pkgs.testers.testEqualContents

The following is the current implementation of this function.

testEqualContents = {
    assertion,
    actual,
    expected,
  }: runCommand "equal-contents-${lib.strings.toLower assertion}" {
    inherit assertion actual expected;
    nativeBuildInputs = [ diffoscopeMinimal ];
  } ''
    echo "Checking:"
    printf '%s\n' "$assertion"
    if ! diffoscope --no-progress --text-color=always --exclude-directory-metadata=no -- "$actual" "$expected"
    then
      echo
      echo 'Contents must be equal, but were not!'
      echo
      echo "+: expected,   at $expected"
      echo "-: unexpected, at $actual"
      false
    else
      echo "expected $expected and actual $actual match."
      echo OK
      touch -- "$out"
    fi
  '';