NAME¶
Capture::Tiny - Capture STDOUT and STDERR from Perl, XS or external programs
VERSION¶
version 0.25
SYNOPSIS¶
use Capture::Tiny ':all';
# capture from external command
($stdout, $stderr, $exit) = capture {
system( $cmd, @args );
};
# capture from arbitrary code (Perl or external)
($stdout, $stderr, @result) = capture {
# your code here
};
# capture partial or merged output
$stdout = capture_stdout { ... };
$stderr = capture_stderr { ... };
$merged = capture_merged { ... };
# tee output
($stdout, $stderr) = tee {
# your code here
};
$stdout = tee_stdout { ... };
$stderr = tee_stderr { ... };
$merged = tee_merged { ... };
DESCRIPTION¶
Capture::Tiny provides a simple, portable way to capture almost anything sent to
STDOUT or STDERR, regardless of whether it comes from Perl, from XS code or
from an external program. Optionally, output can be teed so that it is
captured while being passed through to the original filehandles. Yes, it even
works on Windows (usually). Stop guessing which of a dozen capturing modules
to use in any particular situation and just use this one.
USAGE¶
The following functions are available. None are exported by default.
capture¶
($stdout, $stderr, @result) = capture \&code;
$stdout = capture \&code;
The "capture" function takes a code reference and returns what is sent
to STDOUT and STDERR as well as any return values from the code reference. In
scalar context, it returns only STDOUT. If no output was received for a
filehandle, it returns an empty string for that filehandle. Regardless of
calling context, all output is captured -- nothing is passed to the existing
filehandles.
It is prototyped to take a subroutine reference as an argument. Thus, it can be
called in block form:
($stdout, $stderr) = capture {
# your code here ...
};
Note that the coderef is evaluated in list context. If you wish to force scalar
context on the return value, you must use the "scalar" keyword.
($stdout, $stderr, $count) = capture {
my @list = qw/one two three/;
return scalar @list; # $count will be 3
};
Also note that within the coderef, the @_ variable will be empty. So don't use
arguments from a surrounding subroutine without copying them to an array
first:
sub wont_work {
my ($stdout, $stderr) = capture { do_stuff( @_ ) }; # WRONG
...
}
sub will_work {
my @args = @_;
my ($stdout, $stderr) = capture { do_stuff( @args ) }; # RIGHT
...
}
Captures are normally done to an anonymous temporary filehandle. To capture via
a named file (e.g. to externally monitor a long-running capture), provide
custom filehandles as a trailing list of option pairs:
my $out_fh = IO::File->new("out.txt", "w+");
my $err_fh = IO::File->new("out.txt", "w+");
capture { ... } stdout => $out_fh, stderr => $err_fh;
The filehandles must be read/write and seekable. Modifying the files or
filehandles during a capture operation will give unpredictable results.
Existing IO layers on them may be changed by the capture.
When called in void context, "capture" saves memory and time by not
reading back from the capture handles.
capture_stdout¶
($stdout, @result) = capture_stdout \&code;
$stdout = capture_stdout \&code;
The "capture_stdout" function works just like "capture"
except only STDOUT is captured. STDERR is not captured.
capture_stderr¶
($stderr, @result) = capture_stderr \&code;
$stderr = capture_stderr \&code;
The "capture_stderr" function works just like "capture"
except only STDERR is captured. STDOUT is not captured.
capture_merged¶
($merged, @result) = capture_merged \&code;
$merged = capture_merged \&code;
The "capture_merged" function works just like "capture"
except STDOUT and STDERR are merged. (Technically, STDERR is redirected to the
same capturing handle as STDOUT before executing the function.)
Caution: STDOUT and STDERR output in the merged result are not guaranteed to be
properly ordered due to buffering.
tee¶
($stdout, $stderr, @result) = tee \&code;
$stdout = tee \&code;
The "tee" function works just like "capture", except that
output is captured as well as passed on to the original STDOUT and STDERR.
When called in void context, "tee" saves memory and time by not
reading back from the capture handles, except when the original STDOUT OR
STDERR were tied or opened to a scalar handle.
tee_stdout¶
($stdout, @result) = tee_stdout \&code;
$stdout = tee_stdout \&code;
The "tee_stdout" function works just like "tee" except only
STDOUT is teed. STDERR is not teed (output goes to STDERR as usual).
tee_stderr¶
($stderr, @result) = tee_stderr \&code;
$stderr = tee_stderr \&code;
The "tee_stderr" function works just like "tee" except only
STDERR is teed. STDOUT is not teed (output goes to STDOUT as usual).
tee_merged¶
($merged, @result) = tee_merged \&code;
$merged = tee_merged \&code;
The "tee_merged" function works just like "capture_merged"
except that output is captured as well as passed on to STDOUT.
Caution: STDOUT and STDERR output in the merged result are not guaranteed to be
properly ordered due to buffering.
LIMITATIONS¶
Portability¶
Portability is a goal, not a guarantee. "tee" requires fork, except on
Windows where "system(1, @cmd)" is used instead. Not tested on any
particularly esoteric platforms yet. See the CPAN Testers Matrix
<
http://matrix.cpantesters.org/?dist=Capture-Tiny> for test result by
platform.
PerlIO layers¶
Capture::Tiny does it's best to preserve PerlIO layers such as ':utf8' or
':crlf' when capturing (only for Perl 5.8.1+) . Layers should be applied to
STDOUT or STDERR
before the call to "capture" or
"tee". This may not work for tied filehandles (see below).
