Stop mounting ISO files in Linux with “-t iso9660”

Google “How do I mount an ISO image in Linux” and most of the links still say to use “-t iso9660”. For example:

mount -t iso9660 -o loop,ro diskimage.iso /mnt/iso

That worked fine 10 years ago, but these days not all ISOs use ISO9660 file systems. Many use the UDF (Universal Disk Format) file system, and if you specify ISO9660 when mounting a UDF ISO file, subtle problems can occur. For instance, file names that contain upper case letters on a UDF file system will appear in lower case when that ISO is mounted using ISO9660.

On any modern Linux distro mount is smart enough to figure out what type of file system to use when mounting an ISO file, so it’s perfectly fine to let mount infer the type, e.g.:

mount -o loop,ro diskimage.iso /mnt/iso

Here’s an example of what happens when you try to mount a type UDF ISO as type ISO9660. Note that the case of the file names changes to all lower case when mounting as iso9660, which in this case causes subtle errors to occur within the software.

[~]$ blkid /srv/isos/specsfs/SPECsfs2014-1.0.iso
/srv/isos/specsfs/SPECsfs2014-1.0.iso: UUID="2014-10-22-15-52-41-00" LABEL="SPEC_SFS2014" TYPE="udf"

[~]$ mount -t iso9660 -o loop,ro /srv/isos/specsfs/SPECsfs2014-1.0.iso /mnt/iso
[~]$ cd /mnt/iso
[/mnt/iso]$ ls
benchmarks.xml    netmist_modify     redistributable_sources
binaries          netmist_modify.c   sfs2014result.css
copyright.txt     netmist_monitor    sfs_ext_mon
docs              netmist_monitor.c  sfsmanager
import.c          netmist_pro.in     sfs_rc
license.txt       netmist_proj       spec_license.txt
makefile          netmist.sln        specreport
map_share_script  notice             submission_template.xml
mempool.c         pdsm               token_config_file
mix_table.c       pdsmlib.c          win32lib
netmist.c         rcschangelog.txt   workload.c
netmist.h         readme.txt

[/mnt/iso]$ cd
[~]$ umount /mnt/iso
[~]$ mount -o loop,ro /srv/isos/specsfs/SPECsfs2014-1.0.iso /mnt/iso
[~]$ cd /mnt/iso
[/mnt/iso]$ ls
benchmarks.xml    netmist_modify     redistributable_sources
binaries          netmist_modify.c   sfs2014result.css
copyright.txt     netmist_monitor    sfs_ext_mon
docs              netmist_monitor.c  SfsManager
import.c          netmist_pro.in     sfs_rc
license.txt       netmist_proj       SPEC_LICENSE.txt
makefile          netmist.sln        SpecReport
Map_share_script  NOTICE             submission_template.xml
mempool.c         pdsm               token_config_file
mix_table.c       pdsmlib.c          win32lib
netmist.c         rcschangelog.txt   workload.c
netmist.h         README.txt
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Get Ansible’s “pip” method to install the right version of Django

I was using Ansible to set up a bunch of Scientific Linux 6.6 servers running Django and I wanted to use a specific version of Django, version 1.6.5, on all servers.

Ansible makes this easy with the “pip” module:

  - name: Install pip package from yum
    yum: name={{ item }} state=present
    with_items:
    - python-pip
    - python-setuptools

  - name: Install Django 1.6.5
    pip: name=django version=1.6.5 state=present

This works great if you’re installing on a clean, empty server, but if you’re upgrading a server that had an older version of Django on it (1.6.4 in my case) Ansible will act as if it’s installing 1.6.5, but when it’s done I still had version 1.6.4.

If I try using straight PIP commands I get this:

$ pip install django==1.6.5
Downloading/unpacking django==1.6.5
  Running setup.py egg_info for package django
    warning: no previously-included files matching '__pycache__' found under directory '*'
    warning: no previously-included files matching '*.py[co]' found under directory '*'
  Requested django==1.6.5, but installing version 1.6.4
Installing collected packages: django
  Found existing installation: Django 1.6.4
    Uninstalling Django:
      Successfully uninstalled Django
  Running setup.py install for django
    warning: no previously-included files matching '__pycache__' found under directory '*'
    warning: no previously-included files matching '*.py[co]' found under directory '*'
    changing mode of /usr/bin/django-admin.py to 755
Successfully installed django
Cleaning up...

