shell export command

 export [-fn] [name[=word]] ...

 export -p

        The supplied names are marked for automatic export to the environment of   subsequently  executed  commands. If  the  -f  option is given,  the   names refer to functions.  If no names are given, or if the -p option is supplied, a list of all names that are exported in this shell is printed.   The -n option causes the export property to be removed from each name.

        If  a  variable  name is followed by =word, the value of the variable is set to word.  export returns an exit status of 0 unless an invalid option is encountered, one of the names is not a valid shell variable name, or -f is supplied with a name that is not a function.

例子：父脚本使用export命令导出变量和函数，是之能在其调用的子脚本中访问。注意被导出的变量不同于传统意义上的全局变量，因为子脚本只是获得了一份父脚本导出变量的拷贝。

父脚本source.sh：

AA=11
export AA
BB=22
export -n BB
#export -p     

my_func()
{
 echo AA = $AA
 echo BB = $BB
 return
}
export -f my_func

if [ -f sub_shell.sh ]
then 

echo

echo call sub shell!

echo

./sub_shell.sh

fi

echo source.sh\'s AA BB:

my_func

子脚本sub_shell.sh

echo sub_shell now!
my_func
AA=33

运行结果：

call sub shell!

sub_shell now!

AA = 11

BB =

source.sh's AA BB:

AA = 11

BB = 22

由上面的例子可以看到，父脚本使用export命令导出了变量AA,BB 函数my_func. 然后在子脚本中访问，可以看到export -n导出的变量在子脚本得到的对应拷贝中会被置为空值， 导出函数需要使用export -f. 由于子脚本访问的是父脚本导出变量的拷贝，所以对于他们的修改，对于父脚本不可见。