[Novalug] What is the name of a simple command|

[Michael Henry]

Mackenzie Morgan wrote:
>  Of course, if your goal was just to add bar's contents to the end of 
foo's
>  and have it all stay in foo, just tweak the above for that:
>  cat foo bar > foo
>  or
>  cat bar >> foo
The natural-seeming first invocation, ``cat foo bar > foo``
unfortunately doesn't work.  Before ``cat`` executes, the shell has
already clobbered ``foo`` with the redirection operator.  GNU ``cat`` is
smart enough to detect this problem and warn you after the fact::

    $ echo "contents of foo" > foo
    $ echo "contents of bar" > bar
    $ cat foo bar > foo
    cat: foo: input file is output file
    $ cat foo
    contents of bar

But notice that the original contents of ``foo`` have been clobbered,
and only the contents of ``bar`` made it into ``foo``.  The second
invocation, ``cat bar >> foo``, properly appends ``bar`` to ``foo``.

The reason ``cat`` warns you about this problem despite the fact that
it's too late to avoid clobbering your input file is to avoid infinitely
long files:

    $ cat bar foo >> foo
    cat: foo: input file is output file

If ``cat`` were unable to notice that ``foo`` was both input and output,
it could create an infinitely long file.  The contents of ``bar`` would
first be placed into ``foo``, then ``foo`` would be opened for input and
the most recently written ``contents of bar`` would be read from ``foo``
and re-written to ``foo``.  But now, ``foo`` contains another copy of
``contents of bar``, so it would be copied again, ad infinitum.

Michael Henry