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Re: advanced scripting problems - or wrong approach?

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  Re: advanced scripting problems - or wrong approach? Greg Wooledge <greg@wooledge.org> - 2024-06-01 16:10 +0200
    Re: advanced scripting problems - or wrong approach? David Christensen <dpchrist@holgerdanske.com> - 2024-06-03 18:00 +0200

#269804 — Re: advanced scripting problems - or wrong approach?

On Sat, Jun 01, 2024 at 09:20:59AM +0200, DdB wrote:
> > #!/bin/bash -e
> > 
> > coproc { bash; }
> > exec 5<&${COPROC[0]} 6>&${COPROC[1]}
> > fd=5
> > 
> > echo "ls" >&6
> > while IFS= read -ru $fd line
> > do
> >     printf '%s\n' "$line"
> > done
> > 
> > printf "%s\n" "sleep 3;exit" >&6
> > while IFS= read -ru $fd line
> > do
> >     printf '%s\n' "$line"
> > done
> > 
> > exec 5<&- 6>&-
> > 
> > wait
> > echo waited, done
> 
> i get the output from ls, but then the thing is hanging indefinitely,
> apparently not reaching the exit line. :(

Your first while loop never terminates.  "while read ..." continues
running until read returns a nonzero exit status, either due to an
error or EOF.  Your coproc never returns EOF, so the "while read"
loop just keeps waiting for the next line of output from ls.

If you're going to communicate with a long-running process that can
return multiple lines of output per line of input, then you have
three choices:

  1) Arrange for some way to communicate how many lines, or bytes,
     of output are going to be given.

  2) Send a terminator line (or byte sequence) of some kind that
     indicates "end of current data set".

  3) Give up and assume the end of the data set after a certain amount
     of time has elapsed with no new output arriving.  (This is usually
     not the best choice.)

These same design issues occur in any kind of network communication, too.
Imagine an IMAP client or something, which holds open a network connection
to its IMAP server.  The client asks for the body of an email message,
but needs to keep the connection open afterward so that it can ask for
more things later.  The server has to be able to send the message back
without closing the connection at the end.  Therefore, the IMAP protocol
needs some way to "encapsulate" each server response, so the client
knows when it has received the full message.

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#269874

On 6/2/24 21:35, DdB wrote:
> Am 02.06.2024 um 02:41 schrieb DdB:
>> Will share my findings, once i made more progress...
> 
> Here is what i've got before utilizing it:
> 
> 
>> datakanja@PBuster-NFox:/mnt/tmp$ cat test
>> #!/bin/bash -e
>> # testing usefulness of coprocess to control host and backup machine from a single script.
>> # beware: do not use subprocesses or pipes, as that will confuse the pipes setup by coproc!
>> # At this point, this interface may not be very flexible
>> # but trying to follow best practices for using coproc in bash scripts
>> # todo (deferred): how to handle stderr inside coproc?
>> # todo (deferred): what, if coproc dies unexpectedly?
>>
>> 	# setting up the coprocess:
>> 	stdout_to_ssh_stdin=5 # arbitrary choice outside the range of used file desciptors
>> 	stdin_from_ssh_stdout=6
>>
>> 	coproc SSH { bash; } # for testing purposes, i refrain from really involving ssh just yet and replace it with bash:
>>
>> 	# save filedescriptors by duplicating them:
>> 	eval "exec ${stdin_from_ssh_stdout}<&${SSH[0]} ${stdout_to_ssh_stdin}>&${SSH[1]}"
>> 	echo The PID of the coproc is: $SSH_PID # possibly useful for inspection
>>
>> unique_eof_delimirer="<EOF>"
>> line=""
>> # collect the output available and print it locally (synchonous):
>> function print-immediate-output () {
>> 	while IFS= read -r -u "${stdin_from_ssh_stdout}" line
>> 	do
>> 	    if [[ "${line:0-5:5}" == "$unique_eof_delimirer" ]] # currently, the length is fixed
>> 	    then
>> 		line="${line%<EOF>}"
>> 		if [[ ! -z $line ]]
>> 		then
>> 		    printf '%s\n' "$line"
>> 		fi
>> 		break
>> 	    fi
>> 	    printf '%s\n' "$line"
>> 	done
>> }
>>
>> # send a single command via ssh and print output locally
>> function send-single-ssh-command () {
>> 	printf '%s\n' "$@" >&"${stdout_to_ssh_stdin}"
>> 	printf '%s\n' "echo '"$unique_eof_delimirer"'" >&"${stdout_to_ssh_stdin}"
>> 	print-immediate-output
>> }
>>
>>
>> send-single-ssh-command "find . -maxdepth 1 -name [a-z]\*" # more or less a standard command, that succeeds
>> send-single-ssh-command "ls nothin" # more or less a standard command, that fails
>>
>> 	# tearing down the coprocess:
>> 	printf "%s\n" "exit" >&"${stdout_to_ssh_stdin}" # not interested in any more output (probably none)
>> 	wait
>> 	# Descriptors must be closed to prevent leaking.
>> 	eval "exec ${stdin_from_ssh_stdout}<&- ${stdout_to_ssh_stdin}>-"
>>
>> 	echo "waited for the coproc to end gracefully, done"
>>
>> datakanja@PBuster-NFox:/mnt/tmp$ ./test
>> The PID of the coproc is: 28154
>> ./test
>> ./out
>> ls: Zugriff auf 'nothin' nicht möglich: Datei oder Verzeichnis nicht gefunden
>> waited for the coproc to end gracefully, done
>> datakanja@PBuster-NFox:/mnt/tmp$


"test" is both a program and a shell builtin.  I suggest that you pick 
another, non-keyword, name for your script.


I suggest adding the Bash option "-u' (nounset).


Your file descriptor duplication, redirection, etc., seems overly 
complex.  Would not it be easier to use the coproc handles directly?

2024-06-03 08:49:41 dpchrist@laalaa ~/sandbox/bash
$ nl coproc-demo
      1	#!/usr/bin/env bash
      2	# $Id: coproc-demo,v 1.3 2024/06/03 15:49:36 dpchrist Exp $
      3	set -e
      4	set -u
      5	coproc COPROC { bash ; }
      6	echo 'echo "hello, world!"' >&"${COPROC[1]}"
      7	read -r reply <&"${COPROC[0]}"
      8	echo $reply
      9	echo "exit" >&"${COPROC[1]}"
     10	wait $COPROC_PID

2024-06-03 08:49:44 dpchrist@laalaa ~/sandbox/bash
$ bash -x coproc-demo
+ set -e
+ set -u
+ echo 'echo "hello, world!"'
+ bash
+ read -r reply
+ echo hello, 'world!'
hello, world!
+ echo exit
+ wait 4229


David

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