I have a couple almost 30GB .tar.gz files on a RHEL box. I made copies of them on my PC and tried to get the contents out with iZarc, but it gave me a CRC error.

Is there a way to check the original file on the server to see if the original is truly corrupt too? I dont even know how to extract the data in one foul swoop through a linux command line, or if there's like a test mode i can run against the file that will save me some time before I try to extract the whole thing.

To get the contents out in linux CLI:
This will verify that the file is not corrupted

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so will this only do the verify, without extracting the contents to the disk? I'm really not sure how big this would be extracted so not sure how much disk space i would need for it to work anyway.

You can run

tar -t file.tar.gz to list the contents of the archive. You can also take a md5sum hash of the original and compare it to the new one to see if the hashes match. If they're different then you know the new one is corrupt.

To extract a slew of tar.gz files in a directory just do
tar -xvf *.tar.gz

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I'll try the tar -t file.tar.gz, as that sounds like it will just give me a listing of all the files and tell me if they're able to be extracted or not, if i decide i want to do that.

or did i misunderstand what that command will do?

I reckon you'll need the z flag as well to tell it it's a gzipped file

tar -tzf file.tar.gz

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Thanks Chris. Your command is giving me some output to the screen now. It listed a bunch of files, but then it gave me a

tar: Skipping to next header

and hasn't produced any more output to the screen for a while. I'll let it go till it kicks me back to a prompt, but i'm guessing this may mean my file is corrupted and I won't be able to get all the data out of it?

Here are the last few things it said before taking me back to the prompt

guessing the means the file is corrupted and I wont be able to get everything out of it?

Possibly. Do you have access to the original tar.gz?

Run
On the original and on the one you're working on. See if they match. If they don't then we know it's corrupt.

Those results in my last post were from the original file on the server, not my local copy.

Please forbear while I suggest a few things that might or might not be obvious to you:
Perhaps the .tar.gz filename lies, perhaps it's just a .tar file. Or really a .tar.bz2 file.
If you're comfortable with looking at bits of binary files, I would suggest dumping the first 512 bytes with 'dd bs=512 count=1 < filename.tar.gz | od -t c' and verifying that it really is a gzip file. A gzip file, for example, typically contains the name of the uncompressed file in the first few bytes of the compressed file.

If the file is truly corrupted, recovering data is nearly impossible, since errors in decompression affect every subsequent byte of output -- the decompression basically goes off the rails. That said, for the particular case of FTP ASCII transfers of gzip files, I have had success in recovering some data (see http://bukys.com/services/recovery/), but it basically requires some custom coding, so it's a consulting project not a turnkey operation, so you're probably not interested unless this data is extremely critical to you.

I have also seen cases where a Mac decompressor complains after a file reaches a certain size (perhaps it was at 3GB or 4GB). But if that were the problem, just a "tar ztv" would have shown partial success before failing later. If decompression fails immediately, with no output at all, that suggests to me that the file is corrupted or misidentified from the near the beginning. One more stupid question: if you run the "file" command, e.g "file filename.tar.gz", what does it identify it as?