bash中特殊的变量赋值

看例子就好：

unset X Y Z

#${variable?value} - Complain if undefined

cat ${jack?"Please define jack, and try again"}

#${variable-default} - Use default if undefined

echo X is: $X

echo X is: ${X-default}

X=new

echo X is: ${X-default}

echo X is: $X

#${variable+value} - Change if defined

echo Y is: $Y

echo Y is: ${Y+"Current value of Y is $Y"}

Y="test"

echo Y is: $Y

echo Y is: ${Y+"Current value of Y is $Y"}

echo Y is: $Y

#${variable=value} - define if undefined

echo Z is: $Z

echo Z is: ${Z=default}

Z="test"

echo Z is: ${Z=default}

echo Z is: $Z