if

VAL="value"

if [ $VAL = "value" ]; then
    echo "true"
fi
実行結果
true

if-else

VAL=false

if [ $VAL ]; then
    echo "true"
else
    echo "false"
fi
実行結果
false

if-else if-else

VAL="value_02"

if [ $VAL = "value_01" ]; then
    echo "value1"
elif [ $VAL = "value_02" ]; then
    echo "value2"
else
    echo "value3"
fi
実行結果
value2