bash 为什么 $$ 返回与父进程相同的 ID？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/21063765/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Why is $$ returning the same id as the parent process?
提问by ruanhao
I have problem with Bash, and I don't know why.
Under shell, I enter:
我对 Bash 有问题，我不知道为什么。
在 shell 下，我输入：
echo $$ ## print 2433
(echo $$) ## also print 2433
(./getpid) ## print 2602
"getpid" is a C program to get current pid, like:
“getpid”是一个获取当前pid的C程序，例如：
int main() { printf("%d", (int)getpid()); return 0; }
int main() { printf("%d", (int)getpid()); return 0; }
What confuses me is that:
让我困惑的是：
- I think "(command)" is a sub-process (am i right?), and i think its pid should be different with its parent pid, but they are the same, why...
- when i use my program to show pid between parenthesis, the pid it shows is different, is it right?
- is '$$' something like macro?
- 我认为“（命令）”是一个子进程（对吗？），我认为它的 pid 应该与其父 pid 不同，但它们是相同的，为什么...
- 当我使用我的程序在括号之间显示pid时，它显示的pid不同，对吗？
- '$$' 类似于宏吗？
Can you help me?
你能帮助我吗？
回答by chepner
$$is defined to return the process ID of the parent in a subshell; from the man page under "Special Parameters":
$$被定义为在子shell中返回父进程ID；从“特殊参数”下的手册页：
$ Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the current shell, not the subshell.
$ 扩展为 shell 的进程 ID。在 () 子shell 中，它扩展为当前shell 的进程ID，而不是子shell。
In bash4, you can get the process ID of the child with BASHPID.
在bash4，你可以得到与孩子的进程ID BASHPID。
~ $ echo $$
17601
~ $ ( echo $$; echo $BASHPID )
17601
17634
回答by craken
You can use one of the following.
您可以使用以下方法之一。
$!is the PID of the last backgrounded process.kill -0 $PIDchecks whether it's still running.$$is the PID of the current shell.
$!是最后一个后台进程的PID。kill -0 $PID检查它是否仍在运行。$$是当前 shell 的 PID。
回答by Niels Keurentjes
- Parentheses invoke a subshell in Bash. Since it's only a subshell it might have the same PID - depends on implementation.
- The C program you invoke is a separate process, which has its own unique PID - doesn't matter if it's in a subshell or not.
$$is an alias in Bash to the current script PID. See differences between$$and$BASHPIDhere, and right above that the additional variable$BASH_SUBSHELLwhich contains the nesting level.
回答by Alex
Try getppid()if you want your C program to print your shell's PID.
尝试getppid()是否希望 C 程序打印 shell 的 PID。
回答by Don-Pierre Halfaway
If you were asking how to get the PID of a known command it would resemble something like this:
如果您询问如何获取已知命令的 PID，它将类似于以下内容：
If you had issued the command below #The command issued was ***
如果你发出了下面的命令#发出的命令是***
dd if=/dev/diskx of=/dev/disky
dd if=/dev/diskx of=/dev/disky
Then you would use:
然后你会使用：
PIDs=$(ps | grep dd | grep if | cut -b 1-5)
What happens here is it pipes all needed unique characters to a field and that field can be echoed using
这里发生的事情是它将所有需要的唯一字符输送到一个字段，并且该字段可以使用
echo $PIDs
回声 $PIDs
回答by ARTEM LAPKIN
this one univesal way to get correct pid
这是获得正确 pid 的一种通用方法
pid=$(cut -d' ' -f4 < /proc/self/stat)
pid=$(cut -d' ' -f4 < /proc/self/stat)
same nice worked for sub
对 sub 也很好用
SUB(){
pid=$(cut -d' ' -f4 < /proc/self/stat)
echo "$$ != $pid"
}
echo "pid = $$"
(SUB)
check output
检查输出
pid = 8099
8099 != 8100
