feat(cli): added pre-flight shell check

Addresses #314.

packages/perseus-cli/src/errors.rs

@@ -5,6 +5,11 @@ use thiserror::Error;
 /// All errors that can be returned by the CLI.
 #[derive(Error, Debug)]
 pub enum Error {
+    #[error("couldn't find your system shell (sh on unix and powershell on windows), which is required to run the perseus cli")]
+    ShellNotPresent {
+        #[source]
+        source: std::io::Error,
+    },
     #[error("couldn't find `cargo`, which is a dependency of this cli (set 'PERSEUS_CARGO_PATH' to another location if you've installed it elsewhere)")]
     CargoNotPresent {
         #[source]

packages/perseus-cli/src/prepare.rs

@@ -22,6 +22,22 @@ pub fn check_env(global_opts: &Opts) -> Result<(), Error> {
     #[cfg(windows)]
     let shell_param = "-command";
 
+    // Check for the shell before anything else (#314)
+    let shell_res = Command::new(shell_exec)
+        .arg("--version")
+        .output()
+        .map_err(|err| Error::ShellNotPresent { source: err })?;
+    let exit_code = match shell_res.status.code() {
+        Some(exit_code) => exit_code,
+        None if shell_res.status.success() => 0,
+        None => 1,
+    };
+    if exit_code != 0 {
+        return Err(Error::ShellNotPresent {
+            source: std::io::Error::new(std::io::ErrorKind::NotFound, "non-zero exit code"),
+        });
+    }
+
     // Check for `cargo`
     let cargo_cmd = global_opts.cargo_engine_path.to_string() + " --version";
     let cargo_res = Command::new(shell_exec)