linux - BASH:只有当函数被管道传输时，全局变量才在函数中不可更新(简单示例)

Question

这闻起来有问题，但也许有人可以解释一下:

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，去掉| cat 并且有效!

#!/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

管道创建子外壳。在bash manual中说子外壳不能修改其 parent 的环境。请参阅这些链接:

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

http://wiki.bash-hackers.org/scripting/processtree#actions_that_create_a_subshell