为什么这个Bash函数在一个git别名内执行两次，为什么添加`exit`修复它？

问题描述

如果我未能显式调用 那是因为 git 处理别名的方式：

赋予别名

 
 [别名] 
 myalias =！字符串 
 

其中 string 代码，当调用 git myalias args 其中 args 是一个可能为空的参数列表 git 将执行：

 
 sh  - c' string $ @'' string '  args   
 
 
 
 
例如：
 
 
  [别名] 
 banana =！echo$ 1，$ 2，SNIP
  
并呼叫
  git banana one'two two'three 
  
  git 将执行： 
 
 
  sh -c' echo$ 1，$ 2，SNIP$ @''echo$ 1，$ 2，SNIP'one'two two'three 
  
，所以输出将是：
 
 
  one，two two，SNIP one two two three 
  
在您的情况下， 
 
  [别名] 
 encrypt-for =！g（）{echo \once\;}; $ 1; 
  
并呼叫 
 
 
  git encrypt-for g 
  
  git  
 $ $ $ $ $ $ $ $ $ $ $ $ $ $; '''g（）{echoonce;}; $ 1;'g 
  

让我用等价的形式重写它：

  sh -c'g（）{echoonce;}; $ 1; $ @' -  g 
  

我只替换了'g（） {echoonce;}; $ 1;'部分（即 sh 的 $ 0 的位置参数，并且不会在这里扮演任何角色）通过一个伪参数 - 。应该清楚，它就像执行：

  g（）{echoonce;}; g; g 
  

所以你会看到：

 一次
一次
  

补救措施： t使用参数！只需使用：

  [别名] 
 encrypt-for =！g（ ）{echoonce;}; 
  

现在，如果您确实想要使用参数，请确保在给定的尾随参数不在所有。一种可能性是添加一个尾随注释字符，如下所示：

  [别名] 
 encrypt-for =！g （）{echoonce;}; $ 1＃
  

也可以将所有内容包装在一个函数中：

$ $ p $ $ $ $ $ {\
case $ 1 in \
（github）echo github ;; \
（twitter）echo twitter ;; \
（facebook）echo facebook ;; \\ \\ $;
（*）echo>& 2 \错误，未知$ 1\; exit 1 ;; \
esac \
}; main

---

希望您明白 git 正在用别名做的事情！它真的将$ @附加到别名字符串，并用此字符串和给定参数调用 sh -c 。

If I fail to explicitly call exit for certain function-based Bash scripts then there are additional unexpected executions for some functions. What is causing this? The behavior was first noticed while making a git alias as part of answering another user's question on StackOverflow. That alias was composed of this script (which runs the function twice instead of once):

#!/usr/bin/env bash

github(){
        echo github;
};

twitter(){
        echo twitter;
};

facebook(){
        echo facebook;
};

if [[ $(type -t "$1") == "function" ]];
then
        "$1";
else
        echo "There is no defined function for $1";
fi;

But this slightly modified script executes as expected (runs the function only once):

#!/usr/bin/env bash

github(){
        echo github;
};

twitter(){
        echo twitter;
};

facebook(){
        echo facebook;
};

if [[ $(type -t "$1") == "function" ]];
then
        "$1";
        exit 0;
else
        echo "There is no defined function for $1";
        exit 1;
fi;

---

Here is exactly what is happening when I run those scripts via a git alias (added set command for debugging purposes only):

$ git config --global alias.encrypt-for '!set -evu -o pipefail;github(){ echo github;};twitter(){ echo twitter;};facebook(){ echo facebook;};if [[ $(type -t "$1") == "function" ]];then "$1"; exit 0; else echo "There is no defined function for $1"; exit 1; fi;'
$ git encrypt-for "github"
type -t "$1"
github

$ git config --global alias.encrypt-for '!set -evu -o pipefail;github(){ echo github;};twitter(){ echo twitter;};facebook(){ echo facebook;};if [[ $(type -t "$1") == "function" ]];then "$1"; else echo "There is no defined function for $1"; fi;'
$ git encrypt-for "github"
type -t "$1"
github
github

The output from set -x:

$ git encrypt-for "github"
++ type -t github
+ [[ function == \f\u\n\c\t\i\o\n ]]
+ github
+ echo github
github
+ github
+ echo github
github

The output from replacing echo github with echo "I am echo in github" as a way of ruling out the echo command as the source of the second function execution:

$ git encrypt-for "github"
++ type -t github
+ [[ function == \f\u\n\c\t\i\o\n ]]
+ github
+ echo 'I am echo in github'
I am echo in github
+ github
+ echo 'I am echo in github'
I am echo in github

---

The following is the simplest version of the alias/script which gives the undesired behavior of double execution:

g(){
    echo "once";
};
$1;

And this is the resulting output from executing the simplified alias/script (which has the incorrect behavior of executing twice):

$ git config --global alias.encrypt-for '!g(){ echo "once";};$1;'
$ git encrypt-for g
once
once

解决方案

That's because of the way git handles aliases:

Given an alias

[alias]
    myalias = !string

where string is any string that represents some code, when calling git myalias args where args is a (possibly empty) list of arguments, git will execute:

    sh -c 'string "$@"' 'string' args

For example:

[alias]
    banana = !echo "$1,$2,SNIP "

and calling

git banana one 'two two' three

git will execute:

sh -c 'echo "$1,$2,SNIP " "$@"' 'echo "$1,$2,SNIP "' one 'two two' three

and so the output will be:

one,two two,SNIP one two two three

In your case,

[alias]
    encrypt-for = "!g(){ echo \"once\";};$1;"

and calling

git encrypt-for g

git will execute:

sh -c 'g(){ echo "once";};$1;"$@"' 'g(){ echo "once";};$1;' g

For clarity, let me rewrite this in an equivalent form:

sh -c 'g(){ echo "once";};$1;"$@"' - g

I only replaced the 'g(){ echo "once";};$1;' part (that will be sh's $0's positional parameter and will not play any role here) by a dummy argument -. It should be clear that it's like executing:

g(){ echo "once";};g;g

so you'll see:

once
once

To remedy this: don't use parameters! just use:

[alias]
    encrypt-for = "!g(){ echo "once";};"

Now, if you really want to use parameters, make sure that the trailing parameters given are not executed at all. One possibility is to add a trailing comment character like so:

[alias]
    encrypt-for = "!g(){ echo "once";};$1 #"

For your full example, a cleaner way could also be to wrap everything in a function:

[alias]
    encrypt-for = "!main() {\
        case $1 in \
            (github) echo github;; \
            (twitter) echo twitter;; \
            (facebook) echo facebook;; \
            (*) echo >&2 \"error, unknown $1"\; exit 1;; \
        esac \
    }; main"

---

Hopefully you understood what git is doing under the hood with aliases! it really appends "$@" to the alias string and calls sh -c with this string and the given arguments.

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