How to Compare Numbers in Bash

Quick Answer: Compare Numbers in Bash

To compare numbers, use the -eq operator for equality: [ $a -eq 5 ]. For greater/less than, use -gt and -lt. Always quote variables to prevent errors: [ "$var" -gt 10 ]. The test operators are the standard, most portable method.

Quick Comparison: Number Comparison Methods

Method	Syntax	Best For	Speed
Test operators	[ $a -eq 5 ]	Most cases	Fastest
Arithmetic (())	(( a == 5 ))	Bash only	Medium
let command	let result=a>5	Legacy code	Medium
expr	expr $a = 5	Portability	Slower

Bottom line: Use test operators with -eq, -gt, -lt for all comparisons.

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Numeric Comparison Operators

Bash provides several operators for comparing numbers. These are used in if statements, while loops, and other conditionals to make decisions based on numeric values.

Method 1: Using Test Operators (Standard)

Here are the primary numeric comparison operators in Bash:

Operator	Meaning	Example
-eq	Equal	[ $a -eq 5 ]
-ne	Not equal	[ $a -ne 5 ]
-lt	Less than	[ $a -lt 10 ]
-le	Less than or equal	[ $a -le 10 ]
-gt	Greater than	[ $a -gt 0 ]
-ge	Greater than or equal	[ $a -ge 0 ]

These operators work with the [ ] test command or in arithmetic expressions with (( )).

Using the Test Command [ ]

The traditional way to compare numbers uses square brackets:

#!/bin/bash

AGE=25

# Check if age equals 25
if [ $AGE -eq 25 ]; then
  echo "You are 25 years old"
fi

# Check if age is greater than 18
if [ $AGE -gt 18 ]; then
  echo "You are an adult"
fi

# Check if age is less than 65
if [ $AGE -lt 65 ]; then
  echo "You are working age"
fi

Using Arithmetic Expressions (( ))

Modern Bash prefers the (( )) syntax, which is cleaner and uses familiar operators like >, <, ==:

#!/bin/bash

AGE=25

# Using (( )) - feels more like other programming languages
if (( AGE == 25 )); then
  echo "You are 25 years old"
fi

if (( AGE > 18 )); then
  echo "You are an adult"
fi

if (( AGE < 65 )); then
  echo "You are working age"
fi

# Operators in (( ))
# ==, !=, <, <=, >, >=

Practical Example: Age Verification

Here’s a script that validates age input and provides different messages:

#!/bin/bash

read -p "Enter your age: " age

# Check if input is a number
if ! [[ "$age" =~ ^[0-9]+$ ]]; then
  echo "ERROR: Please enter a valid number"
  exit 1
fi

# Categorize by age
if (( age < 13 )); then
  echo "You are a child"
elif (( age < 18 )); then
  echo "You are a teenager"
elif (( age < 65 )); then
  echo "You are an adult"
else
  echo "You are a senior"
fi

# Check specific ranges
if (( age >= 18 && age <= 21 )); then
  echo "You are in the young adult category"
fi

Comparing Multiple Conditions

Combine numeric comparisons with && and ||:

#!/bin/bash

TEMPERATURE=$1
HUMIDITY=$2

# Check if temperature is in safe range AND humidity is acceptable
if (( TEMPERATURE > 0 && TEMPERATURE < 40 )) && (( HUMIDITY < 80 )); then
  echo "Environment is safe"
else
  echo "Environment out of safe range"
fi

# Check if either condition indicates danger
if (( TEMPERATURE > 100 || HUMIDITY > 95 )); then
  echo "WARNING: Dangerous conditions detected!"
  exit 1
fi

Real-World Example: Disk Space Monitor

#!/bin/bash

# Get disk usage percentage for root filesystem
DISK_USAGE=$(df / | tail -1 | awk '{print $5}' | sed 's/%//')

echo "Disk usage: ${DISK_USAGE}%"

# Warn if disk is getting full
if (( DISK_USAGE > 90 )); then
  echo "CRITICAL: Disk almost full!"
  exit 1
elif (( DISK_USAGE > 75 )); then
  echo "WARNING: Disk usage is high"
  exit 0
elif (( DISK_USAGE > 50 )); then
  echo "NOTICE: Disk usage is moderate"
  exit 0
else
  echo "OK: Plenty of disk space"
  exit 0
fi

Incrementing and Decrementing

Common numeric operations inside (( )):

#!/bin/bash

# Increment
count=0
((count++))  # count is now 1
((count += 5))  # count is now 6

# Decrement
((count--))  # count is now 5
((count -= 2))  # count is now 3

# Multiplication
((result = count * 10))  # result is now 30

# Division
((result = result / 2))  # result is now 15

echo "Final count: $count, result: $result"

Important Notes

Always quote variables in comparisons:

# CORRECT
if [ "$age" -gt 18 ]; then

# WRONG - will fail if $age is empty
if [ $age -gt 18 ]; then

The difference between [ ] and (( )):

# [ ] uses -eq, -gt, -lt operators
if [ 5 -eq 5 ]; then
  echo "Equal"
fi

# (( )) uses ==, >, < operators
if (( 5 == 5 )); then
  echo "Equal"
fi

Quick Reference

# Equal
[ $a -eq $b ]    # [ ] syntax
(( a == b ))     # (( )) syntax

# Not equal
[ $a -ne $b ]    # [ ] syntax
(( a != b ))     # (( )) syntax

# Less than
[ $a -lt $b ]    # [ ] syntax
(( a < b ))      # (( )) syntax

# Greater than
[ $a -gt $b ]    # [ ] syntax
(( a > b ))      # (( )) syntax

# Less than or equal
[ $a -le $b ]    # [ ] syntax
(( a <= b ))     # (( )) syntax

# Greater than or equal
[ $a -ge $b ]    # [ ] syntax
(( a >= b ))     # (( )) syntax

Summary

Numeric comparisons are essential for making decisions in scripts. Use (( )) in modern Bash for cleaner syntax, or [ ] with -eq, -lt, -gt operators for compatibility with older shells. Always quote variables and remember that these operators only work with integers, not floating-point numbers.