cpiowriter.py

import time
import shlex

class CPIOWriter:
    # https://www.kernel.org/doc/Documentation/early-userspace/buffer-format.txt
    HEADER_FORMAT = "070701{ino:08X}{mode:08X}{uid:08X}{gid:08X}{nlink:08X}{mtime:08X}" \
                    "{filesize:08X}{maj:08X}{min:08X}{rmaj:08X}{rmin:08X}{namesize:08X}{chksum:08X}"
    OTHER_HEADER_ARGS = \
        { "mode": 0o100000 + 0o644 # regular file + perms, see stat(2)
        , "uid": 0
        , "gid": 0
        , "nlink": 1
        , "mtime": int(time.time())
        , "maj": 0
        , "min": 0
        , "rmaj": 0
        , "rmin": 0
        , "chksum": 0 # magic ends '1' so no chksum required
        }

    def __init__(self, file):
        self.file = file
        self.bytes_written = 0
        self.inode_count = 0

    def _write(self, string):
        self.file.write(string)
        self.bytes_written += len(string)

    def _align(self):
        n = self.bytes_written % 4
        if n == 0:
            return
        else:
            self._write("\0" * (4 - n))

    def append_file(self, filename, contents):
        self.inode_count += 1

        self._write(
            self.HEADER_FORMAT.format(
                  ino=self.inode_count
                , filesize=len(contents)
                , namesize=len(filename) + 1
                , **self.OTHER_HEADER_ARGS
            )
        )

        self._write(filename + "\0")
        self._align()
        self._write(contents)
        self._align()

    def append_bash_vars_file(self, filename, dictionary):
        contents = "".join("{}={}\n".format(k, shlex.quote(v)) for k, v in dictionary.items())
        self.append_file(filename=filename, contents=contents)

    def append_trailer(self):
        self.append_file(filename="TRAILER!!!", contents="")
 
if __name__ == "__main__":
    import sys
    w = CPIOWriter(sys.stdout)
    w.append_bash_vars_file("preseed-settings", {"a": "test", "b": "test two", "c": "moo'"})
    w.append_trailer()