diff --git a/src/libutil/logging.cc b/src/libutil/logging.cc
index 904ba6ebe..56bdaf87a 100644
--- a/src/libutil/logging.cc
+++ b/src/libutil/logging.cc
@@ -32,7 +32,8 @@ void Logger::warn(const std::string & msg)
 
 void Logger::writeToStdout(std::string_view s)
 {
-    std::cout << s << "\n";
+    writeFull(STDOUT_FILENO, s);
+    writeFull(STDOUT_FILENO, "\n");
 }
 
 class SimpleLogger : public Logger
@@ -84,7 +85,7 @@ public:
 
     void startActivity(ActivityId act, Verbosity lvl, ActivityType type,
         const std::string & s, const Fields & fields, ActivityId parent)
-    override
+        override
     {
         if (lvl <= verbosity && !s.empty())
             log(lvl, s + "...");
diff --git a/src/libutil/logging.hh b/src/libutil/logging.hh
index 4642c49f7..a68edd15a 100644
--- a/src/libutil/logging.hh
+++ b/src/libutil/logging.hh
@@ -102,11 +102,9 @@ public:
     virtual void writeToStdout(std::string_view s);
 
     template<typename... Args>
-    inline void cout(const std::string & fs, const Args & ... args)
+    inline void cout(const Args & ... args)
     {
-        boost::format f(fs);
-        formatHelper(f, args...);
-        writeToStdout(f.str());
+        writeToStdout(fmt(args...));
     }
 
     virtual std::optional<char> ask(std::string_view s)
diff --git a/src/nix/build.cc b/src/nix/build.cc
index 12b22d999..f4f2ec81d 100644
--- a/src/nix/build.cc
+++ b/src/nix/build.cc
@@ -139,11 +139,11 @@ struct CmdBuild : InstallablesCommand, MixDryRun, MixJSON, MixProfile
             for (auto & buildable : buildables) {
                 std::visit(overloaded {
                     [&](const BuiltPath::Opaque & bo) {
-                        std::cout << store->printStorePath(bo.path) << std::endl;
+                        logger->cout(store->printStorePath(bo.path));
                     },
                     [&](const BuiltPath::Built & bfd) {
                         for (auto & output : bfd.outputs) {
-                            std::cout << store->printStorePath(output.second) << std::endl;
+                            logger->cout(store->printStorePath(output.second));
                         }
                     },
                 }, buildable.path.raw());
diff --git a/src/nix/cat.cc b/src/nix/cat.cc
index 6420a0f79..60aa66ce0 100644
--- a/src/nix/cat.cc
+++ b/src/nix/cat.cc
@@ -17,7 +17,7 @@ struct MixCat : virtual Args
         if (st.type != FSAccessor::Type::tRegular)
             throw Error("path '%1%' is not a regular file", path);
 
-        std::cout << accessor->readFile(path);
+        writeFull(STDOUT_FILENO, accessor->readFile(path));
     }
 };
 
diff --git a/src/nix/describe-stores.cc b/src/nix/describe-stores.cc
index 1dd384c0e..eafcedd1f 100644
--- a/src/nix/describe-stores.cc
+++ b/src/nix/describe-stores.cc
@@ -25,7 +25,7 @@ struct CmdDescribeStores : Command, MixJSON
             res[storeName] = storeConfig->toJSON();
         }
         if (json) {
-            std::cout << res;
+            logger->cout("%s", res);
         } else {
             for (auto & [storeName, storeConfig] : res.items()) {
                 std::cout << "## " << storeName << std::endl << std::endl;
diff --git a/src/nix/diff-closures.cc b/src/nix/diff-closures.cc
index 3489cc132..c7c37b66f 100644
--- a/src/nix/diff-closures.cc
+++ b/src/nix/diff-closures.cc
@@ -97,7 +97,7 @@ void printClosureDiff(
                 items.push_back(fmt("%s → %s", showVersions(removed), showVersions(added)));
             if (showDelta)
                 items.push_back(fmt("%s%+.1f KiB" ANSI_NORMAL, sizeDelta > 0 ? ANSI_RED : ANSI_GREEN, sizeDelta / 1024.0));
-            std::cout << fmt("%s%s: %s\n", indent, name, concatStringsSep(", ", items));
+            logger->cout("%s%s: %s", indent, name, concatStringsSep(", ", items));
         }
     }
 }
diff --git a/src/nix/eval.cc b/src/nix/eval.cc
index a579213fd..209fd3ed2 100644
--- a/src/nix/eval.cc
+++ b/src/nix/eval.cc
@@ -112,11 +112,11 @@ struct CmdEval : MixJSON, InstallableCommand, MixReadOnlyOption
 
