The original OSX EI Capitan added a SIP feature, whether you are not Su, will prevent you from operating in the system directory. If you want to modify the system directory files, such as upgrade bash, you need to modify the/bin/bash, then you must first close the SIP, after the modification, and then open.
(1) Restart the OSX system, then press command+R (2) to display the interface, select Utilities Menu terminal (3) Enter csrutil disable in terminal to close SIP (csrutil enable SIP) (4) restart reboot OSX
SIP full Name Systems Integrity Protection (System integrity Protection), which is to protect system processes, files, documents are not modified by other processes, regardless of whether the root User,sip technology is mainly divided into file system protection, runtime protection, kernel extended signature, File system protection is mainly through the sandbox mechanism to restrict root permissions, run-time protection, mainly to protect the critical process can not be injected by the process code, hanging debugger and kernel debugging, such as kernel extension signature, all third-party kext must be installed to/library/extensions, and force the use of the signature.
Upgrade permissions in Mac Modify system directory