{"id":318,"date":"2026-10-13T13:41:20","date_gmt":"2026-10-13T13:41:20","guid":{"rendered":"http:\/\/www.scmgalaxy.com\/tutorials\/2012\/05\/04\/executing-external-commands-using-perl\/"},"modified":"2026-10-13T13:41:20","modified_gmt":"2026-10-13T13:41:20","slug":"executing-external-commands-using-perl","status":"publish","type":"post","link":"https:\/\/www.devopsschool.com\/blog\/executing-external-commands-using-perl\/","title":{"rendered":"Executing External Commands in Perl: system, exec, backticks, and open Explained"},"content":{"rendered":"\n

Originally published on 2012-05-04; fully revised and updated.<\/em><\/p>\n\n\n\n

If you automate infrastructure or operations tasks with Perl, you will eventually need to call external tools. This guide explains the four classic approaches\u2014system<\/strong>, exec<\/strong>, backticks<\/strong>, and open<\/strong>\u2014so you can choose the right one for each use case and handle errors safely.<\/p>\n\n\n\n

\"execute-external-commands-from-perl\"<\/figure>\n\n\n\n

Quick comparison of Perl command execution methods<\/h2>\n\n\n\n

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.<\/p>\n\n\n\n

Method<\/th>Returns to Perl?<\/th>Captures command output?<\/th>Best for<\/th><\/tr><\/thead>
system()<\/td>Yes<\/td>No (unless command redirects output itself)<\/td>Run a command and keep script control flow<\/td><\/tr>
exec()<\/td>No (on success)<\/td>No<\/td>Replace current Perl process with another program<\/td><\/tr>
Backticks (`cmd`)<\/td>Yes<\/td>Yes (stdout by default)<\/td>Capture command output as a string or list<\/td><\/tr>
open() with pipe<\/td>Yes<\/td>Streaming read\/write via filehandle<\/td>Incremental input\/output with external commands<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Decision flow: choosing the right method<\/h2>\n\n\n\n
\nflowchart TD\n    A[Need to run external command] --> B{Need command output in Perl}\n    B -- yes --> C{Need streaming data}\n    C -- yes --> D[Use open with pipe]\n    C -- no --> E[Use backticks]\n    B -- no --> F{Should Perl keep running}\n    F -- yes --> G[Use system]\n    F -- no --> H[Use exec]\n<\/pre>\n\n\n\n
system(): execute a command and continue<\/h2>\n\n\n\n
Use system()<\/strong> 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.<\/p>\n\n\n
my $rc = system('kubectl'<\/span>, 'get'<\/span>, 'pods'<\/span>, '-n'<\/span>, 'kube-system'<\/span>);\n\nif<\/span> ($rc == -1<\/span>) {\n    die<\/span> \"failed to execute kubectl: $!\"<\/span>;\n}\n\nmy $exit = $rc >> 8<\/span>;\nif<\/span> ($exit != 0<\/span>) {\n    die<\/span> \"kubectl exited with code $exit\"<\/span>;\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
The raw return value is stored in $?<\/strong> (or your captured return variable); for a normal exit code, shift right by 8 ($? >> 8<\/strong>).<\/p>\n\n\n\n
exec(): replace the current Perl process<\/h2>\n\n\n\n
exec()<\/strong> is for handoff scenarios: on success, your Perl process is replaced and does not return. Any code after exec()<\/strong> runs only if the exec call fails.<\/p>\n\n\n
exec('terraform'<\/span>, 'apply'<\/span>, '-auto-approve'<\/span>)\n    or<\/span> die<\/span> \"exec failed: $!\"<\/span>;\n\nprint<\/span> \"This line runs only if exec fails\\n\"<\/span>;<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Backticks: run command and capture output<\/h2>\n\n\n\n
Backticks return command output to Perl. In scalar context, you get one string; in list context, you get lines. By default, this captures stdout<\/strong>, not stderr<\/strong>.<\/p>\n\n\n
my $output = `kubectl get nodes 2<\/span>>&1<\/span>`;\nmy $exit   = $? >> 8<\/span>;\n\nif<\/span> ($exit != 0<\/span>) {\n    die<\/span> \"kubectl failed with code $exit:\\n$output\"<\/span>;\n}\n\nprint<\/span> $output;<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
open() with pipes: stream data in or out<\/h2>\n\n\n\n
