IPC::S...:Simple(3pm) User Contributed Perl Documentation IPC::S...:Simple(3pm)
NAME
IPC::System::Simple - Run commands simply, with detailed diagnostics
SYNOPSIS
use IPC::System::Simple qw(system systemx capture capturex);
system("some_command"); # Command succeeds or dies!
system("some_command",@args); # Succeeds or dies, avoids shell if @args
systemx("some_command",@args); # Succeeds or dies, NEVER uses the shell
# Capture the output of a command (just like backticks). Dies on error.
my $output = capture("some_command");
# Just like backticks in list context. Dies on error.
my @output = capture("some_command");
# As above, but avoids the shell if @args is non-empty
my $output = capture("some_command", @args);
# As above, but NEVER invokes the shell.
my $output = capturex("some_command", @args);
my @output = capturex("some_command", @args);
DESCRIPTION
Calling Perl's in-built "system()" function is easy, determining if it
was successful is hard. Let's face it, $? isn't the nicest variable in
the world to play with, and even if you do check it, producing a well-
formatted error string takes a lot of work.
"IPC::System::Simple" takes the hard work out of calling external
commands. In fact, if you want to be really lazy, you can just write:
use IPC::System::Simple qw(system);
and all of your "system" commands will either succeed (run to completion
and return a zero exit value), or die with rich diagnostic messages.
The "IPC::System::Simple" module also provides a simple replacement to
Perl's backticks operator. Simply write:
use IPC::System::Simple qw(capture);
and then use the "capture()" command just like you'd use backticks. If
there's an error, it will die with a detailed description of what went
wrong. Better still, you can even use "capturex()" to run the
equivalent of backticks, but without the shell:
use IPC::System::Simple qw(capturex);
my $result = capturex($command, @args);
If you want more power than the basic interface, including the ability
to specify which exit values are acceptable, trap errors, or process
diagnostics, then read on!
ADVANCED SYNOPSIS
use IPC::System::Simple qw(
capture capturex system systemx run runx $EXITVAL EXIT_ANY
);
# Run a command, throwing exception on failure
run("some_command");
runx("some_command",@args); # Run a command, avoiding the shell
# Do the same thing, but with the drop-in system replacement.
system("some_command");
systemx("some_command", @args);
# Run a command which must return 0..5, avoid the shell, and get the
# exit value (we could also look at $EXITVAL)
my $exit_value = runx([0..5], "some_command", @args);
# The same, but any exit value will do.
my $exit_value = runx(EXIT_ANY, "some_command", @args);
# Capture output into $result and throw exception on failure
my $result = capture("some_command");
# Check exit value from captured command
print "some_command exited with status $EXITVAL\n";
# Captures into @lines, splitting on $/
my @lines = capture("some_command");
# Run a command which must return 0..5, capture the output into
# @lines, and avoid the shell.
my @lines = capturex([0..5], "some_command", @args);
ADVANCED USAGE
run() and system()
"IPC::System::Simple" provides a subroutine called "run", that executes
a command using the same semantics as Perl's built-in "system":
use IPC::System::Simple qw(run);
run("cat *.txt"); # Execute command via the shell
run("cat","/etc/motd"); # Execute command without shell
The primary difference between Perl's in-built system and the "run"
command is that "run" will throw an exception on failure, and allows a
list of acceptable exit values to be set. See "Exit values" for further
information.
In fact, you can even have "IPC::System::Simple" replace the default
"system" function for your package so it has the same behaviour:
use IPC::System::Simple qw(system);
system("cat *.txt"); # system now succeeds or dies!
"system" and "run" are aliases to each other.
See also "runx(), systemx() and capturex()" for variants of "system()"
and "run()" that never invoke the shell, even with a single argument.
capture()
A second subroutine, named "capture" executes a command with the same
semantics as Perl's built-in backticks (and "qx()"):
use IPC::System::Simple qw(capture);
# Capture text while invoking the shell.
my $file = capture("cat /etc/motd");
my @lines = capture("cat /etc/passwd");
However unlike regular backticks, which always use the shell, "capture"
will bypass the shell when called with multiple arguments:
# Capture text while avoiding the shell.
my $file = capture("cat", "/etc/motd");
my @lines = capture("cat", "/etc/passwd");
See also "runx(), systemx() and capturex()" for a variant of "capture()"
that never invokes the shell, even with a single argument.
runx(), systemx() and capturex()
The "runx()", "systemx()" and "capturex()" commands are identical to the
multi-argument forms of "run()", "system()" and "capture()"
respectively, but never invoke the shell, even when called with a single
argument. These forms are particularly useful when a command's argument
list might be empty, for example:
systemx($cmd, @args);
The use of "systemx()" here guarantees that the shell will never be
invoked, even if @args is empty.
