From 525393246972d14945187602902d5d5e47a9a59e Mon Sep 17 00:00:00 2001
From: Nick Chambers <uplime@spookyinternet.com>
Date: Fri, 19 Apr 2024 11:20:22 -0500
Subject: [PATCH] Demonstrate how to remap stdout

---
 redirection.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)
 create mode 100644 redirection.c

diff --git a/redirection.c b/redirection.c
new file mode 100644
index 0000000..c0cba6c
--- /dev/null
+++ b/redirection.c
@@ -0,0 +1,44 @@
+#include <fcntl.h>
+#include <stdio.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+int main() {
+  // The program that will get run, and the file to write to. It is similar to
+  // the command-line:
+  // echo "Hello, world!" > ./hello.txt
+  char *const prog[] = { "echo", "Hello, world!", NULL };
+  const char *filename = "./hello.txt";
+
+  // Create a child process like normal. The parent process will wait for the
+  // child process, and the child process will execute its program with the
+  // standard output redirected to the file.
+  pid_t pid = fork();
+
+  if(pid == 0) {
+    // Mimicking the behavior of `>` in the shell, open a handle to the file
+    // `hello.txt` for writing. If the file doesn't exist, it will be created,
+    // and if it does exist, truncate the contents of the file.
+    int handle = open(filename, O_CREAT | O_WRONLY | O_TRUNC, S_IRUSR | S_IWUSR);
+
+    // Map the standard output of the child process to the open file handle.
+    // This means that whatever writes on the child's standard output stream
+    // will instead go to the file. The parent's standard output stream will
+    // remain unchanged.
+    dup2(handle, STDOUT_FILENO);
+
+    // Execute the program in the child as normal, now that the standard output
+    // stream has been redirected to the file.
+    execvp(prog[0], prog);
+  } else {
+    // Wait in the parent process for the child to finish running. Once it is
+    // finished, write the child's process and exit information to its own
+    // standard output stream, which was not changed by the child's `dup2`
+    // call.
+    int status = 0;
+    wait(&status);
+    printf("Process %d exited with status %d.\n", pid, status);
+  }
+
+  return 0;
+}