watch

Overview

The watch command executes a program repeatedly and displays its output, allowing you to monitor changes over time. It’s essential for real-time system monitoring and observing command output changes.

Syntax

watch [options] command

Common Options

Option	Description
-n seconds	Update interval (default: 2)
-d	Highlight differences
-t	Turn off header
-b	Beep on command failure
-e	Exit on command failure
-g	Exit when output changes
-c	Interpret ANSI color sequences
-x	Pass command to exec instead of sh
-p	Precise timing

Key Use Cases

1. Monitor system resources
2. Watch file changes
3. Track process status
4. Monitor network connections
5. Observe command output changes

Examples with Explanations

Example 1: Monitor Disk Usage

watch df -h

Updates disk usage display every 2 seconds

Example 2: Watch Process List

watch -n 1 'ps aux | head -10'

Updates process list every second

Example 3: Monitor with Differences

watch -d free -h

Highlights changes in memory usage

Example 4: Watch File Size

watch -n 0.5 'ls -lh largefile.txt'

Monitors file size changes every 0.5 seconds

System Monitoring

1. CPU usage:

   watch -n 1 'cat /proc/loadavg'

2. Memory usage:

   watch -d 'free -h && echo && ps aux --sort=-%mem | head -5'

3. Network connections:

   watch -n 2 'netstat -tuln | grep LISTEN'

File and Directory Monitoring

1. Directory contents:

   watch -d 'ls -la /tmp'

2. File modifications:

   watch -n 1 'stat file.txt | grep Modify'

3. Log file growth:

   watch -d 'wc -l /var/log/syslog'

Process Monitoring

1. Specific process:

   watch -n 1 'ps aux | grep [p]rocess_name'

2. Process tree:

   watch -n 2 'pstree -p'

3. Process resource usage:

   watch -n 1 'top -bn1 | head -15'

Advanced Usage

1. Exit on change:

   watch -g 'ls /tmp | wc -l'

2. Beep on failure:

   watch -b 'ping -c 1 google.com'

3. Precise timing:

   watch -p -n 0.1 'date +%S.%N'

Performance Analysis

• Low CPU overhead
• Configurable update intervals
• Good for long-term monitoring
• Minimal memory usage
• Efficient for repetitive tasks

Related Commands

• tail -f - Follow file changes
• top - Process monitor
• htop - Interactive process viewer
• iostat - I/O statistics
• vmstat - Virtual memory stats

Best Practices

1. Use appropriate update intervals
2. Combine multiple commands with &&
3. Use quotes for complex commands
4. Consider system load impact
5. Use -d to highlight changes

Network Monitoring

1. Active connections:

   watch -n 1 'ss -tuln'

2. Network traffic:

   watch -d 'cat /proc/net/dev'

3. Ping monitoring:

   watch -n 1 'ping -c 1 8.8.8.8 | tail -2'

Service Monitoring

1. Service status:

   watch -n 5 'systemctl status apache2'

2. Port availability:

   watch -n 2 'nc -zv localhost 80'

3. Database connections:

   watch -n 3 'mysql -e "SHOW PROCESSLIST"'

Scripting Applications

1. Automated monitoring:

   #!/bin/bash
   # Monitor until condition met
   watch -g 'test -f /tmp/done.flag' && echo "Process completed"

2. Resource threshold monitoring:

   watch -n 1 'free | awk "NR==2{printf \"%.2f%%\", \$3/\$2*100}"'

Integration Examples

1. With logging:

   watch -n 10 'df -h | tee -a disk_usage.log'

2. Combined monitoring:

   watch -n 1 'echo "=== CPU ===" && uptime && echo "=== Memory ===" && free -h'

3. Alert integration:

   watch -n 30 'df -h | awk "$5 > 90 {print}" | mail -s "Disk Alert" admin'

Color and Formatting

1. Preserve colors:

   watch -c 'ls --color=always'

2. Custom formatting:

   watch -n 1 'printf "\033[2J\033[H"; date; echo; ps aux | head -10'

Error Handling

1. Exit on command failure:

   watch -e 'ping -c 1 unreachable_host'

2. Beep on errors:

   watch -b 'test -f important_file.txt'

3. Continue on errors:

   watch 'command_that_might_fail || echo "Command failed"'

Troubleshooting

1. High CPU usage with short intervals
2. Terminal size limitations
3. Command quoting issues
4. Color display problems
5. Timing precision limitations

Security Considerations

1. Avoid displaying sensitive information
2. Be careful with command injection
3. Monitor resource usage
4. Consider screen locking
5. Validate command inputs

Alternative Approaches

1. Using while loop:

   while true; do
       clear
       command
       sleep 2
   done

2. Using inotify for file watching:

   inotifywait -m /path/to/file

Real-world Examples

1. Development monitoring:

   watch -n 1 'make test 2>&1 | tail -10'

2. Deployment monitoring:

   watch -d 'kubectl get pods'

3. Performance testing:

   watch -n 0.5 'curl -w "%{time_total}\n" -o /dev/null -s http://localhost'

Customization

1. Custom header:

   watch -t 'echo "Custom Monitor - $(date)"; echo; command'

2. Multiple commands:

   watch 'echo "=== Disk ===" && df -h && echo "=== Memory ===" && free -h'