[OT] run command via ssh - problem

[Dan Track]

Hi,

I'm running a command like this:

for i in server1 server2;do ssh root@$i "`hostname`";done.

However the hostname command always outputs the hostname of the server
that the above command is run from. I'd like to know how to run this
hostname command so that it actually runs on server 1, server2 etc..

Thanks
Dan

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[Dan Track]

On Wed, Nov 4, 2009 at 1:10 PM, Dan Track <dan.track@...> wrote:

> Hi,
>
> I'm running a command like this:
>
> for i in server1 server2;do ssh root@$i "`hostname`";done.
>
> However the hostname command always outputs the hostname of the server
> that the above command is run from. I'd like to know how to run this
> hostname command so that it actually runs on server 1, server2 etc..
>
> Thanks
> Dan
>

Sorry just to add the actual script was like this:

for i in server1 server2;do ssh root@$i "DNSNAME=\"basename
\`hostname\`\";echo $DNSNAME";done

Thanks
Dan

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[Joachim Backes]

On 11/04/2009 02:10 PM, Dan Track wrote:

> Hi,
>
> I'm running a command like this:
>
> for i in server1 server2;do ssh root@$i "`hostname`";done.
>
> However the hostname command always outputs the hostname of the server
> that the above command is run from. I'd like to know how to run this
> hostname command so that it actually runs on server 1, server2 etc..
>
> Thanks
> Dan
>

Use the following
for i in server1 server2;do ssh root@$i '`hostname`';done.


Explanation: `hostname`, or $(hostname), is already evaluated on the
source host by your shell (even inside of "...") , not on the target host.
--

Joachim Backes <joachim.backes@...>

http://www.rhrk.uni-kl.de/~backes




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[Bryn M. Reeves-2]

On Wed, 2009-11-04 at 13:10 +0000, Dan Track wrote:
> Hi,
>
> I'm running a command like this:
>
> for i in server1 server2;do ssh root@$i "`hostname`";done.
>
> However the hostname command always outputs the hostname of the server
> that the above command is run from. I'd like to know how to run this
> hostname command so that it actually runs on server 1, server2 etc..

Just remove the backticks and quotes around hostname?

for i in server1 server2;do ssh root@$i hostname;done

The backticks tell the shell (on your machine) to run the command inside
the backticks and replace that part of the command line with the output
of the command so you actually end up with a command line like:

for i in server1 server2;do ssh root@$i mylocalhostname;done.

Regards,
Bryn

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[Dan Track]

On Wed, Nov 4, 2009 at 1:14 PM, Joachim Backes
<joachim.backes@...> wrote:

> On 11/04/2009 02:10 PM, Dan Track wrote:
>>
>> Hi,
>>
>> I'm running a command like this:
>>
>> for i in server1 server2;do ssh root@$i "`hostname`";done.
>>
>> However the hostname command always outputs the hostname of the server
>> that the above command is run from. I'd like to know how to run this
>> hostname command so that it actually runs on server 1, server2 etc..
>>
>> Thanks
>> Dan
>>
>
> Use the following
> for i in server1 server2;do ssh root@$i '`hostname`';done.
>
>
> Explanation: `hostname`, or $(hostname), is already evaluated on the source
> host by your shell (even inside of "...") , not on the target host.
> --
>
> Joachim Backes <joachim.backes@...>
>
> http://www.rhrk.uni-kl.de/~backes
>

Hi,

Thanks for that, any thoughts on how it fits in with my script:

for i in server1 server2;do ssh root@$i
"DNSNAME=\"basename\`hostname\`\";echo $DNSNAME";done

Thanks
Dan

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[Bryn M. Reeves-2]

On Wed, 2009-11-04 at 13:13 +0000, Dan Track wrote:

> On Wed, Nov 4, 2009 at 1:10 PM, Dan Track <dan.track@...> wrote:
> > Hi,
> >
> > I'm running a command like this:
> >
> > for i in server1 server2;do ssh root@$i "`hostname`";done.
> >
> > However the hostname command always outputs the hostname of the server
> > that the above command is run from. I'd like to know how to run this
> > hostname command so that it actually runs on server 1, server2 etc..
> >
> > Thanks
> > Dan
> >
>
> Sorry just to add the actual script was like this:
>
> for i in server1 server2;do ssh root@$i "DNSNAME=\"basename
> \`hostname\`\";echo $DNSNAME";done

Not sure why you're setting a variable here but to have "basename" run
as a command and assign the output to DNSNAME you need to have basename
inside a pair of backticks too.

You'll then hit another problem because you want to have nested
backticks (one pair for basename and another for hostname). Bash
supports '$()' as an alternative to backticks that does allow nesting -
writing $(hostname) is equivalent to `hostname` and allows you to write
$(basename $(hostname)).

