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# 4. Advanced usage

Multiple compressed files can be concatenated. In this case, `gunzip` will extract all members at once. If one member is damaged, other members might still be recovered after removal of the damaged member. Better compression can be usually obtained if all members are decompressed and then recompressed in a single step.

This is an example of concatenating `gzip` files:

    gzip -c file1  > foo.gz
    gzip -c file2 >> foo.gz

Then

    gunzip -c foo

is equivalent to

    cat file1 file2

In case of damage to one member of a ``.gz'` file, other members can still be recovered (if the damaged member is removed). However, you can get better compression by compressing all members at once:

    cat file1 file2 | gzip > foo.gz

compresses better than

    gzip -c file1 file2 > foo.gz

If you want to recompress concatenated files to get better compression, do:

    zcat old.gz | gzip > new.gz

If a compressed file consists of several members, the uncompressed size and CRC reported by the ``--list'` option applies to the last member only. If you need the uncompressed size for all members, you can use:

    zcat file.gz | wc -c

If you wish to create a single archive file with multiple members so that members can later be extracted independently, use an archiver such as `tar` or `zip`. GNU `tar` supports the ``-z'` option to invoke `gzip` transparently. `gzip` is designed as a complement to `tar`, not as a replacement.

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