NAME

Command::Run - Execute external command or code reference

SYNOPSIS

use Command::Run;

# Simple usage
my $result = Command::Run->new(
    command => ['ls', '-l'],
    stderr  => 'redirect',  # merge stderr to stdout
)->run;
print $result->{data};

# Method chaining style
my $runner = Command::Run->new;
$runner->command('cat', '-n')->with(stdin => $data)->run;
print $runner->data;

# Separate stdout/stderr capture
my $result = Command::Run->new(
    command => ['some_command'],
    stderr  => 'capture',
)->run;
print "data: ", $result->{data};
print "error: ", $result->{error};

# Access output via file descriptor path
my $cmd = Command::Run->new(command => ['date']);
$cmd->update;
system("cat", $cmd->path);  # /dev/fd/N

# Code reference execution
my $result = Command::Run->new(
    command => [\&some_function, @args],
    stdin   => $input_data,
)->run;

# Using with() method
my ($out, $err);
Command::Run->new->command("command", @args)
    ->with(stdin => $input, stdout => \$out, stderr => \$err)
    ->run;

VERSION

Version 0.9903

DESCRIPTION

This module provides a simple interface to execute external commands or code references and capture their output.

When a code reference is passed as the first element of the command array, it is called in a forked child process instead of executing an external command. This avoids the overhead of loading Perl and modules for each invocation.

This module inherits from Command::Run::Tmpfile, which provides temporary file functionality. The captured output is stored in this temporary file, accessible via the path method as /dev/fd/N, which can be used as a file argument to external commands.

CONSTRUCTOR

new(%parameters)

Create a new Command::Run object. Parameters are passed as key-value pairs (see "PARAMETERS"):

my $runner = Command::Run->new(
    command => \@command,
    stdin   => $input_data,
    stderr  => 'redirect',
);

# Or use method chaining
my $runner = Command::Run->new->command('ls', '-l');

PARAMETERS

The following parameters can be used with new, with, and run. With new and with, parameters are stored in the object. With run, parameters are temporary and do not modify the object.

command => \@command

The command to execute. Can be an array reference of command and arguments, or a code reference with arguments.

stdin => data

Input data to be fed to the command's STDIN.

stdout => \$scalar

Scalar reference to capture STDOUT.

stderr => \$scalar | 'redirect' | 'capture'

Controls STDERR handling:

\$scalar - Capture STDERR into the referenced variable
'redirect' - Merge STDERR into STDOUT
'capture' - Capture STDERR separately (accessible via error method)
undef (default) - STDERR passes through to terminal

nofork => bool

When true and the command is a code reference, execute the code in the current process without forking. Ignored for external commands. See "NOFORK AND RAW MODE" for details.

raw => bool

When true (with nofork), use :utf8 instead of :encoding(utf8) on I/O temporary files, avoiding encode/decode overhead and PerlIO layer leak. See "NOFORK AND RAW MODE" for details.

my $result = Command::Run->new(
    command => [\&process, @args],
    nofork  => 1,
    raw     => 1,
)->run;

METHODS

command(@command)

Set the command to execute. The argument can be:

External command and arguments: 'ls', '-l'
Code reference and arguments: \&func, @args

Returns the object for method chaining.

with(%parameters)

Set parameters (see "PARAMETERS"). Settings are stored in the object and persist across multiple run calls. Returns the object for method chaining.

my ($out, $err);
Command::Run->new("command")
    ->with(stdin => $data, stdout => \$out, stderr => \$err)
    ->run;

run(%parameters)

Execute the command and return the result hash reference. Accepts the same parameters as with, but parameters are temporary and do not modify the object state.

# All-in-one style
my $result = Command::Run->new->run(
    command => ['cat', '-n'],
    stdin   => $data,
    stderr  => 'redirect',
);

# Reuse runner with different input
my $runner = Command::Run->new('cat');
$runner->run(stdin => $input1);
$runner->run(stdin => $input2);  # object state unchanged

update()

Execute the command and store the output. Returns the object for method chaining.

result()

Return the result hash reference from the last execution.

data()

Return the captured output (stdout) from the last execution.

error()

Return the captured error output (stderr) from the last execution.

path()

Return the file descriptor path (e.g., /dev/fd/3) for the captured output. This can be passed to external commands.

rewind()

Seek to the beginning of the output temp file.

date()

Return the timestamp of the last execution.

RETURN VALUE

The run method returns a hash reference containing:

result: The exit status of the command ($?).
data: The captured output (stdout).
error: The captured error output (stderr, empty string unless stderr is 'capture').
pid: The process ID of the executed command.

NOFORK AND RAW MODE

Overview

When executing Perl code references, the default fork-based execution has two significant costs:

1. Fork overhead: fork() duplicates the entire process, including all loaded modules and data structures.
2. Encoding overhead: I/O between parent and child processes goes through pipes, requiring :encoding(utf8) layers that encode and decode UTF-8 on every read and write.

The nofork option eliminates the fork cost by executing the code reference in the current process. The raw option eliminates the encoding cost by using the :utf8 PerlIO pseudo-layer instead of :encoding(utf8).

Combined, these options can achieve over 30x speedup compared to fork-based execution for lightweight functions with small I/O.

