IPC::Cmd(3) Perl Programmers Reference Guide IPC::Cmd(3)NAMEIPC::Cmd - finding and running system commands made easy
SYNOPSIS
use IPC::Cmd qw[can_run run];
my $full_path = can_run('wget') or warn 'wget is not installed!';
### commands can be arrayrefs or strings ###
my $cmd = "$full_path -b theregister.co.uk";
my $cmd = [$full_path, '-b', 'theregister.co.uk'];
### in scalar context ###
my $buffer;
if( scalar run( command => $cmd,
verbose => 0,
buffer => \$buffer,
timeout => 20 )
) {
print "fetched webpage successfully: $buffer\n";
}
### in list context ###
my( $success, $error_code, $full_buf, $stdout_buf, $stderr_buf ) =
run( command => $cmd, verbose => 0 );
if( $success ) {
print "this is what the command printed:\n";
print join "", @$full_buf;
}
### check for features
print "IPC::Open3 available: " . IPC::Cmd->can_use_ipc_open3;
print "IPC::Run available: " . IPC::Cmd->can_use_ipc_run;
print "Can capture buffer: " . IPC::Cmd->can_capture_buffer;
### don't have IPC::Cmd be verbose, ie don't print to stdout or
### stderr when running commands -- default is '0'
$IPC::Cmd::VERBOSE = 0;
DESCRIPTIONIPC::Cmd allows you to run commands, interactively if desired, platform
independent but have them still work.
The "can_run" function can tell you if a certain binary is installed
and if so where, whereas the "run" function can actually execute any of
the commands you give it and give you a clear return value, as well as
adhere to your verbosity settings.
CLASS METHODS
$ipc_run_version = IPC::Cmd->can_use_ipc_run( [VERBOSE] )
Utility function that tells you if "IPC::Run" is available. If the
verbose flag is passed, it will print diagnostic messages if "IPC::Run"
can not be found or loaded.
$ipc_open3_version = IPC::Cmd->can_use_ipc_open3( [VERBOSE] )
Utility function that tells you if "IPC::Open3" is available. If the
verbose flag is passed, it will print diagnostic messages if
"IPC::Open3" can not be found or loaded.
$bool = IPC::Cmd->can_capture_buffer
Utility function that tells you if "IPC::Cmd" is capable of capturing
buffers in it's current configuration.
FUNCTIONS
$path = can_run( PROGRAM );
"can_run" takes but a single argument: the name of a binary you wish to
locate. "can_run" works much like the unix binary "which" or the bash
command "type", which scans through your path, looking for the
requested binary .
Unlike "which" and "type", this function is platform independent and
will also work on, for example, Win32.
It will return the full path to the binary you asked for if it was
found, or "undef" if it was not.
$ok | ($ok, $err, $full_buf, $stdout_buff, $stderr_buff) = run( command =>
COMMAND, [verbose => BOOL, buffer => \$SCALAR, timeout => DIGIT] );
"run" takes 4 arguments:
command
This is the command to execute. It may be either a string or an
array reference. This is a required argument.
See CAVEATS for remarks on how commands are parsed and their
limitations.
verbose
This controls whether all output of a command should also be
printed to STDOUT/STDERR or should only be trapped in buffers
(NOTE: buffers require "IPC::Run" to be installed or your system
able to work with "IPC::Open3").
It will default to the global setting of $IPC::Cmd::VERBOSE, which
by default is 0.
buffer
This will hold all the output of a command. It needs to be a
reference to a scalar. Note that this will hold both the STDOUT
and STDERR messages, and you have no way of telling which is which.
If you require this distinction, run the "run" command in list
context and inspect the individual buffers.
Of course, this requires that the underlying call supports buffers.
See the note on buffers right above.
timeout
Sets the maximum time the command is allowed to run before
aborting, using the built-in "alarm()" call. If the timeout is
triggered, the "errorcode" in the return value will be set to an
object of the "IPC::Cmd::TimeOut" class. See the "errorcode"
section below for details.
Defaults to 0, meaning no timeout is set.
"run" will return a simple "true" or "false" when called in scalar
context. In list context, you will be returned a list of the following
items:
success
A simple boolean indicating if the command executed without errors
or not.
error message
If the first element of the return value (success) was 0, then some
error occurred. This second element is the error message the
command you requested exited with, if available. This is generally
a pretty printed value of $? or $@. See "perldoc perlvar" for
details on what they can contain. If the error was a timeout, the
"error message" will be prefixed with the string
"IPC::Cmd::TimeOut", the timeout class.
full_buffer
This is an arrayreference containing all the output the command
generated. Note that buffers are only available if you have
"IPC::Run" installed, or if your system is able to work with
"IPC::Open3" -- See below). This element will be "undef" if this
is not the case.
out_buffer
This is an arrayreference containing all the output sent to STDOUT
the command generated. Note that buffers are only available if you
have "IPC::Run" installed, or if your system is able to work with
"IPC::Open3" -- See below). This element will be "undef" if this
is not the case.
error_buffer
This is an arrayreference containing all the output sent to STDERR
the command generated. Note that buffers are only available if you
have "IPC::Run" installed, or if your system is able to work with
"IPC::Open3" -- See below). This element will be "undef" if this
is not the case.
