[Help-tar] Same text file on different OS packed by tar, the tar file has different md5 value.

[ice black]

Why are tar generating different tar file on different OS for the same text file? Here is how to reproduce the problem:

This is host 1:

US[keli@ tmp]uname -a

Linux Smiles-station-001 4.4.0-040400-generic #201601101930 SMP Mon Jan 11 00:32:41 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

US[keli@ tmp]tar --version

tar (GNU tar) 1.28

Copyright (C) 2014 Free Software Foundation, Inc.

License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.

This is free software: you are free to change and redistribute it.

There is NO WARRANTY, to the extent permitted by law.

Written by John Gilmore and Jay Fenlason.

US[keli@ tmp]echo "hello" >test.txt

US[keli@ tmp]tar -cf test.tar test.txt

US[keli@ tmp]md5sum test.txt test.tar 

b1946ac92492d2347c6235b4d2611184  test.txt

40b61cbcd8c4144fa224f40927be2869  test.tar

This is host 2:

U18.04.S225[keli@ tmp]uname -a

Linux smiles-Precision-T7500 4.15.0-42-generic #45-Ubuntu SMP Thu Nov 15 19:32:57 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

U18.04.S225[keli@ tmp]tar --version

tar (GNU tar) 1.29

Copyright (C) 2015 Free Software Foundation, Inc.

License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.

This is free software: you are free to change and redistribute it.

There is NO WARRANTY, to the extent permitted by law.

Written by John Gilmore and Jay Fenlason.

U18.04.S225[keli@ tmp]tar -cf test.tar test.txt

U18.04.S225[keli@ tmp]md5sum test.txt test.tar 

b1946ac92492d2347c6235b4d2611184  test.txt

dae6737634e6f1c381fae91fae8e1e04  test.tar

It's obvious that the test.txt file have same md5 value, but the tar file generate by same file have different md5 values. I am wondering the reason. Is it related to kerner version or tar version?  Any help? Thanks.