如何将子shell的输出文件描述符重定向到父shell中的输入文件描述符？

Question

(在BASH中)我希望子shell使用非STDOUT非STDERR文件描述符将一些数据传递回父shell.我怎样才能做到这一点？最终我希望将数据保存到父shell的某个变量中.

(
  # The following two lines show the behavior of the subshell.
  # We cannot change them.
  echo "This should go to STDOUT"
  echo "This is the data I want to pass to the parent shell" >&3
)
#...
data_from_subshell=... # Somehow assign the value of &3 of the
                       # subshell to this variable

编辑:子shell运行一个写入STDOUT和&3的黑盒程序.

Answer 1

注意,BASHISM AHEAD(有比posash快得多的 posix shell,例如ash或dash,没有进程替换).

你可以做一个手柄舞,将原始标准输出移动到一个新的描述符,使标准输出可用于管道(从我的头顶):

exec 3>&1 # creates 3 as alias for 1
run_in_subshell() { # just shortcut for the two cases below
    echo "This goes to STDOUT" >&3
    echo "And this goes to THE OTHER FUNCTION"
}

现在你应该可以写:

while read line; do
    process $line
done < <(run_in_subshell)

但<()构造是一种基础.您可以用管道替换它

run_in_subshell | while read line; do
    process $line
done

除了第二个命令也在子shell中运行,因为管道中的所有命令都可以.

Answer 2

当然,最简单的方法是直接在父级中捕获输出

data_from_subshell=$(echo "This is the data I want to pass to the parent shell")

您可以使用命名管道作为从子项读取数据的替代方法

mkfifo /tmp/fifo

现在你可以将孩子重定向到 /tmp/fifo

(
    echo "This should go to STDOUT"
    echo "This is the data I want to pass to the parent shell" >/tmp/fifo
) &

父母可以从那里读

read data_from_subshell </tmp/fifo

另一种方法是使用coproc启动子进程.这将创建一个具有双向管道的子项,并将子项的stdin和stdout重定向到管道描述符.要在子项中同时使用管道和标准输出,必须首先在父项中复制stdout

exec 4>&1 # duplicate stdout for usage in client

coproc SUBSHELL (
    exec 3>&1 1>&4- # redirect fd 3 to pipe, redirect fd 1 to stdout
    (
    echo "This should go to STDOUT"
    echo "This is the data I want to pass to the parent shell" >&3
    )
)

exec 4>&- # close fd 4 in parent
read data <&${SUBSHELL[0]}
echo "Parent: $data"

在Bash 4.0中引入了Coprocesses.