lsof
http://ehaselwanter.com/en/blog/2009/03/13/tomcat-too-many-open-files/
http://alvinalexander.com/blog/post/linux-unix/linux-lsof-command
http://www.thegeekstuff.com/2012/08/lsof-command-examples/
http://unix.stackexchange.com/questions/163351/how-to-find-the-process-with-maximum-file-descriptors
http://www.cyberciti.biz/tips/linux-procfs-file-descriptors.html
grep
http://www.cyberciti.biz/faq/howto-use-grep-command-in-linux-unix/
http://www.computerhope.com/unix/ugrep.htm
http://www.tutorialspoint.com/unix_commands/grep.htm
sudo
http://www.howtogeek.com/111479/htg-explains-whats-the-difference-between-sudo-su/
https://www.linux.com/blog/how-use-sudo-and-su-commands-linux-introduction
https://en.wikipedia.org/wiki/Sudo
https://linuxacademy.com/blog/linux/linux-commands-for-beginners-sudo/
http://www.computerhope.com/unix/sudo.htm
http://www.tutorialspoint.com/unix_commands/sudo.htm
http://www.tecmint.com/su-vs-sudo-and-how-to-configure-sudo-in-linux/
Unix commands
https://www.tutorialspoint.com/unix/index.htm