the standard input, and replacing it with the output of the command. This
command is most useful with filters, such as
sort. Its practical effect
is to pass the block through the specified filter.
Note that by selecting an empty block (or equivalently by having the mark
unset) you can use
Through in order to insert the output of any UN*X
command in your file.
If the optional argument command is not specified, you can enter it on the input line.