/etc/alternatives/

Linux allows different implementation of a single command/utility co-exist. For example you can install both open JDK and sun JDK at a time on Linux and can also make one of them as default. The default selection is made using links in the file system. This works as:-

$which java
/usr/bin/java

$ls -l /usr/bin/java
/usr/bin/java -> /etc/alternatives/java

$ls -l /etc/alternatives/java
/etc/alternatives/java -> /usr/lib/jvm/jre-1.6.0-sun/bin/java

If we type command java on command prompt then /usr/bin/java is invoked. The /usr/java/bin points to /etc/alternatives/java and /etc/alternatives/java points to /usr/lib/jvm/jre-1.6.0-sun/bin/java. So the target of the command java becomes /usr/lib/jvm/jre-1.6.0-sun/bin/java


On Ubuntu, there is a graphical utility which lets you to change/configure the alternatives. But this is missing from the default install of openSUSE. But on the openSUSE this can be easily achieved by command line as -

update-alternatives --config java

This command will list all available options and will ask you to enter to select one of them.

Mounting NTFS drive on openSUSE in read-write mode

Recently, I installed openSUSE 11.4 on my laptop. The laptop is dual boot with windows7. There was a minor issue. The NTFS drive was mounted read only by default. Editing /etc/fstab fixed the issue. Here was the original entry in the fstab:-

/dev/disk/by-id/ata-FUJITSU_MHV2080BH_PL_NW9ZT682CFW0-part2 /windows/D           ntfs-3g    users,gid=users,fmask=133,dmask=022,locale=en_US.UTF-8 0 0

I changed this to as below, to make it working
 
/dev/disk/by-id/ata-FUJITSU_MHV2080BH_PL_NW9ZT682CFW0-part2 /windows/D           ntfs-3g    defaults 0 0

Merging PDF files on Linux

There are many tools out there. gs is one of such command line tools to achieve this. Suppose there are two pdf files as file1.pdf and file2.pdf and we want to merge them into final.pdf, then the command will be -
gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=final.pdf file1.pdf file2.pdf

More than two pdf can also be merged by providing their names in the command line argument.