Jul/090
Linux: Share a directory between local users
I spent a few hours yesterday trying to get this done but ran into several problems, and worst of all, I couldn’t find one single post addressing all permission problems associated with allowing several users access to the same directory for read/write operations.
The best way of creating a shared directory is to create a user and group, and then a home directory. Make other users members of the new group and set permission inheritence to propagate to child objects regardless of the user who is creating or uploading the file.
User name is shareowner and group, sharedhome.
Create a user, group and home directory for the new user:
useradd -d /home/shareowner -g sharedhome -m shareowner
The above command creates a user and group as user’s primary group. I wouldn’t create a password for the user if I don’t want the user to actually be able to logon to the server. Once the home directory is created I will add the existing users to the new group:
usermod -G sharedhome existinguser1 usermod -G sharedhome existinguser2
and so on. Note that I use -G and not -g, as I want users to become additional users of the new group. Now we set permissions for the new directory… I make shareowner the owner, and sharedhome the group that owns the directory:
chown -R shareowner:sharedhome /home/sharedhome
Then I use umask and chmod to force all files in /home/sharedhome to inherit permissions from the top directory:
umask 002 chmod -R g+s /home/sharedhome
That’s it. Now existinguser1 and existinguser2 should have read/write permission to /home/sharedhome directory.
Jul/092
Debian: dmesg output contains “Error: Driver ‘pcspkr’ is already registered, aborting…”
Here is what’s transpiring: the boot sequence attempts to load two different drivers for the integrated PC speaker. To solve the problem install alsa-base package:
aptitude install alsa-base
And then execute the following command:
echo blacklist snd-pcsp >> /etc/modprobe.d/alsa-base-blacklist
Jun/090
Debian: open-iscsi, use iSCSI initiator to connect to a SAN
It is actually quite easy as long as your kernel is 2.6.16 or newer. Debian supports fibre channel out of the box, and for iSCSI you only need to install the open-iscsi package (you can easily taylor this to your distro):
aptitude install open-iscsi
Once the package is installed restart the initiator:
/etc/init.d/open-iscsi restart
To find out the indentifier name take a look at initiatorname.iscsi file:
cat /etc/iscsi/initiatorname.iscsi
You can find the initiator identifier towards the bottom of the file… mine is InitiatorName=iqn.1993-08.org.debian:01:61ddbbf82a70. Once you found the name you should be able to discover the target with the following command (my LeftHandNetworks iSCSI SAN IP is 192.168.1.70):
iscsiadm -m discovery -t sendtargets -p 192.168.1.70
Make a note of the record ID (mine is iqn.2003-10.com.lefthandnetworks:sancrp:3139:debian) and connect to it using the following command:
iscsiadm --mode node --targetname iqn.2003-10.com.lefthandnetworks:sancrp:3139:debian --portal 192.168.1.70:3260 --login
Once you initiated the command your iSCSI target will become visible to the OS as an SCSI disk (/dev/sda). You can now partition, format and then mount the LUN just like any other storage device. Add the target to your fstab and it should be auto-mounted everytime your system boots.
nano /etc/fstab
and add:
/dev/sda1 /iscsi ext3 _netdev 0 0
To have the LUN mounted to /iscsi directory. It didn’t work that well in my case… fstab mounts devices early during startup and before iscsi init, and OS couldn’t find the target to mount. I ended up scripting the initiator and mount… created a file “initiscsi” in /etc/init.d directory and added the following lines:
iscsiadm --mode node --targetname iqn.2003-10.com.lefthandnetworks:sancrp:3139:debian --portal 192.168.1.70:3260 --login sleep 5 mount -t ext3 /dev/sda1 /iscsi
I’m sure there is a proper way to have the session initiated before the mount, but I didn’t care enough to figure it out. Init and mount script will do just fine in my case since my LUN is just a storage device.
That sleep command is to make sure the LUN is visible to the OS before it’s mounted. I added the script to my startup sequence after making it executable:
chmod +x initiscsi update-rc.d initiscsi defaults
This is of course for Debian startup.
If you are using CHAP you will need to edit your iscsid.conf file:
nano /etc/iscsi/iscsid.conf
and update it with correct authentication info:
node.startup = automatic node.session.auth.username = username node.session.auth.password = password discovery.sendtargets.auth.username = username discovery.sendtargets.auth.password = password
Save and close the file, then initiate discovery.
Jun/092
Linux: nohup: Execute a command after exiting a shell
This is quite useful. I used to push jobs to the background and then exit shell, but it wasn’t an easy task considering I have short term memory issues (not a complete retard, just too much on my mind 
). I found this awesome little command to have the commands or scripts run right after I’m off the wagon!
nohup
That’s it… simply run your comman with it:
nohup command
Don’t forget the ampersand at the end. Use absolute path for scripts:
nohup ~/script &
If your scripts requires additional options to run, use a quote right before the ampersand:
nohup ~/script "start" &
Note that you don’t need to use this for your wget command… wget comes with a built-in switch (b):
wget -b
Have fun!
Jun/090
Linux: Convert .bin/.cue images to .iso
Install bchunk, then:
bchunk filename.bin filename.cue filename.iso
To mount:
mount -o loop filename.iso mountpoint
