Re^2: How to pass data as STDIN to Capture::Tiny

Hi Ken,

Just to pick up on what the AM mentioned: properly closeing a piped open is important so one can catch all possible errors (as opposed to closeing regular files, where in my experience things go wrong much less often). I see you used autodie, but as far as I can tell it doesn't catch errors when the filehandle is implicitly closed when it goes out of scope.

In your code, if I replace the command with something that returns a nonzero exit code, and add the close at the end of the sub, I get confusing results: inside of capture, the error doesn't seem to get caught at all, and outside of capture, I get the confusing error message "Can't close(GLOB(0x8e2b62)) filehandle: '' at ...". (In fact, if I remember correctly, strange interactions with autodie and piped opens is one of the reasons I started avoiding autodie.)

So here's the same run_external_command code, without autodie but with the minimum error handling:

my $extcmd = 'cat';
open my $cmd_pipe, '|-', $extcmd or die "open $extcmd: $!";
print $cmd_pipe "password\n";
close $cmd_pipe or die "close $extcmd: ".($! ? $! : "\$?=$?");
[download]

However, nowadays I very much prefer to use more better-suited modules, one of my favorites is IPC::Run3. For the OP:

use IPC::Run3 'run3';
my $stdin = "password\n";
my @extcmd = ('cat');
run3 \@extcmd, \$stdin, \my $stdout, \my $stderr
    or die "run3 failed";
$? and die "run3: \$?=$?";
print "stdout: <<$stdout>>\n";
print "stderr: <<$stderr>>\n";
[download]

Regards,
-- Hauke D

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Re^3: How to pass data as STDIN to Capture::Tiny by kcott (Archbishop) on Jan 03, 2017 at 06:22 UTC
G'day Hauke, Thanks for the feedback. In my original test code (unpublished), I did have `close $cmd_pipe;` [download] and I started to add some error checking. However, realising that I had no idea what the OP's external command was nor what error codes he might be looking for, I ripped that all out and just posted the part answering the original question. I probably should have left the `close` statement. I don't know what autodie is doing under the hood; however, given that `open my $cmd_pipe, '\|-', $external_command;` [download] performs a fork, I suspect it's checking that (in its `:threads` category) and not doing anything in the `:system` category. The only mention of pipe that I can see in autodie is that it's in the `:ipc` category. There's also one instance in IPC::System::Simple that I found somewhat humourous: "Implementing the `capture` command involves dark and terrible magicks involving pipes, and one of them has sprung a leak." [That's `&IPC::System::Simple::capture`, not `&Capture::Tiny::capture`.] "perlipc: Using open() for IPC" has more information on error checking: "Be careful to check the return values from both open() and close(). If you're writing to a pipe, you should also trap SIGPIPE. ..." See also: "perlvar: Error Variables". "However, nowadays I very much prefer to use more better-suited modules, ..." Yes, Capture::Tiny wouldn't have been my first choice for this task; however, the OP mentions it four times in his single-paragraph post, so that's what I used. — Ken	[reply] [d/l] [select]
Re^4: How to pass data as STDIN to Capture::Tiny by haukex (Archbishop) on Jan 03, 2017 at 14:17 UTC
Hi Ken, Usually I tend to code defensively, especially when it comes to running external commands, I like to know every single time something goes wrong - I'm one of those people who usually looks at all the mails I get from e.g. cron :-) Having had problems with autodie in the past, I tend to avoid it nowadays, and write the error handling myself. Unfortunately, at the moment I don't recall the specific problems (other than this particular case) I had with it, just that it was "spooky action-at-a-distance" type stuff. On the other hand, I've also had problems with piped opens, such as it not working when given a Readonly variable (which I reported back then). The other reason I don't use piped opens anymore is that I've found that I'm usually not just interested in the command's STDOUT, but also its STDERR. (One of my modules, IPC::Run3::Shell, even includes a `fail_on_stderr` option, which considers any output to STDERR an error condition, something I've used frequently.) So having been burned a little by autodie, Readonly, and piped opens, I instead, respectively, write the error handling myself, give the user just enough rope to shoot themselves in the foot, and use modules like IPC::System::Simple, Capture::Tiny, and IPC::Run3 instead :-) Regards, -- Hauke D	[reply] [d/l]