Why does this rsync call not work properly?

Question

I made a simple script to perform backup of my NAS. I do it as follows:

date="$(date +%Y-%m-%d)"
base="/backup"
newest="$(ls -1td "$base"/*-*-*/ | head -n 1)"
today="$base/$date"
dirs="pub,data,software"

if [ -d "$today" ]; then
  echo "Backup already exists"
  exit 1
fi

echo $newest
echo $dirs

mkdir "$today"
rsync -a --delete --info=progress2 "/myzpool/{$dirs}" "$today" --link-dest="$newest"

So what it is supposed to do is to create a folder with the name of the current date and then copy the folders /myzpool/pub, /myzpool/data and /myzpool/software into this newly created backup folder. But surprisingly, this does not work! rsync aborts with the error rsync: link_stat "/myzpool/{pub,data,software}" failed: No such file or directory.

I don't understand why this happens, because I use a very similar script to backup my virtual server via SSH:

dirs="srv,opt,home,root,etc"
mkdir "$today"
rsync -caz --compress-level=9 -e "ssh -i ~/.ssh/myserver.conf" --delete --info=progress2 "[email protected]:/{$dirs}" "$today" --link-dest="$newest"

and with the virtual server it works just fine. So what is the difference between the two command lines, and how could I fix my script, such that it also works locally?

So why then does it work when backing up my virtual web server via rsync + ssh? — T. Pluess, May 24 '19 at 07:31
In the `bash` shell, `"{$dirs}"` would expand to `"{srv,opt,home,root,etc}"`. As far as I know, `rsync` would then try to access that as the literal name of the file to sync. `rsync` does no special processing of curly braces. There are examples of using brace expansions in the `rsync` manual, but those examples relies on the shell to do the expansion, and `bash` simply won't expand `{$dirs}` like you'd want due to the order in which expansions happens (brace expansions happen before variable expansions). — Kusalananda, May 24 '19 at 07:50
I understand that the braces are expanded as you described, this is clear. But I still don't understand why I then `"[email protected]:/{$dirs}" ` is valid, while `"/myzpool/{$dirs}"` isn't. In both cases the brace expansion happens, but `rsync` doesn't accept the first command line (copying local files), while it does accept the latter (copying from a remote server). — T. Pluess, May 24 '19 at 09:29
Quoted brace expensing works for `rsync` in this case because there is another level of shell evaluation - on the remote server. — roaima, Sep 20 '19 at 19:37

score -2 · Answer 1 · answered Sep 20 '19 at 18:18

I just had a similar problem and have determined that Brace Expansion does work inside Command Substitution.

The problem is that Brace Expansion occurs before anything else so the rsync needs to be run like eval rsync ... so that the Brace Expansion is done a second time.

See this answer for more details...

Why does this rsync call not work properly?

1 Answers1