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Re: advanced scripting problems - or wrong approach?

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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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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

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