Add support for exiting with an exit code

This allows barnacle to be used as an init container or some other
process that simply requires barnacle to render the configuration file
but not execute another process.

It is mutually exclusive with providing a command to barnacle

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "barnacle"
-version = "0.2.0"
+version = "0.3.0"
 edition = "2021"
 authors = ["Bert JW Regeer <[email protected]>"]
 license = "ISC"

src/cli.rs

@@ -13,9 +13,13 @@ pub struct Cli {
     pub output: PathBuf,
 
     /// The command + arguments to execute once template is rendered
-    #[arg(required(true))]
+    #[arg(group = "command_or_exit", required(true))]
     pub command: Vec<String>,
 
+    /// Exit with error code, exclusive with a command to exeute
+    #[arg(short, long, group = "command_or_exit", required(true))]
+    pub exit: Option<u8>,
+
     /// Turn debugging information on
     #[arg(short, long, action = clap::ArgAction::Count)]
     pub verbose: u8,

src/main.rs

@@ -40,5 +40,11 @@ fn main() {
 
     fs::write(&matches.output, output).expect("Failed to write output file");
 
+    // If we have an exit code, use it
+    if let Some(exit) = matches.exit {
+        std::process::exit(exit.into());
+    }
+
+    // Otherwise we execute the command provided on the CLI
     exec(&matches.command);
 }