Linux中if-else条件判断语句

if [expression];
then
    # 执行语句
elif 
    # 执行语句
else
    # 执行语句
fi

num1=2
num2=10
if [ $num1 -gt $num2 ];then # $num1前面和$num2的后面都要有空格
    echo 'the first is larger.'
elif [ $num1 -eq $num2 ];then
    echo 'the 2 numbers are equal.'
else   
    echo 'the second is larger.'
fi

file=test.txt
if [ -f $file ];then
    echo $file "exists."
else
    echo $file "doesn't exist."
fi