shellcheck: Do not call git apply if there is no diff output

`git apply` fails if there isn't a valid patch on stdin, empty stdin is not a
valid patch. Git errors out with `error: unrecognized input` message. This will
only call git apply if stdin is *not* empty so we can avoid this error
scenario.

Signed-off-by: Manuel Mendez <[email protected]>

Author: mmlb

Makefile

@@ -31,7 +31,7 @@ lint: out/linters/shellcheck-$(SHELLCHECK_VERSION)-$(LINT_ARCH)/shellcheck out/l
 .PHONY: fix
 fix: out/linters/shellcheck-$(SHELLCHECK_VERSION)-$(LINT_ARCH)/shellcheck out/linters/golangci-lint-$(GOLINT_VERSION)-$(LINT_ARCH)
 	out/linters/golangci-lint-$(GOLINT_VERSION)-$(LINT_ARCH) run --fix
-	out/linters/shellcheck-$(SHELLCHECK_VERSION)-$(LINT_ARCH)/shellcheck $(shell find . -name "*.sh") -f diff | git apply -p2 -
+	out/linters/shellcheck-$(SHELLCHECK_VERSION)-$(LINT_ARCH)/shellcheck $(shell find . -name "*.sh") -f diff | { read -t 1 line || exit 0; { echo "$$line" && cat; } | git apply -p2; }
 
 out/linters/shellcheck-$(SHELLCHECK_VERSION)-$(LINT_ARCH)/shellcheck:
 	mkdir -p out/linters

lint-install.go

@@ -266,8 +266,12 @@ func shellLintCmd(_ string, level string, fix bool) string {
 	suffix := ""
 
 	if fix {
-		// patch(1) doesn't support patching from stdin on all platforms, so we use git apply instead
-		suffix = " -f diff | git apply -p2 -"
+		// Use git apply instead of patch(1) because it doesn't support patching from stdin on all platforms.
+		// Everything after the first | is so we don't call git apply if there's nothing to apply, which would cause git apply to return an error.
+		// This is the cross platform way to do what gnu xargs does with its --no-run-if-empty switch.
+		// read -t 1 will read the first line from stdin but timeout after 1 second if empty
+		// if its not empty it will run the rest of the line after && which echoes the line and uses cat to pipe the rest to git
+		suffix = ` -f diff | { read -t 1 line || exit 0; { echo "$$line" && cat; } | git apply -p2; }`
 	} else if level == "warn" {
 		suffix = " || true"
 	}