5 Essential Linux Commands for Real‑Time Performance Monitoring

The article introduces five Linux commands useful for monitoring system performance: vmstat, iostat, free, df, and sar. It explains what each command reports, common options, example usages, and the meaning of the columns in their output.

1. vmstat – reports virtual memory statistics

vmstat prints detailed reports on memory, swap, I/O, and CPU activity, including used/available memory, swap in/out, disk reads/writes, and CPU idle time.

Example: vmstat 5 Useful options:

-a – show active and inactive memory

-s – display event counters and memory statistics

-S – output in KB instead of blocks

5 – refresh every 5 seconds

Each column meaning:

procs: process statistics

r – processes running or ready to run

b – processes waiting for I/O

memory: memory statistics

swpd – used swap space (KB)

free – free memory (KB)

buff – memory used as buffers (KB)

cache – memory used as cache (KB)

swap: swap statistics

si – KB/s read from swap

so – KB/s written to swap

io: I/O statistics

bi – blocks received from a block device (blocks/s)

bo – blocks sent to a block device (blocks/s)

system: system statistics

in – interrupts per second

cs – context switches per second

cpu: CPU statistics

us – user CPU time %

sy – system CPU time %

id – idle CPU %

wa – I/O wait %

st – stolen time % (in virtualized environments)

2. iostat – reports CPU and I/O statistics

iostat monitors and displays CPU utilization and disk I/O metrics such as CPU load, IOPS, and read/write throughput.

Common options:

c – show CPU usage

t – add timestamps

x – extended statistics (service time, wait count)

d – detailed per‑disk/partition statistics

p – statistics for a specific device

Example (refresh every 5 seconds for device sda): iostat -d -p sda 5 Sample output includes average CPU usage line (avg‑cpu) and per‑device statistics (tps, kB_read/s, kB_wrtn/s, etc.).

3. free – shows available and used memory

free displays total, used, and free amounts of physical and swap memory, providing a quick overview of memory availability.

Example: free -h Useful options:

b – display in bytes

k – display in KB

m – display in MB

h – human‑readable format (GB, MB, etc.)

4. df – reports filesystem disk space usage

df shows filesystem names, total, used, and available space, and usage percentage.

Common options:

-h – human‑readable format (K, M, G)

-T – display filesystem type

-i – show inode usage instead of space

-a – include all filesystems

-x type – exclude filesystems of a given type

-total – show total space at the end

Example: df -hT Columns: Filesystem, Type, 1K‑blocks, Used, Available, Use%, Mounted on.

5. sar – collects and reports system activity

sar gathers statistics on CPU, memory, I/O, network, and more over a period of time, useful for identifying performance issues.

Syntax: sar [options] [interval [count]] Common options:

-u – CPU utilization

-r – memory usage

-b – disk I/O

-n DEV – network interface statistics

-q – run queue and load average

-A – all available statistics

-s HH:MM:SS – start time

-e HH:MM:SS – end time

-f FILE – read from a sar data file

-o OUTPUT_FILE – write statistics to a file

Example (sample CPU usage every 5 seconds, 60 times):

sar -u 5 60