问题
So this read is executed after the pipeline, which means that the output of the echo gets read into str - but because it is after a pipe, the contents of str are now in a subshell that cannot be read by the parent shell. My questions is - what happens in to the contents of str ? Does the pipe create a subshell, and then once the content are read into str , does the parent process kill the child process and str is erased - or does the contents of str live on somewhere outside the shell. Like how do we see what is in the subshells ? Can we access subshells from the parent shells?
echo hello | read str
echo $str
回答1:
In your example, $str exists inside a subshell and by default, it disappears once the line has finished. A child process cannot modify its parent.
Aside from changing the shell option lastpipe, you can also change the code to avoid using a pipe. In this case, you could use:
read str < <(your command)
# or
str=$(your command)
Both of these create subshells too, but $str is assigned to in the parent process.
回答2:
The value of ${str} only exists during the lifetime of the subshell.
Bash 4.x has an option shopt -s lastpipe to run the last command of a pipeline in the parent shell, like ksh93 does by default. The value of $str will then persist.
来源：https://stackoverflow.com/questions/46946888/bash-pipe-creates-a-subshell