#crontab

Daughter asked if there was a way to set up her new inkjet printer to print once a week to prevent the printer heads from drying out, much like her last printer had done. She really doesn’t want to buy a printer again just because she didn’t use it enough. I said “Absolutely. I can just set up a cronjob to print to it from my Linux computer and schedule that to print once a week. Problem…

06 April 2023 Akhirnya setelah downgrade beberapa package di Centos 7, bisa juga melakukan mounting ke Harddisk + plus setup VM untuk running backup tiap harinya.

09 April 2023 Penambahan monitorix, buat identifikasi. kalau ternyata dijam jam tersebut loadnya tinggi. ya musti dipindah jadwalnya..

ssh -N -L [SOURCE PORT]:127.0.0.1:[DESTINATION PORT] [USERNAME]@[IP-ADDRESS-SERVER] -p [PORT SSH]

Crontab là tiện ích trong hệ điều hành Linux cho phép bạn lên lịch chạy các lệnh hoặc script tự động vào thời gian định sẵn. Đây là công cụ không thể thiếu để tự động hóa công việc như backup dữ liệu, kiểm tra hệ thống, gửi email định kỳ… giúp tiết kiệm thời gian và tăng hiệu suất quản trị server.

Đọc chi tiết: Crontab là gì?

One of the most frequently used tools in the hands of a Linux-servers administrator is Cron. This is a utility that allows you to execute commands or run scripts according to a predetermined schedule. And there are a huge variety of tasks for which Cron is used. In this article, we will take a closer look at Cron, discuss the best practices for using it and the most common mistakes, and also look at a number of illustrative examples.

I’m lazy. On a scale of 1-10, maybe around a 5-6 in terms of laziness. I’ll do things if they need to be done, but I don’t want to waste extra energy doing things when there’s an easier, lazier way to accomplish the same task. And that’s specifically what this post is about. 

The task: Run a packet capture for roughly a day. Then stop the packet capture, save the pcap file, and start a new packet capture.

I’ve been going into the server room, sitting on the floor with the packet capture laptop, and doing this stop, save, restart process for about a week now. Its not a particularly painful process. But it can definitely benefit from improvement. (isn’t that the definition of improvement?) 

Now let’s get to the fun part. First, ingredients: 1 PC with RJ45 port. 1 ethernet cable. Optional: 1 USB to RJ45 adapter. Another ethernet cable. 

Prerequisites: You’ll need to already have a mirrored port on your switch. This is just about how to schedule the packet captures. Secondly, for the optional portion, you’ll need to configure a port on a switch, to use the RJ45 adapter for the PC to give it an IP you can manage it on.

Let’s look at the crontabs now:

05 12 * * * cd /root/pcap; /usr/bin/timeout 1435m /usr/sbin/tcpdump -i eno1 -w outfile 03 12 * * * cd /root/pcap; /bin/bash renameToToday.sh

Oh yeah, gotta set up this crontab as root. so sudo crontab -e. Now, as root, we change directory to the pcap directory, use the timeout command, run the following for 1435 minutes, which is 5 minutes short of 1 day. Run tcpdump on the eno1 interface and write the pcap output to the file: outfile.

This script would run from 12:05pm until noon the next day.

3 minutes of nothingness

Then go to the pcap directory again, and run the script renameToToday.sh

And that script looks like:

#!/bin/bash today=$(date +"%b%d") mv outfile $today.pcap

And since today is October 18th, it would rename the file outfile to Oct18.pcap

2 minutes later