Popen getting pid of newly run process

Question

I want to run some application in background and later kill it by pid.

pipe = IO.popen(\"firefox \'some_url\' 2>&1 &\")
pipe.pid

Answer 1

Since you are running it in the background (command &), you get the interpreter's PID:

>> pipe = IO.popen("xcalc &")
>> pipe.pid 
=> 11204

$ ps awx | grep "pts/8"
11204 pts/8    Z+     0:00 [sh] <defunct>
11205 pts/8    S+     0:00 xcalc

Drop the &:

>> pipe = IO.popen("xcalc")
>> pipe.pid
=> 11206

$ ps awx | grep "pts/8"
11206 pts/8    S      0:00 xcalc

For the additional issue with the redirection, see @kares' answer

Answer 2

it's not just about running it in the background but also due 2>&1

redirecting err/out causes IO.popen to put another process in front of your actual process ( pipe.pid won't be correct)

here's a detailed insight: http://unethicalblogger.com/2011/11/12/popen-can-suck-it.html

possible fix for this could be using exec e.g. IO.popen("exec firefox 'some_url' 2>&1")