tar.gz of the same file is different?

I download stuff from the internet and would like to checksum.

The download script appear to extract itself already.

So I try to archive it back and try to run a simplemd5sum.

But it’s not the same md5 sum !!!

Experiment

I download the package flyway

$ ls
flyway-7.7.0

Then I going to make a tar ball and also zipping with tar command

$ tar -cf flyway-7.7.0.tar flyway-7.7.0
$ tar -cf flyway-7.7.0-2.tar flyway-7.7.0
$ gzip flyway-7.7.0.tar
$ gzip flyway-7.7.0-2.tar
$ md5sum *.gz
9e7df6ead10ba08509c522699f9abb9f  flyway-7.7.0-2.tar.gz
4364ea9a31b542c9f93b0ccc6dc93fcd  flyway-7.7.0.tar.gz

This also different !

Reason

I clean up a bit.

$ rm *.gz
$ tar -cf flyway-7.7.0.tar flyway-7.7.0
$ tar -cf flyway-7.7.0-2.tar flyway-7.7.0

I try to gzip it again but this time with -n flag

$ gzip -n flyway-7.7.0.tar
$ gzip -n flyway-7.7.0-2.tar
$ md5sum *.gz
66e04e783300b4297ba18bf263e7c767  flyway-7.7.0-2.tar.gz
66e04e783300b4297ba18bf263e7c767  flyway-7.7.0.tar.gz

Here we are. The -n flag is

-n, --no-name
do not save or restore the original name and timestamp

This is because the default option include name and timestamp.

So it make the file sum change !

Hope this help.

Cheers !