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Vultr profile image Sanskriti Harmukh
Sanskriti Harmukh for Vultr

Posted on with Aashish Chaurasiya Originally published at docs.vultr.com

Automating Bash Scripts on Linux Using Cron and Systemd

#linux #bash #cron #systemd

Automating Bash scripts on Linux is essential for system administration tasks, such as installing applications and monitoring system logs. Cron is a job scheduler that runs tasks at specific intervals, while systemd is a service manager that handles applications through service profiles. This guide walks through writing two Bash scripts — one for log monitoring and one for backups — then automating them with a cron job and running one as a persistent systemd service. By the end, you'll have both a scheduled cron job and a managed systemd service running your own Bash scripts automatically.

Prerequisites

Before you begin:

  • Have access to a Ubuntu server as your management workstation.
  • Access the server using SSH as a non-root sudo user.
  • Install a LAMP stack (Apache, MySQL, PHP).

Create Bash Scripts

Bash scripts support multiple commands and functions that define how tasks run in a system. Follow the steps below to create two Bash scripts: one to back up critical web application files, and one to monitor system logs for errors.

1. Switch to your user's home directory:

$ cd
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2. Create a new log-monitor.sh script:

$ nano log-monitor.sh
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3. Add the following contents to the log-monitor.sh file:

#!/bin/bash

LOG_FILE="/var/log/syslog" 
ERROR_LOG="/opt/data/error_log.txt"
WARNING_LOG="/opt/data/warning_log.txt"

tail -F "$LOG_FILE" | while read line; do
    if echo "$line" | grep -q "error"; then
        echo "$line" >> "$ERROR_LOG"
    elif echo "$line" | grep -q "warning"; then
        echo "$line" >> "$WARNING_LOG"
    fi
done
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Save and close the file. This script creates error_log.txt and warning_log.txt files in the /opt/data directory, filtering results from /var/log/syslog using a while loop. It writes each new line to the respective error or warning file as it appears.

4. Create another script named backup.sh:

$ nano backup.sh
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5. Add the following contents to the file:

#!/bin/bash

DB_USER="user"
DB_PASSWORD="new-password"
DB_NAME="mysql"

DB_BACKUP_DIR="mysql_backup"
WEB_FILES_DIR="web_backup"

mkdir -p "$DB_BACKUP_DIR"
mkdir -p "$WEB_FILES_DIR"

DATE=$(date +"%Y%m%d_%H%M%S")

mysqldump -u"$DB_USER" -p"$DB_PASSWORD" "$DB_NAME" > "$DB_BACKUP_DIR/db_backup_$DATE.sql"
echo "MySQL backup completed: $DB_BACKUP_DIR/db_backup_$DATE.sql"

tar -czf "$WEB_FILES_DIR/web_backup_$DATE.tar.gz" /var/www/html
echo "Web application backup completed: $WEB_FILES_DIR/web_backup_$DATE.tar.gz"
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Save and close the file. This script connects to the MySQL database server using the DB_... variables, then creates mysql_backup and web_backup directories to store the database and web application backups.

6. Move the Bash scripts to a system-wide directory like /opt:

$ sudo cp backup.sh log-monitor.sh /opt/
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7. Create a new data directory under the system-wide directory:

$ sudo mkdir -p /opt/data
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8. Change the directory permissions to 777 to allow all users to write files:

$ sudo chmod 777 /opt/data
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Automate the Bash Script Using Cron

Cron creates jobs you can use to schedule and automatically run a Bash script at specific intervals. Follow the steps below to automate the backup.sh script to back up the MySQL database and web application files daily.

1. Open the crontab editor:

$ crontab -e
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Select your desired text editor when prompted — for example, select 1 to choose the nano text editor.

2. Add the following directive at the end of the file:

0 2 * * * /opt/backup.sh
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Save and close the file. This cron job runs backup.sh daily at 2:00 A.M. In the job:

  • 0: Runs the script at the start of a minute. Set a value between 0 and 59.
  • 2: Runs the script at 2:00 A.M. Specify an hour between 0 and 23.
  • *: Runs the script every day of the month. Specify a day between 1 and 31.
  • *: Runs the script every month. Specify a month between 1 and 12.
  • *: Runs the script daily. Specify a day of the week between 0 and 7.

3. List all cron jobs and verify the new task is available:

$ crontab -l
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Output:

# 
# For more information see the manual pages of crontab(5) and cron(8)
# 
# m h  dom mon dow   command

0 2 * * * ~/backup.sh
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Automate the Bash Script as a System Service Using Systemd

Systemd starts system services automatically at boot and offers options for stopping and restarting them. Follow the steps below to set up the log-monitoring script as a system service.

1. Create a new system service file:

$ sudo nano /etc/systemd/system/logmonitor.service
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2. Add the following contents to the logmonitor.service file:

[Unit]
Description= Automatic System Logs Monitoring and Filtering
After=network.target

[Service]
ExecStart=bash /opt/log-monitor.sh
Restart=always

[Install]
WantedBy=multi-user.target
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Save and close the file. This service configuration runs the log-monitor.sh script from /opt to monitor and filter log entries in /var/log/syslog.

3. Reload systemd to apply the service file changes:

$ sudo systemctl daemon-reload
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4. Enable the new system service to automatically start at boot:

$ sudo systemctl enable logmonitor.service
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Output:

Created symlink /etc/systemd/system/multi-user.target.wants/logmonitor.service → /etc/systemd/system/logmonitor.service.
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5. Start the system service:

$ sudo systemctl start logmonitor
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6. View the system service status and verify it's running:

$ sudo systemctl status logmonitor
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Output:

● logmonitor.service - Automatic System Logs Monitoring and Filtering
 Loaded: loaded (/etc/systemd/system/logmonitor.service; enabled; preset: enabled)
 Active: active (running) since Wed 2024-11-06 13:20:07 UTC; 11s ago
Main PID: 14293 (bash)
   Tasks: 3 (limit: 1062)
  Memory: 784.0K (peak: 1.5M)
     CPU: 40ms
  CGroup: /system.slice/logmonitor.service
          ├─14293 bash /opt/log-monitor.sh
          ├─14308 tail -F /var/log/syslog
          └─14309 bash /opt/log-monitor.sh

Nov 06 13:20:07 Linux-server systemd[1]: Started logmonitor.service - Automatic System Logs Monitoring and Filtering.
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7. Run the following commands to simulate new error and warning entries in /var/log/syslog:

$ logger -p user.err "This is a simulated error log message from MySQL"
$ logger -p user.err "This is a simulated warning log message from Apache"
$ logger -p user.err "This is a simulated error log message from PHP"
$ logger -p user.err "This is a simulated Warning log message from MySQL"
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8. List all files in /opt/data and verify that error_log.txt and warning_log.txt are available:

$ ls /opt/data
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Output:

error_log.txt warning_log.txt
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9. View the error_log.txt file and verify that only error entries are present:

$ cat error.log
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Output:

2024-11-06T13:33:48.010096+00:00 Server linuxuser: This is a simulated error log message from MySQL
2024-11-06T13:33:48.010096+00:00 Server linuxuser: This is a simulated error log message from PHP
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Next Steps

  • Add more granular filters (by service name or severity) to log-monitor.sh
  • Extend backup.sh with retention logic to prune old backups automatically
  • Ship backups to remote or off-site storage so they survive a full server loss
  • Add alerting (email or webhook) when new entries land in error_log.txt

For the full guide with additional tips, visit the original article on Vultr Docs.

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