How do I run this script?

Save as service-watchdog.sh, set SERVICE name, run chmod +x service-watchdog.sh, then execute ./service-watchdog.sh.

Does this work on macOS?

No. systemctl is Linux systemd only. macOS uses launchd — use launchctl list instead.

Restart a Service If It Stopped

Q: How do I check if a service is running in bash?

Run systemctl is-active servicename. Exit code 0 means running; non-zero means stopped or failed.

Q: How do I restart a service automatically in Linux?

Wrap systemctl is-active and systemctl start in an if statement, then schedule the script with cron every minute.

The Script

Save as service-watchdog.sh. Set SERVICE to any unit from systemctl list-units --type=service. If you use sudo systemctl start inside the script (as below), configure passwordless sudo for that command for your cron user, or run the watchdog as root.

bash

#!/bin/bash

CHECK="✓"
CROSS="✗"

# --- Configuration ---
SERVICE="nginx"                              # Change to your service name
LOG_FILE="/var/log/service-watchdog.log"
DATE=$(date '+%Y-%m-%d %H:%M:%S')
NOTIFY_EMAIL=""                              # Optional: you@example.com

# --- Check if service is running ---
if systemctl is-active --quiet "$SERVICE"; then
  echo "$CHECK [$DATE] $SERVICE is running"
else
  echo "$CROSS [$DATE] $SERVICE is NOT running — attempting restart..."
  echo "$CROSS [$DATE] $SERVICE DOWN — restarting" >> "$LOG_FILE"

  # --- Attempt restart ---
  if sudo systemctl start "$SERVICE"; then
    echo "$CHECK [$DATE] $SERVICE restarted successfully" | tee -a "$LOG_FILE"

    # --- Optional: send email notification ---
    if [ -n "$NOTIFY_EMAIL" ]; then
      echo "$SERVICE was down and has been restarted on $(hostname) at $DATE" \
        | mail -s "[RECOVERED] $SERVICE restarted" "$NOTIFY_EMAIL"
    fi
  else
    echo "$CROSS [$DATE] $SERVICE FAILED to restart — manual intervention needed" \
      | tee -a "$LOG_FILE"

    if [ -n "$NOTIFY_EMAIL" ]; then
      echo "$SERVICE failed to restart on $(hostname) at $DATE. Check: journalctl -u $SERVICE" \
        | mail -s "[CRITICAL] $SERVICE restart failed" "$NOTIFY_EMAIL"
    fi
  fi
fi

Schedule this as a 1-minute cron job

- - - - /home/user/service-watchdog.sh To suppress cron email output:
- - - - /home/user/service-watchdog.sh >> /var/log/watchdog-cron.log 2>&1

Cron and sudo

Cron runs with a minimal environment and no TTY. If sudo systemctl start prompts for a password, the job fails silently. Prefer a sudoers rule for your cron user (NOPASSWD for /bin/systemctl start yourservice) or run without sudo when the cron user already has permission to manage the unit.

How It Works

systemctl is-active --quiet "$SERVICE" asks systemd whether the unit is active. With --quiet, there is no output—only an exit status: 0 means running (script prints the ✓ path and exits), anything else means inactive, failed, or missing (non-zero), so the script enters the restart branch.

When the service is down, the script logs a line by appending to LOG_FILE. On success, echo ... | tee -a "$LOG_FILE" prints the recovered message and appends it to the log—the operator sees immediate feedback while cron still has an audit trail.

Failure handling is two levels deep: first, is the unit running? Second, did systemctl start succeed? If the restart command fails (bad unit name, dependency error, masked service), if sudo systemctl start is false—you get ✓/recovery email only after a verified start; otherwise the script emits a ✗ critical message, logs via tee -a, and sends a [CRITICAL] email pointing you at journalctl -u $SERVICE instead of implying the service is healthy.

Step-by-Step Setup

Step 1 — Create the script file

bash

nano service-watchdog.sh

Paste the script above, then press Ctrl+X → Y → Enter to save.

