shell 中调用另一个shell (fork, exec, source)

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FROM : http://mindream.wang.blog.163.com/blog/static/2325122220084624318692/

    fork ( /directory/script.sh)

    fork是最普通的, 就是直接在脚本里面用/directory/script.sh来调用script.sh这个脚本.

    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中的环境变量不会带回

parent-shell

    exec (exec /directory/script.sh)

    exec与fork不同，不需要新开一个sub-shell来执行被调用的脚本. 被调用的脚本与父脚本在同一个shell内执行。但

是使用exec调用一个新脚本以后, 父脚本中exec行之后的内容就不会再执行了。这是exec和source的区别

    source (source /directory/script.sh 也可以用点命令，即 ./directory/script.sh)

    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. 所以被调用的脚本中声明的

变量和环境变量, 都可以在主脚本中得到和使用.

可以通过下面这两个脚本来体会三种调用方式的不同:

1.sh

    #!/bin/bash
    A=B
    echo "PID for 1.sh before exec/source/fork:$$"
    export A
    echo "1.sh: \$A is $A"
    case $1 in
            exec)
                    echo "using exec…"
                    exec ./2.sh ;;
            source)
                    echo "using source…"
                    . ./2.sh ;;
            *)
                    echo "using fork by default…"
                    ./2.sh ;;
    esac
    echo "PID for 1.sh after exec/source/fork:$$"
    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"





    执行情况：

    $ ./1.sh
    PID for 1.sh before exec/source/fork:5845364
    1.sh: $A is B
    using fork by default…
    PID for 2.sh: 5242940
    2.sh get $A=B from 1.sh
    2.sh: $A is C
    PID for 1.sh after exec/source/fork:5845364
    1.sh: $A is B
    $ ./1.sh exec
    PID for 1.sh before exec/source/fork:5562668
    1.sh: $A is B
    using exec…
    PID for 2.sh: 5562668
    2.sh get $A=B from 1.sh
    2.sh: $A is C
    $ ./1.sh source
    PID for 1.sh before exec/source/fork:5156894
    1.sh: $A is B
    using source…
    PID for 2.sh: 5156894
    2.sh get $A=B from 1.sh
    2.sh: $A is C
    PID for 1.sh after exec/source/fork:5156894
    1.sh: $A is C
    $