in reply to How to print the contents of system function to a .txt file

@output = system ("....")
to collect the output of the command you want to use the backticks @output = `...` or @output = qx{...} not a system function.
Please check Quote-Like-Operators.
However, to know if your command "java -jar cmdline-jmxclient-0.10.3.jar - localhost: +1100 com.ogs.red5.client.manager:name=Manager getConnectedAgents" works is a different story altogehter.

If you tell me, I'll forget.
If you show me, I'll remember.
if you involve me, I'll understand.
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