usrlocalbinpip download -r requirementstxt -d packages --trusted-host pypidoubancom

This command downloads all the packages listed in the "requirements.txt" file and saves them to the "packages" directory. The "--trusted-host" option is used to indicate that the specified host (in this case, "pypi.douban.com") is trusted, which is necessary for some network configurations. The "/usr/local/bin/pip" specifies the location of the pip executable.