[转载] 环境变量的继承,fork、source、exec区别差异
背景
原文
http://zhu8337797.blog.163.com/blog/static/1706175492011671158505/
fork
使用 fork 方式运行 script 时, 就是让 shell(parent process) 产生一个 child
process 去执行该 script, 当 child process 结束后, 会返回 parent process,
但 parent process 的环境是不会因 child process 的改变而改变的.
source
使用 source 方式运行 script 时, 就是让 script 在当前 process 内执行, 而不
是产生一个 child process 来执行. 由于所有执行结果均于当前 process 内完成,
若 script 的环境有所改变, 当然也会改变当前 process 环境了.
exec
使用 exec 方式运行script时, 它和 source 一样, 也是让 script 在当前process
内执行, 但是 process 内的原代码剩下部分将被终止. 同样, process 内的环境随
script 改变而改变.
结论:通常如果我们执行时,都是默认为fork的。大家可以通过pstree命令看看关于父子进程的关系。如上,如果想让父进程得到子进程的环境变量,就是source方式了。
测试脚本练习
[root@orathiz tmp]# vi parent.sh
#!/bin/bash
A=B
echo "PID is parent.sh before child.sh :$$"
export A
echo "parent.sh: \$A is $A"
case $1 in
fork)
echo "using fork by default..."
./child.sh ;;
source)
echo "using source..."
. ./child.sh ;;
exec)
echo "using exec..."
exec ./child.sh ;;
esac
echo "PID is parent.sh after child.sh :$$"
echo "parent.sh: \$A is $A"
[root@orathiz tmp]# vi child.sh
#!/bin/bash
echo "PID for child.sh:$$"
echo "child.sh get \$A=$A from parent.sh"
A=C
export A
echo "child.sh: \$A is $A"
分别执行./parent.sh fork、source和exec得到的结果如下:
[root@orathiz tmp]# ./parent.sh fork
PID is parent.sh before child.sh :25679
parent.sh: $A is B
using fork by default...
PID for child.sh:25680
child.sh get $A=B from parent.sh
child.sh: $A is C
PID is parent.sh after child.sh :25679
parent.sh: $A is B
[root@orathiz tmp]# ./parent.sh source
PID is parent.sh before child.sh :25731
parent.sh: $A is B
using source...
PID for child.sh:25731
child.sh get $A=B from parent.sh
child.sh: $A is C
PID is parent.sh after child.sh :25731
parent.sh: $A is C
[root@orathiz tmp]# ./parent.sh exec
PID is parent.sh before child.sh :25959
parent.sh: $A is B
using exec...
PID for child.sh:25959
child.sh get $A=B from parent.sh
child.sh: $A is C
查看某个进程的环境变量
cat /proc/42638/environ|tr '\0' '\n'
参考
https://site.douban.com/196781/widget/notes/12220452/note/261008964/
http://zhu8337797.blog.163.com/blog/static/1706175492011671158505/