Add pacman install check

src/main.rs

@@ -6,12 +6,19 @@
     You should have received a copy of the GNU General Public License along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */
 
-fn main() -> Result<(), Box<dyn std::error::Error>> {
+mod pacman_install_check;
+
+fn main() {
     println!("{}", copyright_notice());
 
-    Ok(())
+    if let Err(e) = pacman_install_check::run() {
+        eprintln!("{}", e);
+        return;
+    }
+
+    println!("Here we do things");
 }
 
 fn copyright_notice() -> &'static str {
-    "Rust Arch Linux Updater Copyright (C) 2024 Luke Harding <luke@lukeh990.io>\nThis  program comes with ABSOLUTELY NO WARRANTY\nThis is free software, and you are welcome to redistribute it under certain conditions"
+    "Rust Arch Linux Updater Copyright (C) 2024 Luke Harding <luke@lukeh990.io>\nThis  program comes with ABSOLUTELY NO WARRANTY\nThis is free software, and you are welcome to redistribute it under certain conditions\n"
 }

src/pacman_install_check.rs

@@ -0,0 +1,30 @@
+/*
+    Rust Arch Linux Updater
+    Copyright (C) 2024 Luke Harding <luke@lukeh990.io>
+    This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
+    This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+    You should have received a copy of the GNU General Public License along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+
+use std::fmt;
+use std::fmt::Formatter;
+use std::path::Path;
+
+#[derive(Debug, Clone)]
+pub struct PacmanInstalledError;
+
+impl fmt::Display for PacmanInstalledError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(f, "pacman pkg manager not installed.")
+    }
+}
+
+pub fn run() -> Result<(), PacmanInstalledError> {
+    let exists = Path::new("/usr/bin/pacman").exists();
+
+    if exists {
+        Ok(())
+    } else {
+        Err(PacmanInstalledError)
+    }
+}