How Nofork Works

In nofork mode, _execute_nofork temporarily redirects the real STDOUT, STDERR, and STDIN file descriptors to temporary files using dup, executes the code reference, then restores them:

# Simplified flow:
open $save, '>&', \*STDOUT;           # save original
open STDOUT, '>&', $tmpfile;          # redirect to tmpfile
$code->(@args);                       # execute code ref
open STDOUT, '>&', $save;             # restore original
$tmpfile->seek(0, 0);
$output = do { local $/; <$tmpfile> }; # read captured output

The code reference sees real STDOUT/STDIN file descriptors (not tied handles), so it behaves identically to the fork path from the callee's perspective. @ARGV, $0, and $_ are protected with local to prevent side effects.

How Raw Mode Works

The raw option controls which PerlIO layer is applied to the temporary files used for I/O redirection:

# Normal mode (raw => 0):
binmode $tmpfile, ':encoding(utf8)';  # full encode/decode

# Raw mode (raw => 1):
binmode $tmpfile, ':utf8';            # flag only, no conversion

In the normal fork path, :encoding(utf8) is necessary because data crosses process boundaries through pipes as byte streams. But in nofork mode, caller and callee share the same Perl interpreter, so Perl's internal string format (which is already UTF-8 internally) can be passed directly. The :utf8 layer simply sets Perl's UTF-8 flag on strings read from the file without performing actual byte-level conversion.

PerlIO Encoding Leak

There is an additional reason to prefer :utf8 over :encoding(utf8) in long-running processes. Repeatedly pushing and popping the :encoding(utf8) layer (which happens on each nofork execution when opening and closing temporary files) causes a cumulative performance degradation in Perl's PerlIO subsystem. This affects all PerlIO operations in the process, not just the ones using the encoding layer.

In benchmarks, nofork with :encoding(utf8) is actually slower than fork after many iterations, due to this leak. Raw mode avoids the issue entirely.

# Benchmark: code ref with stdin (100-byte input, 1000 iterations)
fork:                  399/s (baseline)
nofork + :encoding:    316/s (0.8x — slower than fork!)
nofork + :utf8 (raw): 13,433/s (34x faster)

Zero-Modification Callee Integration

A key advantage of this mechanism is that callee modules typically require no modification to work with nofork+raw mode.

Many Perl modules use use open pragma or equivalent to set up encoding layers on standard I/O:

package App::ansicolumn;
use open IO => ':utf8', ':std';    # sets :encoding(utf8) on STDIO

This works transparently because of execution order. When using nofork mode with method chaining:

require App::ansicolumn;           # (1) module loaded here
Command::Run->new
    ->command(\&ansicolumn, @args)
    ->with(stdin => $text, nofork => 1, raw => 1)
    ->update                       # (2) STDOUT redirected here
    ->data;

At step (1), require loads the module and use open ':std' applies :encoding(utf8) to the original STDOUT. At step (2), _execute_nofork redirects STDOUT to a fresh temporary file with :utf8 layer. The callee's encoding setup has already fired on the original STDOUT and does not affect the redirected one.

This means existing modules like App::ansicolumn and App::ansifold work unchanged with nofork+raw mode, achieving significant speedups with zero code changes on the callee side.

Note: If a module's encoding setup runs lazily (e.g., inside the called function rather than at module load time), the encoding layer would be applied to the redirected STDOUT, conflicting with raw mode. In such cases, the Getopt::EX::raw module can be used to intercept and replace :encoding(utf8) with :raw:utf8 at the callee side.

Caller Protection

Nofork mode executes the code reference in the same process, so care is needed to prevent the callee from corrupting the caller's state. The following protections are applied:

local $_;: Prevents the callee from modifying the caller's $_. This is critical when the caller aliases $_ to important data (e.g., greple's local *_ = shift to alias $_ to the content buffer). Without this protection, a callee's while (<>) loop would set $_ to undef at EOF, destroying the caller's data.
local @ARGV: Prevents the callee from modifying the caller's @ARGV.
$0 save/restore: Prevents the callee from permanently changing the program name.

COMPARISON WITH SIMILAR MODULES

There are many modules on CPAN for executing external commands. This module is designed to be simple and lightweight, with minimal dependencies.

This module was originally developed as App::cdif::Command and has been used in production as part of the App::cdif distribution since 2014. It has also been adopted by several unrelated modules, which motivated its release as an independent distribution.

IPC::Run: Full-featured module for running processes with support for pipelines, pseudo-ttys, and timeouts. Very powerful but large (135KB+) with non-core dependencies (IO::Pty). Overkill for simple command execution.
Capture::Tiny: Excellent for capturing STDOUT/STDERR from Perl code or external commands. Does not provide stdin input functionality.
IPC::Run3: Simpler than IPC::Run, handles stdin/stdout/stderr. Good alternative but does not support code reference execution.
Command::Runner: Modern interface with timeout support and code reference execution. Has non-core dependencies.
Proc::Simple: Simple process management with background execution support. Focused on process control rather than I/O capture.

Command::Run differs from these modules in several ways:

Core modules only - No non-core dependencies
Code reference support - Execute Perl code with $0 and @ARGV setup
File descriptor path - Output accessible via /dev/fd/N
Minimal footprint - About 200 lines of code
Method chaining - Fluent interface for readability

AUTHOR

Kazumasa Utashiro

LICENSE

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

To install Command::Run, copy and paste the appropriate command in to your terminal.

cpanm

cpanm Command::Run

CPAN shell

perl -MCPAN -e shell
install Command::Run

For more information on module installation, please visit the detailed CPAN module installation guide.

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