linux bash中的$ RANDOM在启动时总是一样的 [英] $RANDOM in linux bash will alway be the same upon bootup

问题描述

我遇到了这个问题，我很困惑.

I ran into this problem and I'm so confused.

在Linux启动过程的一开始，我在bash中使用$ RANDOM作为测试字符串.非常有趣的是，我观察到$ RANDOM在每次启动Linux时都是相同的.就我而言，是18869.

I am using $RANDOM in bash as a test string at the very beginning of the linux bootup process. Very interestingly, I observed that $RANDOM will be the same every time linux booted up. In my case, it's 18869.

我只是在文件中回显$ RANDOM.我在每次启动时都观察到了相同的数字.之后，$ RANDOM似乎会返回真实"随机数.

I was simply echoing $RANDOM to a file. I observed the same number in every boot up. After that, $RANDOM seems to return "real" random numbers.

echo "$RANDOM is a test string" >> /tmp/test

有人可以解释其中的原因吗?

Can anyone explain the reason in this?

推荐答案

这是Bash 4.2源代码中 variables.c 的初始种子算法:

Here is the initial seed algorithm from variables.c in the Bash 4.2 source code:

static void                                                                                               
seedrand ()                                                                                               
{                                                                                                         
  struct timeval tv;                                                                                      
                                                                                                          
  gettimeofday (&tv, NULL);                                                                               
  sbrand (tv.tv_sec ^ tv.tv_usec ^ getpid ());                                                            
}  

换句话说，它基于pid和当前时间(以秒和微秒为单位)(系统精度约为1/100秒)来生成种子.

In other words, it generates the seed based on pid and current time in seconds and microseconds (with a system accuracy of ~1/100th second).

如果在可预测的小型系统上检查 $ RANDOM ，则在初始化系统时钟之前，您每次可能会看到相同的值.

If you check $RANDOM on a small, predictable system, before the system clock is initialized, you're likely to see the same value every time.

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