cmd: Add --diff option for caddy fmt

cmd/commandfuncs.go

@@ -32,6 +32,7 @@ import (
 	"sort"
 	"strings"
 
+	"github.com/aryann/difflib"
 	"github.com/caddyserver/caddy/v2"
 	"github.com/caddyserver/caddy/v2/caddyconfig"
 	"github.com/caddyserver/caddy/v2/caddyconfig/caddyfile"
@@ -569,6 +570,20 @@ func cmdFmt(fl Flags) (int, error) {
 		if err := os.WriteFile(formatCmdConfigFile, output, 0600); err != nil {
 			return caddy.ExitCodeFailedStartup, fmt.Errorf("overwriting formatted file: %v", err)
 		}
+	} else if fl.Bool("diff") {
+		diff := difflib.Diff(
+			strings.Split(string(input), "\n"),
+			strings.Split(string(output), "\n"))
+		for _, d := range diff {
+			switch d.Delta {
+			case difflib.Common:
+				fmt.Printf("  %s\n", d.Payload)
+			case difflib.LeftOnly:
+				fmt.Printf("- %s\n", d.Payload)
+			case difflib.RightOnly:
+				fmt.Printf("+ %s\n", d.Payload)
+			}
+		}
 	} else {
 		fmt.Print(string(output))
 	}

cmd/commands.go

@@ -282,12 +282,18 @@ human readability. It prints the result to stdout.
 If --overwrite is specified, the output will be written to the config file
 directly instead of printing it.
 
+If --diff is specified, the output will be compared against the input, and
+lines will be prefixed with '-' and '+' where they differ. Note that
+unchanged lines are prefixed with two spaces for alignment, and that this
+is not a valid patch format.
+
 If you wish you use stdin instead of a regular file, use - as the path.
 When reading from stdin, the --overwrite flag has no effect: the result
 is always printed to stdout.`,
 		Flags: func() *flag.FlagSet {
 			fs := flag.NewFlagSet("fmt", flag.ExitOnError)
 			fs.Bool("overwrite", false, "Overwrite the input file with the results")
+			fs.Bool("diff", false, "Print the differences between the input file and the formatted output")
 			return fs
 		}(),
 	})