sub : subs) {
-                    auto log = sub->getBuildLog(path);
+        for (auto & b : installable->toBuildable(true)) {
+
+            for (auto & sub : subs) {
+                auto log = b.second.drvPath != "" ? sub->getBuildLog(b.second.drvPath) : nullptr;
+                if (!log) {
+                    log = sub->getBuildLog(b.first);
                     if (!log) continue;
-                    std::cout << *log;
-                    found = true;
-                    break;
                 }
-                if (!found)
-                    throw Error("build log of path '%s' is not available", path);
+                stopProgressBar();
+                printInfo("got build log for '%s' from '%s'", b.first, sub->getUri());
+                std::cout << *log;
+                return;
             }
         }
+
+        throw Error("build log of '%s' is not available", installable->what());
     }
 };
 
diff --git a/src/nix/main.cc b/src/nix/main.cc
index 9781b5854..ec9b58b20 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -8,6 +8,7 @@
 #include "shared.hh"
 #include "store-api.hh"
 #include "progress-bar.hh"
+#include "finally.hh"
 
 extern std::string chrootHelperName;
 
@@ -84,6 +85,8 @@ void mainWrapped(int argc, char * * argv)
 
     if (!args.command) args.showHelpAndExit();
 
+    Finally f([]() { stopProgressBar(); });
+
     if (isatty(STDERR_FILENO))
         startProgressBar();
 
diff --git a/src/nix/progress-bar.cc b/src/nix/progress-bar.cc
index f48cfa13b..9bc405239 100644
--- a/src/nix/progress-bar.cc
+++ b/src/nix/progress-bar.cc
@@ -116,7 +116,7 @@ public:
     {
         auto state(state_.lock());
         if (!state->active) return;
-        state->active = true;
+        state->active = false;
         std::string status = getStatus(*state);
         writeToStderr("\r\e[K");
         if (status != "")