ui: Add VMTEST_NO_UI environment variable

This disable the terminal drawing UI in vmtest. Previous workaround was
`vmtest ... | cat -`. But that's only obvious to people who are deeply
familiar with linux/unix. So add a more mundane environment variable and
document it.

Author: danobi

src/main.rs

@@ -13,8 +13,12 @@ use regex::Regex;
 
 use vmtest::{Config, Target, Ui, VMConfig, Vmtest};
 
+const HELP_ENV_VARS: &str = r#"Environment variables:
+  VMTEST_NO_UI    Set to disable UI  [default: unset]
+"#;
+
 #[derive(Parser, Debug)]
-#[clap(version)]
+#[clap(version, disable_colored_help=true, after_help=HELP_ENV_VARS)]
 struct Args {
     /// Path to config file
     #[clap(short, long)]

src/ui.rs

@@ -1,4 +1,5 @@
 use std::cmp::min;
+use std::env;
 use std::sync::mpsc::{channel, Receiver};
 use std::thread;
 
@@ -26,9 +27,18 @@ struct Stage {
     expand: bool,
 }
 
+/// Returns whether the terminal is attended or not
+fn is_attended(term: &Term) -> bool {
+    if env::var_os("VMTEST_NO_UI").is_some() {
+        return false;
+    }
+
+    term.features().is_attended()
+}
+
 /// Helper to clear lines depending on whether or not a tty is attached
 fn clear_last_lines(term: &Term, n: usize) {
-    if term.features().is_attended() {
+    if is_attended(term) {
         term.clear_last_lines(n).unwrap();
     }
 }
@@ -68,7 +78,7 @@ impl Stage {
     /// We kinda hack this to return 1 if we're not writing to terminal.
     /// Should really find a cleaner solution.
     fn window_size(&self) -> usize {
-        if self.term.features().is_attended() {
+        if is_attended(&self.term) {
             min(self.lines.len(), WINDOW_LENGTH)
         } else {
             min(self.lines.len(), 1)
@@ -91,7 +101,7 @@ impl Stage {
 
         // Compute new visible lines
         let trimmed_line = line.trim_end();
-        let styled_line = if self.term.features().is_attended() {
+        let styled_line = if is_attended(&self.term) {
             // If output is attended, we do custom window with our own styling.
             // Therefore, we need to strip away any existing formatting.
             let stripped = strip_ansi_codes(trimmed_line);
@@ -133,7 +143,7 @@ impl Stage {
 impl Drop for Stage {
     fn drop(&mut self) {
         clear_last_lines(&self.term, self.window_size());
-        if self.expand && self.term.features().is_attended() {
+        if self.expand && is_attended(&self.term) {
             for line in &self.lines {
                 self.term
                     .write_line(line)