Capture::Tiny - Capture STDOUT and STDERR from Perl, XS or external programs
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 { ... };
$merged = tee_merged { ... };
User Contributed Perl Documentation Capture::Tiny(3)
NAME
Capture::Tiny - Capture STDOUT and STDERR from Perl, XS or external
programs
VERSION
version 0.48
SYNOPSIS
use Capture::Tiny ':all';
# capture from external command
($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 its 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.
Forking inside a capture
Forks aren't portable. The behavior of filehandles during a fork is
even less so. If Capture::Tiny detects that a fork has occurred within
a capture, it will shortcut in the child process and return empty
strings for captures. Other problems may occur in the child or parent,
as well. Forking in a capture block is not recommended.
Using threads
Filehandles are global. Mixing up I/O and captures in different
threads without coordination is going to cause problems. Besides,
threads are officially discouraged.
Dropping privileges during a capture
If you drop privileges during a capture, temporary files created to
facilitate the capture may not be cleaned up afterwards.
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 internally for "tee". By default,
Capture::Tiny will timeout with an error if such 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. NOTE, this does not timeout the code
reference being captured -- this only prevents Capture::Tiny itself
from hanging your process waiting for its child processes to be ready
to proceed.
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
+---------------+---------------------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+---------------------------------+
|Availability | library/perl-5/capture-tiny-532 |
+---------------+---------------------------------+
|Stability | Volatile |
+---------------+---------------------------------+
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.
o IO::Capture
o IO::Capture::Extended
o IO::CaptureOutput
o IPC::Capture
o IPC::Cmd
o IPC::Open2
o IPC::Open3
o IPC::Open3::Simple
o IPC::Open3::Utils
o IPC::Run
o IPC::Run::SafeHandles
o IPC::Run::Simple
o IPC::Run3
o IPC::System::Simple
o Tee
o IO::Tee
o File::Tee
o Filter::Handle
o Tie::STDERR
o Tie::STDOUT
o 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
o Dagfinn Ilmari Mannsker <ilmari@ilmari.org>
o David E. Wheeler <david@justatheory.com>
o fecundf <not.com+github@gmail.com>
o Graham Knop <haarg@haarg.org>
o Peter Rabbitson <ribasushi@cpan.org>
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
NOTES
Source code for open source software components in Oracle Solaris can
be found at https://www.oracle.com/downloads/opensource/solaris-source-
code-downloads.html.
This software was built from source available at
https://github.com/oracle/solaris-userland. The original community
source was downloaded from
http://www.cpan.org/authors/id/D/DA/DAGOLDEN/Capture-Tiny-0.48.tar.gz.
Further information about this software can be found on the open source
community website at http://search.cpan.org/dist/Capture-Tiny/.
perl v5.32.0 2018-04-22 Capture::Tiny(3)