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Re: output redirection with process substitution asynchronous?

From: Marc Herbert
Subject: Re: output redirection with process substitution asynchronous?
Date: Mon, 07 Dec 2009 16:25:00 +0000
User-agent: Thunderbird (X11/20090320)

Marc Herbert wrote:
> What is wrong with the following:
> prefix_with_date () 
> { 
>     while read; do
>         printf '%s: %s\n' "$(date)" "$REPLY";
>     done
> }
> seq 4 | prefix_with_date
> ls | prefix_with_date

Sorry I missed the fact that you want to run your commands in the current shell.

There are no real coroutines in shell. The current shell process does
not know how to schedule two pieces of code. So each time two
pieces of code communicate through a pipe they have to be run in two
*concurrent* processes (typically: subshells).  The producer and
consumer of a pipe must run independently of each other. Whether you
are using process substitution or the more usual and portable pipe "|"
does not matter here: you need concurrency.

So at least one of: 1. your prefixer code, 2. your unknown command
has to run in a independent process, "asynchronous" with the current

Since you absolutely want your unknown commands to run in the current
shell, then it is your "prefixing" code that has to run in a
concurrent process.

Now I do not really see any other way to avoid the ugliness of
concurrently printing on stdout than to "wait" for your concurrent
prefixer to complete, more or less like you did.

A variant is to ask socat to handle the cleanup actions for you like

    local P=/tmp/dataorig.$$
    socat -u  PIPE:${P}  SYSTEM:'while read; do echo "$(date):\\ $REPLY"; done' 
    until [ -e ${P} ]; do sleep 1; done
    $@ > ${P}
    wait $socatPID

prefix_with_date  seq 5

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