You cannot use `sudo cat` to open a file with root privileges, because `sudo cat > foo` means "open file foo with your current privileges, then run `sudo cat` passing the file to it", and the whole root thing only happens after you already tried and failed to open the file.

I seem to get this wrong roughly once a week.

Cat won’t write the file. If you mean `command | sudo cat > file.txt`, that won’t work because the redirection is still happening in the non-root shell. You could do `command | sudo sh -c "cat > file.txt"` but that’s rather verbose.

I thought the same thing. What's wrong with this approach?

  cat myfile.txt | awk '{do_stuff()}' | sort -u | column --table > new-myfile.txt