diff --git a/default.nix b/default.nix
index 5625e99..1c1b319 100644
--- a/default.nix
+++ b/default.nix
@@ -171,6 +171,6 @@ in rec {
       pkgs.yq-go
     ]}
     export SHELL=${pkgs.bash}/bin/bash
-    exec ${./tests}/entrypoint.sh "$@"
+    exec ${pkgs.bash}/bin/bash ${./tests}/entrypoint.sh "$@"
   '';
 }
diff --git a/tests/cases/01-create-network.sh b/tests/cases/01-create-network.sh
index 02b9bc2..28a0f4d 100644
--- a/tests/cases/01-create-network.sh
+++ b/tests/cases/01-create-network.sh
@@ -1,3 +1,6 @@
+# shellcheck source=../utils/with-tmp-for-case.sh
+source "$UTILS"/with-tmp-for-case.sh
+
 mkdir a
 mkdir b
 mkdir c
diff --git a/tests/entrypoint.sh b/tests/entrypoint.sh
old mode 100755
new mode 100644
index e4ea2e2..2591e8b
--- a/tests/entrypoint.sh
+++ b/tests/entrypoint.sh
@@ -4,6 +4,8 @@ set -e
 cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null
 root=$(pwd)
 
+export UTILS="$root"/utils
+
 REGEXS=()
 
 while [[ $# -gt 0 ]]; do
@@ -39,6 +41,11 @@ if [ -z "$KEEP_TMP" ]; then trap 'rm -rf $TMPDIR' EXIT; fi
 export TMPDIR
 echo "tmp dir is $TMPDIR"
 
+# Blackhole these directories so that tests don't accidentally use the host's
+# real ones.
+export XDG_RUNTIME_DIR=/dev/null
+export XDG_DATA_HOME=/dev/null
+
 test_files=$(
     find ./cases -type f -name '*.sh' \
     | sed "s|^\./cases/||" \
@@ -57,12 +64,7 @@ for file in $test_files; do
     [ -z "$VERBOSE" ] && output="$TMPDIR/$file.log" || output=/dev/stdout
 
     (
-        export TMPDIR="$TMPDIR/$file.tmp"
-        export XDG_RUNTIME_DIR="$TMPDIR/.run"
-        export XDG_DATA_HOME="$TMPDIR/.data"
-
-        mkdir -p "$TMPDIR" "$XDG_RUNTIME_DIR" "$XDG_DATA_HOME"
-        cd "$TMPDIR"
+        export TEST_CASE_FILE="$file"
 
         if ! $SHELL -e -x "$root/cases/$file" >"$output" 2>&1; then
             echo "$file FAILED"
diff --git a/tests/utils/with-tmp-for-case.sh b/tests/utils/with-tmp-for-case.sh
new file mode 100644
index 0000000..72abcd8
--- /dev/null
+++ b/tests/utils/with-tmp-for-case.sh
@@ -0,0 +1,9 @@
+set -e
+
+TMPDIR="$TMPDIR/$TEST_CASE_FILE.tmp"
+XDG_RUNTIME_DIR="$TMPDIR/.run"
+XDG_DATA_HOME="$TMPDIR/.data"
+
+mkdir -p "$TMPDIR" "$XDG_RUNTIME_DIR" "$XDG_DATA_HOME"
+
+cd "$TMPDIR"