How to check if a Unix .tar.gz file is a valid file without uncompressing?

you could probably use the gzip -t option to test the files integrity

http://linux.about.com/od/commands/l/blcmdl1_gzip.htm

from: http://unix.ittoolbox.com/groups/technical-functional/shellscript-l/how-to-test-file-integrity-of-targz-1138880

To test the gzip file is not corrupt:

gunzip -t file.tar.gz

To test the tar file inside is not corrupt:

gunzip -c file.tar.gz | tar -t > /dev/null

As part of the backup you could probably just run the latter command and check the value of $? afterwards for a 0 (success) value. If either the tar or the gzip has an issue, $? will have a non zero value.

What about just getting a listing of the tarball and throw away the output, rather than decompressing the file?

tar -tzf my_tar.tar.gz >/dev/null

Edited as per comment. Thanks zrajm!

Edit as per comment. Thanks Frozen Flame! This test in no way implies integrity of the data. Because it was designed as a tape archival utility most implementations of tar will allow multiple copies of the same file!