From 53abbbaee95f63330419411474ba8e8b6d558ccc Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Sat, 28 Mar 2026 03:27:46 +0000
Subject: [PATCH] Validate command node type and handle multiple results with
 debug log

Agent-Logs-Url: https://github.com/mikefarah/yq/sessions/928aabc5-ad71-41d8-94ab-403942e3f92d

Co-authored-by: mikefarah <1151925+mikefarah@users.noreply.github.com>
---
 .../doc/operators/headers/system-operators.md |  2 +-
 pkg/yqlib/operator_system.go                  | 26 ++++++++++++++-----
 pkg/yqlib/operator_system_test.go             |  7 +++++
 3 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/pkg/yqlib/doc/operators/headers/system-operators.md b/pkg/yqlib/doc/operators/headers/system-operators.md
index 2df8fdaa..9b84e8df 100644
--- a/pkg/yqlib/doc/operators/headers/system-operators.md
+++ b/pkg/yqlib/doc/operators/headers/system-operators.md
@@ -7,7 +7,7 @@ The `system` operator allows you to run an external command and use its output a
 ## Usage
 
 ```bash
-yq --null-input --enable-system-operator '.field = system("command"; "arg1")'
+yq --enable-system-operator --null-input '.field = system("command"; "arg1")'
 ```
 
 The operator takes:
diff --git a/pkg/yqlib/operator_system.go b/pkg/yqlib/operator_system.go
index f2e8c9b7..2fb58869 100644
--- a/pkg/yqlib/operator_system.go
+++ b/pkg/yqlib/operator_system.go
@@ -22,6 +22,20 @@ func resolveSystemArgs(argsNode *CandidateNode) []string {
 	return nil
 }
 
+func resolveCommandNode(commandNodes Context) (string, error) {
+	if commandNodes.MatchingNodes.Front() == nil {
+		return "", fmt.Errorf("system operator: command expression returned no results")
+	}
+	if commandNodes.MatchingNodes.Len() > 1 {
+		log.Debugf("system operator: command expression returned %d results, using first", commandNodes.MatchingNodes.Len())
+	}
+	cmdNode := commandNodes.MatchingNodes.Front().Value.(*CandidateNode)
+	if cmdNode.Kind != ScalarNode || cmdNode.Tag == "!!null" {
+		return "", fmt.Errorf("system operator: command must be a string scalar")
+	}
+	return cmdNode.Value, nil
+}
+
 func systemOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	if !ConfiguredSecurityPreferences.EnableSystemOps {
 		log.Warning("system operator is disabled, use --enable-system-operator flag to enable")
@@ -51,10 +65,10 @@ func systemOperator(d *dataTreeNavigator, context Context, expressionNode *Expre
 			if err != nil {
 				return Context{}, err
 			}
-			if commandNodes.MatchingNodes.Front() == nil {
-				return Context{}, fmt.Errorf("system operator: command expression returned no results")
+			command, err = resolveCommandNode(commandNodes)
+			if err != nil {
+				return Context{}, err
 			}
-			command = commandNodes.MatchingNodes.Front().Value.(*CandidateNode).Value
 
 			argsNodes, err := d.GetMatchingNodes(nodeContext, block.RHS)
 			if err != nil {
@@ -68,10 +82,10 @@ func systemOperator(d *dataTreeNavigator, context Context, expressionNode *Expre
 			if err != nil {
 				return Context{}, err
 			}
-			if commandNodes.MatchingNodes.Front() == nil {
-				return Context{}, fmt.Errorf("system operator: command expression returned no results")
+			command, err = resolveCommandNode(commandNodes)
+			if err != nil {
+				return Context{}, err
 			}
-			command = commandNodes.MatchingNodes.Front().Value.(*CandidateNode).Value
 		}
 
 		var stdin bytes.Buffer
diff --git a/pkg/yqlib/operator_system_test.go b/pkg/yqlib/operator_system_test.go
index 385e1561..867e1921 100644
--- a/pkg/yqlib/operator_system_test.go
+++ b/pkg/yqlib/operator_system_test.go
@@ -98,6 +98,13 @@ func TestSystemOperatorEnabledScenarios(t *testing.T) {
 			expression:    `.a = system("` + falsePath + `")`,
 			expectedError: "system command '" + falsePath + "' failed: exit status 1",
 		},
+		{
+			description:   "Null command returns error",
+			skipDoc:       true,
+			document:      "a: hello",
+			expression:    `.a = system(null)`,
+			expectedError: "system operator: command must be a string scalar",
+		},
 	}
 
 	for _, tt := range scenarios {