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

November 18, 2014

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 '' 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:

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.)

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, 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.

Written on 18 November 2014.
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Last modified: Tue Nov 18 23:26:13 2014
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