Userspace: Add basic tee command

userspace/CMakeLists.txt

@@ -9,6 +9,7 @@ set(USERSPACE_PROJECTS
 	init
 	ls
 	Shell
+	tee
 	test
 	u8sum
 	yes

userspace/tee/CMakeLists.txt

@@ -0,0 +1,16 @@
+cmake_minimum_required(VERSION 3.26)
+
+project(tee CXX)
+
+set(SOURCES
+	main.cpp
+)
+
+add_executable(tee ${SOURCES})
+target_compile_options(tee PUBLIC -O2 -g)
+target_link_libraries(tee PUBLIC libc)
+
+add_custom_target(tee-install
+	COMMAND cp ${CMAKE_CURRENT_BINARY_DIR}/tee ${BANAN_BIN}/
+	DEPENDS tee
+)

userspace/tee/main.cpp

@@ -0,0 +1,64 @@
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+
+#define MAX_FILES 20
+#define BUF_SIZE 1024
+
+int main(int argc, char** argv)
+{
+    int files[MAX_FILES] {};
+    size_t file_count = 0;
+
+    int arg = 1;
+
+    int oflag = O_WRONLY | O_CREAT;
+    if (arg < argc && strcmp(argv[arg], "-a") == 0)
+    {
+        oflag |= O_APPEND;
+        arg++;
+    }
+    else
+    {
+        oflag |= O_TRUNC;
+    }
+
+    for (int i = arg; i < argc; i++)
+    {
+        files[file_count] = open(argv[i], oflag, 0644);
+        if (files[file_count] == -1)
+            perror(argv[i]);
+        else
+            file_count++;
+        
+        if (file_count >= MAX_FILES)
+        {
+            fprintf(stderr, "only up to %d files are supported\n", MAX_FILES);
+            break;
+        }
+    }
+
+    char* buffer = (char*)malloc(BUF_SIZE);
+    for (;;)
+    {
+        ssize_t nread = read(STDIN_FILENO, buffer, BUF_SIZE);
+        if (nread == -1)
+            perror("stdin");
+        if (nread <= 0)
+            break;
+        write(STDOUT_FILENO, buffer, nread);
+        for (size_t i = 0; i < file_count; i++)
+            write(files[i], buffer, nread);
+    }
+    free(buffer);
+
+    if (ferror(stdin)) 
+        perror("stdin");
+
+    for (size_t i = 0; i < file_count; i++)
+        close(files[i]);
+
+    return 0;
+}