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Comparison Operators: Compare Values

Scripts need to compare values. Is this equal to that? Is this greater than that?

Here's the thing: Comparisons are how scripts make decisions. Learn them. Use them.

Numeric Comparisons​

[ $a -eq $b ]                    # Equal
[ $a -ne $b ]                    # Not equal
[ $a -lt $b ]                    # Less than
[ $a -le $b ]                    # Less than or equal
[ $a -gt $b ]                    # Greater than
[ $a -ge $b ]                    # Greater than or equal

Memory trick: -eq = equal, -ne = not equal, -lt = less than, -gt = greater than.

Real example:

a=10
b=20

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

My take: Use these for numbers. They work. Simple.

String Comparisons​

[ "$str1" = "$str2" ]            # Equal
[ "$str1" != "$str2" ]           # Not equal
[ -z "$str" ]                    # Empty string
[ -n "$str" ]                    # Non-empty string

Important: Always quote strings. [ "$name" = "John" ] not [ $name = "John" ].

Real example:

if [ "$USER" = "root" ]; then
    echo "Running as root"
fi

if [ -z "$BACKUP_DIR" ]; then
    BACKUP_DIR="/backup"
fi

My take: Quote strings. Always. Spaces break things without quotes.

Common Patterns​

Check if Equal​

if [ "$value" = "expected" ]; then
    echo "Match"
fi

Check if Not Empty​

if [ -n "$variable" ]; then
    echo "Variable is set"
fi

Check if Empty​

if [ -z "$variable" ]; then
    echo "Variable is empty"
fi

Common Mistakes (I've Made These)​

  1. Using = for numbers: Use -eq for numbers, = for strings.

  2. Not quoting strings: [ $name = "John" ] breaks with spaces. Use [ "$name" = "John" ].

  3. Wrong operator: -lt for less than, -gt for greater than. Don't mix them up.

  4. Spaces in brackets: [condition] doesn't work. Use [ condition ]. Spaces required.

  5. Using ==: = works. == also works, but = is more portable.

Real-World Examples​

Validate Input​

if [ "$1" = "start" ]; then
    start_service
elif [ "$1" = "stop" ]; then
    stop_service
else
    echo "Usage: $0 {start|stop}"
fi

Check Number Range​

if [ $age -ge 18 ] && [ $age -le 65 ]; then
    echo "Valid age"
fi

Check if Variable Set​

if [ -z "$PORT" ]; then
    PORT=8080
fi

What's Next?​

Now that you can compare values, scripts can make decisions. Or learn about Logical Operators to combine conditions.

Personal note: When I started, I'd use = for everything. Then I learned the difference. Numbers use -eq. Strings use =. Small detail. Big difference.