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IPC::Run3 - run a subprocess with input/ouput redirection


version 0.045


    use IPC::Run3;    # Exports run3() by default

    run3 \@cmd, \$in, \$out, \$err;


This module allows you to run a subprocess and redirect stdin, stdout, and/or stderr to files and perl data structures. It aims to satisfy 99% of the need for using system, qx, and open3 with a simple, extremely Perlish API.

Speed, simplicity, and portability are paramount. (That's speed of Perl code; which is often much slower than the kind of buffered I/O that this module uses to spool input to and output from the child command.)

run3($cmd, $stdin, $stdout, $stderr, \%options)

All parameters after $cmd are optional.

The parameters $stdin, $stdout and $stderr indicate how the child's corresponding filehandle (STDIN, STDOUT and STDERR, resp.) will be redirected. Because the redirects come last, this allows STDOUT and STDERR to default to the parent's by just not specifying them -- a common use case.

run3 throws an exception if the wrapped system call returned -1 or anything went wrong with run3's processing of filehandles. Otherwise it returns true. It leaves $? intact for inspection of exit and wait status.

Note that a true return value from run3 doesn't mean that the command had a successful exit code. Hence you should always check $?.

See "%options" for an option to handle the case of system returning -1 yourself.


Usually $cmd will be an ARRAY reference and the child is invoked via

  system @$cmd;

But $cmd may also be a string in which case the child is invoked via

  system $cmd;

(cf. "system" in perlfunc for the difference and the pitfalls of using the latter form).

$stdin, $stdout, $stderr

The parameters $stdin, $stdout and $stderr can take one of the following forms:

undef (or not specified at all)

The child inherits the corresponding filehandle from the parent.

  run3 \@cmd, $stdin;                   # child writes to same STDOUT and STDERR as parent
  run3 \@cmd, undef, $stdout, $stderr;  # child reads from same STDIN as parent

The child's filehandle is redirected from or to the local equivalent of /dev/null (as returned by File::Spec->devnull()).

  run3 \@cmd, \undef, $stdout, $stderr; # child reads from /dev/null
a simple scalar

The parameter is taken to be the name of a file to read from or write to. In the latter case, the file will be opened via

  open FH, ">", ...

i.e. it is created if it doesn't exist and truncated otherwise. Note that the file is opened by the parent which will croak in case of failure.

  run3 \@cmd, \undef, "out.txt";        # child writes to file "out.txt"
a filehandle (either a reference to a GLOB or an IO::Handle)

The filehandle is inherited by the child.

  open my $fh, ">", "out.txt";
  print $fh "prologue\n";
  run3 \@cmd, \undef, $fh;              # child writes to $fh
  print $fh "epilogue\n";
  close $fh;
a SCALAR reference

The referenced scalar is treated as a string to be read from or written to. In the latter case, the previous content of the string is overwritten.

  my $out;
  run3 \@cmd, \undef, \$out;           # child writes into string
  run3 \@cmd, \<<EOF;                  # child reads from string (can use "here" notation)
an ARRAY reference

For $stdin, the elements of @$stdin are simply spooled to the child.

For $stdout or $stderr, the child's corresponding file descriptor is read line by line (as determined by the current setting of $/) into @$stdout or @$stderr, resp. The previous content of the array is overwritten.

  my @lines;
  run3 \@cmd, \undef, \@lines;         # child writes into array
a CODE reference

For $stdin, &$stdin will be called repeatedly (with no arguments) and the return values are spooled to the child. &$stdin must signal the end of input by returning undef.

For $stdout or $stderr, the child's corresponding file descriptor is read line by line (as determined by the current setting of $/) and &$stdout or &$stderr, resp., is called with the contents of the line. Note that there's no end-of-file indication.

  my $i = 0;
  sub producer {
    return $i < 10 ? "line".$i++."\n" : undef;

  run3 \@cmd, \&producer;              # child reads 10 lines

Note that this form of redirecting the child's I/O doesn't imply any form of concurrency between parent and child - run3()'s method of operation is the same no matter which form of redirection you specify.

If the same value is passed for $stdout and $stderr, then the child will write both STDOUT and STDERR to the same filehandle. In general, this means that

    run3 \@cmd, \undef, "foo.txt", "foo.txt";
    run3 \@cmd, \undef, \$both, \$both;

will DWIM and pass a single file handle to the child for both STDOUT and STDERR, collecting all into file "foo.txt" or $both.


The last parameter, \%options, must be a hash reference if present.

Currently the following keys are supported:

binmode_stdin, binmode_stdout, binmode_stderr

The value must a "layer" as described in "binmode" in perlfunc. If specified the corresponding parameter $stdin, $stdout or $stderr, resp., operates with the given layer.

