How to analyze the state of a Linux machine without monitoring if it 'feels bad'? How to identify the causes of problems?

To analyze the state of a Linux machine without monitoring when its performance deteriorates, you can use standard system utilities:

• top or htop — view CPU load, memory, processes.
• free -m — check RAM and swap usage.
• vmstat — get statistics on processes, memory, input/output.
• iostat — check disk load.
• dmesg — view system messages and kernel errors.
• ps aux --sort=-%mem or ps aux --sort=-%cpu — find processes consuming a lot of resources.
• netstat -tulnp or ss -tulnp — check network connections.
• df -h — check disk space usage.

By analyzing this data, you can identify bottlenecks: for example, high CPU, lack of memory, disk or network issues. This will help understand the cause of the machine's "poor health" and take appropriate measures.