sh: Implement our own execvp() while we wait for libc.

Of course, this is a very primitive execvp with hardcoded paths.
If it were decent, it would be integrated into libc instantly.

apps/src/sh.c

@@ -16,6 +16,37 @@ typedef struct
     size_t capacity;
 } command;
 
+char* shell_concat_path(const char* dirname, const char* basename)
+{
+    char* buf = malloc(strlen(basename) + strlen(dirname) + 6);
+    strlcpy(buf, dirname, strlen(dirname) + 1);
+    strncat(buf, basename, strlen(basename));
+    return buf;
+}
+
+void shell_execvp(char* pathname, char* const argv[])
+{
+    char* new_path;
+    if (access(pathname, F_OK) == 0)
+    {
+        execv(pathname, argv);
+        return;
+    }
+    if (pathname[0] == '/') return; // We do not want to lookup absolute paths
+    new_path = shell_concat_path("/bin/", pathname);
+    if (access(new_path, F_OK) == 0)
+    {
+        execv(new_path, argv);
+        return;
+    }
+    free(new_path);
+    new_path = shell_concat_path("/usr/bin/", pathname);
+    execv(new_path, argv);
+    int saved = errno;
+    free(new_path);
+    errno = saved;
+}
+
 void show_prompt()
 {
     if (WEXITSTATUS(status)) { printf("%d [%ld]> ", WEXITSTATUS(status), getpid()); }
@@ -105,23 +136,9 @@ void command_execute(command* cmd)
         return;
     }
     if (child == 0)
-    {
-        if (cmd->buffer[0] != '/' && access(cmd->buffer, F_OK) < 0) // FIXME: Race condition.
-        {
-            if (errno == ENOENT)
-            { // Try in /bin
-                char* buf = malloc(cmd->size + 6);
-                strlcpy(buf, "/bin/", 6);
-                strncat(buf, cmd->buffer, cmd->size);
-                char* argv[] = {buf, NULL};
-                execv(argv[0], argv);
-            }
-        }
-        else
     {
         char* argv[] = {cmd->buffer, NULL};
-            execv(argv[0], argv);
-        }
+        shell_execvp(argv[0], argv);
         perror(cmd->buffer);
         exit(127);
     }