How to use both pipes and prevent shell expansion in perl system function? - perl

Question

If multiple arguments are passed to perl's system function then the shell expansion will not work:
# COMMAND
$ perl -e 'my $s="*"; system("echo", "$s" )'
# RESULT
*
If the command is passed as an one argument then the expansion will work:
# COMMAND
$ perl -e 'my $s="echo *"; system("$s")'
# RESULT
Desktop Documents Downloads
The system function also allows to using multiple commands and connect them using pipes. This only works when argument is passed as an one command:
# COMMAND
$ perl -e 'my $s="echo * | cat -n"; system("$s")'
# RESULT
1 Desktop Documents Downloads
How can I combine mentioned commands and use both pipes and prevent shell expansion?
I have tried:
# COMMAND
$ perl -e 'my $s="echo"; system("$s", "* | cat -n")'
# RESULT
* | cat -n
but this did not work because of reasons that I've described above (multiple arguments are not expanded). The result that I want is:
1 *
EDIT:
The problem that I'm actually facing is that when I use following command:
system("echo \"$email_message\" | mailx -s \"$email_subject\" $recipient");
Then the $email_message is expanded and it will break mailx if it contains some characters that are further expanded by shell.

Answer 1

system has three calling conventions:
system($SHELL_CMD)
system($PROG, #ARGS) # #ARGS>0
system( { $PROG } $NAME, #ARGS ) # #ARGS>=0
The first passes a command to the shell. It's equivalent to
system('/bin/sh', '-c', $SHELL_CMD)
The other two execute the program $PROG. system never prevents shell expansion or performs any escaping. There's simply no shell involved.
So your question is about building a shell command. If you were at the prompt, you might use
echo \* | cat -n
or
echo '*' | cat -n
to pass *. You need a function that performs the job of escaping * before interpolating it. Fortunately, one already exists: String::ShellQuote's shell_quote.
$ perl -e'
use String::ShellQuote qw( shell_quote );
my $s = "*";
my $cmd1 = shell_quote("printf", q{%s\n}, $s);
my $cmd2 = "cat -n";
my $cmd = "$cmd1 | $cmd2";
print("Executing <<$cmd>>\n");
system($cmd);
'
Executing <<printf '%s\n' '*' | cat -n>>
1 *
I used printf instead of echo since it's very hard to handle arguments starting with - in echo. Most programs accept -- to separate options from non-options, but not my echo.
All these complications beg the question: Why are you shelling out to send an email? It's usually much harder to handle errors from external programs than from libraries.

Answer 2

You can use open to pipe directly to mailx, without your content being interpreted by the shell:
open( my $mail, "|-", "mailx", "-s", $email_subject, $recipient );
say $mail $email_message;
close $mail;
More details can be found in open section of perlipc.