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12th Feb 2017

Tar's Force-Local Switch

or Why Can't tar Cope With Colons?

The Problem

You're probably here because of one of these two error messages:

tar: Cannot connect to hostname: resolve failed
tar: hostname:filename: Cannot open: Input/output error

Don't worry, it's easily fixed, with a little bit of understanding of Unix history.

tar force-local switch

Back in the days of yore...

Back in the old days of Unix, there was a command called "rsh". If you were around in those days, you will know it; if you were not, well, it was similar to "ssh", but without any of the good bits. Basically, you could set up a file called $HOME/.rhosts, or a global file named /etc/hosts.equiv, and any named host (or user:host combination) would be allowed to log in to your account. They were simpler, more trusting times.

If you want an idea of how old and bad it is note the "BUGS" section on the FreeBSD man page: "The rlogin utility will be replaced by telnet(1) in the near future." So yeah, pretty old. And bad.

Why is this relevant today?

Well, certain implementations of "tar" were able to use this facility. You could extract an archive from a remote machine, for example:

john@saturn$ tar -xvf venus:misc/myfiles.tar
tar: blocksize = 15
x myfiles, 0 bytes, 0 tape blocks
x myfiles/b.txt, 29 bytes, 1 tape blocks
x myfiles/e.txt, 29 bytes, 1 tape blocks
x myfiles/d.txt, 29 bytes, 1 tape blocks
x myfiles/c.txt, 29 bytes, 1 tape blocks
x myfiles/f.txt, 29 bytes, 1 tape blocks
x myfiles/a.txt, 29 bytes, 1 tape blocks

I must admit, I've not yet managed to find a combination of machines with this awful combination, but it must look 95% like the above.

Why do I care?

The main reason that you are likely to care, is if you are trying to deal with a tar file with a colon in its name:

$ tar cf my:files.tar *
tar: Cannot connect to my: resolve failed

Or even (with a massive delay as it tries to connect to Google):

$ tar cf google.com:files.tar *
ssh: connect to host google.com port 22: Network is unreachable
tar: google.com\:files.tar: Cannot open: Input/output error
tar: Error is not recoverable: exiting now

So what tar is trying to do, is connect to a server called "my" and create an archive called "files.tar". In the second case, it's trying to connect to google.com and create an archive there. That is quite unlikely to succeed, unless you happen to have an rlogin account on google.com (hint: Nobody has one of those!)

The Solution

Along with tar's ability to interpret a "hostname:filename" argument as a host/file pair, is its "--force-local" switch, which disables this behaviour.

$ tar --force-local -cf google.com:files.tar *

This tells tar not to try to be so damned clever, and just take the filename it was given, without trying to convert it into a hostname:filename pair.

Even More Gotchas

Before you think you're out of the woods already; a lot of people have bad habits with tar syntax, even though they might not realise it.

Most people will either use "tar cvf foo.tar file1 file2" or "tar -cvf foo.tar file1 file2". tar does nobody any favours by accepting arguments either with or without the "minus". It makes tar syntax harder to think about. See, for example, https://xkcd.com/1168/: XKCD on tar

So we have to think about the switches. Be very precise. You can combine single-character switches (like "cvf") together, but the "f" is really a "-f filename" argument. So the two need to go together.

So it's worth being in the habit of using the minuses, and make sure that you keep the "-f" next to the actual filename that you want tar to use. For example: "tar -cvf --force-local foo.tar file1 file2" is not right, because it will attempt to create an archive file called "--force-local" containing the files foo.tar, file1 and file2.


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og:image credit: Unknown. Please contact me if source is known.

Steve Parker - Linux / DevOps Consultant
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