Why won't this bash script to drop caches work?

Question

I am trying to make a bash script to clear my RAM caches.

Here's what I have so far:

#! /bin/bash

free -m
sync
sudo -s
echo 3 > /proc/sys/vm/drop_caches

But it doesn't fully work. The first three commands work perfectly, at the fourth command it exits.

Please explain.

Answer 1

sudo -s creates a login shell session. Then after you exit that it, you return to being a regular user. Then it executes the 4th line, but you aren't root at that point so it fails.

You can't simply stick a sudo in front of the echo statement since that's a built-in command.

So try this instead:

#!/bin/bash

free -m
sync
sudo sh -c "echo 3 > /proc/sys/vm/drop_caches"

---

From man sudo:

-s [command]
     The -s (shell) option runs the shell specified by the SHELL 
environment variable if it is set or the shell as specified in passwd(5).
If a command is specified, it is passed to the shell for execution. 
Otherwise, an interactive shell is executed.