         else if (raw) {
             stopProgressBar();
-            std::cout << *state->coerceToString(noPos, *v, context, "while generating the eval command output");
+            writeFull(STDOUT_FILENO, *state->coerceToString(noPos, *v, context, "while generating the eval command output"));
         }
 
         else if (json) {
-            std::cout << printValueAsJSON(*state, true, *v, pos, context, false).dump() << std::endl;
+            logger->cout("%s", printValueAsJSON(*state, true, *v, pos, context, false));
         }
 
         else {
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 053a9c9e1..3fe093fc7 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -952,7 +952,7 @@ struct CmdFlakeArchive : FlakeCommand, MixJSON, MixDryRun
                 {"path", store->printStorePath(flake.flake.sourceInfo->storePath)},
                 {"inputs", traverse(*flake.lockFile.root)},
             };
-            std::cout << jsonRoot.dump() << std::endl;
+            logger->cout("%s", jsonRoot);
         } else {
             traverse(*flake.lockFile.root);
         }
diff --git a/src/nix/log.cc b/src/nix/log.cc
index a0598ca13..0c9f778f0 100644
--- a/src/nix/log.cc
+++ b/src/nix/log.cc
@@ -53,7 +53,7 @@ struct CmdLog : InstallableCommand
             if (!log) continue;
             stopProgressBar();
             printInfo("got build log for '%s' from '%s'", installable->what(), logSub.getUri());
-            std::cout << *log;
+            writeFull(STDOUT_FILENO, *log);
             return;
         }
 
diff --git a/src/nix/ls.cc b/src/nix/ls.cc
index e964b01b3..c990a303c 100644
--- a/src/nix/ls.cc
+++ b/src/nix/ls.cc
@@ -93,7 +93,7 @@ struct MixLs : virtual Args, MixJSON
         if (json) {
             if (showDirectory)
                 throw UsageError("'--directory' is useless with '--json'");
-            std::cout << listNar(accessor, path, recursive);
+            logger->cout("%s", listNar(accessor, path, recursive));
         } else
             listText(accessor);
     }
diff --git a/src/nix/main.cc b/src/nix/main.cc
index d3d2f5b16..53bf649d4 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -292,7 +292,7 @@ void mainWrapped(int argc, char * * argv)
     NixArgs args;
 
     if (argc == 2 && std::string(argv[1]) == "__dump-args") {
-        std::cout << args.toJSON().dump() << "\n";
+        logger->cout("%s", args.toJSON());
         return;
     }
 
@@ -312,7 +312,7 @@ void mainWrapped(int argc, char * * argv)
             b["doc"] = trim(stripIndentation(primOp->doc));
             res[state.symbols[builtin.name]] = std::move(b);
         }
-        std::cout << res.dump() << "\n";
+        logger->cout("%s", res);
         return;
     }
 
