Depending on what you want to do you can use 1,2 or 3
from https://www.kernel.org/doc/Documentation/sysctl/vm.txt
drop_caches
Writing to this will cause the kernel to drop clean caches, dentries and inodes from memory, causing that memory to become free.
To free pagecache:
echo 1 > /proc/sys/vm/drop_caches To free dentries and inodes:
echo 2 > /proc/sys/vm/drop_caches To free pagecache, dentries and inodes:
echo 3 > /proc/sys/vm/drop_caches As this is a non-destructive operation and dirty objects are not freeable, the user should run 'sync' first.
If you want to run it with sudo, ( thanks for Evhz's comment ):
sudo sh -c "echo 1 > /proc/sys/vm/drop_caches" # or 2, 3 per your needs. 
