Introduced in OS X El Capitan and also present in current Mac operating system, MacOS Sierra has an inbuilt security technology to prevent potentially malicious software from modifying protected files and folders on your Mac. It is known as System Integrity Protection or SIP. System Integrity protection restricts the root user account and limits the actions that the root user can perform on protected parts of the Mac operating system.Due to SIP, you won’t be able to perform many tasks including editing plist files. Before SIP, the root user (or Administrator user) had no permission restrictions, so it could access any system folder or app on your Mac. You inherited root-level access when you entered your administrator name and password to install a software on Mac and it allowed the software to modify or overwrite any system file or app.
With SIP following parts of the system have received protection:
Apps that are pre-installed with OS X
Paths and apps that third-party apps and installers can continue to write to include:
Here are a few things you can do to bypass SIP:
System Integrity Protection also helps prevent software from selecting a startup disk. To select a startup disk, choose System Preferences from the Apple menu, then click Startup Disk. Or hold down the Option key while you restart, then choose from the list of startup disks
To edit a plist file, restart Mac in Recovery Mode (Hold both Control and R buttons) and then select Terminal from Options. Once you have the terminal window open, run the command csrutil disable. Restart Mac and you’ll be able to edit plist files.