ubuntu fdisk is full

I can Not use the chinese input method, so bad, but I have to memorize some stuff about using ubuntu,

yesterday I found i can not login my ubuntu system, i donnot know why ,so search any Q/A in website, some advice listed:

1. environment is wrong, directory in etc/profile, etc/env, etc/bashrc

2.system disk is full

I am pretty sure my question is 2. so i went to recovery mode to check my diskspace,but under root user any file and  command can not use, file is readonly ,command no authority,fuck.so search website again , oh ,god there is a command can change this:"mount -o remount, rw /",now i can use everything

second, where to found my big file which stuck the whole memory, just cd /

df

ls -l

and i found directory /var/log/cups is so so big,

so i use rm to del these log files .and then i can login my ubuntu again ,till now ,I made it .