*After Release of Zabbix 5.2 I began switching the first agent to *5.2 and to zabbix agent2.

It seems the handling of double dollar is wrong. Described in https://www.zabbix.com/documentation/current/manual/config/items/userparameters

Steps to reproduce:

1. Add config file userparameter_diskstats.conf contains lines like this:
   UserParameter=custom.vfs.dev.read.merged[*],awk '{print $$1}' /sys/class/block/$1/stat
2. restart zabbix-agent2
3. zabbix_agent2 -t "custom.vfs.dev.read.merged[dm-1]"

Result:

custom.vfs.dev.read.merged[dm-1]                                 [s| 2696802        0 116307270  8722504  3530852        0 197744664 22953464        0  2932116 31721108]

Expected:

custom.vfs.dev.read.merged[dm-1]                                 [s| 0]

Some experiments with less "complicated" expressions like "echo $$1" which should result in "$1" showed me that is seems the double dollar isn't handled (correctly).

I don't know (yet) if is is a problem of 5.2 or a problem of zabbix agent 2.