How to Compare Strings in Bash

Bash scripting often requires comparing strings for equality or difference. Understanding how quoting, command substitution, and the test command interact is crucial for accurate string comparisons. This guide will delve into these concepts and demonstrate how to effectively compare strings in bash.

Understanding String Quoting in Bash

Quoting determines how Bash interprets characters within a string. There are three primary quoting mechanisms:

• Escape Character (): The backslash preserves the literal value of the next character, except for newline. For example, " represents a literal double quote.

• Double Quotes ("): Double quotes preserve the literal value of most characters, with exceptions for $ (variable expansion), `(command substitution),` (escape character), and sometimes ! (history expansion).

• Single Quotes ('): Single quotes preserve the literal value of all characters within the quotes without any exceptions. This makes them ideal for representing strings literally.

Command Substitution and String Variables

Command substitution allows using the output of a command as part of a string. The preferred syntax is $(command). For instance, STATUS=$(echo "test") assigns the output of the echo command (which is “test”) to the variable STATUS. While functional, this is equivalent to the simpler STATUS="test".

Using the test Command for String Comparison

The test command (or its alias [) provides a way to perform comparisons. The = operator compares strings for equality.

Consider the following example:

STATUS="test"
if [ "$STATUS" = "test" ]; then
  exit 0
fi

This code snippet checks if the value of STATUS is equal to “test”. Crucially, quoting $STATUS prevents word splitting and ensures that the comparison works correctly, even if the string contains spaces. Using the POSIX compliant = instead of == enhances script portability. The if statement and exit 0 are redundant since [ already returns an exit code indicating success (0) or failure (1).

Comparing Strings with Escaped Quotes

To compare a string variable against a literal string containing escaped quotes, ensure consistent quoting:

STATUS=""test""  # Assigning a string with escaped double quotes

if [ "$STATUS" = ""test"" ]; then # Correct comparison
    echo "Match"
fi


if [ "$STATUS" = '"test"' ]; then # Correct comparison using single quotes around literal
  echo "Match using single quotes"
fi

Both comparisons are valid and will evaluate to true. Remember that using single quotes around the literal string provides a cleaner way to represent strings with double quotes without needing to escape them.

Best Practices for String Comparison

• Always quote variables: This prevents word splitting and globbing issues.

• Use = for portability: While == works in Bash, = is the POSIX standard for string comparison.

• Understand quoting rules: Choose the appropriate quoting mechanism (single or double quotes) based on whether you need variable expansion or literal interpretation.

By adhering to these guidelines, you can write robust and reliable Bash scripts that perform accurate string comparisons.