Modifying filehandles before capturing¶
Generally speaking, you should do little or no manipulation of the standard IO
filehandles prior to using Capture::Tiny. In particular, closing, reopening,
localizing or tying standard filehandles prior to capture may cause a variety
of unexpected, undesirable and/or unreliable behaviors, as described below.
Capture::Tiny does its best to compensate for these situations, but the
results may not be what you desire.
Closed filehandles
Capture::Tiny will work even if STDIN, STDOUT or STDERR have been previously
closed. However, since they will be reopened to capture or tee output, any
code within the captured block that depends on finding them closed will, of
course, not find them to be closed. If they started closed, Capture::Tiny will
close them again when the capture block finishes.
Note that this reopening will happen even for STDIN or a filehandle not being
captured to ensure that the filehandle used for capture is not opened to file
descriptor 0, as this causes problems on various platforms.
Prior to Perl 5.12, closed STDIN combined with PERL_UNICODE=D leaks filehandles
and also breaks
tee() for undiagnosed reasons. So don't do that.
Localized filehandles
If code localizes any of Perl's standard filehandles before capturing, the
capture will affect the localized filehandles and not the original ones.
External system calls are not affected by localizing a filehandle in Perl and
will continue to send output to the original filehandles (which will thus not
be captured).
Scalar filehandles
If STDOUT or STDERR are reopened to scalar filehandles prior to the call to
"capture" or "tee", then Capture::Tiny will override the
output filehandle for the duration of the "capture" or
"tee" call and then, for "tee", send captured output to
the output filehandle after the capture is complete. (Requires Perl 5.8)
Capture::Tiny attempts to preserve the semantics of STDIN opened to a scalar
reference, but note that external processes will not be able to read from such
a handle. Capture::Tiny tries to ensure that external processes will read from
the null device instead, but this is not guaranteed.
Tied output filehandles
If STDOUT or STDERR are tied prior to the call to "capture" or
"tee", then Capture::Tiny will attempt to override the tie for the
duration of the "capture" or "tee" call and then send
captured output to the tied filehandle after the capture is complete.
(Requires Perl 5.8)
Capture::Tiny may not succeed resending UTF-8 encoded data to a tied STDOUT or
STDERR filehandle. Characters may appear as bytes. If the tied filehandle is
based on Tie::StdHandle, then Capture::Tiny will attempt to determine
appropriate layers like ":utf8" from the underlying filehandle and
do the right thing.
Tied input filehandle
Capture::Tiny attempts to preserve the semantics of tied STDIN, but this
requires Perl 5.8 and is not entirely predictable. External processes will not
be able to read from such a handle.
Unless having STDIN tied is crucial, it may be safest to localize STDIN when
capturing:
my ($out, $err) = do { local *STDIN; capture { ... } };
Modifying filehandles during a capture¶
Attempting to modify STDIN, STDOUT or STDERR
during "capture"
or "tee" is almost certainly going to cause problems. Don't do that.
No support for Perl 5.8.0¶
It's just too buggy when it comes to layers and UTF-8. Perl 5.8.1 or later is
recommended.
Limited support for Perl 5.6¶
Perl 5.6 predates PerlIO. UTF-8 data may not be captured correctly.
ENVIRONMENT¶
PERL_CAPTURE_TINY_TIMEOUT¶
Capture::Tiny uses subprocesses for "tee". By default, Capture::Tiny
will timeout with an error if the subprocesses are not ready to receive data
within 30 seconds (or whatever is the value of $Capture::Tiny::TIMEOUT). An
alternate timeout may be specified by setting the
"PERL_CAPTURE_TINY_TIMEOUT" environment variable. Setting it to zero
will disable timeouts.
SEE ALSO¶
This module was, inspired by IO::CaptureOutput, which provides similar
functionality without the ability to tee output and with more complicated code
and API. IO::CaptureOutput does not handle layers or most of the unusual cases
described in the "Limitations" section and I no longer recommend it.
There are many other CPAN modules that provide some sort of output capture,
albeit with various limitations that make them appropriate only in particular
circumstances. I'm probably missing some. The long list is provided to show
why I felt Capture::Tiny was necessary.
- •
- IO::Capture
- •
- IO::Capture::Extended
- •
- IO::CaptureOutput
- •
- IPC::Capture
- •
- IPC::Cmd
- •
- IPC::Open2
- •
- IPC::Open3
- •
- IPC::Open3::Simple
- •
- IPC::Open3::Utils
- •
- IPC::Run
- •
- IPC::Run::SafeHandles
- •
- IPC::Run::Simple
- •
- IPC::Run3
- •
- IPC::System::Simple
- •
- Tee
- •
- IO::Tee
- •
- File::Tee
- •
- Filter::Handle
- •
- Tie::STDERR
- •
- Tie::STDOUT
- •
- Test::Output
SUPPORT¶
Bugs / Feature Requests¶
Please report any bugs or feature requests through the issue tracker at
<
https://github.com/dagolden/Capture-Tiny/issues>. You will be notified
automatically of any progress on your issue.
Source Code¶
This is open source software. The code repository is available for public review
and contribution under the terms of the license.
<
https://github.com/dagolden/Capture-Tiny>
git clone https://github.com/dagolden/Capture-Tiny.git
AUTHOR¶
David Golden <dagolden@cpan.org>
CONTRIBUTORS¶
- •
- Dagfinn Ilmari Mannsaaker <ilmari@ilmari.org>
- •
- David E. Wheeler <david@justatheory.com>
COPYRIGHT AND LICENSE¶
This software is Copyright (c) 2009 by David Golden.
This is free software, licensed under:
The Apache License, Version 2.0, January 2004