diff --git a/src/libexpr/primops/fetchGit.cc b/src/libexpr/primops/fetchGit.cc
index 7ef3b3823..9d0c64291 100644
--- a/src/libexpr/primops/fetchGit.cc
+++ b/src/libexpr/primops/fetchGit.cc
@@ -4,6 +4,7 @@
 #include "store-api.hh"
 #include "pathlocks.hh"
 #include "hash.hh"
+#include "tarfile.hh"
 
 #include <sys/time.h>
 
@@ -164,14 +165,16 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
         if (e.errNo != ENOENT) throw;
     }
 
-    // FIXME: should pipe this, or find some better way to extract a
-    // revision.
-    auto tar = runProgram("git", true, { "-C", cacheDir, "archive", gitInfo.rev });
+    auto source = sinkToSource([&](Sink & sink) {
+        RunOptions gitOptions("git", { "-C", cacheDir, "archive", gitInfo.rev });
+        gitOptions.standardOut = &sink;
+        runProgram2(gitOptions);
+    });
 
     Path tmpDir = createTempDir();
     AutoDelete delTmpDir(tmpDir, true);
 
-    runProgram("tar", true, { "x", "-C", tmpDir }, tar);
+    unpackTarfile(*source, tmpDir);
 
     gitInfo.storePath = store->addToStore(name, tmpDir);
 
diff --git a/src/libstore/builtins/unpack-channel.cc b/src/libstore/builtins/unpack-channel.cc
index 5fc68cd66..d18e3ddaf 100644
--- a/src/libstore/builtins/unpack-channel.cc
+++ b/src/libstore/builtins/unpack-channel.cc
@@ -1,5 +1,4 @@
 #include "builtins.hh"
-#include "compression.hh"
 #include "tarfile.hh"
 
 namespace nix {
@@ -18,16 +17,7 @@ void builtinUnpackChannel(const BasicDerivation & drv)
 
     createDirs(out);
 
-    auto source = sinkToSource([&](Sink & sink) {
-        auto decompressor =
-            hasSuffix(src, ".bz2") ? makeDecompressionSink("bzip2", sink) :
-            hasSuffix(src, ".xz") ? makeDecompressionSink("xz", sink) :
-            makeDecompressionSink("none", sink);
-        readFile(src, *decompressor);
-        decompressor->finish();
-    });
-
-    unpackTarfile(*source, out);
+    unpackTarfile(src, out);
 
     auto entries = readDirectory(out);
     if (entries.size() != 1)
diff --git a/src/libstore/download.cc b/src/libstore/download.cc
index e80663dff..61e88c5c1 100644
--- a/src/libstore/download.cc
+++ b/src/libstore/download.cc
@@ -8,6 +8,7 @@
 #include "compression.hh"
 #include "pathlocks.hh"
 #include "finally.hh"
+#include "tarfile.hh"
 
 #ifdef ENABLE_S3
 #include <aws/core/client/ClientConfiguration.h>
@@ -903,12 +904,15 @@ CachedDownloadResult Downloader::downloadCached(
                 unpackedStorePath = "";
         }
         if (unpackedStorePath.empty()) {
-            printInfo(format("unpacking '%1%'...") % url);
+            printInfo("unpacking '%s'...", url);
             Path tmpDir = createTempDir();
             AutoDelete autoDelete(tmpDir, true);
-            // FIXME: this requires GNU tar for decompression.
-            runProgram("tar", true, {"xf", store->toRealPath(storePath), "-C", tmpDir, "--strip-components", "1"});
-            unpackedStorePath = store->addToStore(name, tmpDir, true, htSHA256, defaultPathFilter, NoRepair);
+            unpackTarfile(store->toRealPath(storePath), tmpDir, baseNameOf(url));
+            auto members = readDirectory(tmpDir);
+            if (members.size() != 1)
+                throw nix::Error("tarball '%s' contains an unexpected number of top-level files", url);
+            auto topDir = tmpDir + "/" + members.begin()->name;
+            unpackedStorePath = store->addToStore(name, topDir, true, htSHA256, defaultPathFilter, NoRepair);
         }
         replaceSymlink(unpackedStorePath, unpackedLink);
         storePath = unpackedStorePath;
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 128e287f3..5780c93a6 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -77,7 +77,6 @@ struct BufferedSource : Source
 
     size_t read(unsigned char * data, size_t len) override;
 
-
     bool hasData();
 
 protected:
diff --git a/src/libutil/tarfile.cc b/src/libutil/tarfile.cc
index ae6d512bd..f7d3ad417 100644
--- a/src/libutil/tarfile.cc
+++ b/src/libutil/tarfile.cc
@@ -1,4 +1,5 @@
 #include "rust-ffi.hh"
+#include "compression.hh"
 
 extern "C" {
     rust::CBox2<rust::Result<std::tuple<>>> unpack_tarfile(rust::Source source, rust::StringSlice dest_dir);
@@ -6,9 +7,28 @@ extern "C" {
 
 namespace nix {
 
-void unpackTarfile(Source & source, Path destDir)
+void unpackTarfile(Source & source, const Path & destDir)
 {
     unpack_tarfile(source, destDir).use()->unwrap();
 }
 
+void unpackTarfile(const Path & tarFile, const Path & destDir,
+    std::optional<std::string> baseName)
+{
+    if (!baseName) baseName = baseNameOf(tarFile);
+
+    auto source = sinkToSource([&](Sink & sink) {
+        // FIXME: look at first few bytes to determine compression type.
+        auto decompressor =
+            // FIXME: add .gz support
+            hasSuffix(*baseName, ".bz2") ? makeDecompressionSink("bzip2", sink) :
+            hasSuffix(*baseName, ".xz") ? makeDecompressionSink("xz", sink) :
+            makeDecompressionSink("none", sink);
+        readFile(tarFile, *decompressor);
+        decompressor->finish();
+    });
+
+    unpackTarfile(*source, destDir);
+}
+
 }
diff --git a/src/libutil/tarfile.hh b/src/libutil/tarfile.hh
index c3e95fb0c..ce0911e2a 100644
--- a/src/libutil/tarfile.hh
+++ b/src/libutil/tarfile.hh
@@ -2,6 +2,9 @@
 
 namespace nix {
 
-void unpackTarfile(Source & source, Path destDir);
+void unpackTarfile(Source & source, const Path & destDir);
+
+void unpackTarfile(const Path & tarFile, const Path & destDir,
+    std::optional<std::string> baseName = {});
 
 }
diff --git a/src/nix-prefetch-url/nix-prefetch-url.cc b/src/nix-prefetch-url/nix-prefetch-url.cc
index f54706a8a..78c883833 100644
--- a/src/nix-prefetch-url/nix-prefetch-url.cc
+++ b/src/nix-prefetch-url/nix-prefetch-url.cc
@@ -9,6 +9,7 @@
 #include "legacy.hh"
 #include "finally.hh"
 #include "progress-bar.hh"
+#include "tarfile.hh"
 
 #include <iostream>
 
@@ -192,8 +193,7 @@ static int _main(int argc, char * * argv)
                 if (hasSuffix(baseNameOf(uri), ".zip"))
                     runProgram("unzip", true, {"-qq", tmpFile, "-d", unpacked});
                 else
-                    // FIXME: this requires GNU tar for decompression.
-                    runProgram("tar", true, {"xf", tmpFile, "-C", unpacked});
+                    unpackTarfile(tmpFile, unpacked, baseNameOf(uri));
 
                 /* If the archive unpacks to a single file/directory, then use
                    that as the top-level. */