Memory Management and System Performance Commands

1. Memory Management Commands

Free Command:
- free -h: Display memory usage in human-readable format (KB, MB, GB).
```
free -h
```
- free -g: Display memory usage in gigabytes.
```
free -g
```
- free -m: Display memory usage in megabytes.
```
free -m
```
- free -k: Display memory usage in kilobytes.
```
free -k
```

Vmstat Command:
- vmstat: Display basic system performance stats (memory, CPU, processes, etc.).
```
vmstat
```
- vmstat -a: Show active memory and swap usage.
```
vmstat -a
```
- vmstat -d: Display disk statistics.
```
vmstat -d
```
- vmstat -s: Display summary of memory, swap, and disk statistics.
```
vmstat -s
```
- vmstat 4 5: Display performance stats every 4 seconds for 5 iterations.
```
vmstat 4 5
```
- vmstat -s m 4 5: Display memory statistics every 4 seconds for 5 iterations.
```
vmstat -s m 4 5
```
Iostat Command:
- iostat: Display input/output statistics for devices and partitions.
```
iostat
```
- Install iostat if not installed:
```
yum install sysstat
```
- iostat 4 5: Display I/O statistics every 4 seconds for 5 iterations.
```
iostat 4 5
```

Listing Open Files with lsof
- Install lsof Command (if not installed):
```
yum install lsof
```
- To list open files by the user root:
```
lsof -u root
```
- To list network files (all types of network connections):
```
lsof -N -i
```
- To list open UDP network connections:
```
lsof -n -i UDP
```
- To list open TCP network connections:
```
lsof -n -i TCP
```
- To list open TCP port 22 (usually SSH):
```
lsof -n -i TCP:22
```
- To list open UDP ports 22, 443, and 90:
```
lsof -n -i UDP:22,443,90
```
- To list open UDP ports in the range 80-220:
```
lsof -n -i UDP:80-220
```
- To list open files for process ID 6029:
```
lsof -n -p 6029
```
- To list open IPv4 network connections:
```
lsof -n -i 4
```
- To list open IPv6 network connections:
```
lsof -n -i 6
```
- To get process IDs (PIDs) of files opened by the user Armour:
```
lsof -t -u Armour
```

To kill processes opened by the user Armour using the process IDs (PIDs) retrieved from lsof:
```
kill -9 $(lsof -t -u Armour)
```