@@ -321,14 +321,14 @@ void mainWrapped(int argc, char * * argv)
         if (completions) {
             switch (completionType) {
             case ctNormal:
-                std::cout << "normal\n"; break;
+                logger->cout("normal"); break;
             case ctFilenames:
-                std::cout << "filenames\n"; break;
+                logger->cout("filenames"); break;
             case ctAttrs:
-                std::cout << "attrs\n"; break;
+                logger->cout("attrs"); break;
             }
             for (auto & s : *completions)
-                std::cout << s.completion << "\t" << trim(s.description) << "\n";
+                logger->cout(s.completion + "\t" + trim(s.description));
         }
     });
 
diff --git a/src/nix/make-content-addressed.cc b/src/nix/make-content-addressed.cc
index d86b90fc7..6693c55ac 100644
--- a/src/nix/make-content-addressed.cc
+++ b/src/nix/make-content-addressed.cc
@@ -45,7 +45,7 @@ struct CmdMakeContentAddressed : virtual CopyCommand, virtual StorePathsCommand,
             }
             auto json = json::object();
             json["rewrites"] = jsonRewrites;
-            std::cout << json.dump();
+            logger->cout("%s", json);
         } else {
             for (auto & path : storePaths) {
                 auto i = remappings.find(path);
diff --git a/src/nix/prefetch.cc b/src/nix/prefetch.cc
index fc3823406..51c8a3319 100644
--- a/src/nix/prefetch.cc
+++ b/src/nix/prefetch.cc
@@ -234,9 +234,9 @@ static int main_nix_prefetch_url(int argc, char * * argv)
         if (!printPath)
             printInfo("path is '%s'", store->printStorePath(storePath));
 
-        std::cout << printHash16or32(hash) << std::endl;
+        logger->cout(printHash16or32(hash));
         if (printPath)
-            std::cout << store->printStorePath(storePath) << std::endl;
+            logger->cout(store->printStorePath(storePath));
 
         return 0;
     }
diff --git a/src/nix/profile.cc b/src/nix/profile.cc
index 711fbe2f0..eef33b3d9 100644
--- a/src/nix/profile.cc
+++ b/src/nix/profile.cc
@@ -228,12 +228,12 @@ struct ProfileManifest
 
         while (i != prevElems.end() || j != curElems.end()) {
             if (j != curElems.end() && (i == prevElems.end() || i->describe() > j->describe())) {
-                std::cout << fmt("%s%s: ∅ -> %s\n", indent, j->describe(), j->versions());
+                logger->cout("%s%s: ∅ -> %s", indent, j->describe(), j->versions());
                 changes = true;
                 ++j;
             }
             else if (i != prevElems.end() && (j == curElems.end() || i->describe() < j->describe())) {
-                std::cout << fmt("%s%s: %s -> ∅\n", indent, i->describe(), i->versions());
+                logger->cout("%s%s: %s -> ∅", indent, i->describe(), i->versions());
                 changes = true;
                 ++i;
             }
@@ -241,7 +241,7 @@ struct ProfileManifest
                 auto v1 = i->versions();
                 auto v2 = j->versions();
                 if (v1 != v2) {
-                    std::cout << fmt("%s%s: %s -> %s\n", indent, i->describe(), v1, v2);
+                    logger->cout("%s%s: %s -> %s", indent, i->describe(), v1, v2);
                     changes = true;
                 }
                 ++i;
@@ -250,7 +250,7 @@ struct ProfileManifest
         }
 
         if (!changes)
-            std::cout << fmt("%sNo changes.\n", indent);
+            logger->cout("%sNo changes.", indent);
     }
 };
 
@@ -640,9 +640,9 @@ struct CmdProfileDiffClosures : virtual StoreCommand, MixDefaultProfile
 
         for (auto & gen : gens) {
             if (prevGen) {
-                if (!first) std::cout << "\n";
+                if (!first) logger->cout("");
                 first = false;
-                std::cout << fmt("Version %d -> %d:\n", prevGen->number, gen.number);
+                logger->cout("Version %d -> %d:", prevGen->number, gen.number);
                 printClosureDiff(store,
                     store->followLinksToStorePath(prevGen->path),
                     store->followLinksToStorePath(gen.path),
@@ -678,10 +678,10 @@ struct CmdProfileHistory : virtual StoreCommand, EvalCommand, MixDefaultProfile
         for (auto & gen : gens) {
             ProfileManifest manifest(*getEvalState(), gen.path);
 
