Bash: How to Compare Strings
In Bash scripting, comparing strings is a common task, especially when dealing with conditional logic and decision-making. Bash provides several ways to compare strings, allowing you to determine equality, inequality, and other relationships between string values.
Basic String Comparison Operators
Bash offers several operators for comparing strings:
1. Equality (==)
2. Inequality (!=)
3. Greater than (>)
4. Less than (<)
These operators are typically used within conditional statements like if or while.
Comparing Strings for Equality
To check if two strings are equal, use the == operator within an if statement.
1. Basic Equality Check:
string1="hello"
string2="hello"
if [ "$string1" == "$string2" ]; then
echo "Strings are equal"
else
echo "Strings are not equal"
fiOutput:
Strings are equal-
This checks if
string1is equal tostring2.
2. Inequality Check:
Use the != operator to check if strings are not equal.
string1="hello"
string2="world"
if [ "$string1" != "$string2" ]; then
echo "Strings are not equal"
else
echo "Strings are equal"
fiOutput:
Strings are not equalComparing Strings for Greater or Less Than
Bash allows you to compare strings lexicographically (alphabetically) using > and < operators.
3. Greater Than Comparison:
string1="apple"
string2="banana"
if [[ "$string1" > "$string2" ]]; then
echo "$string1 is greater than $string2"
else
echo "$string1 is not greater than $string2"
fiOutput:
apple is not greater than banana4. Less Than Comparison:
string1="apple"
string2="banana"
if [[ "$string1" < "$string2" ]]; then
echo "$string1 is less than $string2"
else
echo "$string1 is not less than $string2"
fiOutput:
apple is less than banana
Using test and [ for String Comparisons
Bash provides the test command and its shorthand [ for string comparisons, which are commonly used in scripts.
5. Using test for Equality:
string1="hello"
string2="hello"
if test "$string1" = "$string2"; then
echo "Strings are equal"
else
echo "Strings are not equal"
fiOutput:
Strings are equal
6. Using [ for Equality:
string1="hello"
string2="world"
if [ "$string1" = "$string2" ]; then
echo "Strings are equal"
else
echo "Strings are not equal"
fiOutput:
Strings are not equalCase-Insensitive String Comparison
To compare strings without considering case, you can convert both strings to the same case using tr or ,, (for lowercase) and ^^ (for uppercase) within Bash.
7. Converting to Lowercase for Comparison:
string1="Hello"
string2="hello"
if [ "${string1,,}" == "${string2,,}" ]; then
echo "Strings are equal (case-insensitive)"
else
echo "Strings are not equal"
fiOutput:
Strings are equal (case-insensitive)8. Converting to Uppercase for Comparison:
string1="Hello"
string2="HELLO"
if [ "${string1^^}" == "${string2^^}" ]; then
echo "Strings are equal (case-insensitive)"
else
echo "Strings are not equal"
fiOutput:
Strings are equal (case-insensitive)Checking if a String is Empty
You might also need to check if a string is empty or not.
9. Check if a String is Empty:
string=""
if [ -z "$string" ]; then
echo "String is empty"
else
echo "String is not empty"
fiOutput:
String is empty10. Check if a String is Not Empty:
string="hello"
if [ -n "$string" ]; then
echo "String is not empty"
else
echo "String is empty"
fiOutput:
String is not emptyBest Practices for String Comparisons in Bash
- Always quote variables in comparisons to avoid issues with spaces or special characters.
-
Use
[[...]]for advanced string comparisons, such as using>and<for lexicographic ordering. - Convert strings to a consistent case (all lowercase or all uppercase) when performing case-insensitive comparisons.
Official References
Understanding how to compare strings in Bash is crucial for effective scripting, especially in conditional logic. By mastering these comparison techniques, you can build more dynamic and responsive Bash scripts.
Jorge García
Fullstack developer