Well, you have a need based on your specific circumstances. You should be able to take your requirements and write a loadable builtin to do what you need it to do.

Then we see whether or not that translates into a general need.

Hi, all.

There is a need for something like a mkpipe bash builtin.
Sometimes there is need to execute some commands in a subshell and we need to collect an output from stdout and stderr(and sometimes from even more file descriptors; e.g. our script need to collect an output from 4 fd - 1, 2, 3 and 4 - where 1 is used for a data, 2 for errors, 3 is used for error which should be sent to a client and 4 for debug messages). Thus, simple:

data=$(commands here)

isn't enough for this case. We can use files as a temporary storage for an output:

({commands here} >data.out 2>err.out 3>err_client.out 4>debug.out)

But with read-only fs this isn't work.

Now we use the next workaround:

# Create a pipe
#  $1 - read fd
#  $2 - write fd
mkpipe()
{
        local PID1 PID2
        local FD_READ FD_WRITE

        FD_READ=$1
        FD_WRITE=$2
        tail -f /dev/null | tail -f /dev/null &
        PID2=$!
        PID1=`jobs -p %+`
        eval "exec $FD_READ</proc/$PID2/fd/0 $FD_WRITE>/proc/$PID1/fd/1"
        disown $PID1
        kill $PID1 $PID2
}

Example of use:

dbg_out()
{
        echo "DBG: $@" >&4
}

# Read all data from a pipe
# $1 - variable name
read_all()
{
        while eval "read -t0 ${1}_"; do
                eval "read ${1}_"
                eval "$1=\"\${${1}}\${${1}_}
\""
        done
}
exec 4>&2
dbg_out start

mkpipe 5 6
({
# HERE SOME REAL OPERATIONS ...
# ...
dbg_out inner msg
dbg_out inner msg again
} 4>&6)

read_all line <&5
echo "$line"

dbg_out finish