将 bash 翻译成 python； 从内容中提取特定字节（十六进制）

Question

I'm trying to transform a bash script to python, and I have a piece of code that I want to understands better. The script:

magic="$(readhex "$file" $end 6)" || break
test "$magic" = 070701 || test "$magic" = 070702 || break
namesize=0x$(readhex "$initramfs" $((end + 94)) 8)
filesize=0x$(readhex "$initramfs" $((end + 54)) 8)
end=$(((end + 110)))
end=$(((end + namesize + 3) & ~3))
end=$(((end + filesize + 3) & ~3))

Looks like is reading some hexdata from a file(or content), and getting some sizes of attributes.

with open("initramfs", "rb") as f:
    byte = f.read(1)
    while byte != "":
        byte = f.read(1)

I believe I need to use seek, but it is not very clear what magic should represent.

Answer 1

Magic bytes are like a file signature used to determine the type of file. The first 6 bytes 070701 and 070702 tell the software reading the file that it is a CPIO archive, a common method of packing up the initramfs it appears to be reading. If it was a JPEG, for example, it would be FFD8.

The rest of this is reading addresses and moving to them. A CPIO archive will have a table of the addresses of the files it contains and their metadata (name, size etc.). Writing a method of unpacking archives is non-trivial and poses a potential security risk unless thoroughly tested. There's lots of off the shelf Python libraries you can use to do this that will give you nice APIs with which to access the contents of various archive formats.

Should you wish to proceed you are correct in your assertion, you would use seek then read in to "end" bytes. The magic part is only testing it's the correct type of file that has been passed to it.