Exception handling
In the case where the command returns an unexpected status, both "run"
and "capture" will throw an exception, which if not caught will
terminate your program with an error.
Capturing the exception is easy:
eval {
run("cat *.txt");
};
if ($@) {
print "Something went wrong - $@\n";
}
See the diagnostics section below for more details.
Exception cases
"IPC::System::Simple" considers the following to be unexpected, and
worthy of exception:
• Failing to start entirely (eg, command not found, permission
denied).
• Returning an exit value other than zero (but see below).
• Being killed by a signal.
• Being passed tainted data (in taint mode).
Exit values
Traditionally, system commands return a zero status for success and a
non-zero status for failure. "IPC::System::Simple" will default to
throwing an exception if a non-zero exit value is returned.
You may specify a range of values which are considered acceptable exit
values by passing an array reference as the first argument. The special
constant "EXIT_ANY" can be used to allow any exit value to be returned.
use IPC::System::Simple qw(run system capture EXIT_ANY);
run( [0..5], "cat *.txt"); # Exit values 0-5 are OK
system( [0..5], "cat *.txt"); # This works the same way
my @lines = capture( EXIT_ANY, "cat *.txt"); # Any exit is fine.
The "run" and replacement "system" subroutines returns the exit value of
the process:
my $exit_value = run( [0..5], "cat *.txt");
# OR:
my $exit_value = system( [0..5] "cat *.txt");
print "Program exited with value $exit_value\n";
$EXITVAL
The exit value of any command executed by "IPC::System::Simple" can
always be retrieved from the $IPC::System::Simple::EXITVAL variable:
This is particularly useful when inspecting results from "capture",
which returns the captured text from the command.
use IPC::System::Simple qw(capture $EXITVAL EXIT_ANY);
my @enemies_defeated = capture(EXIT_ANY, "defeat_evil", "/dev/mordor");
print "Program exited with value $EXITVAL\n";
$EXITVAL will be set to "-1" if the command did not exit normally (eg,
being terminated by a signal) or did not start. In this situation an
exception will also be thrown.
WINDOWS-SPECIFIC NOTES
The "run" subroutine make available the full 32-bit exit value on Win32
systems. This has been true since "IPC::System::Simple" v0.06 when
called with multiple arguments, and since v1.25 when called with a
single argument. This is different from the previous versions of
"IPC::System::Simple" and from Perl's in-build "system()" function,
which can only handle 8-bit return values.
The "capture" subroutine always returns the 32-bit exit value under
Windows. The "capture" subroutine also never uses the shell, even when
passed a single argument.
The "run" subroutine always uses a shell when passed a single argument.
On NT systems, it uses "cmd.exe" in the system root, and on non-NT
systems it uses "command.com" in the system root.
As of "IPC::System::Simple" v1.25, the "runx" and "capturex"
subroutines, as well as multiple-argument calls to the "run" and
"capture" subroutines, have their arguments properly quoted, so that
arugments with spaces and the like work properly. Unfortunately, this
breaks any attempt to invoke the shell itself. If you really need to
execute "cmd.exe" or "command.com", use the single-argument form. For
single-argument calls to "run" and "capture", the argument must be
properly shell-quoted in advance of the call.
Versions of "IPC::System::Simple" before v0.09 would not search the
"PATH" environment variable when the multi-argument form of "run()" was
called. Versions from v0.09 onwards correctly search the path provided
the command is provided including the extension (eg, "notepad.exe"
rather than just "notepad", or "gvim.bat" rather than just "gvim"). If
no extension is provided, ".exe" is assumed.
Signals are not supported on Windows systems. Sending a signal to a
Windows process will usually cause it to exit with the signal number
used.
DIAGNOSTICS
"%s" failed to start: "%s"
The command specified did not even start. It may not exist, or you
may not have permission to use it. The reason it could not start
(as determined from $!) will be provided.
"%s" unexpectedly returned exit value %d
The command ran successfully, but returned an exit value we did not
expect. The value returned is reported.
"%s" died to signal "%s" (%d) %s
The command was killed by a signal. The name of the signal will be
reported, or "UNKNOWN" if it cannot be determined. The signal
number is always reported. If we detected that the process dumped
core, then the string "and dumped core" is appended.
IPC::System::Simple::%s called with no arguments
You attempted to call "run" or "capture" but did not provide any
arguments at all. At the very lease you need to supply a command to
run.
IPC::System::Simple::%s called with no command
You called "run" or "capture" with a list of acceptable exit values,
but no actual command.
IPC::System::Simple::%s called with tainted argument "%s"
You called "run" or "capture" with tainted (untrusted) arguments,
which is almost certainly a bad idea. To untaint your arguments
you'll need to pass your data through a regular expression and use
the resulting match variables. See "Laundering and Detecting
Tainted Data" in perlsec for more information.
