add dry-run option

Closes #2

src/cli.rs

@@ -41,6 +41,9 @@ pub struct Args {
     #[clap(short = 'o', long)]
     pub output: Option<PathBuf>,
 
+    #[clap(long)]
+    pub dry_run: bool,
+
     #[clap(value_parser)]
     pub sections: Vec<String>,
 }

src/cli/switch.rs

@@ -16,6 +16,7 @@
 
 use crate::{cli::Args, domain};
 use std::fs::File;
+use std::io::{stdout, Write};
 
 #[derive(Debug)]
 pub enum Error {
@@ -36,8 +37,15 @@ impl std::error::Error for Error {}
 
 pub fn execute(args: &Args) -> Result<(), Error> {
     let content = std::fs::read_to_string(&args.file).map_err(|_| Error::OpenFile)?;
-    let writer =
-        File::create(args.output.as_ref().unwrap_or(&args.file)).map_err(|_| Error::OpenFile)?;
+
+    let writer: Box<dyn Write> = if args.dry_run {
+        Box::new(stdout())
+    } else {
+        Box::new(
+            File::create(args.output.as_ref().unwrap_or(&args.file))
+                .map_err(|_| Error::OpenFile)?,
+        )
+    };
 
     domain::switch::execute(domain::switch::Request {
         content: &content,