Running commands on bootup on unknown Linux-based machine only by placing files

Question

I have a Zmodo DVR that runs some kind of Linux-based OS and which (I think) has an ARM processor. I can place files in its filesystem using its updater tool, replacing anything with the same name, but I cannot read or execute any commands or access the DVR in any other way.

So where could I place some files that would make the machine run a shell script or something when it boots up?

Update: I know the path of its HTTP web root, but it apparently doesn't support PHP. I also see an executable file in an update for it that I'm guessing it runs at bootup. I would replace it with a shell script that runs some commands then runs the original executable, but I'm afraid to do that because if the shell script fails and prevents it from booting, my DVR is bricked forever.

P.S. This is not for malicious purposes. I'm trying to make this thing network-accessible.

Answer 1

Traditionally you have file /etc/rc.local which looks like this:

#!/bin/sh -e
#
# rc.local - executed at the end of each multiuser runlevel
#
# Make sure that the script will "exit 0" on success or any other
# value on error.

It should be enough to add your shell script there