Backticks within child process

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Backticks within child process by zentara (Cardinal) on Jul 09, 2012 at 16:56 UTC
It's because you have 3 independent processes all trying to write to STDOUT simultaneously. Try having each fork open a filehandle, and write to it's own file, that should be orderly. I'm not really a human, but I play one on earth. Old Perl Programmer Haiku ................... flash japh	[reply]
Re: Backticks within child process by onelesd (Pilgrim) on Jul 09, 2012 at 17:03 UTC
It would help if you pasted your output, and also what you expected to see. I suspect you wanted to see something like: `hello [ps output] world hello [ps output] world hello [ps output] world` [download] But instead you are seeing something like: `hello hello hello [ps output] [ps output] [ps output]world world world` [download] You answered your own question- the 3 forks you did are running async.	[reply] [d/l] [select]
Re: Backticks within child process by frozenwithjoy (Priest) on Jul 09, 2012 at 16:50 UTC
What do you mean by "jumbled"? What does it look like? Out of curiosity, does your problem go away if you put `sleep 1;` before your `if` statement?	[reply] [d/l] [select]
Re: Backticks within child process by locked_user sundialsvc4 (Abbot) on Jul 09, 2012 at 17:53 UTC
Think of “your terminal” as an output file, which in fact it is. Three processes, all running independently of one another, are simultaneously writing lines of text to that file. In what order, therefore, will the lines of text appear? God only knows. It would get even more tricky if the character-strings that each process was writing did not end in a newline character. As it happens, most programming languages buffer character-output such that they only write complete text-lines at a time. In the worst case, the characters being output by the various competitors could be intermingled.
Re: Backticks within child process by Anonymous Monk on Jul 09, 2012 at 19:09 UTC
When the "ps" line is commented out, I see: hello world hello world hello world When I remove the comment I get hello hello hello my@prompt:~$ world world world I think I understand it now. I was thrown off by the fact that the "ps" line doesn't do any printing, so I didn't understand how it managed to "jumble" the output. I didn't understand that the time that command takes to run was was important. Thanks for all your input.	[reply]
Re^2: Backticks within child process by dave_the_m (Monsignor) on Jul 09, 2012 at 23:58 UTC
One extra fact: when perl executes the backticks, it flushes the STDOUT buffer: that's why when you include the backticks, the half-lines get flushed to STDOUT, appearing to get jumbled up with half lines from the other processes. Dave.	[reply]
Re: Backticks within child process by The Code Captain (Initiate) on Jul 09, 2012 at 17:49 UTC
If you don't want to sleep, you could also just collect the output together, rather than printing it in different stages. When you print to a file (and STDOUT is still a file) you are asking perl and the OS to handle the output for you which may not always work in the way you expect. Normally that's not a problem, but if you have different processes all trying to access the same resource (STDOUT for each process is competing for the same terminal session) it probably won't happen quite as you expect. So ... you could use something like: for (1..3) { my $fork_pid = fork(); if (! $fork_pid) { my ($Output); $Output="hello\t"; $Output.=`ps au`; $Output.="world\n"; print $Output; exit 0; } } [download] This keeps the print a single item which is probably going to be written in one go (and hence stay together) as you want. Of course I am sure that your real code is doing something much more interesting than just printing output :o)	[reply] [d/l]