从perl执行if else shell命令

Question

I have never run into this scenario before so I could use a little help. I am trying to issue an if/else statement from a shell command but within perl. I've tried system and exec with no success. Here's a sample of the code

if ($os eq 'Ubuntu'){
    system('if ! dpkg -l curl &> /dev/null; then apt-get install curl -y; fi');
    foreach my $mod (keys %deb){
        eval "use $mod";
        if($@){
            system("apt-get install $deb{$mod} -y");
            eval "use $mod";
        }
    }
}

If I run the command if ! dpkg -l curl &> /dev/null; then apt-get install curl -y; fi if ! dpkg -l curl &> /dev/null; then apt-get install curl -y; fi if ! dpkg -l curl &> /dev/null; then apt-get install curl -y; fi in shell it works fine but executing it from perl is skips the if part and goes right on to installing it.

Answer 1

The if works perfectly fine.

$ perl -e'system("if ! true ; then echo ok ; fi")'

$ perl -e'system("if ! false ; then echo ok ; fi")'
ok

There is a problem, though. What you posted is a bash command, but system(EXPR) is short for system('/bin/sh', '-c', EXPR) . Simply replacing

system(EXPR)

with

system('/bin/bash', '-c', EXPR)

should do the trick.

Answer 2

You can avoid getting into the details of if syntax and also simplify it a bit by using || instead of negation. To try it out on the command line

perl -we 'system("rpm -q NO_PACK &> /dev/null || echo \"install package\"")'

I can only test on RHEL right now thus rpm . However, on RHEL I find this to work fine, too

perl -we 'system("if ! rpm -q NO >& /dev/null ; then echo install; fi")'

It is the syntax in the command that conflicts with the shell invoked by system , which is dash on Ubuntu (itself being a link to sh ). Thanks to ikegami and ThisSuitIsBlackNot for comments on this. In particular, the redirection &> does not work since that is specifically bash 's feature. Then you can change to the generic > /dev/null 2>&1 which sends STDOUT stream to /dev/null and redirects STDERR stream (2) to the same place where STDOUT (1) goes

system('dpkg -l curl > /dev/null 2>&1 || apt-get install curl -y');

On a RHEL system the sh is a link to bash , thus &> works. Note that the 2&>1 redirection works as well. However, in my opinion a better solution is to specify which shell you want to use.

The system allows one to not invoke the shell, by supplying arguments in a list. Then the program given as the first argument is run directly, without triggering the shell, with the rest of the list being passed to it. This is the way to get the right shell to run your command, as you can specify that shell as the first argument in the list. This is shown in the answer by ikegami .

Answer 3

The issue seemed a little problematic so I went about another way to solve it. Here is what I ended up doing which gives me the same end result

if ($os eq 'Ubuntu'){
    my $check = `dpkg --get-selections | grep "^curl *" | awk '{print \$1}'`;
    if(!$check){ system('apt-get install curl -y > /dev/null'); }
        foreach my $mod (keys %deb){
            eval "use $mod";
            if($@){
            system("apt-get install $deb{$mod} -y");
            eval "use $mod";
        }
    }
}