bash pid 和 $$ 之间的区别

发布于 2024-12-11 05:33:00 字数 1000 浏览 0 评论 0原文

我是 bash 脚本初学者，我有一个“作业”要做。我弄清楚了大部分内容，但有一部分说我必须回显父 bash 的 pid 和我将运行的两个子 shell 的 pid。所以我上网查了一下，发现了这个（Linux文档项目）：

#!/bin/bash4

echo "\$\$ outside of subshell = $$"                              # 9602
echo "\$BASH_SUBSHELL  outside of subshell = $BASH_SUBSHELL"      # 0
echo "\$BASHPID outside of subshell = $BASHPID"                   # 9602

echo

( echo "\$\$ inside of subshell = $$"                             # 9602
  echo "\$BASH_SUBSHELL inside of subshell = $BASH_SUBSHELL"      # 1
  echo "\$BASHPID inside of subshell = $BASHPID" )                # 9603
  # Note that $$ returns PID of parent process.

所以这里是我的问题：

1）第一个回显打印什么？这是父bash的pid吗？

2）为什么第二个echo打印出0？

3) $BASH_SUBSHELL 是命令还是变量？

4）我在 Mac 上做所有事情，过几天我会在 Linux 机器上尝试所有这些，但是每当我运行此脚本时 $BASHPID 不会返回任何内容，我只是得到一个新行。这是因为我在 Mac 上运行它，而 $BASHPID 在 Mac 上不起作用吗？

原文

I'm a bash scripting beginner, and I have a "homework" to do. I figured most of the stuff out but there is a part which says that I have to echo the pid of the parent bash and the pid of the two subshells that I will be running. So I looked online and found this (The Linux documentation project):

#!/bin/bash4

echo "\$\$ outside of subshell = $"                              # 9602
echo "\$BASH_SUBSHELL  outside of subshell = $BASH_SUBSHELL"      # 0
echo "\$BASHPID outside of subshell = $BASHPID"                   # 9602

echo

( echo "\$\$ inside of subshell = $"                             # 9602
  echo "\$BASH_SUBSHELL inside of subshell = $BASH_SUBSHELL"      # 1
  echo "\$BASHPID inside of subshell = $BASHPID" )                # 9603
  # Note that $ returns PID of parent process.

So here are my questions:

1) What does the first echo print? Is this the pid of the parent bash?

2) Why does the 2nd echo print out 0?

3) Is $BASH_SUBSHELL a command or a variable?

4) I'm doing everything on a mac, I will try all of this on a Linux machine in some days but
whenever I run this script $BASHPID doesn't return anything, I just get a new line. Is this because I'm running this on a mac and $BASHPID doesn't work on a mac?

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我只土不豪 2024-12-18 05:33:00

查看文档，它看起来像：

$$ 表示脚本文件正在运行的进程 ID。对于任何给定的脚本，当它运行时，它将只有一个“主”进程 ID。无论您调用多少个子 shell，$$ 都将始终返回与脚本关联的第一个进程 ID。 BASHPID 将显示当前 bash 实例的进程 ID，因此在子 shell 中它将与可能调用它的“顶级”bash 不同。
BASH_SUBSHELL 表示您所在的“子 shell 级别”。如果您不处于任何子 shell 级别，则您的级别为零。如果您在主程序中启动一个子 shell，则该子 shell 级别为 1。如果您在该子 shell 中启动一个子 shell，则该级别将为 2，依此类推。
BASH_SUBSHELL 是一个变量。
也许您的 bash 版本不支持 BASHPID ？我怀疑这是一个“Mac”问题。

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陌若浮生 2024-12-18 05:33:00

最好熟悉 bash(1)：

   BASHPID
          Expands to the process ID of the current bash process.
          This differs from $ under certain circumstances, such
          as subshells that do not require bash to be re-
          initialized.
   [...]
   BASH_SUBSHELL
          Incremented by one each time a subshell or subshell
          environment is spawned.  The initial value is 0.

$BASHPID 是通过 bash-4.0-alpha。如果您运行 bash --version，您可以了解您正在使用的 bash(1) 版本。

如果您要执行大量 bash(1) 工作，您还需要以下内容：

It'd be best to get well-acquainted with bash(1):

   BASHPID
          Expands to the process ID of the current bash process.
          This differs from $ under certain circumstances, such
          as subshells that do not require bash to be re-
          initialized.
   [...]
   BASH_SUBSHELL
          Incremented by one each time a subshell or subshell
          environment is spawned.  The initial value is 0.

$BASHPID was introduced with bash-4.0-alpha. If you run bash --version you can find out what version of bash(1) you're using.

If you're going to be doing much bash(1) work, you'll also need the following:

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