Hex Dump - Compression of Duplicate Lines

Compression of Duplicate Lines

In the Unix programs od and hexdump, not all lines of display output that contain the same data as the previous line are shown; instead, a line containing just one asterisk is displayed. For example, a block of all zeros is printed as:

0000000 0000 0000 0000 0000 0000 0000 0000 0000 * 0000030

This compression feature makes a useful tool for inspecting large files or complete devices for irregularities. In a modern Linux system, it is convenient to scan an entire hard drive to check if it is all blank:

# hexdump /dev/sda (replace sda with the proper name for the device to be scanned)

The -v option causes hexdump and od to display all input data, explicitly:

0000000 0000 0000 0000 0000 0000 0000 0000 0000 0000010 0000 0000 0000 0000 0000 0000 0000 0000 0000020 0000 0000 0000 0000 0000 0000 0000 0000

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