Unable to run any command on mac terminal

I have changed by mistake the mode of /usr to 644 on my mac from terminal so now non of my process is accessible.

I can't use sudo and I am unable to find any way to login as root user so I will be able to change /usr's mode back to 777.

Please give me any suggestion

Here is the error it shows when I open new tab in terminal.

could not create a new process and open a pseudo-tty

I followed trojanfoe's suggestion. Here are the steps to fix this issue:

• On system boot press Command+R it will take you to OS X Recovery
• Click on Repair Disk Utility
• Select your disk where os is installed
• Click on repair file permissions

It will repair your permissions now you just need to restart your mac.