project with relay and log file from computer

Hello everyone! I'm a new guy here, and I'd love some help with my project. I already have software with interface and everything. In addition, this program has several files: 'LOG FILE'. I want, that in a certain field of LOG FILE, where there is a field in which a number is different than 0 (zero), then the arduino Will light a lamp (which works at 220 / 230volts) via relay. Thanks in advance!

If the file does not live on an SD card in a shield attached to the Arduino, which file, which field, and what the field contains are NOT Arduino programming issues.

What the PC program sends to the serial port is important, but that is entirely up to you.

So, we can sum up your Arduino requirements, as expressed so far as:

When the Arduino gets serial data, it should turn a relay on.

Well, that won't take more than a dozen lines of code, half of which will be { or }.

Have a look at the examples in Serial Input Basics - simple reliable ways to receive data.

You may also find this Python - Arduino demo useful.