Perl system function - what it is?

Perl System:

system command invokes shell execution by default inside the brackets (). incorrect meta characters may lead errors in shell execution. However we have methods to byepass the shell execution without errors. using system we can able to pass more than one arguments as list for the execution.

we can perform most command line execution using system command.system perform the commands to execute and it waits for the completion of execution and then it returns the output to perl.


Single Argument System Commands:

it interacts with direct shell execution

Method 1: 

#prints the string on shell

system("echo Hello Perl");

#  Get errors in syslog

system("tail /var/fslog | grep -i ERROR");

# open notepad to edit a file

system("notepad fslog.txt");

Multiple  Argument System Commands:

1.it byepass the shell execution and it avoids the meta characters interpretations and exploit Errors,
2.it is secured and less open security issues.
3.in this execution, first argument is commands remaing all are considered as arguments.


Method 1:

system('ls', '*.txt');

Method 2:[Perl built-in glob function]

system('ls', glob(’*.txt’) );

Method 3

system('ls', @files);

Error Tracking With $?:

$? is the special variable used for capture the system output. this output will be as like below

1. -1
2.  0
3.  anything else

-1 will indicate the process is unable start due to some problem 0 indicate command was successful

If $? is anythingelse, we have to do number of bit-masking and bit-shifting operations to extract values

Method 1:

system("commands");

if ($? == -1) {
print "Cant run $!\n";
} elsif ($? == 0) {
print "successfull\n";
} else {
print "Exit number is ", $? >> 8, "\n";
print "Signal value is ", $? & 127, "\n";
print "Dumped value\n" if $? & 128;
}

Method 2:

use POSIX qw(WIFEXITED WEXITSTATUS WIFSIGNALED WTERMSIG);

system("command");

if (WIFEXITED($?)) {
print "normal exit value ",
WEXITSTATUS($?),"\n";
} elsif (WIFSIGNALED($?)) {
print "Killed the execution ",WTERMSIG($?),"\n";
} else {
warn "it is not normal exit.\n";

}

Method 3:

eval {
system(’backup_files’);
system(’delete_files’);
};

if ($@) {
print "Error commands: $@\n";
}

Method 4:

eval {
use autodie qw(system);
system('touch a.txt');
system('rm a.txt');
};

if ($@) {
print "Errors : $@\n";
}

we can  specify  return values as  first argument using IPC::System::Simple module.


1.returns 0, 1 ,2:

system( [0,1,2], "command");

2.captures exit value:

my $exit = system( [0,1,2], ’command’);

3.range operator:

my $exit = system( [0..2], ’command’);