How to run free command by crontab

Question

I want to run this command:

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

by cron. That way I can free my RAM and cache in an Ubuntu system.

I'm using these steps:

# crontab -e 2 * * * * my_user_name /usr/bin/free free && sync && echo 3 > /proc/sys/vm/drop_$

save & exit

# service cron restart

But, when I run free -m nothing changes. For reference, it looks something like this:

# free -m total used free shared buffers cached Mem: 7976 5064 2911 5 169 3478 -/+ buffers/cache: 1416 6560 Swap: 8112 1264 6848

BTW, you shouldn't need service cron restart after crontab -e. crontab -e will automatically notify cron that something has changed. If you found the crontab file, and manually went to it and edited it, you would need to do something to tell cron to reread the modified file (so don't do that — just use crontab -e). — G-Man Says 'Reinstate Monica'
– G-Man Says 'Reinstate Monica', Commented Oct 16, 2015 at 5:02

mjturner · Accepted Answer · 2015-08-12 07:48:37Z

3

I'm not sure why you want to drop the caches every hour - that is going to kill performance.

There are a few problems with what you've done:

You need to edit the crontab of root, as you cannot write to /proc/sys/vm/drop_caches as a non-root user.
Your crontab entry is not correct. It should be:
```
2 * * * * /usr/bin/free && sync && echo 3 > /proc/sys/vm/drop_caches 
```
The fields, in order, are: minute, hour, day of month, month, day of week, command. You don't need to restart cron after editing a crontab.

answered Aug 12, 2015 at 7:28

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