diff --git a/command/operator_api.go b/command/operator_api.go
index b7812aad2ef1..36dd375b241b 100644
--- a/command/operator_api.go
+++ b/command/operator_api.go
@@ -18,6 +18,10 @@ import (
 	"github.com/posener/complete"
 )
 
+// Stdin represents the system's standard input, but it's declared as a
+// variable here to allow tests to override it with a regular file.
+var Stdin = os.Stdin
+
 type OperatorAPICommand struct {
 	Meta
 
@@ -141,13 +145,13 @@ func (c *OperatorAPICommand) Run(args []string) int {
 
 	// Opportunistically read from stdin and POST unless method has been
 	// explicitly set.
-	stat, _ := os.Stdin.Stat()
+	stat, _ := Stdin.Stat()
 	if (stat.Mode() & os.ModeCharDevice) == 0 {
 		verbose("* Reading request body from stdin.")
 
 		// Load stdin into a *bytes.Reader so that http.NewRequest can set the
 		// correct Content-Length value.
-		b, err := ioutil.ReadAll(os.Stdin)
+		b, err := ioutil.ReadAll(Stdin)
 		if err != nil {
 			c.Ui.Error(fmt.Sprintf("Error reading stdin: %v", err))
 			return 1
diff --git a/command/operator_api_test.go b/command/operator_api_test.go
index 9a17d31bd619..b71e65787f07 100644
--- a/command/operator_api_test.go
+++ b/command/operator_api_test.go
@@ -2,7 +2,6 @@ package command
 
 import (
 	"bytes"
-	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
@@ -176,7 +175,7 @@ func Test_pathToURL(t *testing.T) {
 // TestOperatorAPICommand_ContentLength tests that requests have the proper
 // ContentLength set.
 //
-// Don't run in parallel since it messes with os.Stdin.
+// Don't run it in parallel as it modifies the package's Stdin variable.
 func TestOperatorAPICommand_ContentLength(t *testing.T) {
 	contentLength := make(chan int, 1)
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -184,23 +183,20 @@ func TestOperatorAPICommand_ContentLength(t *testing.T) {
 	}))
 	defer ts.Close()
 
-	// Setup os.Stdin to a known value.
-	// The command stats stdin, so we can't mock it as another io.Reader.
-	// https://stackoverflow.com/a/46365584
+	// Setup a temp file to act as stdin.
 	input := []byte("test-input")
-	tmpfile, err := ioutil.TempFile("", "example")
+	fakeStdin, err := os.CreateTemp("", "fake-stdin")
 	require.NoError(t, err)
-	defer os.Remove(tmpfile.Name())
+	defer os.Remove(fakeStdin.Name())
 
-	_, err = tmpfile.Write(input)
+	_, err = fakeStdin.Write(input)
 	require.NoError(t, err)
-	_, err = tmpfile.Seek(0, 0)
+	_, err = fakeStdin.Seek(0, 0)
 	require.NoError(t, err)
 
-	oldStdin := os.Stdin
-	defer func() { os.Stdin = oldStdin }()
-
-	os.Stdin = tmpfile
+	// Override the package's Stdin variable for testing.
+	Stdin = fakeStdin
+	defer func() { Stdin = os.Stdin }()
 
 	// Setup command.
 	buf := bytes.NewBuffer(nil)