如何在不创建子shell的情况下将命令的输出存储在变量中[Bash< v4] [英] How to store the output of command in a variable without creating a subshell [Bash <v4]
问题描述
ksh 具有一个非常有趣的构造来执行此操作，此答案中对此进行了详细说明: https://stackoverflow.com/a/11172617/636849
ksh has a really interesting construct to do this, detailed in this answer: https://stackoverflow.com/a/11172617/636849
从Bash 4.0开始，有一个内置的 mapfile 内置命令可以解决此问题: http://www.gnu.org/software/bash/manual/html_node/Bash-Builtins.html
Since Bash 4.0, there is a builtin mapfile builtin command that should solve this problem: http://www.gnu.org/software/bash/manual/html_node/Bash-Builtins.html
但是奇怪的是，它似乎不适用于进程替换:
But strangely, it doesn't seem to work with process substitution:
foo () { echo ${BASH_SUBSHELL}; }
mapfile -t foo_output <(foo) # FAIL: hang forever here
subshell_depth=${foo_output[0]} # should be 0
但是如何在Bash v3.2中做到这一点?
But how to do this in Bash v3.2 ?
推荐答案
这是另一种方法，它与众不同，因此需要单独的答案.我认为该方法无Subshell且无bash子进程:
Here's another way to do it, which is different enough that it warrants a separate answer. I think this method is subshell-free and bash sub-process free:
ubuntu@ubuntu:~$ bar () { echo "$BASH_SUBSHELL $BASHPID"; }
ubuntu@ubuntu:~$ bar
0 8215
ubuntu@ubuntu:~$ mkfifo /tmp/myfifo
ubuntu@ubuntu:~$ exec 3<> /tmp/myfifo
ubuntu@ubuntu:~$ unlink /tmp/myfifo
ubuntu@ubuntu:~$ bar 1>&3
ubuntu@ubuntu:~$ read -u3 a
ubuntu@ubuntu:~$ echo $a
0 8215
ubuntu@ubuntu:~$ exec 3>&-
ubuntu@ubuntu:~$
这里的窍门是使用 exec 在带有FD的读写模式下打开FIFO，这似乎具有使FIFO成为非阻塞状态的副作用.然后，您可以将命令重定向到FD，而不会阻止它，然后读取FD.
The trick here is to use exec to open the FIFO in read-write mode with an FD, which seems to have the side-effect of making the FIFO non-blocking. Then you can redirect your command to the FD without it blocking, then read the FD.
请注意，FIFO将是一个有限大小的缓冲区，可能在4K左右，因此，如果您的命令产生的输出超过此数量，它将最终再次阻塞.
Note that the FIFO will be a limited-size buffer, probably around 4K, so if your command produces more output than this, it will end up blocking again.
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