For backward compatibility, a true value that doesn't start with ":" (e.g. a number) is interpreted as ":raw". If the value is false or not specified, the default is ":crlf" on Windows and ":raw" otherwise.

Don't expect that values other than the built-in layers ":raw", ":crlf", and (on newer Perls) ":bytes", ":utf8", ":encoding(...)" will work.

append_stdout, append_stderr

If their value is true then the corresponding parameter $stdout or $stderr, resp., will append the child's output to the existing "contents" of the redirector. This only makes sense if the redirector is a simple scalar (the corresponding file is opened in append mode), a SCALAR reference (the output is appended to the previous contents of the string) or an ARRAY reference (the output is pushed onto the previous contents of the array).


If this is true run3 does not throw an exception if system returns -1 (cf. "system" in perlfunc for possible failure scenarios.), but returns true instead. In this case $? has the value -1 and $! contains the errno of the failing system call.



For each redirector $stdin, $stdout, and $stderr, run3() furnishes a filehandle:

  • if the redirector already specifies a filehandle it just uses that

  • if the redirector specifies a filename, run3() opens the file in the appropriate mode

  • in all other cases, run3() opens a temporary file (using tempfile)


If run3() opened a temporary file for $stdin in step (1), it writes the data using the specified method (either from a string, an array or returnd by a function) to the temporary file and rewinds it.


run3() saves the parent's STDIN, STDOUT and STDERR by duplicating them to new filehandles. It duplicates the filehandles from step (1) to STDIN, STDOUT and STDERR, resp.


run3() runs the child by invoking system with $cmd as specified above.


run3() restores the parent's STDIN, STDOUT and STDERR saved in step (3).


If run3() opened a temporary file for $stdout or $stderr in step (1), it rewinds it and reads back its contents using the specified method (either to a string, an array or by calling a function).


run3() closes all filehandles that it opened explicitly in step (1).

Note that when using temporary files, run3() tries to amortize the overhead by reusing them (i.e. it keeps them open and rewinds and truncates them before the next operation).


Often uses intermediate files (determined by File::Temp, and thus by the File::Spec defaults and the TMPDIR env. variable) for speed, portability and simplicity.

Use extrem caution when using run3 in a threaded environment if concurrent calls of run3 are possible. Most likely, I/O from different invocations will get mixed up. The reason is that in most thread implementations all threads in a process share the same STDIN/STDOUT/STDERR. Known failures are Perl ithreads on Linux and Win32. Note that fork on Win32 is emulated via Win32 threads and hence I/O mix up is possible between forked children here (run3 is "fork safe" on Unix, though).


To enable debugging use the IPCRUN3DEBUG environment variable to a non-zero integer value:

  $ IPCRUN3DEBUG=1 myapp


To enable profiling, set IPCRUN3PROFILE to a number to enable emitting profile information to STDERR (1 to get timestamps, 2 to get a summary report at the END of the program, 3 to get mini reports after each run) or to a filename to emit raw data to a file for later analysis.


Here's how it stacks up to existing APIs:

compared to system(), qx'', open "...|", open "|..."

  • better: redirects more than one file descriptor

  • better: returns TRUE on success, FALSE on failure

  • better: throws an error if problems occur in the parent process (or the pre-exec child)

  • better: allows a very perlish interface to Perl data structures and subroutines

  • better: allows 1 word invocations to avoid the shell easily:

     run3 ["foo"];  # does not invoke shell
  • worse: does not return the exit code, leaves it in $?

compared to open2(), open3()

  • better: no lengthy, error prone polling/select loop needed

  • better: hides OS dependencies

  • better: allows SCALAR, ARRAY, and CODE references to source and sink I/O

  • better: I/O parameter order is like open3() (not like open2()).

  • worse: does not allow interaction with the subprocess

compared to IPC::Run::run()

  • better: smaller, lower overhead, simpler, more portable

  • better: no select() loop portability issues

  • better: does not fall prey to Perl closure leaks

  • worse: does not allow interaction with the subprocess (which IPC::Run::run() allows by redirecting subroutines)

  • worse: lacks many features of IPC::Run::run() (filters, pipes, redirects, pty support)


Copyright 2003, R. Barrie Slaymaker, Jr., All Rights Reserved


You may use this module under the terms of the BSD, Artistic, or GPL licenses, any version.


Barrie Slaymaker <>

Ricardo SIGNES <> performed routine maintenance since 2010, thanks to help from the following ticket and/or patch submitters: Jody Belka, Roderich Schupp, David Morel, Jeff Lavallee, and anonymous others.