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Thursday, December 29, 2011

The power of find - exec and friends (grep, sed)

This is quite old post. I started to write it in October 2010, but could not properly polish it for a long time.

Today I would like to present an example of usage of find (and grep) rooted in Crux. Let say that I want to find all packages in version 20100511 (this was true scenario when I wanted to update e17 related ports). Translating into less Crux specific language it means that I had to find all Pkgfile files (simple find), which had string 20100511 (simple grep). I needed only file names not a matching string so I used -l option for grep.

find . -name Pkgfile -exec grep -l 20100511 {} \;
I not only needed to find all old files but  to updated them as well (to version 1.0.0.beta that time). I used the same find but exchanged grep to sed (with option -i for  "in place").

find . -name Pkgfile -exec sed -i 's/20100511/1.0.0.beta/ {} \;
Let push our example one step further. I wanted to find dependence for packages, therefore I ran following command.

grep -i depen `find . -name Pkgfile -exec grep -l beta {} \;
What used  previous command to create a list of files to hunted through for word beta. (I wasn't sure if word "dependence" begun lower or upper case so used option -i for --ignore-case). 

In UNIX world there are always more than one way of doing things and in our scenario the find -exec can be replace with a separate command xargs. Xargs might be very useful in many cases because can be use to create unix command from standard input. Using xargs rather then find -exec my first example would be:

find . -name Pkgfile  | xargs grep -l beta $1 
Let use xargs for another task related to above example. In my scenario I had not only to update the version, but also to change the sources of the packages. To do that I used find, xargs and sed in a for loop.

for file in `find . -name Pkgfile  | xargs grep -l beta $1 2&> /dev/null` ; \
do \
 sed -i 's/\/crux\/distfiles/\/releases/' $file; \
The above command might be one liner, but can be paste line by line. It used the command from the previous example to create the $file array consist of names of files with word "beta". Elements from $file were use as input for sed command.

Wednesday, December 21, 2011

Mount LVM from domU in dom0

Mounting logical volumes (LVM) from domX (guest) within dom0 (XenServer) is very not recommender and suggested. But sometimes it might be the only options, and it's cool hacker (hacker in good old meaning, not cracker) trick. So to mount your volume you should take following steps:
  1. xe vm-disk-list vm=test4 - list disk of the VM.
  2. xe vm-list - find info (UUID) about dom0.
  3. xe vbd-create device=xvda unpluggable=true vdi-uuid=79a7a556-a6ba-48cf-8c82-30fa5bb9597c vm-uuid=36895434-e6d7-4fea-8271-d5477ca23c6d - create unpluggable VBD (xvda) with disk (uuid from point 1) on dom0 (uuid from point 2).
  4. xe vbd-plug uuid=0a4151b6-2b59-2fdb-c0a9-492520a8d52c - plug created vbd.
  5. mount /dev/xvda1 /mnt - mount disk (any 'physical partition').
  6. vi /etc/lvm/lvm.conf - if you need anything on LVM you have to edit lvm.conf 
  7. # Ignore /dev/xvd* devices to prevent deadlocking when live-snapshotting
    # dom0-attached LVHD VDIs
    #filter = "r|/dev/xvd.|","r|/dev/VG_Xen.*/LV.*|"
  8. vgchange -a y test4 - activate new vg.
  9. mount /dev/test4/root /test/mnt - mount partition.