Shell: Remember previous working dir

...and allow switching back to it with `cd -`

Partially addresses SerenityOS#397

Shell/main.cpp

@@ -83,10 +83,16 @@ static int sh_cd(int argc, char** argv)
  if (argc == 1) {
  strcpy(pathbuf, g.home.characters());
  } else {
- if (argv[1][0] == '/')
+ if (strcmp(argv[1], "-") == 0) {
+ char* oldpwd = getenv("OLDPWD");
+ size_t len = strlen(oldpwd);
+ ASSERT(len + 1 <= PATH_MAX);
+ memcpy(pathbuf, oldpwd, len + 1);
+ } else if (argv[1][0] == '/') {
  memcpy(pathbuf, argv[1], strlen(argv[1]) + 1);
- else
+ } else {
  sprintf(pathbuf, "%s/%s", g.cwd.characters(), argv[1]);
+ }
  }
 
  FileSystemPath canonical_path(pathbuf);
@@ -111,7 +117,9 @@ static int sh_cd(int argc, char** argv)
  printf("chdir(%s) failed: %s\n", path, strerror(errno));
  return 1;
  }
+ setenv("OLDPWD", g.cwd.characters(), 1);
  g.cwd = canonical_path.string();
+ setenv("PWD", g.cwd.characters(), 1);
  return 0;
 }
 
@@ -635,6 +643,7 @@ int main(int argc, char** argv)
  {
  auto* cwd = getcwd(nullptr, 0);
  g.cwd = cwd;
+ setenv("PWD", cwd, 1);
  free(cwd);
  }