diff --git a/testscript/testdata/long_diff.txt b/testscript/testdata/long_diff.txt
new file mode 100644
index 00000000..431ab8eb
--- /dev/null
+++ b/testscript/testdata/long_diff.txt
@@ -0,0 +1,167 @@
+# Verify the behaviour of cmp in the case of a small diff between
+# two large files
+
+unquote dir/script.txt
+! testscript dir
+env
+cmpenv stdout stdout.golden
+
+-- dir/script.txt --
+>cmp a b
+>
+>-- a --
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>-- b --
+>b
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+-- stdout.golden --
+
+> cmp a b
+[diff -a +b]
+-a
++b
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+
+FAIL: $$WORK${/}dir${/}script.txt:1: a and b differ
diff --git a/testscript/testscript.go b/testscript/testscript.go
index 495a6e0a..75a6c9a6 100644
--- a/testscript/testscript.go
+++ b/testscript/testscript.go
@@ -299,6 +299,7 @@ func (ts *TestScript) setup() string {
 			"devnull=" + os.DevNull,
 			"/=" + string(os.PathSeparator),
 			":=" + string(os.PathListSeparator),
+			"$=$",
 
 			// If we are collecting coverage profiles for merging into the main one,
 			// ensure the environment variable is forwarded to sub-processes.