delete bytes from file

Loriel · #1 (**permalink**) 04-12-2006

I'm doing a bit of hex editing with dd and I can replace values fairly simply. However, I've run across a situation where I need to delete bytes in the file and I'm not sure how to do that. For example:

Input file has:
1234567890

Output needs to be:
123abc90

I tried this:
printf 1234567890 > /tmp/test
printf abc | dd bs=1 seek=0x03 count=5 conv=notrunc of=/tmp/test

The output is:
123abc7890

Other things I've tried:
1) setting ibs and obs to different sizes
2) not using the notrunc option - that truncates everything after the info I'm inserting

The actual file I'm editing is a Mac OS X binary, so I can't actually use text manipulation - it has to be hex editing, which is why I'm using dd. I'm not stuck on using dd if something else will get the job done, but it's what I know. I only use Unix for a few scripting projects to make my life easier, so I'm probably just missing a common command that most people would know. Any suggestions?

Perderabo · #2 (**permalink**) 04-12-2006

Code:

$ printf 1234567890 > test
$ dd if=test bs=1 count=3 of=test2
3+0 records in
3+0 records out
3 bytes transferred in 1 secs (3 bytes/sec)
$ printf abc >> test2
$ dd if=test bs=1 skip=8 seek=7 of=test2
2+0 records in
2+0 records out
2 bytes transferred in 1 secs (2 bytes/sec)
$ cat test2 ; echo
123abc90
$

Loriel · #3 (**permalink**) 04-12-2006

Thanks for the quick reply! That almost works for me. I had to change the seek to 6, however, because it leaves a null character after 123abc. It doesn't show up with cat, but a hexdump will show it. Anyways, thanks for the help!

Loriel · #4 (**permalink**) 04-12-2006

It's me again - I should have tested on the binary file. While this solution works, it takes a long time if the file is bigger than a few kb in size. The app I'm trying to patch is 11MB, so it's still going an hour later. I have a few of these patches to do and I don't have a week, unfortunately.

Is there another faster way to destructively replace x bytes with y bytes if y<x?

Perderabo · #5 (**permalink**) 04-12-2006

Show the exact commands that you used.

Loriel · #6 (**permalink**) 04-13-2006

The actual code has 17 offsets to process, but for my test run, I just used two. Also, keep in mind it never made it through the first cycle. I know the cycle was working (the apptest2 file kept growing, kb-by-slow-kb), but it's just too slow considering the total app size is 11 MB.

Code:

offsets=(0x5766ee 0xb020ae)
length1=(8 8)
length2=(5 5)
replace=(HOCUS POKUS)

for ((i=0;$i<${#offsets[@]};i++)); do
# Enter in the correct value
    printf ${replace[$i]} | dd bs=1 count="${length2[$i]}" seek="${offsets[$i]}" conv=notrunc of=apptest
# Create the infile
    dd if=apptest bs=1 count="$((${offsets[$i]}+${length2[$i]}))" of=apptest2
# Splice infile into original
    dd bs=1 skip="$((${offsets[$i]}+${length1[$i]}))" seek="$((${offsets[$i]}+${length2[$i]}))" of=apptest2
# Replace original so we can cycle
    mv apptest2 apptest
done

I tried using the printf approach to appending the infile, but that changes the filetype of the end result, which ruins the application. I didn't know if I could use dd to append to the end, so I simply changed the order to enter the correct value first, then cut up the file. That part of the loop works in a fraction of a second. It's the infile (and I suspect the splicing, as well) that takes so long.

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