diff --git a/t/README b/t/README
index 2aceb67c78..1a78982da1 100644
--- a/t/README
+++ b/t/README
@@ -500,6 +500,10 @@ library for your script to use.
    <expected> file.  This behaves like "cmp" but produces more
    helpful output when the test is run with "-v" option.
 
+ - test_line_count (= | -lt | -ge | ...) <length> <file>
+
+   Check whether a file has the length it is expected to.
+
  - test_path_is_file <file> [<diagnosis>]
    test_path_is_dir <dir> [<diagnosis>]
    test_path_is_missing <path> [<diagnosis>]
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 87308f5a9b..a417bdfed1 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -598,6 +598,28 @@ test_path_is_missing () {
 	fi
 }
 
+# test_line_count checks that a file has the number of lines it
+# ought to. For example:
+#
+#	test_expect_success 'produce exactly one line of output' '
+#		do something >output &&
+#		test_line_count = 1 output
+#	'
+#
+# is like "test $(wc -l <output) = 1" except that it passes the
+# output through when the number of lines is wrong.
+
+test_line_count () {
+	if test $# != 3
+	then
+		error "bug in the test script: not 3 parameters to test_line_count"
+	elif ! test $(wc -l <"$3") "$1" "$2"
+	then
+		echo "test_line_count: line count for $3 !$1 $2"
+		cat "$3"
+		return 1
+	fi
+}
 
 # This is not among top-level (test_expect_success | test_expect_failure)
 # but is a prefix that can be used in the test script, like: