Comparing Strings and Numbers in Bash

Comparing variables is a fundamental operation in shell scripting, and Bash provides several ways to compare both strings and numbers. Selecting the right operators is crucial for writing correct and robust scripts. In this article, we'll walk through the syntax and usage of comparison operators like -eq, -lt, !=, and == for both numeric and string comparisons in Bash.

Table of Contents

  1. Numeric Comparisons in Bash
  2. String Comparisons in Bash
  3. Common Pitfalls
  4. Practical Examples
  5. Summary

Numeric Comparisons in Bash

Bash uses specific operators for integer comparisons inside test brackets [ ... ] or the test command. Here are the most commonly used ones:

Operator Meaning
-eq equal
-ne not equal
-lt less than
-le less than or equal to
-gt greater than
-ge greater than or equal to

Syntax Example:

a=5
b=10

if [ "$a" -lt "$b" ]; then
  echo "$a is less than $b"
fi

Tip: Always quote your variables to prevent script errors if they are empty or contain spaces.

String Comparisons in Bash

Bash provides different operators for string comparison:

Operator Meaning
= or == Strings are equal
!= Strings are not equal
< String is less than (ASCII order)
> String is greater than (ASCII)
-z String is null (zero length)
-n String is not null

Syntax Example:

str1="hello"
str2="world"

if [ "$str1" != "$str2" ]; then
  echo "Strings are different."
fi

Note: In POSIX sh, use = for equality, not ==. In Bash, both can work inside [ ... ], but == is generally reserved for [[ ... ]].

Common Pitfalls

  • Do not mix up numeric and string operators. For example, using == for numeric comparison does not work as expected.
  • Do not forget to quote variables. Unquoted empty strings can lead to syntax errors.
  • Use double brackets [[ ... ]] for advanced string comparison in Bash. Double brackets offer enhanced functionality, like pattern matching.

Practical Examples

1. Comparing Integers

num1=7
num2=15

if [ "$num1" -eq "$num2" ]; then
  echo "They are equal."
elif [ "$num1" -lt "$num2" ]; then
  echo "$num1 is less than $num2."
else
  echo "$num1 is greater than $num2."
fi

2. Comparing Strings

word1="bash"
word2="shell"

if [ "$word1" == "$word2" ]; then
  echo "Words are the same."
else
  echo "Words are different."
fi

3. Check for Null or Empty String

input=""

if [ -z "$input" ]; then
  echo "Input is empty."
fi

4. Lexicographical String Comparison

str1="apple"
str2="banana"

if [[ "$str1" < "$str2" ]]; then
  echo "$str1 comes before $str2"
fi

Summary

Use operators like -eq, -lt, !=, and == to compare numbers and strings in Bash. Remember:

  • Use -eq, -lt, etc., for numeric comparisons.
  • Use !=, == (or =), <, and > for string comparisons (prefer == and != for equality and inequality).
  • Always quote your variables for safety.
  • Use [[ ... ]] for advanced or pattern-based string testing in Bash-specific scripts.

Mastering these comparisons will help you write more reliable and readable Bash scripts!

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