Creating a script for modifying several users crontabs

Question

I run a seed-box like local machine with Ubuntu 14.04LTS setup with multi-user rtorrent/rutorrent.

I installed that through a script that also installed crontab entries for each user so rtorrent & irrsi would start automatically at boot.

Load is too high (especially on the network) if all users rtorrent/irrsi are running at the same time. So I have started to

su - user
password
crontab -e

& then comment these rtorrent/irrsi lines out for users A-D & uncomment them for users E-H.

Later at night, when I return from work, I do the same but uncomment these lines for users A-D & comment these lines out for users E-H.

All this editing has made me starting to wonder if there wouldn't be an easier way to accomplish this... Would there be a way of creating a script that adds a # in front of all the lines in the crontab of all the users I specify. And then maybe a second script to remove that # (if present).

EDIT: each users' crontab contains only these 2 lines.

@reboot sleep 10; /usr/local/bin/rtcheck irssi rtorrent
*/5 * * * * /usr/local/bin/rtcheck irssi rtorrent

When I don't want them to be run at reboot or later, when cron finds them not running, I just comment out these lines so they look like this:

#@reboot sleep 10; /usr/local/bin/rtcheck irssi rtorrent
#*/5 * * * * /usr/local/bin/rtcheck irssi rtorrent

making it effectively neither irssi nor rtorrent start a boot nor get started later when cron finds them not running.

Answer 1

Probably the simplest way of doing this with cron is to:

• start the torrent service at time T1 for each user in user group A
• stop the torrent service at time T2>T1 for users in group A
• start the torrent service at time T2>T1 for users in group B
• stop the torrent service at time T3>T2 for users in group B

Choose times, e.g.

• T1 is anywhere between 0:01 am and 1:59 pm on D-day
• T2 is 2:00 pm on D-day
• T3 is 11:59 pm on D-day

You can change that anyway you like to make time sharing equitable for all group A and B users.

For user "username" in group A:
1) Edit existing crontab entries by commenting them out. Don't erase them just comment them out so they do not fire.
2) Add new crontab entries:

# => either start daemon, while checking that (i) startup takes place between 0:01 am
#+ and 2:00pm and (i) no 'rtorrent' process owned by "username" already runs. 
*/10 0-14 * * * [ -z $(/bin/ps -eF | /usr/bin/awk '/rtorrent/ && /username/') ] && /sbin/start-stop-daemon -S /usr/local/bin/rtorrent  
# => or start client-app based on similar conditions
*/10 0-14 * * * [ -z $(/bin/ps -eF | /usr/bin/awk '/rtorrent/ && /username/') ] && /usr/bin/env DISPLAY=:0.0 /usr/local/bin/rtorrent  

# stop 'rtorrent' process if it exists and is owned by "username"
#+ Check every 5 minutes between 2pm and midnight
*/5 14-24 * * * /usr/bin/kill "$(/bin/ps -eF | /usr/bin/awk '/rtorrent/ && /username/ printf "%s",$2')"  > /dev/null 2>&1

Details of the command starting yr daemon will depend on a number of factors, e.g. whether the client starting rtorrent is a single user machine or whether several user can simultaneously run their own instance of rtorrent. In this case you may want to save the process pid in a run-file for each user so you can manage those processes as a unction of traffic load. Look at this to actually start the rtorrent process identifying user, socket and pid.

I could not tried this solution, so it is a bit of a foray in the dark as far as the correct start procedure is concerned. In particular I assume that yr /home/username/.rtorrent.rc is adequately configured, and that irssi gives you no trouble as detailed elsewhere.

HTH. Please report.