#!/bin/bash
 
# ideone boilerplate - we can't write files in the home directory;
# so create a temporary directory for our files instead
t=$(mktemp -d -t ideone.XXXXXXXXXXXX) || exit
trap 'rm -rf "$t"' ERR EXIT
cd "$t"
 
cat <<\____ >gz.py
import struct
 
 
def gzinfo(filename):
    # Copy+paste from gzip.py line 16
    FTEXT, FHCRC, FEXTRA, FNAME, FCOMMENT = 1, 2, 4, 8, 16
 
    with open(filename, 'rb') as fp:
        # Basically copy+paste from GzipFile module line 429f
        magic = fp.read(2)
        if magic == b'':
            return False
 
        if magic != b'\037\213':
            raise ValueError('Not a gzipped file (%r)' % magic)
 
        method, flag, _last_mtime = struct.unpack("<BBIxx", fp.read(8))
 
        if method != 8:
            raise ValueError('Unknown compression method')
 
        if flag & FEXTRA:
            # Read & discard the extra field, if present
            extra_len, = struct.unpack("<H", fp.read(2))
            fp.read(extra_len)
        if flag & FNAME:
            fname = []
            while True:
                s = fp.read(1)
                if not s or s==b'\000':
                    break
                fname.append(s.decode('latin-1'))
            return ''.join(fname)
 
def main():
    from sys import argv
    for filename in argv[1:]:
        print(filename, gzinfo(filename))
 
if __name__ == '__main__':
    main()
____
 
echo hello >hello.txt
gzip -9 hello.txt
mv hello.txt.gz test.gz
 
python3 ./gz.py ./test.gz /dev/null

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