IPC::System::Simple::%s called with tainted environment $ENV{%s}
You called "run" or "capture" but part of your environment was
tainted (untrusted). You should either delete the named environment
variable before calling "run", or set it to an untainted value
(usually one set inside your program). See "Cleaning Up Your Path"
in perlsec for more information.
Error in IPC::System::Simple plumbing: "%s" - "%s"
Implementing the "capture" command involves dark and terrible
magicks involving pipes, and one of them has sprung a leak. This
could be due to a lack of file descriptors, although there are other
possibilities.
If you are able to reproduce this error, you are encouraged to
submit a bug report according to the "Reporting bugs" section below.
Internal error in IPC::System::Simple: "%s"
You've found a bug in "IPC::System::Simple". Please check to see if
an updated version of "IPC::System::Simple" is available. If not,
please file a bug report according to the "Reporting bugs" section
below.
IPC::System::Simple::%s called with undefined command
You've passed the undefined value as a command to be executed.
While this is a very Zen-like action, it's not supported by Perl's
current implementation.
DEPENDENCIES
This module depends upon Win32::Process when used on Win32 system.
"Win32::Process" is bundled as a core module in ActivePerl 5.6 and
above.
There are no non-core dependencies on non-Win32 systems.
COMPARISON TO OTHER APIs
Perl provides a range of in-built functions for handling external
commands, and CPAN provides even more. The "IPC::System::Simple"
differentiates itself from other options by providing:
Extremely detailed diagnostics
The diagnostics produced by "IPC::System::Simple" are designed to
provide as much information as possible. Rather than requiring the
developer to inspect $?, "IPC::System::Simple" does the hard work
for you.
If an odd exit status is provided, you're informed of what it is.
If a signal kills your process, you are informed of both its name
and number. If tainted data or environment prevents your command
from running, you are informed of exactly which data or
environmental variable is tainted.
Exceptions on failure
"IPC::System::Simple" takes an aggressive approach to error
handling. Rather than allow commands to fail silently, exceptions
are thrown when unexpected results are seen. This allows for easy
development using a try/catch style, and avoids the possibility of
accidentally continuing after a failed command.
Easy access to exit status
The "run", "system" and "capture" commands all set $EXITVAL, making
it easy to determine the exit status of a command. Additionally,
the "system" and "run" interfaces return the exit status.
Consistent interfaces
When called with multiple arguments, the "run", "system" and
"capture" interfaces never invoke the shell. This differs from the
in-built Perl "system" command which may invoke the shell under
Windows when called with multiple arguments. It differs from the
in-built Perl backticks operator which always invokes the shell.
BUGS
When "system" is exported, the exotic form "system { $cmd } @args" is
not supported. Attemping to use the exotic form is a syntax error.
This affects the calling package only. Use "CORE::system" if you need
it, or consider using the autodie module to replace "system" with
lexical scope.
Core dumps are only checked for when a process dies due to a signal. It
is not believed there are any systems where processes can dump core
without dying to a signal.
"WIFSTOPPED" status is not checked, as perl never spawns processes with
the "WUNTRACED" option.
Signals are not supported under Win32 systems, since they don't work at
all like Unix signals. Win32 signals cause commands to exit with a
given exit value, which this modules does capture.
Reporting bugs
Before reporting a bug, please check to ensure you are using the most
recent version of "IPC::System::Simple". Your problem may have already
been fixed in a new release.
You can find the "IPC::System::Simple" bug-tracker at
<http://rt.cpan.org/Public/Dist/Display.html?Name=IPC-System-Simple> .
Please check to see if your bug has already been reported; if in doubt,
report yours anyway.
Submitting a patch and/or failing test case will greatly expedite the
fixing of bugs.
FEEDBACK
If you find this module useful, please consider rating it on the CPAN
Ratings service at
<http://cpanratings.perl.org/rate/?distribution=IPC-System-Simple> .
The module author loves to hear how "IPC::System::Simple" has made your
life better (or worse). Feedback can be sent to
<pjf@perltraining.com.au>.
SEE ALSO
autodie uses "IPC::System::Simple" to provide succeed-or-die
replacements to "system" (and other built-ins) with lexical scope.
POSIX, IPC::Run::Simple, perlipc, perlport, IPC::Run, IPC::Run3,
Win32::Process
AUTHOR
Paul Fenwick <pjf@cpan.org>
COPYRIGHT AND LICENSE
Copyright (C) 2006-2008 by Paul Fenwick
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself, either Perl version 5.6.0 or, at
your option, any later version of Perl 5 you may have available.
perl v5.34.0 2022-10-13 IPC::S...:Simple(3pm)
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