Encode and Decoder QR Code in Command Line

You can encode and decode QR code in command line by using ZXing.

Download jar file of zxing
http://repo1.maven.org/maven2/com/google/zxing/core/3.1.0/core-3.1.0.jar
http://repo1.maven.org/maven2/com/google/zxing/javase/3.1.0/javase-3.1.0.jar

Decoder QR code with an image file (img.png)
java -cp "core-3.1.0.jar:javase-3.1.0.jar" com.google.zxing.client.j2se.CommandLineRunner img.png

Encode QR code
java -cp "core-3.1.0.jar;javase-3.1.0.jar" com.google.zxing.client.j2se.CommandLineEncoder --image_format=GIF --output=output.gif "YHChien: Text for encode here"

Mount An Image with Multiple Partitions

If an image is from a hard disk, it may contains multiple partitions for mount. You access an image  with multiple partitions by following commands:

List partitions for a disk image

kpartx -l disk_image

Add and show mapper for a disk image

kpartx -av disk_image

Mount a loop device (e.g. the second partition)

mount /dev/mapper/loop0p2 /mnt

Delete mapper for a disk image

kpart -d disk_image