Linux – Write n bytes from a file into another in Bash

bashbash-scriptingbinary filesfile managementlinux

Hello how can I write n bytes from one file into a new file starting from the k position using Bash?

  • For example if n=60, k=1 and file size=100 then: the 2nd file would consist of from the 1st byte up to the 60th byte and would be 60 bytes in size
  • For example if n=40, k=61 and file size=100 then: the 2nd file would consist of from the 61st byte up to the 100th byte and would be 40 bytes in size

Possibly we are working with binary files not ASCII files, so the concatenation of 2 halves should be identical to the original file!

(Is it possible with dd ?)

Best Answer

Yes. Per the dd man page, you are looking for something like:

dd bs=1 count=60 if=_filename_1_ of=_filename_2_
dd bs=1 skip=60 count=40 if=_filename_1_ of=_filename_2_

where _filename_n_ is replaced with an actual filename.

bs=1 means that count and skip are byte counts. skip is how many to skip; count is how many to copy. Edit byte counts start from 0, not 1. Therefore, to start at the first byte, use skip=0 (or leave skip unspecified).

As a bash function, you could use:

# copy_nk(n, k, infile, outfile)
copy_nk() {
    dd bs=1 count="$1" skip="$2" ${3:+if="$3"} ${4:+of="$4"}
}

and then call it as 

copy_nk 60 0 infile.txt outfile.txt

(with k=0 because byte numbers start at zero).

With the ${3:+...}, you can leave off the output file or the input file. E.g., 

cat infile.txt | copy_nk 60 0 > outfile.txt
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