Linux tcp tuning:
http://www-didc.lbl.gov/TCP-tuning/linux.html
Generate random numbers on linux:
dd if=/dev/urandom bs=1 count=128 | od -x
Solaris:
psrinfo -v
lists the processors on a box
Re-format external USB hard drive (Debian)
Motive: I wanted a backup solution that preserved file permissions when using rsync with the
-a option.
My backup drive, a Western Digital usb hard drive, had a vfat file system - which I found didn't preserve file permissions.
The following describes how to convert from vfat to an
ext3 file system.
Remember, if you follow these instructions all your existing data will be erased. Be sure, be very sure you want to re-format your existing drive.
Here goes...
When you plug the drive in check
dmesg to find out which /dev device it's been mapped to. I found mine under /dev/sda1.
Then you'll need to partition the disk:
fdisk /dev/sda1
This will give you a command prompt interface. Type
p to get a list of the current partitions.
I had 4 partitions, so I deleted them all using the
d command.
Then create your partition(s). I created one great big extended partition using the
n command. It asks you a few questions about block sizes etc, I chose the defaults.
Then I typed
w to write the partition data to the disk. Then I quit fdisk.
Then I created the filesystem:
mkfs.ext3 /dev/sda1
It asked me a few questions, I excepted all the defaults.
Then you're ready to mount the device:
mount /dev/sda1 /mnt/usbdrive
You'll have to create the folder /mnt/usbdrive yourself.
So that the disk gets mounted on reboot add the following line to /etc/fstab:
/dev/sda1 /mnt/usbdrive ext3 defaults 0 0
This also means you can simply type:
mount /dev/sda1
when mounting the disk from the command line.
This
link was useful.
Recovering a screwed NTFS disk
I created an image file from the corrupt disk using
dd -> dd if=/dev/hdd of=imagename.img conv=noerror
I wanted to use an image file so that I could mess around without harming the data on the original corrupt disk.
I used
testdisk to restore the image file's partition information (follow interactive menu system).
I was then able to mount the disk:
mount -t ntfs imagename.img /mnt/restoredntfs -o loop,offset=32256
Becasue I was mounting from an image file, I had to mount to a loop device. Thanks NASA, apparently.
According to testdisk my partition was situated at the 63rd cylinder -> 63 * 512 (assuming 512 bytes per block) = 32256 bytes.
X11
How to run x windows apps using X11 forwarding.
- Enable X11 forwarding on the client (in my case a debian distro). To do this reconfigure the sshd daemon, by changing the line
X11Forwarding no in /etc/sshd_config to X11Forwarding yes. Then restart the sshd daemon: /etc/init.d/ssh restart.
- From the server (in my case a Powerbook) login into the linux box (but make sure you do this from an xterm session), using ssh (don't forget the -X):
ssh -X linuxhostname. From the client (debian ssh prompt) run an app: xcalc, ddd etc. The window should appear on the server (Powerbook).
Note, no need to use
xhost +. The above solution is much more secure, both for client and server. All communication is authenticated and encrypted. And you don't have to set/export your display on the client (debian)!
awk
To print a timestamp (UTC) in a locale readable string do the following:
echo "1136211843" | awk '{print strftime("%c", $1)}'
prints:
Mon Jan 2 09:24:03 2006
RPM
To check the version of an installed rpm package, use the following command (this example looks at procps, which includes the ps command):
rpm -qa | grep procps
returns:
procps-2.0.7-11.21AS.4
To list the files in an uninstalled package:
rpm -qpl packagename.rpm
Vim
Setting a text file to unix format from DOS using vim:
:set ff=unix
To find out what format a file is:
:set ff?
du
Lists the size of each directory/file in the current directory, and prints the sizes in human readable form:
du -h --max-depth=1
Prolong the life of your external USB drive
I have an external usb powered hard drive (one of
these) connected to my server permanently, as part of my backup solution. To prolong the life of the disk, I spin it down after my overnight using the following command:
sdparm --command=stop /dev/sda
Note: your device might be mounted somewhere other than
/dev/sda.
Renaming a large number of hidden files
The following command searches the current directory, recursively, for all hidden
mp3 files, appending a
.blah suffix:
find . | egrep "/\..*mp3$" | while read e ; do mv "$e" "$e.blah"; done
Filenames
can have spaces in them!
XEmacs
To get rid of that annoying flashing when making a mistake in XEmacs:
M-x
set-variable
visible-bell
Value:
nil
to top
