Update

src/examples/shell.c

+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+#include <syscall.h>
+
+static void read_line (char line[], size_t);
+static bool backspace (char **pos, char line[]);
+
+int
+main (void)
+{
+  printf ("Shell starting...\n");
+  for (;;)
+    {
+      char command[80];
+
+      /* Read command. */
+      printf ("--");
+      read_line (command, sizeof command);
+
+      /* Execute command. */
+      if (!strcmp (command, "exit"))
+        break;
+      else if (!memcmp (command, "cd ", 3))
+        {
+          if (!chdir (command + 3))
+            printf ("\"%s\": chdir failed\n", command + 3);
+        }
+      else if (command[0] == '\0')
+        {
+          /* Empty command. */
+        }
+      else
+        {
+          pid_t pid = exec (command);
+          if (pid != PID_ERROR)
+            printf ("\"%s\": exit code %d\n", command, wait (pid));
+          else
+            printf ("exec failed\n");
+        }
+    }
+
+  printf ("Shell exiting.");
+  return EXIT_SUCCESS;
+}
+
+/* Reads a line of input from the user into LINE, which has room
+   for SIZE bytes.  Handles backspace and Ctrl+U in the ways
+   expected by Unix users.  On return, LINE will always be
+   null-terminated and will not end in a new-line character. */
+static void
+read_line (char line[], size_t size)
+{
+  char *pos = line;
+  for (;;)
+    {
+      char c;
+      read (STDIN_FILENO, &c, 1);
+
+      switch (c)
+        {
+        case '\r':
+          *pos = '\0';
+          putchar ('\n');
+          return;
+
+        case '\b':
+          backspace (&pos, line);
+          break;
+
+        case ('U' - 'A') + 1:       /* Ctrl+U. */
+          while (backspace (&pos, line))
+            continue;
+          break;
+
+        default:
+          /* Add character to line. */
+          if (pos < line + size - 1)
+            {
+              putchar (c);
+              *pos++ = c;
+            }
+          break;
+        }
+    }
+}
+
+/* If *POS is past the beginning of LINE, backs up one character
+   position.  Returns true if successful, false if nothing was
+   done. */
+static bool
+backspace (char **pos, char line[])
+{
+  if (*pos > line)
+    {
+      /* Back up cursor, overwrite character, back up
+         again. */
+      printf ("\b \b");
+      (*pos)--;
+      return true;
+    }
+  else
+    return false;
+}