writing (large) *fixed-length* files (to tape) using 'dd', et al

writing (large) *fixed-length* files (to tape) using 'dd', et al

Post by Tim Hoogasian, Senior Network Adm » Thu, 04 Mar 1993 10:14:46


I need to be able to create pad out a variable-length file to a (larger)
fixed length when writing the file to tape.  (To answer the "why?" question
would require a _lengthy_ response.)  For argument's sake, I want to be
able to create a file of exactly 1Mb (1024Kb).

The following sequence (on a Sun) will work:

# create the padding
$ dd if=/dev/zero of=1MbFile bs=100k count=10  (or, of course, bs=1000k, count=1)

# pad out the original file to a bit over 1 Mb
$ cat {orig-file} 1MbFile > tempfile

# write the larger file out to tape
$ dd if=tempfile of=/dev/{tapedevice} bs=100k count=10

Problem: the file /dev/zero is not universal.  It shows up on our Suns and on the
RS/6000, but not on our HP's or DEC's.  (/dev/zero just gives you a stream of
nulls as long as you "read" from it - like the way /dev/null never "fills up".)

Is there a platform independent way of doing what /dev/zero does?  Or at any
rate, what I *want* to do, rather than just the implementation-specific details?



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