IPC::Cmd(3perl) - Man pages

IPC::Cmd(3perl) Perl Programmers Reference Guide IPC::Cmd(3perl)

NAME
IPC::Cmd - finding and running system commands made easy

SYNOPSIS
use IPC::Cmd qw[can_run run run_forked];

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_message, $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;
}

### run_forked example ###
my $result = run_forked("$full_path -q -O - theregister.co.uk", {'timeout' => 20});
if ($result->{'exit_code'} eq 0 && !$result->{'timeout'}) {
print "this is what wget returned:\n";
print $result->{'stdout'};
}

### 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;

DESCRIPTION
IPC::Cmd allows you to run commands platform independently,
interactively if desired, 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.

$bool = IPC::Cmd->can_use_run_forked
Utility function that tells you if "IPC::Cmd" is capable of providing
"run_forked" on the current platform.

FUNCTIONS
$path = can_run( PROGRAM );
"can_run" takes only one 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.

If called in a scalar context it will return the full path to the binary
you asked for if it was found, or "undef" if it was not.

If called in a list context and the global variable $INSTANCES is a true
value, it will return a list of the full paths to instances of the
binary where found in "PATH", or an empty list if it was not found.

$ok | ($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 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 "error message" 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 array reference 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). Otherwise, this element will be "undef".

out_buffer
This is an array reference containing all the output sent to STDOUT
the command generated. The notes from "full_buffer" apply.

error_buffer
This is an arrayreference containing all the output sent to STDERR
the command generated. The notes from "full_buffer" apply.

See the "HOW IT WORKS" section below to see how "IPC::Cmd" decides what
modules or function calls to use when issuing a command.

$hashref = run_forked( COMMAND, { child_stdin => SCALAR, timeout => DIGIT,
stdout_handler => CODEREF, stderr_handler => CODEREF} );
"run_forked" is used to execute some program or a coderef, optionally
feed it with some input, get its return code and output (both stdout and
stderr into separate buffers). In addition, it allows to terminate the
program if it takes too long to finish.

The important and distinguishing feature of run_forked is execution
timeout which at first seems to be quite a simple task but if you think
that the program which you're spawning might spawn some children itself
(which in their turn could do the same and so on) it turns out to be not
a simple issue.

"run_forked" is designed to survive and successfully terminate almost
any long running task, even a fork bomb in case your system has the
resources to survive during given timeout.

This is achieved by creating separate watchdog process which spawns the
specified program in a separate process session and supervises it:
optionally feeds it with input, stores its exit code, stdout and stderr,
terminates it in case it runs longer than specified.

Invocation requires the command to be executed or a coderef and
optionally a hashref of options:

"timeout"
Specify in seconds how long to run the command before it is killed
with SIG_KILL (9), which effectively terminates it and all of its
children (direct or indirect).

"child_stdin"
Specify some text that will be passed into the "STDIN" of the
executed program.

"stdout_handler"
Coderef of a subroutine to call when a portion of data is received
on STDOUT from the executing program.

"stderr_handler"
Coderef of a subroutine to call when a portion of data is received
on STDERR from the executing program.

"wait_loop_callback"
Coderef of a subroutine to call inside of the main waiting loop
(while "run_forked" waits for the external to finish or fail). It
is useful to stop running external process before it ends by itself,
e.g.

my $r = run_forked("some external command", {
'wait_loop_callback' => sub {
if (condition) {
kill(1, $$);
}
},
'terminate_on_signal' => 'HUP',
});

Combined with "stdout_handler" and "stderr_handler" allows
terminating external command based on its output. Could also be used
as a timer without engaging with alarm (signals).

Remember that this code could be called every millisecond (depending
on the output which external command generates), so try to make it
as lightweight as possible.

"discard_output"
Discards the buffering of the standard output and standard errors
for return by run_forked(). With this option you have to use the
std*_handlers to read what the command outputs. Useful for commands
that send a lot of output.

"terminate_on_parent_sudden_death"
Enable this option if you wish all spawned processes to be killed if
the initially spawned process (the parent) is killed or dies without
waiting for child processes.

"run_forked" will return a HASHREF with the following keys:

"exit_code"
The exit code of the executed program.

"timeout"
The number of seconds the program ran for before being terminated,
or 0 if no timeout occurred.

"stdout"
Holds the standard output of the executed command (or empty string
if there was no STDOUT output or if "discard_output" was used; it's
always defined!)

"stderr"
Holds the standard error of the executed command (or empty string if
there was no STDERR output or if "discard_output" was used; it's
always defined!)

"merged"
Holds the standard output and error of the executed command merged
into one stream (or empty string if there was no output at all or if
"discard_output" was used; it's always defined!)

"err_msg"
Holds some explanation in the case of an error.

$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
separate arguments to the "echo" function.

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,
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.

$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.

$IPC::Cmd::INSTANCES
This variable controls whether "can_run" will return all instances of
the binary it finds in the "PATH" when called in a list context.

Defaults to false, set to true to enable the described behaviour.

$IPC::Cmd::ALLOW_NULL_ARGS
This variable controls whether "run" will remove any empty/null
arguments it finds in command arguments.

Defaults to false, so it will remove null arguments. Set to true to
allow them.

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 an 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 lose the significance of
the whitespace. Therefore, "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 are extra or spurious whitespaces 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, e.g. 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 '13' on STDOUT and '24' on STDERR,
instead of

1
2
3
4

This has been recorded in rt.cpan.org as bug #37532: Unable to
interleave STDOUT and STDERR.