You need to erase a hard disk and it should be impossible to recover any data from it.
You want to dispose an old computer system and want to be shure nobody can access your data.
In Linux you can use the shred command to overwrite all data 3 times (or more if you want to)
sudo shred -v /dev/sdX
Please do not use sdcX because that is a partition and you want to erase everything, not just one partition.
The following command will set all bits to 0 after the last iteration.
sudo shred -v -n1 -z /dev/sdX
There also exists a gui tool called bleachbit.
Deleting all information from a HD will take a long time. If you do not want to sell your computer it might be easier and much quicker to physical destroy the hard disk with a hammer.
About the Author
Wim Peeters is electronics engineer with an additional master in IT and over 30 years of experience including time spent in support, development, consulting, training and database administration. Wim has worked with SQL Server since version 6.5. He has developed in C/C++, Java and C# on Windows and Linux in different European countries and different European languages. He writes knowledge base articles to solve IT problems and publishes them on the Lubby Knowledge Platform where he is one of the most important contributors and the main developer.