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

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\" )\         


        

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 <>
     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.