I'm not sure if this is related to Linux only and cannot be prevented, but I'll post in case there's a solution for this on PIVX side.

Describe the issue:

pivxd gets killed when you cast a command during the initial loading block index phase, before the wallet loads up.

Can you reliably reproduce the issue?
Yes.

If so, please list the steps to reproduce below:

1. Start the pivxd with the ./pivxd command (it will return PIVX server starting)
2. At the moment when tail -f ~/.pivx/debug.log command returns init message: Loading block index..., type any command, for example ./pivx-cli getbestblockhash, return message will be error: couldn't connect to server and pivxd will be killed.
3. To confirm that pivxd got killed, ./pivx-cli stop command returns error: couldn't connect to server instead of the PIVX server stopping.

What version of PIVX Core are you using?
4.3.0

OS: Ubuntu Linux 20.04