the parser doesn't support 'arithmetic expansion' constructs in the command you tried

[ijash]

while entering:

tar cf - /folder-with-big-files -P | pv -s $(($(du -sk /folder-with-big-files | awk '{print $1}') * 1024)) | gzip > big-files.tar.gz

it outputs:

the parser doesn't support 'arithmetic expansion' constructs in the command you tried. you may report a bug to have this added, if one doesn't already exist

So, as the output says, i report it.

thank you