Spawn a thread to capture and process the output

commandtest/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "commandtest"
+version = "0.1.0"

commandtest/src/main.rs

@@ -1,5 +1,6 @@
 use std::ffi::OsString;
 use std::fs::File;
+use std::io::{BufRead, BufReader, Write};
 use std::path::PathBuf;
 use std::process::Command;
 
@@ -18,15 +19,41 @@ fn search_in_path(name: &str) -> Option<PathBuf> {
 fn main() {
     // this requires always running with `cargo run`
     let base = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
-    let log_file = File::create("script.log").unwrap();
     let the_script = base.join("src").join("the_script.sh");
     let bash = search_in_path("bash").unwrap();
 
     let mut cmd = Command::new(bash)
         .arg(the_script)
-        .stdout(log_file)
+        .stdout(std::process::Stdio::piped())
         .spawn()
         .unwrap();
 
+    let stdout = cmd.stdout.take().unwrap();
+    let writer_pid = std::thread::spawn(move || {
+        let reader = BufReader::new(stdout);
+        let lines = reader.lines();
+        let mut log_file = File::create("script.log").unwrap();
+        let mut in_warning = false;
+        let mut result = Vec::new();
+
+        for line in lines {
+            let line = line.unwrap();
+            log_file.write(line.as_bytes()).unwrap();
+            log_file.write(b"\n").unwrap(); // 'cause lines() eat it
+
+            if line.starts_with("WARNING:") {
+                in_warning = true;
+            } else if line.starts_with("   ") && in_warning {
+                result.push(line);
+            } else if in_warning {
+                in_warning = false;
+            }
+        }
+
+        result
+    });
+
     cmd.wait().unwrap();
+    let warnings = writer_pid.join().unwrap();
+    println!("Warnings:\n{:?}", warnings);
 }

commandtest/src/the_script.sh

@@ -1,11 +1,19 @@
 #!/usr/bin/env bash
 
+for loop in {1..1000}
+do
 	echo "Hello, I'm a script!"
 	echo "I write stuff in the output."
 	echo "Everything should go to a file."
-echo "But also, you need to capture this:"
+	echo "But also, you need to capture warnings:"
+
+	if (( $loop%7 == 0)); then
 		echo "WARNING: This is a warning"
 		echo "   It continues if the line starts with spaces"
 		echo "   And keeps going till there are no more spaces-prefixes"
+	fi
+
 	echo "Like this."
 	echo "Then you're good to go."
+	echo ""
+done