Question

See the example below.

<concat destfile="dest-file">
    <fileset dir="dir1" />
    <!-- how to append the output of the executable below without creating a temporary file for that output? -->
    <apply executable="command1">...</apply>
</concat>

Answer 1

The apply and exec tasks support redirectors so you can do something like:

<apply executable="command1" >
    <redirector append="true" output="file.log" />
</apply>