How do I create a file that has X bytes of 0, Y bytes of the number 7 (i.e. 0x07 in hex, 00000111 in binary), Z random bytes and T bytes of the number 25(0x19 in hex, 00011001 in binary). Finally, this file should be of size X+Y+Z+T bytes.
Install 'ghex' from the software center.
Go to a terminal and run
Open ghex, then open the 'hexfile'.
Press insert, then type the bytes you want.
You might be able to do save something like this in a file, change the varibles (as instructed), and make it executable (