How to Compare Two Strings in Bash

Comparing strings is a fundamental operation in any scripting language, and Bash is no exception. This guide provides a comprehensive overview of how to effectively compare two strings in Bash, covering various methods and scenarios. Understanding these techniques will empower you to write more robust and efficient Bash scripts.

Using the = and != Operators with Single Square Brackets

The simplest way to compare two strings in Bash is using the = and != operators within single square brackets [ ]. This method performs a literal string comparison.

string1="hello"
string2="world"

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


if [ "$string1" != "$string2" ]; then
   echo "Strings are not equal"
else
    echo "Strings are equal"
fi

Important: Always quote your variables within single square brackets. Unquoted variables can lead to unexpected behavior, especially if they contain spaces or special characters. For example, if string1 was empty and unquoted, the if statement would become if [ = "world" ], which would result in a syntax error. Quoting ensures the script interprets the variables correctly. The image below illustrates the correct syntax.

Using the == and != Operators with Double Square Brackets

Double square brackets [[ ]] offer a more advanced way to compare strings. They provide pattern matching capabilities and are generally recommended over single square brackets for string comparisons.

string1="hello"
string2="hello*"

if [[ "$string1" == "$string2" ]]; then
  echo "Strings match the pattern"
else
  echo "Strings do not match the pattern"
fi

In this example, string2 contains a wildcard character *, indicating that it should match any string starting with “hello.” This functionality is not available with single square brackets.

Understanding Potential Issues: Control Characters and String Comparison

Sometimes, strings can contain hidden control characters that interfere with comparisons. These characters, often generated by command-line tools with colored output, can cause seemingly identical strings to appear different to Bash.

For instance, if a string contains ANSI escape codes for color, a simple comparison might fail. To address this, you can use tools like sed or perl to strip these control characters before comparing the strings. The following image shows an example of how control characters can affect the output of commands.

• Using sed:

cleaned_string=$(echo "$string" | sed -r "s/x1B[([0-9]{1,2}(;[0-9]{1,2})?)?[m|K]//g")

• Using perl:

cleaned_string=$(echo "$string" | perl -pe 's/e[?.*?[@-~]//g')

Conclusion

Accurately comparing strings in Bash scripts is crucial for their proper functioning. By understanding the nuances of single and double square brackets, quoting variables, and handling potential issues like control characters, you can write more reliable and predictable scripts. Remember to always sanitize your input and choose the comparison method that best suits your specific needs.