Writing a simple bash script to automate tasks using cron

1. Task Definition

Today, we’ll write a real script that checks a website’s availability and logs the result to a log file, then automates its execution using cron.

What are we trying to do?

We’ll be creating a bash script that:

1. Checks the availability of a specified website using ping.
2. Logs the result of the check into a log file, including the time and date.
3. Runs this script automatically every 5 minutes using cron.

Why is this important?

In the real world, monitoring the availability of websites and servers is one of the key tasks for system administrators. For example, if your website (or server) suddenly stops responding, you’d want to know about it as soon as possible. This kind of scenario is not just useful for monitoring but also for job interviews or work practice.

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2. Step 1: Writing a Simple Bash Script

Let's start by writing the basic bash script. Here's a step-by-step explanation of what it will do:

1. We'll set the website address to check.
2. Use ping to try sending a request to the server.
3. Check if an accessible response was returned.
4. Save the result to a log file.

Let's create the file site_check.sh:

#!/bin/bash

# Define the website address
WEBSITE="example.com"

# File to log results
LOG_FILE="/var/log/site_status.log"

# Check website availability using ping
if ping -c 1 $WEBSITE &> /dev/null; then
    # If the site is reachable
    echo "$(date): $WEBSITE is reachable" >> $LOG_FILE
else
    # If the site is unreachable
    echo "$(date): $WEBSITE is unreachable" >> $LOG_FILE
fi

Code Explanation:

1. #!/bin/bash — tells that this script should be executed using Bash.
2. WEBSITE="example.com" — the address of the website to check. You can replace example.com with any site or server you want to monitor.
3. ping -c 1 $WEBSITE — sends one (-c 1) packet to the specified website. If the site responds, the command exits with code 0. If not, it exits with an error.
4. &> /dev/null — redirects standard output and errors to the "black hole" (we don't want to see the ping output in the console).
5. $(date) — adds the current date and time to the message.
6. >> $LOG_FILE — appends the result to the log file.

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3. Step 2: Running the script manually

Before automating the script execution, let's make sure it works.

1. Save the script to a file named site_check.sh.

2. Make it executable:

   chmod +x site_check.sh
   

3. Run it:

   sudo ./site_check.sh
   

After executing the script, open the file /var/log/site_status.log to ensure the result of the check is logged. Use the following command:

cat /var/log/site_status.log

If everything works, you should see a line like this:

Mon Oct 30 14:35:22 UTC 2023: example.com is reachable

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4. Step 3: Setting Up Automatic Execution with cron

We already know that cron lets us schedule tasks. Now we'll configure cron so our script runs every 5 minutes.

Editing crontab

Open the crontab editor:

crontab -e

Add the following line:

*/5 * * * * /path/to/site_check.sh

Explanation:

• */5 — specifies that the task should run every 5 minutes.
• /path/to/site_check.sh — full path to our script. Make sure it's correct. For example, if the script is in the user's home directory, the path might look like /home/your_username/site_check.sh.

After saving the changes, cron will start running the script every 5 minutes. To confirm the task has been added, run:

crontab -l

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5. Step 4: Checking cron functionality

Now let's verify if the automation works. Wait for 5-10 minutes and open the log file again:

cat /var/log/site_status.log

You should see new entries being updated every 5 minutes. For example:

Mon Oct 30 14:35:22 UTC 2023: example.com is reachable
Mon Oct 30 14:40:22 UTC 2023: example.com is reachable
Mon Oct 30 14:45:22 UTC 2023: example.com is reachable

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6. Typical Questions and Errors Breakdown

1. Ping returns 'Permission denied' error

   If you're running the script as a regular user, it might not have enough permissions. Run the script with sudo or add execute permissions.

2. Log file isn't being created

   Make sure the path to the log file (/var/log/site_status.log) is correct and your user has write permissions in that directory. If the file doesn't exist, create it manually using:

   sudo touch /var/log/site_status.log
   sudo chmod 666 /var/log/site_status.log
   

3. cron isn't running the script

   Check if the cron service is active:

   sudo systemctl status cron
   

   It should have the status active (running). If it's stopped, start it:

   sudo systemctl start cron
   

4. The script runs manually but doesn't execute via cron

   Make sure the path to the script and all used files is specified completely. In cron jobs, environment variables (e.g., $PATH) might be different. It's better to use absolute paths.

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7. Improvements and Additional Tasks

Website Filtering

Expand the script to check the availability of multiple websites. For example:

WEBSITES=("example.com" "google.com" "stackoverflow.com")
for SITE in ${WEBSITES[@]}; do
    if ping -c 1 $SITE &> /dev/null; then
        echo "$(date): $SITE is reachable" >> $LOG_FILE
    else
        echo "$(date): $SITE is unreachable" >> $LOG_FILE
    fi
done

Sending Notifications

Add sending notifications if a website is unavailable. For example, using the mail command to send an email:

# Email notification
if ! ping -c 1 $WEBSITE &> /dev/null; then
    echo "$WEBSITE is down!" | mail -s "Website Check Alert" your_email@example.com
fi

Cyclic Logging

Limit the log file size to, say, 1 MB. If the file gets too large, rename it (like an archive) and start a new log.

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Congrats! Now you know how to use bash scripts and cron to automate real-world tasks. These skills are especially valuable for DevOps engineers and system administrators.