-            if (!first) std::cout << "\n";
+            if (!first) logger->cout("");
             first = false;
 
-            std::cout << fmt("Version %s%d" ANSI_NORMAL " (%s)%s:\n",
+            logger->cout("Version %s%d" ANSI_NORMAL " (%s)%s:",
                 gen.number == curGen ? ANSI_GREEN : ANSI_BOLD,
                 gen.number,
                 std::put_time(std::gmtime(&gen.creationTime), "%Y-%m-%d"),
diff --git a/src/nix/realisation.cc b/src/nix/realisation.cc
index c9a7157cd..0d3466515 100644
--- a/src/nix/realisation.cc
+++ b/src/nix/realisation.cc
@@ -65,18 +65,16 @@ struct CmdRealisationInfo : BuiltPathsCommand, MixJSON
 
                 res.push_back(currentPath);
             }
-            std::cout << res.dump();
+            logger->cout("%s", res);
         }
         else {
             for (auto & path : realisations) {
                 if (auto realisation = std::get_if<Realisation>(&path.raw)) {
-                    std::cout <<
-                        realisation->id.to_string() << " " <<
-                        store->printStorePath(realisation->outPath);
+                    logger->cout("%s %s",
+                        realisation->id.to_string(),
+                        store->printStorePath(realisation->outPath));
                 } else
-                    std::cout << store->printStorePath(path.path());
-
-                std::cout << std::endl;
+                    logger->cout("%s", store->printStorePath(path.path()));
             }
         }
     }
diff --git a/src/nix/search.cc b/src/nix/search.cc
index 4fa1e7837..2e38f7e4b 100644
--- a/src/nix/search.cc
+++ b/src/nix/search.cc
@@ -196,9 +196,8 @@ struct CmdSearch : InstallableCommand, MixJSON
         for (auto & cursor : installable->getCursors(*state))
             visit(*cursor, cursor->getAttrPath(), true);
 
-        if (json) {
-            std::cout << jsonOut->dump() << std::endl;
-        }
+        if (json)
+            logger->cout("%s", *jsonOut);
 
         if (!json && !results)
             throw Error("no results for the given search term(s)!");
diff --git a/src/nix/show-derivation.cc b/src/nix/show-derivation.cc
index d1a516cad..520e8b1ce 100644
--- a/src/nix/show-derivation.cc
+++ b/src/nix/show-derivation.cc
@@ -57,7 +57,7 @@ struct CmdShowDerivation : InstallablesCommand
             jsonRoot[store->printStorePath(drvPath)] =
                 store->readDerivation(drvPath).toJSON(*store);
         }
-        std::cout << jsonRoot.dump(2) << std::endl;
+        logger->cout(jsonRoot.dump(2));
     }
 };
 
diff --git a/src/nix/sigs.cc b/src/nix/sigs.cc
index 3d659d6d2..ee27e3725 100644
--- a/src/nix/sigs.cc
+++ b/src/nix/sigs.cc
@@ -173,7 +173,7 @@ struct CmdKeyGenerateSecret : Command
         if (!keyName)
             throw UsageError("required argument '--key-name' is missing");
 
-        std::cout << SecretKey::generate(*keyName).to_string();
+        writeFull(STDOUT_FILENO, SecretKey::generate(*keyName).to_string());
     }
 };
 
@@ -194,7 +194,7 @@ struct CmdKeyConvertSecretToPublic : Command
     void run() override
     {
         SecretKey secretKey(drainFD(STDIN_FILENO));
-        std::cout << secretKey.toPublicKey().to_string();
+        writeFull(STDOUT_FILENO, secretKey.toPublicKey().to_string());
     }
 };