Call command-line function in Perl and get output as String

Question

What is the best/easiest way to execute a command line function in Perl, such that I can get the output as a String?

What I'm actually trying to do is call a Java program from within a PL/Perl function in PostgreSQL, and I want the output String, however at the moment it seems to just be returning 0.

Here is a simple piece of code to explain what I am after:

CREATE OR REPLACE FUNCTION perl_func()
  RETURNS character varying AS
$BODY$    
  return system("java -version");
$BODY$
  LANGUAGE plperlu VOLATILE
  COST 100;

system returns the result code of executing the program. So it's a good thing that it returns 0. That just means you'll have to use backticks per Jack Maney's answer. — Axeman
– Axeman, Commented Apr 18, 2012 at 14:37

score 4 · Accepted Answer · 2012-04-18 14:41:54Z

4

You can use backticks. Quoting from perldoc perlop:

*qx/STRING/

*`STRING`

A string which is (possibly) interpolated and then executed as a system command with /bin/sh or its equivalent. Shell wildcards, pipes, and redirections will be honored. The collected standard output of the command is returned; standard error is unaffected. In scalar context, it comes back as a single (potentially multi-line) string, or undef if the command failed. In list context, returns a list of lines (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.

You can't use system for this, since it just returns the return value of the argument (when run as a shell command). This should work:

CREATE OR REPLACE FUNCTION perl_func()
  RETURNS character varying AS
$BODY$
  my $output=`java -version`;
  chomp($output);
  return $output;
$BODY$
  LANGUAGE plperlu VOLATILE
  COST 100;

Note that the output of a backticked command usually includes a trailing newline, so it's often useful to get rid of that via chomp.

edited Apr 18, 2012 at 14:41

answered Apr 18, 2012 at 14:26

user554546

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2 Comments

Larry Over a year ago

Actually, this seems to return an error: Can't modify quoted execution (``, qx) in chomp at line 1, near "java -version)” And when trying it without the chomp, it just returns an empty cell. Any ideas? Thanks!

user554546 Over a year ago

Hmmm...try the chomp separately (as done in the edited code above).

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