Change content of a file in block or byte level

October 19, 2024 - 1 minutes read - 168 words

Since wrote Things learnt from recent preparation of 1T data for testing, I had figured out how to do that on the following weekends. Today I couldn’t get the steps to do so immediately. It is better to note down for my next time usage. The detailed steps is as following

Find the location in the file
Get the block contains contents to modify
Extract the block to a file and modify it
Replace the target block

scripts:

# test file test.ndjson in /dev/sdc
# line contains aaa need to replace with same length of spaces
> grep -oba aaa test.ndjson
16697:aaa
> lsblk -o NAME,PHY-SEC,LOG-SEC,SIZE
NAME PHY-SEC LOG-SEC   SIZE
sda      512     512 388.4M
sdb     4096     512     4G
sdc     4096     512     1T

# backup the file to verify and diff
> cp test.ndjson test2.ndjson
# 16697/4096=4.07
dd if=/home/jack/tmp/test.ndjson bs=4096 skip=4 count=1 > 5block.json
# replace line ["aaa": 1, with spaces
# use sed or vim edit 5block.json

# replace the block 5
dd if=5block.json of=test.ndjson  bs=4096 count=1 seek=4

dd
linux