Master Linux System Monitoring: Essential Commands for CPU, Memory, Network, and I/O

Below is a curated list of Linux commands and explanations for monitoring various aspects of a server, including CPU, memory, network, I/O, processes, and system status.

1. CPU

cat /proc/cpuinfo
# Physical CPU count
cat /proc/cpuinfo | grep 'physical id' | sort | uniq | wc -l
# Cores per CPU
cat /proc/cpuinfo | grep 'core id' | sort | uniq | wc -l
# Logical CPUs
cat /proc/cpuinfo | grep 'processor' | sort | uniq | wc -l
mpstat
mpstat 2 10

2. Memory

cat /proc/meminfo
free -gt
df -hT
du -csh ./*
ipcs   # shared memory / queues / semaphores

Common commands to monitor memory usage:

free

vmstat

top

dstat -m

2.1 free

free -h
              total        used        free      shared   buffers   cached
Mem:          7.7G        6.2G        1.5G        17M       33M      184M
-/+ buffers/cache:   6.0G        1.7G
Swap:         24G        581M        23G

Explanation of columns: total: total physical memory. used: memory in use (includes cached memory). free: unused memory. shared: shared memory size. buffers: memory used by buffer cache. cached: memory used by page cache.

The second line ( -/+ buffers/cache) shows the memory actually used by applications (used – buffers – cached) and the memory that could be reclaimed (free + buffers + cached).

2.2 vmstat

Virtual memory statistics provide an overview of processes, memory, swap, I/O, and CPU usage.

vmstat -SM 1 100   # interval 1s, 100 samples, values in MB

procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  0     0    470    188   1154    0    0     0     4    0    0 99  0  0
 ... (additional rows omitted for brevity)

Key columns: r: runnable processes (waiting for CPU). b: processes blocked for I/O or swap. swpd: memory swapped out. free: idle physical memory. buff: buffer cache (disk block cache). cache: page cache (file data cache). si / so: swap in/out rates. bi / bo: block I/O (KB/s). us / sy / id / wa: CPU user, system, idle, and I/O wait percentages.

3. Network

3.1 Interfaces

ifconfig
iftop
ethtool

3.2 Ports

# TCP ports
netstat -ntlp
# UDP ports
netstat -nulp
# UNIX sockets
netstat -nxlp
# All connections
netstat -nalp
lsof -p <PID> -P
lsof -i :5900
sar -n DEV 1
ss
ss -s

3.3 tcpdump

sudo tcpdump -i any udp port 20112 and ip[0x1f:02]=0x4e91 -XNnvvv
sudo tcpdump -i any -XNnvvv

3.4 nethogs

Monitor per‑process network traffic:

nethogs

4. I/O Performance

iotop
iostat -kx 2
vmstat -SM
vmstat 2 10
dstat --top-io --top-bio

5. Processes

top
htop
ps auxf
ps -eLf   # show threads
ls /proc/<PID>/task

5.1 top

Common interactive commands (press h for help): 1: show each CPU. c: display full command line. H: toggle thread view. P: sort by CPU usage. M: sort by memory usage. R: reverse sort order. u: filter by user. k / r: kill or renice a process.

6. Performance Testing

stress --cpu 8 \
       --io 4  \
       --vm 2  \
       --vm-bytes 128M \
       --timeout 60s

7. Users

w
whoami

8. System Status

uptime
htop
vmstat
mpstat
dstat

9. Hardware Devices

lspci
lscpu
lsblk
lshw -c display
dmidecode

10. Filesystems

# Mount management
mount
umount
cat /etc/fstab
# LVM
pvdisplay
pvs
lvdisplay
lvs
vgdisplay
vgs
df -hT
lsof

11. Kernel & Interrupts

cat /proc/modules
sysctl -a | grep ...
cat /proc/interrupts

12. Logs

dmesg
less /var/log/messages
less /var/log/secure
less /var/log/auth

13. Cron Jobs

crontab -l
crontab -l -u nobody
sudo find /var/spool/cron/ | sudo xargs cat

14. Debugging Tools

14.1 perf

14.2 strace

Print system calls and signals:

strace -p <PID>
strace -p 5191 -f
strace -e trace=signal -p 5191
strace -e trace=open
strace -e trace=file

14.3 ltrace

Print library calls:

ltrace -p <PID>
ltrace -S   # show syscalls

15. Scenario Examples

Scenario 1 – After logging into a server

w        # current users and processes
last      # recent logins and reboots
uptime    # load average
history   # command history

Scenario 2 – Information in /proc

cat /proc/cpuinfo
cat /proc/meminfo
cat /proc/uptime
cat /proc/version
...

Scenario 3 – Run a command in background

nohup <command> >some.log 2>&1 &

Additional Useful Commands

# General monitoring
top
htop
glances
dstat & sar
mpstat
# Performance analysis
perf
# Process handling
ps
pstree -p
pgrep
pkill
pidof
# Network utilities
ip
ifconfig
dig
ping
traceroute
iftop
nload
netstat
vnstat
scp
# Disk I/O
iotop
iostat
# Virtual machines
virt-top
# Permissions
chown
chmod
# Service management
systemctl list-unit-files
# Search
find
locate
# Timing
time