The problem is that retry try to execute a file called '/usr/bin/rsync -av source destination' (including the spaces in the middle).

In order to tell the rsync is the command, and -av source destination are the flags that belong to the rsync command, you'll need to run it the following way:

retry -- /usr/bin/rsync -av source destination

The reason you'll need the -- (double dash) is to signify end of command options and beginning of positional arguments. With the -- before the command, retry will think that -av is one of it's command options, and will throw the following error:

retry: invalid option -- 'a'

The problem is that retry tries to execute a file called '/usr/bin/rsync -av source destination' (including the spaces in the middle).

In order to tell retry that rsync is the command, and -av ,source, and destination are the parameters that belong to the rsync command, you'll need to run it the following way:

retry -- /usr/bin/rsync -av source destination

The reason you'll need the -- (double dash) is to signify the end of command options and beginning of positional arguments. Without the -- before the command, retry will think that -av is one of it's command options, and will throw the following error:

retry: invalid option -- 'a'