Permission denied when redirection with sudo
When you are doing redirection with sudo, such as the following command
# sudo echo 1 > /proc/sys/vm/drop_caches -bash: /proc/sys/vm/drop_caches: Permission denied
You will get permission denied as the redirection will be performed as the user calling sudo, not as the superuser. So if you don't have permission to write to the target file, you will get permission denied.
To solve the problem, use the following method
sudo sh -c 'echo 1 > /proc/sys/vm/drop_caches'