Essential Linux Commands for Real‑Time System Troubleshooting

top – System Overview

The top command provides a snapshot similar to Windows Task Manager. The first line shows current time, uptime, logged‑in users, and load averages for the past 1, 5, and 15 minutes (e.g.,

18:14:58 up 112 days, 1:35, 1 user, load average: 0.00, 0.10, 0.11

). A load average above 1 indicates the system is overloaded.

The second line (

Tasks: 225 total, 1 running, 224 sleeping, 0 stopped, 0 zombie

) details process counts, helping you see how many processes are active, sleeping, stopped, or zombie.

CPU Usage (Cpu line)

The

Cpu(s): 1.8%us, 0.9%sy, 0.0%ni, 97.1%id, 0.1%wa, 0.0%hi, 0.1%si, 0.0%st

line breaks down CPU time:

us : user‑space processes

sy : kernel‑space processes

ni : processes with altered priority

id : idle CPU

wa : I/O wait

hi : hardware interrupts

si : software interrupts

st : stolen time (virtualized environments)

Memory Usage (Mem line)

Mem: 32879852k total, 23633040k used, 9246812k free, 311552k buffers

shows total, used, free memory and kernel buffers. Available memory can be calculated as free + buffers + cached.

Swap Usage (Swap line)

Swap: 4194300k total, 255104k used, 3939196k free, 10422508k cached

reports virtual memory statistics.

free -m – Memory Summary

The free -m command displays memory usage in megabytes, summarizing total, used, and free memory for both RAM and swap.

iostat – I/O Statistics

Usage: iostat [options] [interval] [count]. Example commands:

iostat -d

iostat -d 2 2

iostat -x 1 2

These commands report device‑level I/O statistics and extended metrics.

netstat – Network Connections

Run netstat to view socket states. Pay special attention to the number of ESTABLISHED connections; a persistently high count may indicate resource exhaustion.

Common usages:

Show IPs with the most connections:

netstat -na | grep ESTABLISHED | awk '{print $5}' | awk -F: '{print $1}' | sort | uniq -c

Count TCP connections by state:

netstat -na | awk '/^tcp/ {++S[$NF]} END {for(a in S) print a, S[a]}'

df -h – Disk Space

df -h

displays filesystem usage in a human‑readable format, helping you locate partitions that are near capacity.

du -sh – Directory Size

Use du -sh to get the total size of a directory. Variations such as du --max-depth=2 --block-size=M or ll --block-size=M provide more granular size breakdowns.