We've all run into problems like this:

$ echo 12000 > /proc/sys/vm/dirty_writeback_centisecs
-bash: /proc/sys/vm/dirty_writeback_centisecs: Permission denied

The command fails because the target file is only writeable by root. The fix seems obvious and easy:

$ sudo echo 12000 > /proc/sys/vm/dirty_writeback_centisecs -bash: /proc/sys/vm/dirty_writeback_centisecs: Permission denied

Huh? It still fails. What gives? The reason it fails is that it is the shell that sets up the re-direction before running the command under sudo. The solution is to run the whole pipeline under sudo. There are several ways to do this:

echo 'echo 12000 > /proc/sys/vm/dirty_writeback_centisecs' | sudo sh
sudo sh -c 'echo 12000 > /proc/sys/vm/dirty_writeback_centisecs'
echo 12000 | sudo tee /proc/sys/vm/dirty_writeback_centisecs

This is fine for simple commands, but what if you have a complex command that already includes quotes and shell meta-characters?

Here's what I use for that:

sudo su <<\EOF
echo 12000 > /proc/sys/vm/dirty_writeback_centisecs

Note that the backslash before EOF is important to ensure meta-characters are not expanded.

Finally, here's an example of a command for which I needed to use this technique:

sudo sh  << \EOF
perl -n -e '
use strict;
use warnings;
if (/^([^=]*=)([^\$]*)(.*)/) {
  my $pre = $1;
  my $path = $2;
  my $post = $3;
  (my $newpath = $path) =~ s/usr/usr\/local/;
  $newpath =~ s/://g;
  print "$pre$newpath:$path$post\n"
else {
' < /opt/rh/ruby193/enable > /opt/rh/ruby193/enable.new

I wanted to create a full-disk partition, with optimal alignment, on a 4TB disk under CentOS 6.4 and use it as an LVM PV.

fdisk doesn’t work on disks larger than 2TB so I used parted:

parted -a optimal /dev/sda
(parted) mklabel
Warning: The existing disk label on /dev/sda will be destroyed and all data on this disk will be lost. Do you want to continue?
Yes/No? Yes
(parted) mkpart primary ext2 0% 100%
(parted) set 1 lvm on

We have app servers with smallish local file systems and application data mounted over NFS.

Sometimes I want to find all files matching a particular set of criteria but don't want to traverse the NFS mounts.

Here's how to do it:

find / -group sophosav -print -o -fstype nfs -prune

Ordering is important, as is the explict inclusion of -print. If you omit this, it will print the name(s) of the NFS mounts as well.

Change start location (/) and criteria (-group sophosav) to suit your own purposes.

Here are two ways to update the firmware on Dell iDRAC6 remote access cards.

Both methods require downloading the BIOS from Dell and extracting it from the bundle. For example, this is the 1.70.21 firmware:

mkdir /tmp/dell
cd /tmp/dell
wget http://ftp.dell.com/esm/IDRAC6_FRMW_LX_R299265.BIN

Grab this and extract like this:

cd /tmp/dell
sh IDRAC6_FRMW_LX_R299265.BIN --extract ./idrac6-1.70.21

The firmware image is now in /tmp/dell/idrac6-1.70.21/payload/firmimg.d6

If you are just updating one machine, then the simplest way to perform the update is to use the Dell bmcfwul tool locally. This is supplied in the dell_ie_nitrogen package, and is installed to /usr/libexec/dell_dup/dell_ie_nitrogen/bmcfwul

Install the new firmware like this:

/usr/libexec/dell_dup/dell_ie_nitrogen/bmcfwul -input=/tmp/dell/idrac6-1.70.21/payload/firmimg.d6

If you have several machines to update, the most convenient way to perform the update is with tftp.

First, copy the firmware image to the tftp server, and put it in /tftproot, or wherever the root of your tftp server is located:

scp /tmp/dell/idrac6-1.70.21/payload/firmimg.d6 $ip_of_tftp_server:/tftproot

Then, trigger a firmware upgrade on the machines remotely using either racadm or ssh:

racadm -r host.to.update -u root -p calvin fwupdate -g -u -a $ip_of_tftp_server


ssh host.to.update racadm fwupdate -g -u -a $ip_of_tftp_server

I  ran into an odd issue today – my firewall build script was failing on our account master node.

It turns out that I was trying to use a chain name in iptables that exceeded the maximum length allowed. I wanted to use "REMOTE_ACCOUNT_SLAV ES_ASHEVILLE" (31 chars) and the limit is 30 chars.

You can see this in /usr/include/linux/netfilter_ipv4/ip_tables.h and /usr/include/linux/netfilter/x_tables.h:



This was on CentOS 5.6.

I use the toggl time-tracking service to keep track of the hours I work for my various clients.

toggl make available desktop clients for Windows, Mac, & Linux, but the Linux packages are in .deb format for Ubuntu and, until recently, they did not provide x86_64 packages.

toggl recently released the desktop client as open source so I grabbed it and have built an RPM.

SRPM: TogglDesktop-2.5.1-1.fc12.src.rpm

RPM (Fedora 12, x86_64): TogglDesktop-2.5.1-1.fc12.x86_64.rpm

It seems that several people have been having problems getting Dell OMSA 6.2 to work correctly on CentOS 5.4 x86_64. Specifically, the software does not detect any storage controllers, and therefore also doesn't find any disks. eg.

[root@b034 ~]# omreport storage pdisk controller=0
Invalid controller value. Read, controller=0
No controllers found.

After a little investigation, I found the source of the problem.

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