bash – 如何将第二个进程的stdout重定向到第一个进程的标准？

(编辑：添加简单的使用方案)

它的工作原理如下：

# start first process as a coprocess to the current shell
coproc proc1

# now ${COPROC[0]} contains the number of an open (input) file descriptor
# connected to the output of proc1,and ${COPROC[1]} the number of an
# open (output) file descriptor connected to the input of proc1.


# start second process,connecting its input- and outputstreams
# to the output- and inputstreams of the first process
proc2 <&${COPROC[0]} >&${COPROC[1]}

# wait on the first process to finish.
wait $COPROC_PID

如果你可能有多个协处理器，请给你的进程一个这样的名字：

coproc NAME {
    proc1
}

然后可以使用以前使用COPROC的NAME。

这是一个使用ping函数作为proc1和proc2的完整示例程序：

#!/bin/bash
#
# Example program using a bash coprocess to run two processes
# with their input/output streams 
#


#
# A function which reads lines of input and
# writes them back to standard output with the
# first char cut off,waiting 5s inbetween.
#
# It finishes whenever an empty line is read or written,# or at end-of-file.
#
# The parameter $1 is used in debugging output on stderr.
#
function ping ()
{
    while read 
    do
        local sending
        echo "ping $1: received '$REPLY'" >&2
        [[ -n $REPLY ]] || break
        sleep 5s
        sending=${REPLY:1}
        echo "ping $1: sending '$sending'"  >&2
        echo $sending
        [[ -n $sending ]] || break
    done
    echo "ping $1: end" >&2
}

#
# Start first ping process as a coprocess with name 'p1'.
#

coproc p1 {
    ping 1
}

# send some initial data to p1. (Not needed if one of the processes
# starts writing before first reading.)
echo "Hello World" >&${p1[1]}
sleep 2.5s

#
# Run second ping process,connecting its default input/output
# to the default output/input of p1.
# 
ping 2 <&${p1[0]} >&${p1[1]}

# wait for the coprocess to finish too.
wait $p1_PID

它使用两个调用shell函数而不是外部程序，但它也可以与这些程序一起使用。这是输出(在stderr上)：

ping 1: received 'Hello World'
ping 1: sending 'ello World'
ping 2: received 'ello World'
ping 2: sending 'llo World'
ping 1: received 'llo World'
ping 1: sending 'lo World'
ping 2: received 'lo World'
ping 2: sending 'o World'
ping 1: received 'o World'
ping 1: sending ' World'
ping 2: received 'World'
ping 2: sending 'orld'
ping 1: received 'orld'
ping 1: sending 'rld'
ping 2: received 'rld'
ping 2: sending 'ld'
ping 1: received 'ld'
ping 1: sending 'd'
ping 2: received 'd'
ping 2: sending ''
ping 2: end
ping 1: received ''
ping 1: end