T16: a shell saves only when the running user may run it, as access answers, ACLs too, found at a full path or in one of PATH's full folders, never a relative one; a session runs that same full path, so the check and the start agree

src/term/shell.rs

@@ -77,7 +77,12 @@ pub fn launch(shell: &str, cwd: PathBuf, dir: &Path) -> anyhow::Result<Launch> {
     let path = if shell.is_empty() {
         default_shell()?
     } else {
-        shell.to_string()
+        // The settings checked this path; the session runs no other.
+        let found = crate::settings::program_path(shell)
+            .with_context(|| format!("shell integration: {shell:?} names no program"))?;
+        (found.to_str())
+            .with_context(|| format!("shell integration: {} is not UTF-8", found.display()))?
+            .to_string()
     };
     let name = Path::new(&path)
         .file_stem()

src/term/shell/tests.rs

@@ -247,6 +247,14 @@ fn powershell_reports_its_folder_byte_for_byte_and_keeps_the_status() {
     run.finish();
 }
 
+#[test]
+fn a_shell_named_bare_runs_from_its_full_path() {
+    let started = launch("sh", PathBuf::from("/"), Path::new("/nonexistent")).expect("a launch");
+    let (program, _) = started.program.expect("a program");
+    assert!(Path::new(&program).is_absolute(), "{program}");
+    assert!(launch("no-such-shell", PathBuf::from("/"), Path::new("/")).is_err());
+}
+
 #[test]
 fn an_unknown_shell_starts_without_integration() {
     let dir = PathBuf::from("/nonexistent");