Add help command. Remove compiled files.

programs/CMakeLists.txt

@@ -73,7 +73,8 @@ set(PROGRAMS
     kill.c
     ipcrm.c
     ipcs.c
-    sleep.c)
+    sleep.c
+    help.c)
 foreach(PROGRAM ${PROGRAMS})
   # Prepare the program name.
   string(REPLACE ".c" "" PROGRAM_NAME ${PROGRAM})

programs/help.c

@@ -0,0 +1,32 @@
+///                MentOS, The Mentoring Operating system project
+/// @file help.c
+/// @brief Shows the available commands.
+/// @copyright (c) 2014-2021 This file is distributed under the MIT License.
+/// See LICENSE.md for details.
+
+#include <strerror.h>
+#include <sys/dirent.h>
+#include <sys/unistd.h>
+#include <fcntl.h>
+#include <stdio.h>
+
+int main(int argc, char *argv[])
+{
+    int fd = open("/bin", O_RDONLY | O_DIRECTORY, 0);
+    if (fd == -1) {
+        printf("%s: cannot access '/bin': %s\n\n", argv[0], strerror(errno));
+        return 1;
+    }
+    dirent_t dent;
+    int per_line = 0;
+    while (getdents(fd, &dent, sizeof(dirent_t)) == sizeof(dirent_t)) {
+        printf("%10s ", dent.d_name);
+        if (++per_line == 6) {
+            per_line = 0;
+            putchar('\n');
+        }
+    }
+    putchar('\n');
+    close(fd);
+    return 0;
+}