Command Line Arg contains Variable and more

VinsWorldcom has asked for the wisdom of the Perl Monks concerning the following question:

I've found some documentation on using the perl -s switch to get variables from the command line, but that's not really what I want. I'd like to take a command line argument that is a string that includes a command and a variable in the Perl script to print.

#!/usr/bin/perl

use strict;

my $host = 'localhost';

# Use 1 of the following lines:
#my $output = eval($ARGV[0]);
my $output = $ARGV[0];

print "COMMAND:  $output\n";
system($output);
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Now call with:

script.pl "echo hello"
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Output is:

{C} > script.pl "echo hello"
COMMAND:  echo hello
hello
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However, try:

{C} > test "echo $host"
COMMAND:  echo $host
$host
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I'd like "$host" that is printed to actually be "localhost" as is set in the script (my $host = 'localhost';) that is output. I had a feeling the "eval" would be needed, but that only works when the argument passed is the variable itself (ie: "$host", not "echo $host" and I need it to be the later since I'd like to pass more complex commands other than just 'echo').

Comment on Command Line Arg contains Variable and more Select or Download Code

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Re: Command Line Arg contains Variable and more by ikegami (Patriarch) on Dec 11, 2009 at 21:52 UTC
I had a feeling the "eval" would be needed, If your argument was Perl code, sure. `$ perl -e' my $host = "localhost"; eval $ARGV[0]; ' 'system("echo", $host)' localhost` [download] But your argument is a shell command. That means you'll need to parse the argument as if it was a shell command an interpolate the appropriate variables. It would be easier to use a template. `$ perl -MTemplate -e' Template->new()->process( \$ARGV[0], { host => "localhost" }, \my $cmd ); system($cmd) ' 'echo [% host %]' localhost` [download]	[reply] [d/l] [select]
Re: Command Line Arg contains Variable and more (pass the buck) by ikegami (Patriarch) on Dec 11, 2009 at 22:10 UTC
You could also create environment variables that the shell will be happy to interpolate for you. `$ perl -e'$ENV{host}="localhost"; system $ARGV[0]' 'echo $host' localhost` [download] The above is basically the same as `$ host=localhost sh -c 'echo $host' localhost` [download]	[reply] [d/l] [select]
Re: Command Line Arg contains Variable and more by gman (Friar) on Dec 11, 2009 at 21:13 UTC
Not sure this will work in every arg passed Also, doing something like this is probably not the safest #!/usr/bin/perl use warnings; use strict; my $string = $ARGV[0]; #print "$string"; my $rc = `$string`; print $rc; [download]	[reply] [d/l]
Re^2: Command Line Arg contains Variable and more by ikegami (Patriarch) on Dec 11, 2009 at 21:57 UTC
Changing `system $cmd;` [download] to print `$cmd`; [download] is never a good idea. All it does is introduce memory overhead and lag. It definitely doesn't help the OP interpolate a Perl variable into a shell command.	[reply] [d/l] [select]