Expressions are tested whether they are True or False using the if statement.

#!/bin/bash
 
x=0
 
if [[ ! $x ]]; then
    echo "empty value"
elif [[ $x =~ [a-zA-Z] ]]; then
    echo "$x not an integer"
else 
    if [[ $x -gt 0 ]]; then
        echo "$x > 0"
    elif [[ $x -lt 0 ]]; then
        echo " $x < 0"
    elif [[ $x -eq 0 ]]; then
        echo "$x == 0"
    fi
fi

Otherwise, expressions are tested whether they have a specific value using the case statement.

#!/bin/bash
 
x=0
 
case $x in
    "") 
        echo "empty value"
    ;;
    0)
        echo "$x == 0"
    ;;
    *)
        echo "unknown value"
    ;;
esac