Unix question du jour: tar [Matthew's Journal]

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Is it a bug that while (GNU) tar -cjf foo.tar.bz2 creates foo.tar.bz2, tar -cjf user@host.name.fqdn:/path/to/foo.tar.bz2 won't create /path/to/foo.tar.bz2 on the target host, instead failing with ENOENT, or merely a bug in the man-page that it doesn't mention this fact?

ETA: Please Switch NoBoydie, those of you to whom that means something...

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There are 22 comments on this entry. (Reply.)

No. It's not a bug, it's missing functionality. Try tar cjf - ... | ssh user@host.name cat > /path/to/wherever.tar.bz2...

touch /path/to/foo.tar.bz2 before invoking tar works too, AFAICT

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Wait, you mean that tar will connect to a remote host by unspecified means, and is only foiled by the nonexistence of the file on that host? Or have I misunderstood?

I would say that the failure to transparently create a file on a remote host can only be seen as a bug in tar if you're also willing to see it as a bug in absolutely all other Unix programs which take pathnames on their command lines (from cat to oowriter), i.e. essentially extend the Unix pathname format to include a syntax for specifying remote file access, and arrange to be able to support that syntax universally (err, somehow - probably by putting more cleverness in libc).

VMS does do this, but Unix never has done, and it would seem odd for tar in particular to be singled out if you're unhappy with the latter.

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GNU tar uses rsh or something else if you specify e.g. --rsh-command=/usr/bin/ssh

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Hmm, yes, I now see --rsh-command in the man page, but oddly I can still find no text about what it's for or what syntax you should use to invoke it. Presumably that's just because GNU in their unbounded wisdom put all that in the info file instead :-(

I still say it's odd to single out tar from all the other Unix programs that might usefully do transparent remote file access, but now it's odd of GNU rather than of you :-)

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Whatever happened to "Make each program do one thing well", anyway?

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Berkley.

"cat came back from Berkeley waving flags" — Rob Pike

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The //host/path syntax was used by the Unix variant Domain/OS, which is where the special get-out for // in POSIX comes from. I am not sure if this was the basis of the URL syntax, nor am I sure how it relates to the Windows Universal Naming Convention of \\host\share\path.

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I always preferred the Newcastle Connection, which used the /../host/user/path syntax. I only ever saw it running on PNX (Perq), and 4.2Nix (Whitechapel) hosts.

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Mmm. I have one machine which can talk Newcastle Connection, which is a bit useless really...

(ICL 3300, aka the PERQ3a.)

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Back when I collected PERQs (yes, really), a PERQ3 was the only one I never managed to acquire. Not that it was morally a PERQ, though, what with not having bit-sliced CPU, or accessible microprogramming.

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Yeah, I have a PERQ1 with the 16K microcode store too. I really must get round to seeing if it still boots; it's been a decade since I last tried it I think...

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OK, apologies, I see it is meant to do this (with --rsh-command=/usr/bin/ssh). Shouldn't do technical stuff before breakfast.

Doesn't work for me, though (Debian squeeze): and the big advantage of piping through ssh is that you know exactly what it's doing...

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I confess I'm mildly surprised it doesn't create the path user@host.name.fqdn:/path/to/foo.tar.bz2 on the local host. tar has moved on since I last read the man page.

Edited Date: 2011-03-01 08:41 am (UTC)

Isn't that just because the directory user@host.name.fqdn:/path/to doesn't already exist?

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It works just fine, at least with squeeze or better at each end.

richard@araminta:~$ ssh deodand ls -l /home/richard/foo.tar.bz2
ls: cannot access /home/richard/foo.tar.bz2: No such file or directory
richard@araminta:~$ tar cjf deodand:/home/richard/foo.tar.bz2 foo.java
richard@araminta:~$

With lenny at the receiving end it just hangs for me (and I don't have time to figure out why right now).

Perhaps you've omitted some aspect of what you're trying to do from your description.

The remote end is an OSX box. strace showed that tar calls /usr/sbin/rmt on the far end, and that complains that the file doesn't exist (AFAICT from trying to see what was going on by looking at strace output at 2am).

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chymax$ uname -s
Darwin
chymax$ ls -l /usr/sbin/rmt
ls: /usr/sbin/rmt: No such file or directory

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Huh. This is a colleague's machine I'm copying to, so I'm not sure what they've installed on it, but...

hill60:~ matthew$ uname
Darwin
hill60:~ matthew$ ls -l /usr/sbin/rmt 
-r-xr-xr-x  1 root  wheel  38736 Dec  9  2008 /usr/sbin/rmt

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Perhaps they have a later OSX. At any rate, using a (correctly quoted!) version of 3c66b's rune seems like the answer.

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Wow. rmt. There's a blast from the past. (And indeed from OSX's past, as it turns out - it looks like it was in 10.5 but doesn't seem to be in 10.6.)

I like its manpage:

BUGS
People should be discouraged from using this for a remote file access protocol.

(S)

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Wow. I don't think I've ever seen the rmt syntax in the wild. (And certainly not combined with bzip2.)

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