if you are sure that you’ve installed splunk in opt directory
then go to bin directory like this: cd /opt/splunk/bin/
now run this command from your bin directory: ./splunk start or ./splunk restart
root@aryan-VirtualBox:~# cd /opt/splunk/bin/
root@aryan-VirtualBox:/opt/splunk/bin# ./splunk start
The splunk daemon (splunkd) is already running.
If you get stuck, we’re here to help.
Look for answers here: http://docs.splunk.com
The Splunk web interface is at http://aryan-VirtualBox:8000