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TTY Keystroke Injection using Perl


TTY KEYSTROKE INJECTION PERL

TTY Keystroke Injection using Perl

2014-10-30

One of my current projects deals with a program that gathers input from a mag strip reader, which emulates a USB attached keyboard. As a result, the program needs to run inside an active shell on the machine. Know, I know there are other possible methods such as capturing the keyboard input by way of custom drivers for the device, but that's a challenge for another day. I needed to occasionally shutdown the program and start it back up in the original tty session. Of course you cant simply stop and start it via restart script scheduled in cron because when it starts back up, the process will be headless and not capture any keystrokes sent to the active tty. What I needed is a way to send keystrokes to the active tty session.

My solution, TTY Keystroke Injection.

Here is a perl one liner to send one character. I found this online.

$ sudo perl -e '$TIOCSTI = 0x5412; $tty = "/dev/pts/3"; $char = "A"; open($fh, ">", $tty); ioctl($fh, $TIOCSTI, $char)'

The downside of the above command is it only sends a single character at a time (in the example, the A will get sent to /dev/pts/3). So… I wrote a simple perl script allowing me to conduct keystroke injection, passing in a full command, into a different session.

#!/usr/bin/perl
# tty keystroke injection
# version 1.2
# Richard

use Getopt::Long;
my $tty; # The tty session we are about to send keys to
my $command; # The command we are about to send to the target tty

GetOptions ("help|?" => sub{usage(0);}
	       ,"tty=s"  => \$tty
	       ,"cmd=s"  => \$command) or usage (1);

if($tty eq ''){
	print 'ERROR: You must specify a target tty session' . "\n";
	usage(0);
}
if($command eq ''){
	print 'ERROR: You must specify a command' . "\n";
	usage(0);
}
# Questions on how to get the correct value for $TIOCSTI
# is discussed here : http://www.perlmonks.org/?node_id=10920
my $TIOCSTI = 0x5412;
# Open a connection to the TTY
open(TTY,">$tty") or die "ERROR: Cannot open $tty\n";
# Split the command into single characters
my @command = split("",$command . "\n");
# Sleep a second to make sure the key gets sent
sleep(2);
# Loop the arry and send the key to the target TTY
foreach $arr(@command){
  ioctl(TTY,$TIOCSTI,$arr);
  select(undef,undef,undef,0.1);
};
print "Command: $command\nSent to $tty\n";

sub usage{
  my $err = shift and select STDERR;
  print < -cmd 
    -tty   -> The dev path to the targeted tty session
                                  To get a list of active tty session you can use the "w" command
                                  or use the command "tty" to see what your current tty session is
    -cmd      -> This is the command to pass to the target tty session
EOU
  exit $err;
}
To execute the program simply pass the target tty device and the command you want to execute.
$ sudo perl ./tty_key.pl -tty /dev/pts/2 -cmd 'ls -al'

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