diff --git a/bin/git-check-assertions b/bin/git-check-assertions
index 685b591..1195884 100755
--- a/bin/git-check-assertions
+++ b/bin/git-check-assertions
@@ -27,9 +27,12 @@ set -euo pipefail
 # helper functions inspired by bats/bats-assert
 run() {
   set +e
-  output="$("$@" 2>&1)"
-  status=$?
+  tmp_output="$(mktemp)"
+  "$@" 2>&1 | tee "$tmp_output"
+  status=${PIPESTATUS[0]}
   set -e
+  output="$(cat "$tmp_output")"
+  rm -f "$tmp_output"
   printf '%s\n' "$output"
   return 0
 }
diff --git a/default.nix b/default.nix
index 4f9f1bd..c260e4c 100644
--- a/default.nix
+++ b/default.nix
@@ -10,6 +10,7 @@
   resholve,
   shfmt,
   perl,
+  coreutils,
 }:
 
 let
@@ -42,6 +43,7 @@ resholve.mkDerivation {
     gitMinimal
     shfmt
     perl
+    coreutils
   ];
 
   checkPhase = ''
@@ -69,6 +71,7 @@ resholve.mkDerivation {
       gnused
       bash
       perl
+      coreutils
     ];
     execer = [
       # Not true at all, but ¯\_(ツ)_/¯
diff --git a/flake.nix b/flake.nix
index 06dda90..1fabe89 100644
--- a/flake.nix
+++ b/flake.nix
@@ -24,6 +24,7 @@
             ]))
             pkgs.perl
             pkgs.shfmt
+            pkgs.coreutils
           ];
         };
       }