fork exec source的区别

参考：http://www.cnblogs.com/bkygg/p/5023072.html

1：fork  运行的时候开一个sub_shell 执行调用的脚本，sub_shell执行的时候，parent_shell还在。sub_shell执行完毕后返回parent_shell。sub_shell从parent_shell继承环境变量。但是sub_shell中的变量不会带回parent_shell。在子命令执行完后再执行父命令，子级的环境变量不会影响到父级

2：exec 执行子级的命令后，不再执行父级的后面的命令

3：source 执行子级命令后继续执行父级的命令，同时子级设置的环境变量会影响到父级的环境变量

      与fork的区别是不新开一个sub_shell来执行被调用的脚本，而是在同一个shell中执行，所有被调用的脚本中的声明的变量和环境变量，都可以在主脚本中得到使用

例子理解：

1.sh

#!/bin/bash

A=B
#$$表示当前进程的PID
echo "PID for 1.sh before exec/source/fork:$$"
export A
echo "1.sh: \$A is $A"
case $ in
    exec)
         echo "using exec......"
         exec ./.sh;;
    source)
         echo "using source......"
         source ./.sh;;
    *)
         echo "using fork by default......"
         ./.sh;;
esac
echo "PID for 1.sh after exec/source/fork:$$"
echo "1.sh: \$A is $A"
~                    

2.sh

#!/bin/bash
echo "PID for 2.sh:$$"
echo "2.sh get \$A=$A from 1.sh"
A=C
export A
echo "2.sh:\$A is $A"

执行的结果：

[root@lile shell]# ./.sh fork
PID for .sh before exec/source/fork:
.sh: $A is B
using fork by default......
PID for .sh:
.sh get $A=B from .sh
.sh:$A is C
PID for .sh after exec/source/fork:
.sh: $A is B

[root@lile shell]# ./.sh source
PID for .sh before exec/source/fork:
.sh: $A is B
using source......
PID for .sh:
.sh get $A=B from .sh
.sh:$A is C
PID for .sh after exec/source/fork:
.sh: $A is C

[root@lile shell]# ./.sh exec
PID for .sh before exec/source/fork:
.sh: $A is B
using exec......
PID for .sh:
.sh get $A=B from .sh
.sh:$A is C