== Finding free numbers in a range, crudely, with Unix tools

Suppose, not entirely hypothetically, that you have the zone file
for the reverse mapping of a single /24 subnet (in Bind format) and
you want to find a free IP address in that subnet. The first field
in the zone file is the last octet of the IP (ie for '127.0.0.1'
it is '1'), so this problem reduces to finding what numbers are not
used in the file out of the range 1 to 254.

So let's do this really crudely with readily available Unix tools:

.pn prewrap on
>  grep '^[0-9]' ptrzone | awk '{print $1}' | sort >/tmp/used
>  seq 1 254 | sort >/tmp/all
>  comm -13 /tmp/used /tmp/all | sort -n

(We must do a '_sort -n_' only at the end because of [[a stupid GNU
_comm_ decision ../sysadmin/GnuCommMisfeature]].)

We can get rid of one of the two files here by piping directly into
_comm_, but unfortunately we can't get rid of both of them in
standard Bourne shell. In Bash and zsh (and AT&T ksh), we can use
what they each call 'process substitution' to directly embed each
pipeline into the _comm_ arguments:

>  comm -13 <(grep '^[0-9]' ptrzone | awk '{print $1}' | sort) <(seq 1 254 | sort) | sort -n

(This also works in [[_rc_ https://en.wikipedia.org/wiki/Rc]], with a
different syntax.)

Once upon a time I would have happily written the single-line
version. These days my tendencies are towards the multi-line version
unless I'm feeling excessively clever, partly because it's easier
to check the intermediate steps.

(A neater version would condense runs of numbers into ranges, but
I don't think you can do that simply with any existing common Unix
tools. Perl experts can maybe do it in a single line.)

PS: One hazard of a very well stuffed _$HOME/bin_ directory is that
it turns out I already had a command to do this for me (and with
the neater range-based output). Oh well, such things happen.  Perhaps
I should thin out my _$HOME/bin_ on the grounds of disuse of much
of it, but it's always hard to get rid of your personal little
scripts.