A question about the .tar.gz file
 

In the current directory,there are two files:
a.tar.gz b.txt
Now I want to directly add b.txt to a.tar.gz
How can I do that?

Note that you can't release the a.tat.gz then create a new a.tar.gz file.

Use tar's --append option.

But a.tar.gz is a gz file.I don't think any tar options is suitable

.gz means the file was compressed by GZIP
.tar means that N files were archived into one file by TAR
TAR includes commands to do the GZIP (and GUNZIP) all in one command

To do your thing, first use gunzip, which will give you a .tar file. Then you can do the append option suggested by Nylex.

See for more information on tar --append
http://www.gnu.org/software/tar/manu...nd.html#append
that is if you haven't read it.