dd Command – Write File at Various Offsets to Filesystem Partition

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Requirement

I want to write a file at various offsets into the partition

Partition /dev/part2 is mounted at /mypart

I tried the command below:

dd if=/dev/urandom of=/mypart/aaa bs=1024 seek=0 count=15000
dd if=/dev/urandom of=/mypart/aaa bs=1024 seek=15000 count=15000
dd if=/dev/urandom of=/mypart/aaa bs=1024 seek=30000 count=15000

Are they doing what I want to do? Are they writing a file to the partition at offset 0, 15000K and 30000K?

At what offset is the file written if I omit seek from dd

dd if=/dev/urandom of=/mypart/aaa bs=1024 count=15000

Best Answer

You can't "write a file" at an "offset into the partition" using dd this way -- you are just writing data into a file named "aaa" within the mounted file system on that partition.

"seek=" will indeed cause dd to lseek to the given position before beginning its writes -- that means that it will simply create a file called /mypart/aaa and lseek the given number of blocks into that file before writing.

If you omit "seek=", dd will write starting at the beginning of the file named "aaa".

Related Solutions

Dd, seek with one block size, write with another block size

Solution:

dd if='input_file.bin'           \
   of='/dev/sd{X}'               \
   bs={desired write block size} \
   seek={start offset in bytes}  \
   count={write size in bytes}   \
   oflag=seek_bytes              \
   iflag=count_bytes

From the man page:

count_bytes
    treat 'count=N' as a byte count (iflag only)

...

seek_bytes
    treat 'seek=N' as a byte count (oflag only)

This does seem to slow down the transfer a bit, but at least puts it in MB/s, instead of kB/s. Also, be sure to check the man page on your system, as it seems the ones available on the web (i.e. googling 'man dd') don't include these options.

How to grow filesystem to use unallocated space in partition

If gparted only has to extend the partition or filesystem into unused space (immediately following the partition), then it should be safe to let it extend the partition and/or fs.
If, however, it has to MOVE any partitions around to make space for resizing, you'll have to boot with a gparted Live CD
See the man page for resize2fs (which is the command-line tool gparted will use to grow an ext2, ext3, and ext4 filesystem) for more details about resizing those filesystems.

For ext2/3/4, growing a filesystem is generally not a problem and can safely be done while the fs is mounted.

Shrinking a filesystem, however, is more troublesome and should be done while the fs is unmounted. If it's the rootfs, that means booting to a rescue CD/USB/PXE etc.

BTW, both dd and cat are amongst the worst ways to copy a linux system to another hard disk. Use Clonezilla, that's what it's for.

Best Answer

Related Solutions

Dd, seek with one block size, write with another block size

How to grow filesystem to use unallocated space in partition

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