Published on 2020-10-29, by Javed Shaikh
There are many ways to monitor and set alerts for your web server using third party apps but they don't come with free, be it Pingdom or Amazon's CloudWatch. In this post I am going to show how we can write and setup our own alert tool with just few lines of code. However before going further into codes let us see what are the commands and utilities available in Linux to monitor web server health.
All the below commands are very basic and available on all Linux flavors. I am running these commands on Ubuntu 20.04 LTS for below demos
📗 How to check if a website is up and running or down?
📙 How to write a basic shell script?
📗 How to call Python program from shell script?
💡 How to send an email using Python?
There are many commands and utilities available to monitor if your website or app is up and running fine. I have listed few of them. Lets checkout
curl is a very powerful utility to send and receive data over many supported protocols such as HTTP,FTP, SMTP... We are going to use curl with I options to fetch the headers from a website. Based on the header we can identify if server is down or up. Lets try it
USAGE: curl -I SERVER IP
shaikh@ubuntu:~/scripts$ curl -I "https://hashnode.com" HTTP/2 200 date: Thu, 29 Oct 2020 17:35:18 GMT content-type: text/html; charset=utf-8 cache-control: public, s-maxage=1800, max-age=0 etag: W/"4436a-AFOlSSR9igz5hRyOL+SdGTPJfxk" vary: Accept-Encoding x-frame-options: Deny x-powered-by: Next.js cf-cache-status: HIT age: 1020 cf-request-id: 061706b33c0000051308a22000000001
As shown above curl -I returns header and the very first line (HTTP/2 200 ) tells us that webserver for hashnode is up and running. Note that server is up and running as long as the response code is 200,301,302,308. In above example response code is 200 , so its up.
This is another utility in linux used as network down loader. We will use option -S to get the headers from a website. USAGE:
wget -s SERVER : This will fetch and download the headers and pages of a website
wget -S --spider SERVER :This will work exact similar way as above except that it will behave as web spider and wont download the pages.
shaikh@ubuntu:~/scripts$ wget -S --spider http://shaikhu.com URL transformed to HTTPS due to an HSTS policy Spider mode enabled. Check if remote file exists. --2020-10-29 10:47:55-- https://shaikhu.com/ Resolving shaikhu.com (shaikhu.com)... 22.214.171.124 Connecting to shaikhu.com (shaikhu.com)|126.96.36.199|:443... connected. HTTP request sent, awaiting response... HTTP/1.1 302 Found Alt-Svc: h3-29=":443"; ma=2592000 Content-Length: 206 Content-Type: text/plain; charset=utf-8 Date: Thu, 29 Oct 2020 17:47:55 GMT
As shown above the returned response code is 302 for redirection, so our server is up.
Ping is most common utility to check the web server's health. It sends ICMP ECHO_REQUEST to network host to get the up information. We are going to use -C count option to send n number of request packets.
ping -c COUNT SERVER Above command will send COUNT no of request packets to SERVER
shaikh@ubuntu:~/scripts$ ping -c 3 shaikhu.com PING hashnode.network (188.8.131.52) 56(84) bytes of data. 64 bytes from 184.108.40.206 (220.127.116.11): icmp_seq=1 ttl=55 time=38.2 ms 64 bytes from 18.104.22.168 (22.214.171.124): icmp_seq=2 ttl=55 time=34.0 ms 64 bytes from 126.96.36.199 (188.8.131.52): icmp_seq=3 ttl=55 time=36.0 ms --- hashnode.network ping statistics --- 3 packets transmitted, 3 received, 0% packet loss, time 2003ms rtt min/avg/max/mdev = 33.954/36.039/38.178/1.724 ms shaikh@ubuntu:~/scripts$
As shown above we sent 3 packets and received 3 packets as well, so that means our website is up and running
Now that we know which commands and utility to use to know the status of our website, so lets get started. We are going to issue the command
Lets create a new file named alert.sh and save it to your home folder. We can use any of the above command to know the status of a website. I am going to use wget's web spider option to get the details. However you can use any of them.
Before writing any script, note the exact location of bash
shaikh@ubuntu:~/scripts$ which bash /usr/bin/bash
Now lets start writing shell script alert.sh
#!/usr/bin/bash MYHOST="https://YOUR WEBSITE ADDRESS" if wget --spider -S $MYHOST 2>&1 | grep -w "200\|301\|302\|308" > /dev/null then echo "server is up" else #Server is down" /usr/bin/python3 send_alert.py fi
This is a very simple script to monitor your web server and send an email notification if its down. Lets understand what we did line by line.
In the very first line we are providing location of bash which was found in the previous step above then we have saved our website address in a variable named MYHOST From line number 3 we are executing the wget command in an if else statement. If the return code from wget is other than 200,301,302 and 308 then control will move to else block considering server is down or unreachable. In this step you can change the command to ping or curl as you like
In the else block we are executing a Python file named send_alert.py to send an email notification. You can ignore this step if you are sending email using MAILX of linux package within shell script. However I dont want to install any third party library and want to use existing Python library to send the email.
Writing an email using Python is very simple. No third party library is needed to send an email. Here we are using Python's inbuilt library smtplib to send an email. As shown rest of the code is pretty simple. We have defined FROM_EMAIL,Password, TO_EMAIL,header and finally body
Lets see what our send_alert looks like
import smtplib FROM = 'FROM EMAIL ADDRESS' PSWD = 'YOUR PASSWORD' TO = 'TO EMAIL ADDRESS' #Subject and header SUB = 'Subject: Alert for shaikhu.com \n' header = 'To:' + TO + '\n' + 'From: ' + FROM + '\n' + SUB #Message body = """ MAY DAY! MAY DAY!! MAY DAY!!! Your webserver for shaikhu.com is DOWN.""" message = header + body #Define function to send email def sendalert(msg): smtpObj = smtplib.SMTP('smtp.mail.yahoo.com', 587) smtpObj.ehlo() smtpObj.starttls() smtpObj.login(FROM, PSWD) smtpObj.sendmail(FROM,TO,msg) smtpObj.quit() if __name__ == '__main__': sendalert(message)
Congratulations!!🥳. We have just wrote our own script to monitor our server and send us an email notification if it is down. Now all we have to do is put this script on a scheduler or cron table so that the script will run every n no of minutes and keep monitoring our website.
If you want to know how to setup job scheduler using crontab then please do check my article https://shaikhu.com/how-to-schedule-and-manage-tasks-using-crontab
*/5 * * * * sh alert.sh
As per above cron table, our alert.sh will be executed every 5 minutes. That means alert.sh will check our website health every 5 minutes and will send me an email if the website is down.
This is how the email from my script looks like.
Though there are many websites are providing website alert service but not all of them are free. As seen in this article with just few lines of code, you can build your own alert app that will keep monitoring your website every moment and will send an email notifications when its down.
We have learned some important commands and codes such as how to check website is down or up and how to send email using Python and also how to write a basic shell script.
Hope this tutorial helped you learn something new today. Let me know your thoughts in the comment section 😃.
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