Question

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using bash: write bit representation of integer to file

I need to write the size of a file into a binary file. For example:

$ stat -c %s in.txt 
68187

$ stat -c %s in.txt >> out.bin

Instead of writing 68187 as a string to out.bin, i want to write it as a 4 bytes integer to out.bin.

Answer 1

This is what I could come up with:

int=65534
printf "0: %.8x" $int | xxd -r -g0 >>file

Now depending on endianness you might want to swap the byte order:

printf "0: %.8x" $int | sed -E 's/0: (..)(..)(..)(..)/0: \4\3\2\1/' | xxd -r -g0 >>file

Example (decoded, so it’s visible):

printf "0: %.8x" 65534 | sed -E 's/0: (..)(..)(..)(..)/0: \4\3\2\1/' | xxd -r -g0 | xxd
0000000: feff 0000                                ....

This is for unsigned int, if the int is signed and the value is negative you have to compute the two’s complement. Simple math.