Tag: Linux

Specifically Linux related things

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Less is more: colordiff and more or less

In the Unix/Linux/Mac OS X world, less is more. Literally, in that ‘less‘ fully emulates ‘more‘, and figuratively, as it provides useful additional functionality like backwards scrolling. So, you really want to use ‘less’ instead of ‘more’ for paging another command’s output, e.g.

cat a_long_document.txt|less

When used to page the output of colordiff however, ‘less’ displays a mess instead of properly displaying colored output like ‘more’.
The trick is to use ‘less’ with either the -r or -R option (which both repaint the screen), i.e.

colordiff -u file_old.py file_new.py|less -r

or

colordiff -u file_old.py file_new.py|less -R

(try which one works better with your system and terminal)

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Linux: Saving energy using auto-poweroff

I used to have a headless Linux LAN server that ran 24/7, i.e. even when I actually didn’t use it. Now, it powers itself off automatically as soon as it doesn’t detect any running workstations/notebooks (with dynamically assigned IP addresses) in the LAN.

Here’s how to do it:

In /usr/local/bin, create a new bash script named “shutdown_if_no_dhcp_client_in_lan” with the following content:

#!/bin/sh
# Shutdown the box if there aren't any (not ignored)
# DHCP clients in the LAN

# ignore the skype base station
ignored_macaddr="aa:bb:cc:dd:ee:ff"

# dhcp range starts at 192.168.1.$dhcpstart
dhcpstart=32

# dhcp range ends at 192.168.1.$dhcpend
dhcpend=46

for i in $(seq $dhcpstart 1 $dhcpend)
do
  # returns 0, if ip reachable. returns 1, if not reachable
  ping -w 1 -q 192.168.1.$i >/dev/null
  if [ "$?" = "0" ]; then
    # check whether this ip should be ignored
    if [ "`ip neighbor|grep 192.168.1.$i|grep $ignored_macaddr|wc -l`" = "1" ]; then
      echo "192.168.1.$i is reachable but ignored"
    else
      echo "192.168.1.$i is reachable and NOT ignored"
      # exit the program without shutting down the box
      exit;
    fi
  else
    echo "192.168.1.$i is NOT reachable"
  fi
done
# we didn't find any (not ignored) DHCP client in the LAN
# we can thus shutdown this box
echo "warning the users and shutting down the box in 5 minutes"
shutdown -h -P +5 Please save your work now!

(Adjust the script to your LAN environment)

In /etc/crontabs, add:

*/20  * * * * root /usr/local/bin/shutdown_if_no_dhcp_client_in_lan >/dev/null 2>&1

Now the LAN server will shutdown automatically after a while when no workstations/notebooks are running anymore. Note that this can happen 5 to 25 minutes after the last activity and isn’t fault-tolerant at all. The script can easily be improved however.