cli: fmt: always output content when formatting stdin

ci/release/changelogs/next.md

@@ -19,6 +19,7 @@
 - exports: pptx follows standards more closely, addressing warnings from some Powerpoint software [#2645](https://github.com/terrastruct/d2/pull/2645)
 - d2sequence: fix edge case of invalid sequence diagrams [#2660](https://github.com/terrastruct/d2/pull/2660)
 - d2svg: Text may overflow legend bounds when monospace font is used [#2674](https://github.com/terrastruct/d2/pull/2674)
+- fmt: always output content when formatting stdin [#2718](https://github.com/terrastruct/d2/pull/2718)
 
 ---
 

d2cli/fmt.go

@@ -54,6 +54,12 @@ func fmtCmd(ctx context.Context, ms *xmain.State, check bool) (err error) {
 					return err
 				}
 			}
+		} else if !check && inputPath == "-" {
+			// still output unchanged file to prevent deletion in
+			// automated setups
+			if err := ms.WritePath(inputPath, output); err != nil {
+				return err
+			}
 		}
 	}