Linux command behaves strangely in shell command line
What's happening is that $x is executed as a single command. The shell isn't "re-parsing" the contents of $x, and so it's not, in particular, setting up redirection and spawning sort.
What happens is that cat gets handed three arguments: file, | and sort. You don't have a file named | or sort in that directory, so cat can't open them. (I'm guessing that what the error says anyway1.)
You could get the result you expect by running:
eval "$x"I'd avoid using eval if you can though, it can be dangerous.
1Prefix your commands with LC_ALL= to get English error messages before asking for help on international sites.
e.g. LC_ALL= $x.