Note the line “Requested django==1.6.5, but installing version 1.6.4”. Thanks PIP!

It turned out to be a bug in PIP versions earlier than PIP 1.4, not Ansible. A little Googling turned up a page on Stackoverflow that pointed the finger at an old cached copy of 1.6.4 in the build directory, which I found in /tmp/pip-build-root.

I updated my Ansible YAML file to get rid of the temporary directory and now it works fine:

  - name: Install pip package from yum
    yum: name={{ item }} state=present
    with_items:
    - python-pip
    - python-setuptools

  - name: Remove PIP temp directory
    file: path=/tmp/pip-build-root state=absent

  - name: Install Django 1.6.5
    pip: name=django version=1.6.5 state=present

Hope you find this useful.

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Getting rid of the “redirecting to systemctl” message in OpenSUSE

On OpenSUSE systems running systemd all rcX scripts now redirect start, stop, reload, restart, etc. service commands to systemctl. The messages that  used to appear on STDOUT telling you that a command is successful (or not) are now logged, but are no longer displayed on STDOUT.

That I can deal with, but every call to an rcX script now generates the message “redirecting to systemctl” to STDERR. I have a lot of scripts that call rcX scripts, and they interpret STDERR messages as “something just broke”.

The culprit is the new /etc/rc.status script that ships with OpenSUSE. It spews out the “redirecting to systemctl” message to STDERR for every operation that you do. The following command will modify the script and remove this stupid message:

if ( grep -q 'redirecting to systemctl' /etc/rc.status ) ; then
    # Save a copy of the original file
    cp -p /etc/rc.status /etc/rc.status.orig;

    # OpenSUSE 12.1:
    perl -i.bak -pe 's,echo "redirecting to systemctl" >/dev/stderr,,;' /etc/rc.status;

    # OpenSUSE 12.3:
    perl -i.bak -pe 's,echo "redirecting to systemctl \${SYSTEMCTL_OPTIONS} \$1 \${_rc_base}" 1>&2,,;' /etc/rc.status;
fi

This works for OpenSUSE 12.1 and 12.3. I did not have a 12.2 system available to test with.

Hope you find this useful.

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Bring Pidgin’s window into front focus when there’s an inbound IM

I was talking to a co-worker about Pidgin not coming into focus when there’s a new, inbound IM. The Pidgin window used to come into focus, front and center, when I was running Ubuntu/Gnome and when running OpenSUSE/KDE, but when I upgraded my office desktop to Ubuntu/Unity it stopped behaving this way. My co-worker noticed the same behavior with Fedora17/Gnome. A new IM would come in, but the Pidgin IM window would remain in the background, hidden, unseen and unread.

I thought “There has to be a setting that controls this,” and there is…

  • Bring up Pidgin’s Buddy List
  • Click Tools > Plugins
  • Locate the Message Notification plugin and highlight it
  • At the bottom of the Plugins window is a Configure Plugin button. Click it
  • Under Notification Methods check both Raise conversation window and Present conversation window
  • Click Close

That’s it. The next time someone IM’s you, your Pidgin Conversation will pop up in the center of your screen, in front of all of your other windows.

Hope you find this useful.

