Bash Script to Check CPU Usage Easily

Use the command top -bn1 | grep 'Cpu(s)' | awk '{print 100 - $8 "% CPU used"}' inside a Bash script to check current CPU usage percentage.

Examples

InputCPU idle is 90%

Output10% CPU used

InputCPU idle is 75.5%

Output24.5% CPU used

InputCPU idle is 0%

Output100% CPU used

How to Think About It

To check CPU usage, find the percentage of CPU idle time from system commands, then subtract it from 100% to get the used CPU percentage. This gives a simple number showing how busy the CPU is right now.

Algorithm

1

Run a system command to get CPU idle percentage.

2

Extract the idle value from the command output.

3

Calculate CPU usage as 100 minus idle percentage.

4

Print the CPU usage percentage.

Code

bash

#!/bin/bash
cpu_idle=$(top -bn1 | grep 'Cpu(s)' | awk '{print $8}' | sed 's/,//')
cpu_usage=$(echo "100 - $cpu_idle" | bc)
echo "$cpu_usage% CPU used"

Output

10.5% CPU used

Dry Run

Let's trace a CPU idle value of 89.5% through the script

1

Get CPU idle

Command output: Cpu(s): 10.5% us, 0.0% sy, 0.0% ni, 89.5% id, 0.0% wa, 0.0% hi, 0.0% si, 0.0% st Extracted idle: 89.5

2

Calculate usage

100 - 89.5 = 10.5

3

Print result

Output: 10.5% CPU used

Step	Action	Value
1	Extract idle	89.5
2	Calculate usage	10.5
3	Print output	10.5% CPU used

Why This Works

Step 1: Extract CPU idle

The top command shows CPU stats; we use grep and awk to get the idle percentage from its output.

Step 2: Calculate CPU usage

CPU usage is the opposite of idle, so we subtract idle from 100 using bc for decimal math.

Step 3: Display result

The script prints the CPU usage with a percent sign to show how busy the CPU is.

Alternative Approaches

Using vmstat

bash

#!/bin/bash
idle=$(vmstat 1 2|tail -1|awk '{print $15}')
usage=$((100 - idle))
echo "$usage% CPU used"

This uses <code>vmstat</code> which samples CPU idle twice; it may be more accurate but slower.

Using mpstat

bash

#!/bin/bash
idle=$(mpstat 1 1 | grep 'Average' | awk '{print $12}')
usage=$(echo "100 - $idle" | bc)
echo "$usage% CPU used"

Requires <code>mpstat</code> from sysstat package; gives detailed CPU stats.

Complexity: O(1) time, O(1) space

Time Complexity

The script runs a fixed number of commands once, so it takes constant time regardless of CPU load.

Space Complexity

Uses only a few variables to store numbers, so constant space.

Which Approach is Fastest?

Using top is fast and available on most systems; vmstat and mpstat may be slower or require extra packages.

Approach	Time	Space	Best For
top command	O(1)	O(1)	Quick, no extra tools
vmstat command	O(1)	O(1)	More accurate sampling
mpstat command	O(1)	O(1)	Detailed CPU stats, needs sysstat