Executing External Commands in Perl: system, exec, backticks, and open Explained

Originally published on 2012-05-04; fully revised and updated.

If you automate infrastructure or operations tasks with Perl, you will eventually need to call external tools. This guide explains the four classic approaches—system, exec, backticks, and open—so you can choose the right one for each use case and handle errors safely.

Quick comparison of Perl command execution methods

All four methods execute external commands, but they differ in whether your Perl script continues, whether output is captured, and whether you can stream data.

Decision flow: choosing the right method

flowchart TD
    A[Need to run external command] --> B{Need command output in Perl}
    B -- yes --> C{Need streaming data}
    C -- yes --> D[Use open with pipe]
    C -- no --> E[Use backticks]
    B -- no --> F{Should Perl keep running}
    F -- yes --> G[Use system]
    F -- no --> H[Use exec]

system(): execute a command and continue

Use system() when you want to run a command and continue executing Perl code afterward. Prefer passing arguments as a list (not a single string) to avoid unnecessary shell parsing and reduce injection risk.

my $rc = system('kubectl', 'get', 'pods', '-n', 'kube-system');

if ($rc == -1) {
    die "failed to execute kubectl: $!";
}

my $exit = $rc >> 8;
if ($exit != 0) {
    die "kubectl exited with code $exit";
}Code language: PHP (php)

The raw return value is stored in $? (or your captured return variable); for a normal exit code, shift right by 8 ($? >> 8).

exec(): replace the current Perl process

exec() is for handoff scenarios: on success, your Perl process is replaced and does not return. Any code after exec() runs only if the exec call fails.

exec('terraform', 'apply', '-auto-approve')
    or die "exec failed: $!";

print "This line runs only if exec fails\n";Code language: PHP (php)

Backticks: run command and capture output

Backticks return command output to Perl. In scalar context, you get one string; in list context, you get lines. By default, this captures stdout, not stderr.

my $output = `kubectl get nodes 2>&1`;
my $exit   = $? >> 8;

if ($exit != 0) {
    die "kubectl failed with code $exit:\n$output";
}

print $output;Code language: PHP (php)

open() with pipes: stream data in or out

Use open() with pipes when you need to process data incrementally instead of collecting everything at once.

open(my $fh, '-|', 'ps', '-e', '-o', 'pid,comm')
    or die "open for read failed: $!";

while (my $line = readline($fh)) {
    print $line;
}

close($fh) or die "close failed: $!";Code language: PHP (php)

open(my $mail, '|-', '/usr/bin/mailx', '-s', 'test', 'user@localhost')
    or die "open for write failed: $!";

print {$mail} "This is a test message\n";

close($mail) or die "mail command failed: $!";Code language: PHP (php)

Legacy forms like open(“command |”) and open(“| command”) are common in older scripts, but list-style forms are generally safer because they avoid implicit shell parsing.

Exit status and failure decoding

For robust automation, distinguish between “failed to start” and “started but exited with error.”

my $rc = system('some-command');

if ($rc == -1) {
    die "failed to start command: $!";
}
elsif ($rc & 127) {
    printf "command died with signal %d\n", ($rc & 127);
}
else {
    my $exit = $rc >> 8;
    printf "command exited with code %d\n", $exit;
}Code language: PHP (php)

Common pitfalls and best practices

• Prefer list-style arguments for system, exec, and piped open to reduce shell-related risk.
• Always check return values and inspect $? (and $! when execution fails).
• Remember: backticks capture stdout only unless you explicitly redirect stderr.
• Use exec() only when you intentionally want no return to the current Perl script.
• When writing to a piped command, check close() to detect downstream command failures.

Rajesh Kumar

I’m Rajesh Kumar, a DevOps, SRE, DevSecOps, Cloud, and Platform Engineering expert passionate about sharing practical knowledge, real-world experiences, and industry best practices. I have worked at Cotocus and regularly write about technology, travel, investing, health, product reviews, and digital marketing through my various platforms.

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