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Creating differential backups with hard links and rsync

You can use a hard link in Linux to create two file names that both point to the same physical location on a hard disk. For instance, if I type:

> echo xxxx > a
> cp -l a b
> cat a
xxxx
> cat b
xxxx

I create a file named “a” that contains the string “xxxx”. Then I create a hard link “b” that also points to the same spot on the disk. Now if I write to the file “a” whatever I write also appears in file “b” and vice versa:

> echo yyyy > b
> cat b
yyyy
> cat a
yyyy
> echo zzzz > a
> cat a
zzzz
> cat b
zzzz

Copying to a hard link updates the data on the disk that each hard link points to:

> rm a b c
> echo xxxx > a
> echo yyyy > c
> cp -l a b
> cat a b c
xxxx
xxxx
yyyy

“a” and “b” point to the same file on disk, “c” is a separate file. If I copy a file “c” to “b” that also updates “a”:

> cp c b 
> cat a b c
yyyy
yyyy
yyyy
> echo zzzz > c
> cat a b c
yyyy
yyyy
zzzz 

What most people don’t know is that rsync is an exception to this rule. If you use rsync to sync two files, and it sees that the target file is a hard link, it will create a new target file but only if the contents of the two files are not the same:

> rm a
> rm b
> echo xxxx > a
> cp -l a b
> cat a
xxxx
> cat b
xxxx
> echo yyyy > c
> cat c
yyyy
> rsync -av c b
sending incremental file list
c
sent 87 bytes  received 31 bytes  236.00 bytes/sec
total size is 5  speedup is 0.04
> cat b
yyyy
> cat c
yyyy
> cat a
xxxx

File “b” is no longer a hard link of “a”, it’s a new file. If I update “a” it no longer updates “b”:

> echo zzzz > a
> cat a b c
zzzz
yyyy
yyyy

However, if the file that I’m rsync-ing is the same as “b”, then rsync does NOT break the hard link, it leaves the file alone:

> rm a
> rm b
> rm c
> echo xxxx > a
> cp -al a b
> cp -p a c
> cat a b c
xxxx
xxxx
xxxx

At this point “a” and “b” both point to the same file on the disk, which contains the string “xxxx”. “c” is a separate file that also contains the string “xxxx” and has the same permissions and timestamp as “a”.

> rsync -av c b
sending incremental file list
sent 39 bytes  received 12 bytes  102.00 bytes/sec
total size is 5  speedup is 0.10
> cat a b c
xxxx
xxxx
xxxx

At this point I’ve rsynced file “c” to “b”, but since c has the same contents and timestamp as “a” and “b” rsync does nothing at all. It doesn’t break the hard link. If I change “b” it still updates “a”:

> echo yyyy > b
> cat a b c
yyyy
yyyy
xxxx

This is how many modern file system backup programs work. On day 1 you make an rsync copy of your entire file system:

backup@backup_server> DAY1=`date +%Y%m%d%H%M%S`
backup@backup_server> rsync -av -e ssh earl@192.168.1.20:/home/earl/ /var/backups/$DAY1/

On day 2 you make a hard link copy of the backup, then a fresh rsync:

backup@backup_server> DAY2=`date +%Y%m%d%H%M%S`
backup@backup_server> cp -al /var/backups/$DAY1 /var/backups/$DAY2
backup@backup_server> rsync -av -e ssh --delete earl@192.168.1.20:/home/earl/ /var/backups/$DAY2/

“cp -al” makes a hard link copy of the entire /home/earl/ directory structure from the previous day, then rsync runs against the copy of the tree. If a file remains unchanged then rsync does nothing — the file remains a hard link. However, if the file’s contents changed, then rsync will create a new copy of the file in the target directory. If a file was deleted from /home/earl then rsync deletes the hard link from that day’s copy.

In this way, the $DAY1 directory has a snapshot of the /home/earl tree as it existed on day 1, and the $DAY2 directory has a snapshot of the /home/earl tree as it existed on day 2, but only the files that changed take up additional disk space. If you need to find a file as it existed at some point in time you can look at that day’s tree. If you need to restore yesterday’s backup you can rsync the tree from yesterday, but you don’t have to store a copy of all of the data from each day, you only use additional disk space for files that changed or were added.

I use this technique to keep 90 daily backups of a 500GB file system on a 1TB drive.

One caveat: The hard links do use up inodes. If you’re using a file system such as ext3, which has a set number of inodes, you should allocate extra inodes on the backup volume when you create it. If you’re using a file system that can dynamically add inodes, such as ext4, zfs or btrfs, then you don’t need to worry about this.

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