Split and recombine stdout in Sh

Question

I base the following question off How can I split and re-join STDOUT from multiple processes? .

The following Bash command splits the output of command1 into two streams going into command2 and command3 before combining their outputs and piping it into command4.

((command1 | tee >(command2 >&3) | command3) 3>&1) | command4

Graphically this looks as follows:

         command2
        /        \
command1          command4
        \        /
         command3

How would i do this in the Sh shell?

KamilCuk · Accepted Answer · 2019-10-07T08:23:55.840

In place of the process substitution use a fifo with a background process.

fifo=$(mktemp -u)
mkfifo "$fifo"
{
    command2 <"$fifo" &
    command1 | tee "$fifo" | command3
    wait
} | command4

Example tested on busybox on alpine linux in docker:

f() { sed 's/^/'"$1"' /'; } ; 
fifo=$(mktemp -u); mkfifo "$fifo"; 
{ f 2 <"$fifo" & seq 3 | tee "$fifo" | f 3; wait; } | f 4;
rm "$fifo";

will output:

Split and recombine stdout in Sh

1 Answers1