BASH:只有当该函数通过管道传输时,全局变量才能在函数中更新(简单示例)

Question

这有点臭虫,但可能有人可以解释它:

以下脚本不起作用,输出如下:

#!/bin/bash
GLOBAL_VAR="OLD"
myfunc() {
        echo "func before set> $GLOBAL_VAR"
        GLOBAL_VAR="NEW"
        echo "func after set> $GLOBAL_VAR"
}
myfunc | cat
echo "final value> $GLOBAL_VAR"

输出:

func before set> OLD
func after set> NEW
final value> OLD

---

现在,只需脱掉| cat它就行了!

#!/bin/bash
GLOBAL_VAR="OLD"
myfunc() {
        echo "func before set> $GLOBAL_VAR"
        GLOBAL_VAR="NEW"
        echo "func after set> $GLOBAL_VAR"
}
myfunc
echo "final value> $GLOBAL_VAR"

输出:

func before set> OLD
func after set> NEW
final value> NEW

Answer 1

管道创建子壳.在bash手册中说,子壳不能改变父母的环境.看到这些链接:

http://www.gnu.org/software/bash/manual/bashref.html#Pipelines

http://wiki.bash-hackers.org/scripting/processtree#actions_that_create_a_subshel​​l