Redirections are processed from left to right, and in Bash, after pipelines. In

cat numbers.txt | sort < words.txt

the shell first sets up sort’s (future) standard input to be the pipe from cat, but then when it processes < words.txt, changes it to read words.txt.

<< starts a heredoc:

cat numbers.txt | sort << words.txt

will sit waiting for a line saying “words.txt” (since that is specified as the end-of-input marker in your command). See What are the shell's control and redirection operators?

Redirections are processed from left to right, and in Bash, after pipelines (in fact, in any POSIX shell). In

cat numbers.txt | sort < words.txt

the shell first sets up sort’s (future) standard input to be the pipe from cat, but then when it processes < words.txt, changes it to read words.txt.

<< starts a heredoc:

cat numbers.txt | sort << words.txt

will sit waiting for a line saying “words.txt” (since that is specified as the end-of-input marker in your command). See What are the shell's control and redirection operators?

Redirections are processed from left to right. In

cat numbers.txt | sort < words.txt

the shell first sets up sort’s (future) standard input to be the pipe from cat, but then when it processes < words.txt, changes it to read words.txt.

<< starts a heredoc:

cat numbers.txt | sort << words.txt

will sit waiting for a line saying “words.txt” (since that is specified as the end-of-input marker in your command). See What are the shell's control and redirection operators?

Redirections are processed from left to right, and in Bash, after pipelines. In

cat numbers.txt | sort < words.txt

the shell first sets up sort’s (future) standard input to be the pipe from cat, but then when it processes < words.txt, changes it to read words.txt.

<< starts a heredoc:

cat numbers.txt | sort << words.txt

will sit waiting for a line saying “words.txt” (since that is specified as the end-of-input marker in your command). See What are the shell's control and redirection operators?