Too Busy For Words - the PaulWay Blog

I had an idea on Friday for a utility that fills a little niche that I hit regularly. The particular example was wanting to save the iptables configuration after a couple of updates. This is put (on Red Hat standard boxes) in /etc/sysconfig/iptables, and I keep copies named /etc/sysconfig/iptables.yyyymmdd (where yyyymmdd is the current year, month and day) in case a change breaks something and I need to go back to a previous version. Other people use revision control systems like Mercurial for this, with its ability to watch edits to a file that isn't in a pre-set directory. I may be old fashioned here but this method does me fine. Normally, in order to roll the configuration over to the new version you would do:

mv /etc/sysconfig/iptables /etc/sysconfig/iptables.yyyymmdd
iptables_save > /etc/sysconfig.iptables

But what if you'd already done one edit today? Then you'd use a name like /etc/sysconfig/iptables.yyyymmdd.inc, where inc is an increment number or something. And you want that number to increment up until it finds a 'free' number. The usual convention for log files is to roll each file down, so /etc/sysconfig/iptables.yyyymmdd becomes /etc/sysconfig/iptables.yyyymmdd.1, /etc/sysconfig/iptables.yyyymmdd.1 becomes /etc/sysconfig/iptables.yyyymmdd.2 and so forth; I usually end up putting the latest revision at the end of the sequence rather than the earliest.

Now, of course, it would be relatively simple to do that renaming automatically given the base file name. Cafuego coded up a Bash one-liner in half an hour or so, and Debian already has the savelog utility to do just this (a fact I found out much later, not running Debian). However, that only really does half the job. We still end up with:

savelog /etc/sysconfig/iptables
iptables_save > /etc/sysconfig.iptables

That's one repetition of that annoying path too many, with its hostile tab-unfriendly sysconfig directory, for my taste. I realised that what I wanted was something like:

iptables_save | roll /etc/sysconfig.iptables

that would both roll the log file over and then 'redirect' standard input to the target file. Again, a relatively short piece of work in Perl or bash. But do you really want to have to call up all that framework just to roll one file over? I resolved to learn a bit more and do it in C. Not only that, but I'd forswear my usual use of the talloc library and do it as raw as possible.

It took a day, but by the end of it I had coded up the first working version of the code. I presented it to the gallery on the #linux-aus IRC channel on Freenode and Cafuego pointed out that I'd only implemented the all-move-down method, not the move-to-last method. A bit more work that night added that. A bit more work with valgrind found the couple of memory leaks and odd overwrites. More work today put the command-line options processing in place, and corrected the move-to-last method to not only work, but in the process be more efficient.

So, now I release to the wider Linux and Unix community the roll command. You can find the source code at http://mabula.net/code/roll.c and check it out via Subversion through svn://tangram.dnsalias.net/roll/trunk. Comments, criticisms and suggestions as always are welcomed via paulway@mabula.net. Of course, the irony is that I could have written that mv /etc/sysconfig/iptables /etc/sysconfig/iptables.20071123 command by now...