Step 2 — Set your service name

Edit SERVICE="nginx" to the name of the service you want to monitor. The name must match exactly what systemctl uses:

bash

# Find the correct name for your service
systemctl list-units --type=service | grep -i nginx
systemctl list-units --type=service | grep -i mysql

Common service names:

Software	Service name
nginx	nginx
Apache	apache2 (Debian/Ubuntu) or httpd (RHEL/CentOS)
MySQL	mysql or mysqld
PostgreSQL	postgresql
Redis	redis or redis-server
SSH daemon	ssh or sshd

Step 3 — Make it executable and run a manual test

bash

chmod +x service-watchdog.sh
./service-watchdog.sh

When the service is running, you should see:

text

✓ [2026-06-03 14:30:01] nginx is running

To test the restart path, stop the service first:

bash

sudo systemctl stop nginx
./service-watchdog.sh

You should see the ✗ DOWN — restarting line followed by ✓ restarted successfully. Then confirm nginx is back:

bash

systemctl is-active nginx
# active

Step 4 — Set up email alerts (optional)

Edit NOTIFY_EMAIL="" at the top of the script and add your address:

bash

NOTIFY_EMAIL="you@example.com"

The script sends two different emails: one when the service recovers ([RECOVERED]) and one when the restart itself fails ([CRITICAL]). The [CRITICAL] email includes the journalctl command to run so you can investigate immediately.

Step 5 — Schedule with cron every minute

Open your crontab:

bash

crontab -e

Add this line (use your actual path):

bash

# Check every minute — maximum downtime before auto-recovery is 60 seconds
* * * * * /home/user/service-watchdog.sh >> /var/log/watchdog-cron.log 2>&1

The >> /var/log/watchdog-cron.log 2>&1 redirect captures all output so you have a log of every check, every outage, and every recovery.

Cron and sudo permissions

If sudo systemctl start prompts for a password inside cron, the restart fails silently. Add a targeted NOPASSWD rule for your cron user: echo 'cronuser ALL=(ALL) NOPASSWD: /bin/systemctl start nginx' | sudo tee /etc/sudoers.d/watchdog. Replace cronuser and nginx with your actual user and service name.

Variations

Watch multiple services

Use a Bash array of unit names and loop: for each element, call systemctl is-active --quiet "$svc" and restart on failure—the same logic as above, reused per service.

Use systemd’s own watchdog semantics

For long-running apps you control, a unit drop-in can set Restart=on-failure (and related options) so systemd restarts the process without cron. Cron still helps when you want mail, extra logging, or cross-checks across several units.

Slack or webhook alerts

Replace the mail pipeline with curl -X POST -H 'Content-Type: application/json' to your Incoming Webhook URL, sending JSON with hostname, timestamp, and service name for recover vs. failure.

Understanding the Commands

Command	What it does
`systemctl is-active --quiet`	Exit 0 only when the unit is active; gates the restart path
`sudo systemctl start`	Attempts to bring the stopped unit back to active state
`tee -a`	Mirror stdout to terminal while appending to LOG_FILE
`mail`	Optional notifications when NOTIFY_EMAIL is set
`journalctl -u SERVICE`	Inspect why a restart failed (logs from systemd)

Common Mistakes

Cron and sudo

Cron runs with a minimal environment and no TTY. If sudo systemctl start prompts for a password, the job fails silently. Add a sudoers rule for the cron user: cron_user ALL=(ALL) NOPASSWD: /bin/systemctl start servicename.

Frequently Asked Questions

How do I check if a service is running in bash?

Use systemctl is-active: systemctl is-active --quiet nginx returns exit code 0 if running, non-zero if stopped. Use in an if statement: if systemctl is-active --quiet nginx; then ...

How do I restart a service automatically in Linux?

Write a bash script that checks systemctl is-active and runs systemctl start if the service is stopped. Schedule it every minute with cron: * * * * * /home/user/watchdog.sh

What services can I monitor with this script?

Any systemd service: nginx, apache2, mysql, postgresql, redis, docker, ssh, and custom services. Change SERVICE='nginx' to any service name from systemctl list-units.

How do I run a bash script every minute with cron?

Open crontab -e and add: * * * * * /full/path/to/script.sh — the five asterisks mean "every minute of every hour of every day".

What is systemctl is-active?

systemctl is-active servicename checks whether a systemd service is currently running. It returns exit code 0 (success) if active, and 1+ (error) if inactive, failed, or not found.

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Restart a Service If It Stopped

The Script

How It Works

Step-by-Step Setup

Step 1 — Create the script file

Step 2 — Set your service name

Step 3 — Make it executable and run a manual test

Step 4 — Set up email alerts (optional)

Step 5 — Schedule with cron every minute

Variations

Watch multiple services

Use systemd’s own watchdog semantics

Slack or webhook alerts

Understanding the Commands

Common Mistakes

Frequently Asked Questions

How do I check if a service is running in bash?

How do I restart a service automatically in Linux?

What services can I monitor with this script?

How do I run a bash script every minute with cron?

What is systemctl is-active?

Related Snippets

Frequently Asked Questions