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;
}
### 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 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.
- "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.
__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.
$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.
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 loose 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.
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.
Thanks to Petya Kohts for the "run_forked" code.
BUG REPORTS¶
Please report bugs or other issues to <bug-ipc-cmd@rt.cpan.org>.
AUTHOR¶
Original author: Jos Boumans <kane@cpan.org>. Current maintainer: Chris
Williams <bingos@cpan.org>.
COPYRIGHT¶
This library is free software; you may redistribute and/or modify it under the
same terms as Perl itself.