How to Compare Strings in Shell Script

Comparing strings is a fundamental operation in shell scripting, often used for validating user input, processing text data, and controlling program flow. This guide provides a comprehensive overview of various techniques to compare strings in Bash, outlining four distinct methods with practical examples.

Methods for String Comparison in Bash

Bash offers several ways to compare strings, each with its own nuances:

1. Using the test Command

The test command, a built-in utility, evaluates conditional expressions. For string comparison, it uses the following syntax:

test "$string1" = "$string2" 

This command returns true (exit status 0) if the strings are equal and false (exit status 1) otherwise. Note that this comparison is case-sensitive.

Example:

string1="Hello"
string2="hello"

if test "$string1" = "$string2"; then
  echo "Strings are equal"
else
  echo "Strings are not equal"
fi

This script will output “Strings are not equal” because “Hello” and “hello” differ in case.

2. Using the [ Command

The [ command is functionally equivalent to the test command. It also evaluates conditional expressions and follows this syntax:

[ "$string1" = "$string2" ]

Ensure spaces surround the square brackets. The closing bracket ] is an argument to the [ command.

Example:
This example mirrors the test command example and produces the same case-sensitive output.

3. Using the [[ Keyword

The [[ keyword is a Bash built-in, not a command. It offers more advanced features, including pattern matching and regular expression support. For simple string comparison:

[[ "$string1" = "$string2" ]]

Case-Insensitive Comparison with [[ and shopt:

The [[ keyword, combined with the shopt command’s nocasematch option, enables case-insensitive comparison:

shopt -s nocasematch
[[ "$string1" = "$string2" ]]
shopt -u nocasematch

shopt -s nocasematch activates case-insensitive matching, and shopt -u nocasematch deactivates it. This is specific to [[ and doesn’t work with test or [.

4. One-Line String Comparison

For concise comparisons, Bash supports one-line expressions using logical operators && (AND) and || (OR):

[[ "$string1" = "$string2" ]] && echo "Equal" || echo "Not Equal"

This executes the echo "Equal" command if the strings are equal; otherwise, it executes echo "Not Equal". Similar one-liners can be constructed with test and [.

String Comparison Operators

Beyond equality (=), other operators are available for string comparisons:

• !=: Not equal to
• <: Less than (lexicographically)
• >: Greater than (lexicographically)
• -z: String is empty
• -n: String is not empty

Conclusion

This guide detailed How To Compare Strings In Shell Scripts using various methods and operators. Choosing the right method depends on the specific needs of your script, considering factors like case sensitivity and desired complexity. Mastering these techniques allows for effective data handling and control flow in your Bash scripts. Remember to always quote variables containing strings to prevent word splitting and globbing issues.