Use open()<\/strong> with pipes when you need to process data incrementally instead of collecting everything at once.<\/p>\n\n\n
open(my $fh, '-|'<\/span>, 'ps'<\/span>, '-e'<\/span>, '-o'<\/span>, 'pid,comm'<\/span>)\n    or<\/span> die<\/span> \"open for read failed: $!\"<\/span>;\n\nwhile<\/span> (my $line = readline($fh)) {\n    print<\/span> $line;\n}\n\nclose($fh) or<\/span> die<\/span> \"close failed: $!\"<\/span>;<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n
open(my $mail, '|-'<\/span>, '\/usr\/bin\/mailx'<\/span>, '-s'<\/span>, 'test'<\/span>, 'user@localhost'<\/span>)\n    or<\/span> die<\/span> \"open for write failed: $!\"<\/span>;\n\nprint<\/span> {$mail} \"This is a test message\\n\"<\/span>;\n\nclose($mail) or<\/span> die<\/span> \"mail command failed: $!\"<\/span>;<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Legacy forms like open(“command |”)<\/strong> and open(“| command”)<\/strong> are common in older scripts, but list-style forms are generally safer because they avoid implicit shell parsing.<\/p>\n\n\n\n
Exit status and failure decoding<\/h2>\n\n\n\n
For robust automation, distinguish between \u201cfailed to start\u201d and \u201cstarted but exited with error.\u201d<\/p>\n\n\n
my $rc = system('some-command'<\/span>);\n\nif<\/span> ($rc == -1<\/span>) {\n    die<\/span> \"failed to start command: $!\"<\/span>;\n}\nelsif ($rc & 127<\/span>) {\n    printf \"command died with signal %d\\n\"<\/span>, ($rc & 127<\/span>);\n}\nelse<\/span> {\n    my $exit = $rc >> 8<\/span>;\n    printf \"command exited with code %d\\n\"<\/span>, $exit;\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Common pitfalls and best practices<\/h2>\n\n\n\n
  • Prefer list-style arguments for system<\/strong>, exec<\/strong>, and piped open<\/strong> to reduce shell-related risk.<\/li>
  • Always check return values and inspect $?<\/strong> (and $!<\/strong> when execution fails).<\/li>
  • Remember: backticks capture stdout only unless you explicitly redirect stderr.<\/li>
  • Use exec()<\/strong> only when you intentionally want no return to the current Perl script.<\/li>
  • When writing to a piped command, check close()<\/strong> to detect downstream command failures.<\/li><\/ul>\n","protected":false},"excerpt":{"rendered":"
    Perl provides multiple ways to run external commands, and each one behaves differently. This guide shows when to use system, exec, backticks, or open, with safe patterns for output capture and exit-code handling.<\/p>\n","protected":false},"author":1,"featured_media":3828,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_joinchat":[],"footnotes":""},"categories":[14],"tags":[378,1670,2527,2530,2526,2528,1122,545,1467,2529,175,2531,593,213,1927,1452],"class_list":["post-318","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-perl","tag-commands","tag-execute","tag-execute-external-commands","tag-execute-external-commands-from-perl","tag-external","tag-external-commands","tag-guide","tag-how","tag-implement","tag-implement-external-commands-by-using-perl","tag-perl","tag-perl-guide-for-executing-external-commands","tag-process","tag-tutorial","tag-using","tag-ways"],"_links":{"self":[{"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/posts\/318","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/comments?post=318"}],"version-history":[{"count":3,"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/posts\/318\/revisions"}],"predecessor-version":[{"id":79186,"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/posts\/318\/revisions\/79186"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/media\/3828"}],"wp:attachment":[{"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/media?parent=318"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/categories?post=318"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.devopsschool.com\/blog\/wp-json\/wp\/v2\/tags?post=318"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}