Hadoop和RDBMS(Hadoop and RDBMS)

 当您启动子shell时，与反引号一样，bash自行分叉，然后执行您想要运行的命令。 然后你还运行一个管道，导致所有这些都在他们自己的子shell中运行，所以你最终得到了等待管道完成的脚本的“额外”副本，这样它就可以收集输出并将其返回给原始剧本。 
 
 我们将使用(...)在子shell中显式运行进程并使用pgrep命令来做一些实验ps | grep "name" | grep -v grep ps | grep "name" | grep -v grep ps | grep "name" | grep -v grep对我们来说，只是向我们展示了与我们的字符串匹配的进程： 
 
echo "Start"
(pgrep test.sh)
(pgrep test.sh) | wc -l
(pgrep test.sh | wc -l)
echo "end"
 
 这对我来说就是产生输出： 
 
Start
30885
1
2
end
 
 所以我们可以看到，在子shell中运行pgrep test.sh只能找到test.sh的单个实例，即使该子shell是管道本身的一部分。 但是，如果子shell包含一个管道，那么我们会得到等待管道完成的脚本的分叉副本 

When you start a subshell, as with the backticks, bash forks itself, then executes the command you wanted to run. You then also run a pipeline which causes all of those to be run in their own subshells, so you end up with the "extra" copy of the script that's waiting for the pipeline to finish so it can gather up the output and return that to the original script.
 
We'll do a little expermiment using (...) to run processes explicitly in subshells and using the pgrep command which does ps | grep "name" | grep -v grep for us, just showing us the processes that match our string:
 
echo "Start"
(pgrep test.sh)
(pgrep test.sh) | wc -l
(pgrep test.sh | wc -l)
echo "end"
 
which on a run for me produces the output:
 
Start
30885
1
2
end
 
So we can see that running pgrep test.sh in a subshell only finds the single instance of test.sh, even when that subshell is part of a pipeline itself. However, if the subshell contains a pipeline then we get the forked copy of the script waiting for the pipeline to finish