Linux 标准退出状态码
在Linux中，当程序中断后，会返回一个退出码。如果是正常退出，那么这个码将会是零，任何非零的情况都意味着有错误产生。
退出码在我们自己的程序中是可以任意选择并指定的，但有些约定俗成的规矩，部分退出码有明确的含义，尽量不要使用。
我们一般在shell里通过 exit number，像是 exit 42，的方式指定退出码，在标准的输出时是隐藏的，可以通过 echo $? 的方式查询上一条指令的运行状态，这也常常作用在 shell 脚本中对某个指令运行状况的检测。
特殊的退出码
General Error: 1
This is the most used exit code and should be used as a catch-all value for miscellaneous errors.
Misuse of Shell Built-in: 2
Exit code 2 signifies invalid usage of some shell built-in command. Examples of built-in commands include alias, echo, and printf.
Cannot Execute: 126
In this case, the command invoked can’t be executed. This will most likely occur when there’s a permission problem or the command isn’t executable.
Command Not Found: 127
A command couldn’t be found. This may happen, for example, because there was a typo or an issue with our PATH.
Invalid Argument To Exit: 128
The exit command only takes a positive integer as its argument. This means any negative, fractional, or non-numeric values aren’t allowed.
Fatal Error Signal ‘n‘: 128+n
In Linux, programs might send one of 33 different signals. When a program terminates after receiving one of these signals, it will return an error code equal to 128 + signal-number.
For example, when we terminate a program by typing Control-C, we’re effectively sending it a SIGINT signal. This signal has a value of 2, therefore, the program will stop its execution and return an exit code with a value 128 + 2 = 130.
Exit Status Out of Range: 255
Depending on our shell, exit code 255 might mean that the returned exit code is outside of the 0-255 range.
不要使用255以上的退出码。