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At my company, we download a local development database snapshot as a db.dump.tar.gz file. The compression makes sense, but the tarball only contains a single file (db.dump).
Is there any point to archiving a single file, or is .tar.gz just such a common idiom? Why not just .gz?
Advantages of using .tar.gz instead of .gz are that
tar stores more meta-data (UNIX permissions etc.) than gzip.The overhead of using tar is also very small.
If not really needed, I still do not recommend to tar a single file.
There are many useful tools which can access compressed single files directly (such as zcat, zgrep etc. - also existing for bzip2 and xz).
.gz, my first instinct is to tar -zxf foo.gz. Remembering that gzip is even a command takes a few more seconds.
Apr 20, 2016 at 14:26
z (or the - for that matter), most modern tars will automatically detect the file needs to be decompressed.
gzip will store the original file name and time stamp. You can use the -N option when decompressing to restore them.
Apr 22, 2016 at 4:56
You are actually asking only half of the question. The other question being, "Why would I compress a tar file with gzip?". And the answer is not just that gzip makes the file smaller (in most cases):
tar:
gzip:
With only tar you cannot be sure your data was not corrupted. With only gzip you cannot restore user/group ID, modification time and possible not the original filename.
The combination is more powerful than the individual commmands/formats provides, because they complement each other's features.
tar wikipedia page, I misunderstood the description to mean that the checksum was for the whole file.
Apr 21, 2016 at 14:23
There is a quite big advantage to using only-gzipped text files - the contents can be directly accessed with command-line tools like less, zgrep, zcat.
I would say it's likely that the people just don't realise they can use gzip/bzip2/xz without tar. Possibly because they come from a DOS/Windows background where it is normal for compression and archiving to be integrated in a single format (ZIP, RAR, etc).
While there may be slight advantages to using tar in some situations due to the storage of metadata or the ability to add extra files, there are also disadvantages. With a plain gzip/bzip2/xz file you can decompress it and pipe the decompressed data straight to another tool (such as your database) without ever having to store the decompressed data as a file on disk. With a tarball this is harder.
tgz extension. However, the OP's case uses tar.gz - and if these hypothetical ex-Win/DOS users are anything like I was, the first thing they say when looking at such a file is: 'Why does it have 2 extensions?'. Then they google it and quickly get the answer, which specifically explains that tar and compression are distinct. ;-)
Apr 20, 2016 at 20:42
There is an important difference that could make using tar important under some circumstances: Besides the "metadata" that @jofel mentioned in his answer, tar records the filename in the archive. When you extract it, you get the original filename regardless of what the archive is called.
In your case the tar archive and the file it contains have the related names db.dump.tar.gz and db.tar, but suppose you rename the tar file to 20-Apr-16.dump.tgz, or whatever. Untar this with tar xvfz, and you get db.dump. For comparison, unzip 20-Apr-16.dump.gz and you've got 20-Apr-16.dump. (Edit: as pointed out in the comments, gzip also makes a record of the filename; but it's not normally used when unzipping). A tar archive can also contain a relative pathname that puts the extracted file in a subdirectory.
Your use case will dictate whether this kind of filename persistence is needed, or even wanted, or is actually undesirable. But certainly, regardless of compression, a tar archive travels differently from a regular file.
gzip --name explicitly when decompressing), but you don't have to use tar to get filename persistence.
In addition to all the other answers, I've recently struck a scripting situation where only one file was expected, but a previous employee wrote the scripts with the possibility of more than one file being generated. So files were tarred and bzipped, then transferred, and expanded.
When the process grew to the point it made a 4.3 GB file, it rolled over and made a .dump.001 file in addition to a .dump file. All the scripts just kept working.
That is proactive sysadmin laziness defined!
I would tar a single file, to copy it preserving the timestamp (which is easily overlooked in downloads). File permissions and ownership are less important: download is a term that applies to systems which are not well integrated.
Whether tar'd or not, it is standard practice to compress the file to make downloads faster — and avoid running out of diskspace.
Tar is especially useful for multiple files not written to a formal file system, it always has been. If for some reason there is on occasion, only 1 file to be written it is of no real consequence. I can dd my .tar.gz directly to /dev/sdx without regard to partition or file system. It may as well be tape.
It is generally done because the script or process has been copied from heritage code. Of course there is no need to tar if there is only one file, but it leaves room for enhancement to multiple files......
tar -zxvf. But for those who look at the file name and see it doesn't have.tgzas extension, it is perfectly fine to gzip the db dump file. Since I don't know the compression algorithms in detail, I am not sure if tar makes any compression on sparse files like db dump, but for plain text files, direct gzip of the file has a very tiny size advantage over taring first and gzip ing the file.tar.gzto be superior to most other common methods. I recall it was superior to just.tarbut cannot remember if it was better than just.gz. Ironically Window's.cabformat was the best of the methods I tried, which was very unexpected.taris not a compression algorithm, it's an archiving format