fix: converted onlyon param to only_on and added validation step

common/shell-local/config.go

@@ -30,7 +30,7 @@ type Config struct {
 	InlineShebang string `mapstructure:"inline_shebang"`
 
 	// An array of multiple Runtime OSs to run on.
-	OnlyOn []string
+	OnlyOn []string `mapstructure:"only_on"`
 
 	// The file extension to use for the file generated from the inline commands
 	TempfileExtension string `mapstructure:"tempfile_extension"`
@@ -162,6 +162,25 @@ func Validate(config *Config) error {
 				fmt.Errorf("Bad script '%s': %s", path, err))
 		}
 	}
+
+	// Check for properly formatted go os types
+	supported_syslist := []string{"darwin", "freebsd", "linux", "openbsd", "solaris", "windows"}
+	supported_os := false
+	if len(config.OnlyOn) > 0 {
+		for _, provided_os := range config.OnlyOn {
+			for _, go_os := range supported_syslist {
+				if provided_os == go_os {
+					supported_os = true
+					break
+				}
+				if supported_os != true {
+					errs = packer.MultiErrorAppend(errs,
+						fmt.Errorf("Invalid OS specified in only_on: '%s'", provided_os))
+				}
+			}
+		}
+	}
+
 	if config.UseLinuxPathing {
 		for index, script := range config.Scripts {
 			scriptAbsPath, err := filepath.Abs(script)

website/source/docs/post-processors/shell-local.html.md

@@ -91,6 +91,12 @@ Optional parameters:
     **Important:** If you customize this, be sure to include something like the
     `-e` flag, otherwise individual steps failing won't fail the provisioner.
 
+-  `only_on` (array of strings) - This is an array of 
+    [runtime operating systems](https://golang.org/doc/install/source#environment)
+    where `shell-local` will execute. This allows you to execute `shell-local` 
+    *only* on specific operating systems. By default, shell-local will always run 
+    if `only_on` is not set."
+
 -  `use_linux_pathing` (bool) - This is only relevant to windows hosts. If you
    are running Packer in a Windows environment with the Windows Subsystem for
    Linux feature enabled, and would like to invoke a bash script rather than

website/source/docs/provisioners/shell-local.html.md

@@ -108,13 +108,11 @@ Optional parameters:
     **Important:** If you customize this, be sure to include something like the
     `-e` flag, otherwise individual steps failing won't fail the provisioner.
 
--  `onlyon` (array of strings) - This is an array of 
+-  `only_on` (array of strings) - This is an array of 
     [runtime operating systems](https://golang.org/doc/install/source#environment)
-    where `shell-local` will only run on. This allows you to run `shell-local` *only* 
-    on specific compatible operating systems. If specified, shell-local will only
-    execute if runtime operating system is in the list; otherwise it will skip the 
-    `shell-local` command. The default behavior is for `shell-local` to run if 
-    `onlyon` is not specified.
+    where `shell-local` will execute. This allows you to execute `shell-local` 
+    *only* on specific operating systems. By default, shell-local will always run 
+    if `only_on` is not set."
 
 -  `use_linux_pathing` (bool) - This is only relevant to windows hosts. If you
    are running Packer in a Windows environment with the Windows Subsystem for