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I have a secondary backup drive that is usually stored offsite, but sometimes mounted. I'd like to put something in a crontab that automatically clones my backup drive to the secondary backup if it's mounted. I know that I could do something like:

if [ -d $target_dir ]; then rsync -a --delete $src_dir $target_dir; fi

but I'm wondering if there's a way to ask rsync the same thing, without resorting to a shell script? Given that it has 6.02*10^23 command line options, you'd think so...

  • Not to make your job harder, but if you have udev available, maybe have it run the rsync when it notices the drive being attached? – Jeff Schaller Nov 2 '18 at 16:53
  • Why don't you run the command regardless? rsync will simply fail if the hard drive is not mounted. – Katu Nov 2 '18 at 17:00
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    Nice use of Avogadro's number. :) – Lewis M Nov 2 '18 at 17:01
  • @Katu, wouldn't it attempt to create a directory in the root filesystem? – Scott Deerwester Nov 2 '18 at 17:34
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    If you don't mind it erroring out you can do something like rsync /src/. /target/. The trailing /. on the path will cause it to error out if the directory doesn't exist. – Zoredache Nov 2 '18 at 18:53
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Here's a trick that would probably work ... In my example, rsync will succeed if $target_dir is on the 'remote' machine (localhost), else fail (by not running the remote rsync).

i.e.

src_dir=/etc;target_dir=/test/a;rsync -azp --rsync-path="[[ -d $target_dir ]] && rsync" $src_dir localhost:$target_dir

The failure result code should be 12 if the remote directory doesn't exist ... because it never actually ran the remote rsync due to the failure of "[[ -d $target_dir ]]"

12 Error in rsync protocol data stream

The caveat is you've got to use a TCP socket to get it to work, which may be slower.

Another option, via /etc/crontab (that wouldn't require a script), would be something like ..

50 23 * * * root (src_dir="/test/src_dir"; target_dir="/test/target_dir"; [ -d $src_dir ] && [ -d $target_dir ] && rsync -a -v --delete ${src_dir}/ ${target_dir}/) &>> /var/log/test.log

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