# set -x        # what I typed
# echo 'love'   # ...
+ echo love     <--(1) the trace
love            # the output

# echo love?    # note the input contains no quote whatsoever
+ echo 'love?'  <--(2) note the trace contains quotes after returning word
love?           # i.e. failed to find any file

# echo 'love?'  # note the input contains single quotes
+ echo 'love?'  <--(3) traces like (2)
love?

# touch loveu   # we create a file that matches the love? pattern
+ touch loveu

# echo love?    # of course now, the pattern matches the created file now
+ echo loveu    <--(4) indeed it finds it and expands to name
loveu           # the name is echoed

# for i in love love? 'love?'; do echo $i; done
+ for i in love 'love?' ''''love?''''
+ echo love
love
+ for i in love 'love?' ''''love?''''
+ echo 'love?'
love?
+ for i in love 'love?' ''''love?''''
+ echo 'love?'
love?

$ set -x
$ echo foo;bar
+ echo 'foo;bar'

/* A function to print the words of a simple command when set -x is on. */
void
xtrace_print_word_list (list, xtflags)
...
{
  ...
  for (w = list; w; w = w->next)
    {
      t = w->word->word;
      ...
      else if (sh_contains_shell_metas (t))
        {
          x = sh_single_quote (t);
          fprintf (xtrace_fp, "%s%s", x, w->next ? " " : "");
          free (x);
        }

/* Print the value cell of VAR, a shell variable.  Do not print
   the name, nor leading/trailing newline.  If QUOTE is non-zero,
   and the value contains shell metacharacters, quote the value
   in such a way that it can be read back in. */
void
print_var_value (var, quote)
...
{
  ...
  else if (quote && sh_contains_shell_metas (value_cell (var)))
    {
      t = sh_single_quote (value_cell (var));
      printf ("%s", t);
      free (t);
    }