Add env var for --context flag

Author: flopib

README.md

@@ -163,10 +163,15 @@ Examples:
 	# Read the flag usage below for more information on --normalize-manifests.",
 	HELM_DIFF_NORMALIZE_MANIFESTS=true helm diff upgrade my-release datadog/datadog",
 
+  # Set HELM_DIFF_OUTPUT_CONTEXT=n to configure the output context to n lines.
+  # This is equivalent to specifying the --context flag.
+  # Read the flag usage below for more information on --context.
+  HELM_DIFF_OUTPUT_CONTEXT=5 helm diff upgrade my-release datadog/datadog
+
 Flags:
       --allow-unreleased             enables diffing of releases that are not yet deployed via Helm
   -a, --api-versions stringArray     Kubernetes api versions used for Capabilities.APIVersions
-  -C, --context int                  output NUM lines of context around changes (default -1)
+  -C, --context int                  output NUM lines of context around changes (default -1), or use HELM_DIFF_OUTPUT_CONTEXT=num
       --detailed-exitcode            return a non-zero exit code when there are changes
       --devel                        use development versions, too. Equivalent to version '>0.0.0-0'. If --version is set, this is ignored.
       --disable-openapi-validation   disables rendered templates validation against the Kubernetes OpenAPI Schema

cmd/upgrade.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"strconv"
 	"strings"
 
 	jsoniterator "github.com/json-iterator/go"
@@ -112,6 +113,11 @@ func newChartCommand() *cobra.Command {
 			"  # This is equivalent to specifying the --normalize-manifests flag.",
 			"  # Read the flag usage below for more information on --normalize-manifests.",
 			"  HELM_DIFF_NORMALIZE_MANIFESTS=true helm diff upgrade my-release datadog/datadog",
+			"",
+			"# Set HELM_DIFF_OUTPUT_CONTEXT=n to configure the output context to n lines.",
+			"# This is equivalent to specifying the --context flag.",
+			"# Read the flag usage below for more information on --context.",
+			"HELM_DIFF_OUTPUT_CONTEXT=5 helm diff upgrade my-release datadog/datadog",
 		}, "\n"),
 		Args: func(cmd *cobra.Command, args []string) error {
 			return checkArgsLength(len(args), "release name", "chart path")
@@ -144,6 +150,16 @@ func newChartCommand() *cobra.Command {
 				}
 			}
 
+			if diff.OutputContext == -1 && !cmd.Flags().Changed("context") {
+				contextEnvVar := os.Getenv("HELM_DIFF_OUTPUT_CONTEXT")
+				if contextEnvVar != "" {
+					context, err := strconv.Atoi(contextEnvVar)
+					if err == nil {
+						diff.OutputContext = context
+					}
+				}
+			}
+
 			ProcessDiffOptions(cmd.Flags(), &diff.Options)
 
 			diff.release = args[0]