rsync - Nov 2012 - Invoking rsync from a script with null parameters

I'm working on a bash backup script using rsync.
(kubuntu precise 12.04, rsync 3.0.9-1ubuntu1)

To avoid having a number of slightly different rsync commands, I would
like to use shell variables as part of the rsync command.  I.e.:
DRYRUN="-n"
rsync "${DRYRUN}" more parameters ...

This does not work if DRYRUN="" - apparently because this command
becomes
rsync "" more parameters ...
instead of
rsync more parameters ...

and rsync uses the null parameter for something and does not perform as
expected.  It does not generate any error or diagnostic message.

Is there a way to get around this problem - other than coding each
permutation of the command separately?

I'm experimenting with putting the whole rsync command in a string so I
can run it after any null parameters revert to pure white space.  Once I
get the quoting to work (preserving those quotes I still need), this
method should work, but it's less than elegant.

Is this a bug in rsync?  (Shouldn't it at least complain/error exit if
it gets something like this that it doesn't understand?)  If it is,
what's the best way to report it?

Ideally (for me anyway), I would like it to completely ignore any null
parameters, but I don't know what problems that might cause for other
people.

TIA

Joe

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I suspect you are missing a space somewhere and are ending up with 2
parameters stuck together. You can have bash output the rsync command
line it intends to run to make sure or you can just use " " instead of
"" as extra spaces between parameters will have no effect.

On 11/04/12 22:23, Joe wrote:
> I'm working on a bash backup script using rsync. (kubuntu precise
> 12.04, rsync 3.0.9-1ubuntu1)
> 
> To avoid having a number of slightly different rsync commands, I
> would like to use shell variables as part of the rsync command.
> I.e.: DRYRUN="-n" rsync "${DRYRUN}" more parameters ...
> 
> This does not work if DRYRUN="" - apparently because this command
> becomes rsync "" more parameters ... instead of rsync more
> parameters ...
> 
> and rsync uses the null parameter for something and does not
> perform as expected.  It does not generate any error or diagnostic
> message.
> 
> Is there a way to get around this problem - other than coding each 
> permutation of the command separately?
> 
> I'm experimenting with putting the whole rsync command in a string
> so I can run it after any null parameters revert to pure white
> space.  Once I get the quoting to work (preserving those quotes I
> still need), this method should work, but it's less than elegant.
> 
> Is this a bug in rsync?  (Shouldn't it at least complain/error exit
> if it gets something like this that it doesn't understand?)  If it
> is, what's the best way to report it?
> 
> Ideally (for me anyway), I would like it to completely ignore any
> null parameters, but I don't know what problems that might cause
> for other people.
> 
> TIA
> 
> Joe
> 
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Joe wrote (Monday, November 05, 2012 4:23 AM):
> To avoid having a number of slightly different rsync commands, I would
> like to use shell variables as part of the rsync command.  I.e.:
> DRYRUN="-n"
> rsync "${DRYRUN}" more parameters ...
Just omit the quotes (and in this case you can
also omit the curly braces):

 $ rsync $DRYRUN more parameters ...

Have a nice day,
Berny