See the "HOW IT WORKS" Section below to see how "IPC::Cmd" decides what
modules or function calls to use when issuing a command.
$q = QUOTE
Returns the character used for quoting strings on this platform. This
is usually a "'" (single quote) on most systems, but some systems use
different quotes. For example, "Win32" uses """ (double quote).
You can use it as follows:
use IPC::Cmd qw[run QUOTE];
my $cmd = q[echo ] . QUOTE . q[foo bar] . QUOTE;
This makes sure that "foo bar" is treated as a string, rather than two
seperate arguments to the "echo" function.
__END__
HOW IT WORKS
"run" will try to execute your command using the following logic:
· If you have "IPC::Run" installed, and the variable
$IPC::Cmd::USE_IPC_RUN is set to true (See the "GLOBAL VARIABLES"
Section) use that to execute the command. You will have the full
output available in buffers, interactive commands are sure to work
and you are guaranteed to have your verbosity settings honored
cleanly.
· Otherwise, if the variable $IPC::Cmd::USE_IPC_OPEN3 is set to true
(See the "GLOBAL VARIABLES" Section), try to execute the command
using "IPC::Open3". Buffers will be available on all platforms
except "Win32", interactive commands will still execute cleanly,
and also your verbosity settings will be adhered to nicely;
· Otherwise, if you have the verbose argument set to true, we fall
back to a simple system() call. We cannot capture any buffers, but
interactive commands will still work.
· Otherwise we will try and temporarily redirect STDERR and STDOUT,
do a system() call with your command and then re-open STDERR and
STDOUT. This is the method of last resort and will still allow you
to execute your commands cleanly. However, no buffers will be
available.
Global Variables
The behaviour of IPC::Cmd can be altered by changing the following
global variables:
$IPC::Cmd::VERBOSE
This controls whether IPC::Cmd will print any output from the commands
to the screen or not. The default is 0;
$IPC::Cmd::USE_IPC_RUN
This variable controls whether IPC::Cmd will try to use IPC::Run when
available and suitable. Defaults to true if you are on "Win32".
$IPC::Cmd::USE_IPC_OPEN3
This variable controls whether IPC::Cmd will try to use IPC::Open3 when
available and suitable. Defaults to true.
$IPC::Cmd::WARN
This variable controls whether run time warnings should be issued, like
the failure to load an "IPC::*" module you explicitly requested.
Defaults to true. Turn this off at your own risk.
Caveats
Whitespace and IPC::Open3 / system()
When using "IPC::Open3" or "system", if you provide a string as the
"command" argument, it is assumed to be appropriately escaped. You
can use the "QUOTE" constant to use as a portable quote character
(see above). However, if you provide and "Array Reference",
special rules apply:
If your command contains "Special Characters" (< > | &), it will be
internally stringified before executing the command, to avoid that
these special characters are escaped and passed as arguments
instead of retaining their special meaning.
However, if the command contained arguments that contained
whitespace, stringifying the command would loose the significance
of the whitespace. Therefor, "IPC::Cmd" will quote any arguments
containing whitespace in your command if the command is passed as
an arrayref and contains special characters.
Whitespace and IPC::Run
When using "IPC::Run", if you provide a string as the "command"
argument, the string will be split on whitespace to determine the
individual elements of your command. Although this will usually
just Do What You Mean, it may break if you have files or commands
with whitespace in them.
If you do not wish this to happen, you should provide an array
reference, where all parts of your command are already separated
out. Note however, if there's extra or spurious whitespace in
these parts, the parser or underlying code may not interpret it
correctly, and cause an error.
Example: The following code
gzip -cdf foo.tar.gz | tar -xf -
should either be passed as
"gzip -cdf foo.tar.gz | tar -xf -"
or as
['gzip', '-cdf', 'foo.tar.gz', '|', 'tar', '-xf', '-']
But take care not to pass it as, for example
['gzip -cdf foo.tar.gz', '|', 'tar -xf -']
Since this will lead to issues as described above.
IO Redirect
Currently it is too complicated to parse your command for IO
Redirections. For capturing STDOUT or STDERR there is a work around
however, since you can just inspect your buffers for the contents.
Interleaving STDOUT/STDERR
Neither IPC::Run nor IPC::Open3 can interleave STDOUT and STDERR.
For short bursts of output from a program, ie this sample:
for ( 1..4 ) {
$_ % 2 ? print STDOUT $_ : print STDERR $_;
}
IPC::[Run|Open3] will first read all of STDOUT, then all of STDERR,
meaning the output looks like 1 line on each, namely '13' on STDOUT
and '24' on STDERR.
It should have been 1, 2, 3, 4.
This has been recorded in rt.cpan.org as bug #37532: Unable to
interleave STDOUT and STDERR
See Also
"IPC::Run", "IPC::Open3"
ACKNOWLEDGEMENTS
Thanks to James Mastros and Martijn van der Streek for their help in
getting IPC::Open3 to behave nicely.
BUG REPORTS
Please report bugs or other issues to <bug-ipc-cmd@rt.cpan.org>.
AUTHOR
This module by Jos Boumans <kane@cpan.org>.
COPYRIGHT
This library is free software; you may redistribute and/or modify it
under the same terms as Perl itself.
perl v5.10.1 2009-06-23 IPC::Cmd(3)