dd: multiple input files

dd can write to stdout too.

( dd if=file1 bs=1M count=99 skip=1
  dd if=file2 bs=1M count=10  ) > final_output

I don't think you can easily read multiple files in a single dd invocation, but you can append to build the output file in several steps:

dd if=file1 bs=1M count=99 skip=1 of=final_output
dd if=file2 bs=1M count=10 of=final_output oflag=append conv=notrunc

You need to specify both conv=notrunc and oflag=append. The first avoids truncating the output, the second starts writing from the end of the existing file.

Bear in mind that dd is a raw interface to the read(), write() and lseek() system call. You can only use it reliably to extract chunks of data off regular files, block devices and some character devices (like /dev/urandom), that is files for which read(buf, size) is guaranteed to return size as long as the end of the file is not reached.

For pipes, sockets and most character devices (like ttys), you have no such guarantee unless you do read()s of size 1, or use the GNU dd extension iflag=fullblock.

So either:

{
  gdd < file1 bs=1M iflag=fullblock count=99 skip=1
  gdd < file2 bs=1M iflag=fullblock count=10
} > final_output

Or:

M=1048576
{
  dd < file1 bs=1 count="$((99*M))" skip="$M"
  dd < file2 bs=1 count="$((10*M))"
} > final_output

Or with shells with builtin support for a seek operator like ksh93:

M=1048576
{
  command /opt/ast/bin/head -c "$((99*M))" < file1 <#((M))
  command /opt/ast/bin/head -c "$((10*M))" < file2
}

Or zsh (assuming your head supports the -c option here):

zmodload zsh/system &&
{
  sysseek 1048576 && head -c 99M &&
  head -c 10M < file2
} < file1 > final_output