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+title = "Running a Command and Saving Its Output to File in Rust"
+date = 2023-09-01
+
+[taxonomies]
+tags = ["random", "rust", "command", "log"]
++++
+
+I had an issue: I needed to run a command inside Rust, but I needed that all
+its output should go to a file, and I needed to check if there were certain
+phrases in it.
+
+<!-- more -->
+
+So, first step: Create a script that could "replicate" the output of a command,
+with the expected strings to be captured:
+
+```bash
+#!/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 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
+
+	if (( $loop%8 == 0)); then
+		# ERR is just to make sure we find it easily in the logs
+		echo "ERR: Sometimes, I also write in stderr!" >&2
+		echo "ERR: Just for funsies!" >&2
+	fi
+
+	echo "Like this."
+	echo "Then you're good to go."
+	echo ""
+done
+```
+
+What this script does is to print a message over 1,000 times, and sometimes it
+will display a "WARNING" text -- which is the special output I need to capture --
+and sometimes it will print things to stderr.
+
+For the code, what we need to do is:
+
+1. Spawn the command;
+2. Take the stdour (and stderr) from it.
+3. Spawn a thread that will keep listening to the output, doing the search,
+   and writing everything to a file (our log).
+4. The thread returns the list of captured messages, which we can get back
+   when we `.join()` it.
+5. Since I was expecting stderr to be smaller enough, I did the capturing of
+   it after the thread completes (which would also close the file, so we can
+   be sure that we can open it again without any issues).
+
+The first step is quite easy: Just use `std::process::Command` and use the
+`.spawn()` function to create the `Child` controller.
+
+For the second step, we use the `Child` structure and use `.take()` on both
+stdout and stderr. This will give us the file descriptor for both (think about
+them as `File`s).
+
+The third step is quite easy, actualy: `std::thread::spawn()` to create a
+thread, and just read the content from the file descriptors from step 2. In
+this, I used `BufReader`, which gives access to reading the content line by
+line, which is way easier than reading to a buffer and processing it.
+
+```rust
+use std::fs::{File, OpenOptions};
+use std::io::{BufRead, BufReader, Read, Write};
+use std::path::PathBuf;
+use std::process::Command;
+
+fn main() {
+    // this requires always running with `cargo run`
+    let base = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+    let the_script = base.join("src").join("the_script.sh");
+
+    let mut cmd = Command::new("bash")
+        .arg(the_script)
+        .stdout(std::process::Stdio::piped())
+        .stderr(std::process::Stdio::piped())
+        .spawn()
+        .unwrap();
+
+    // capture both the stdout and stderr as File structs (actually FDs, but basically the same
+    // thing)
+    let stdout = cmd.stdout.take().unwrap();
+    let mut stderr = cmd.stderr.take().unwrap();
+
+    // spawn a thread to keep capturing and processing the stdout.
+    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
+    });
+
+    // run the command till it finishes
+    cmd.wait().unwrap();
+
+    // ... and wait till the thread finishes processing the whole output.
+    let warnings = writer_pid.join().unwrap();
+
+    // this is somewhat a hack: Instead of spawning a thread for stderr and trying to fight with
+    // stdout for the lock to be able to write in the log file, we do this after the thread ends
+    // (which closes the file) and then open it again and write the stderr in the end. We do this
+    // 'cause we expect that the stderr is way smaller than stdout and can fit in memory without
+    // any issues.
+    let mut buffer = String::new();
+    stderr.read_to_string(&mut buffer).unwrap();
+
+    let mut file = OpenOptions::new().append(true).open("script.log").unwrap();
+    file.write(buffer.as_bytes()).unwrap();
+
+    // This is purely for diagnostic purposes. We could put the warnings in another file, or pass
+    // it along to something else to process it. Here, we just display them.
+    // Same for stderr: Since we already put them in the file, this is used just to make sure we
+    // are capturing the errors without looking at the file.
+    println!("Warnings:\n{:?}", warnings);
+    println!("ERR:\n{:?}", buffer)
+}
+```