I'm not sure basename is going to do what you want here though - are you
looking for the short host name or the domain name? The basename command
separates components of a path based on the '/' (or whatever the system
defined path separator is). E.g.:

$ DNS=$(basename $(hostname))
$ echo $DNS
breeves.fab.redhat.com

If you just want the short hostname you can pass -s to hostname:

$ ssh pe1950-1.gsslab hostname -s
pe1950-1

Or the domain with -d:

$ ssh pe1950-1.gsslab hostname -d
gsslab.fab.redhat.com

Have a look at the man page for hostname for more options.

Regards,
Bryn.


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[Todd Zullinger]

Dan Track wrote:
> Thanks for that, any thoughts on how it fits in with my script:
>
> for i in server1 server2;do ssh root@$i
> "DNSNAME=\"basename\`hostname\`\";echo $DNSNAME";done

What are you trying to achieve with DNSNAME=\"basename\`hostname\`\";
anyway?  If you want the domainname, dnsdomainname or hostname -d seem
better.  The basename command is for finding the base name of a file.

If the domainname is what you want, something like this would work:

for i in server{1..2}; do
    ssh root@$i 'DNSNAME=$(hostname -d); echo $DNSNAME'
done

I presume you have other steps in between DNSNAME=... and the echo,
otherwise you would just use ssh root@$i hostname -d.

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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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And there it is in neon: Liquor, beer, and wine.



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[Dan Track]

On Wed, Nov 4, 2009 at 1:33 PM, Bryn M. Reeves <bmr@...> wrote:

> On Wed, 2009-11-04 at 13:13 +0000, Dan Track wrote:
>> On Wed, Nov 4, 2009 at 1:10 PM, Dan Track <dan.track@...> wrote:
>> > Hi,
>> >
>> > I'm running a command like this:
>> >
>> > for i in server1 server2;do ssh root@$i "`hostname`";done.
>> >
>> > However the hostname command always outputs the hostname of the server
>> > that the above command is run from. I'd like to know how to run this
>> > hostname command so that it actually runs on server 1, server2 etc..
>> >
>> > Thanks
>> > Dan
>> >
>>
>> Sorry just to add the actual script was like this:
>>
>> for i in server1 server2;do ssh root@$i "DNSNAME=\"basename
>> \`hostname\`\";echo $DNSNAME";done
>
> Not sure why you're setting a variable here but to have "basename" run
> as a command and assign the output to DNSNAME you need to have basename
> inside a pair of backticks too.
>
> You'll then hit another problem because you want to have nested
> backticks (one pair for basename and another for hostname). Bash
> supports '$()' as an alternative to backticks that does allow nesting -
> writing $(hostname) is equivalent to `hostname` and allows you to write
> $(basename $(hostname)).
>
> I'm not sure basename is going to do what you want here though - are you
> looking for the short host name or the domain name? The basename command
> separates components of a path based on the '/' (or whatever the system
> defined path separator is). E.g.:
>
> $ DNS=$(basename $(hostname))
> $ echo $DNS
> breeves.fab.redhat.com
>
> If you just want the short hostname you can pass -s to hostname:
>
> $ ssh pe1950-1.gsslab hostname -s
> pe1950-1
>
> Or the domain with -d:
>
> $ ssh pe1950-1.gsslab hostname -d
> gsslab.fab.redhat.com
>
> Have a look at the man page for hostname for more options.
>
> Regards,
> Bryn.

Hi Bryn,

Many thanks. I tried hostname -s but I keep getting the following:

hostname: Host name lookup failure

This may be because the hostname's are short already e.g just
"server1" instead of "server1.example.com"

I've updated teh script to your recommendations but I still get the
local hosts hostname in teh output instead of the remote servers
hostname. Any other thoughts?

I now run the following:

for i in server1 server2;do ssh root@$i "DNSNAME=$(basename
$(hostname)$);echo $DNSNAME";done

Thanks
Dan

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[Bryn M. Reeves-2]

On Wed, 2009-11-04 at 14:14 +0100, Joachim Backes wrote:

> On 11/04/2009 02:10 PM, Dan Track wrote:
> > Hi,
> >
> > I'm running a command like this:
> >
> > for i in server1 server2;do ssh root@$i "`hostname`";done.
> >
> > However the hostname command always outputs the hostname of the server
> > that the above command is run from. I'd like to know how to run this
> > hostname command so that it actually runs on server 1, server2 etc..
> >
> > Thanks
> > Dan
> >
>
> Use the following
> for i in server1 server2;do ssh root@$i '`hostname`';done.

That will try to execute the name of the remote host as though it was a
command (backticks expand on the remote host and the output of the
hostname command is used as the command line).

$ ssh abox '`hostname`'
bash: abox.example.com: command not found

Bryn.


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[Bryn M. Reeves-2]

On Wed, 2009-11-04 at 13:32 +0000, Dan Track wrote:
> Hi Bryn,
>
> Many thanks. I tried hostname -s but I keep getting the following:
>
> hostname: Host name lookup failure

Possibly your resolver on the servers is not configured to search its
own local domain. Add a line like this to /etc/resolv.conf:

search mylocaldomain.com

Or, if you configure the resolver via dhcp add a directive on the server
to pass this over to clients.

> This may be because the hostname's are short already e.g just
> "server1" instead of "server1.example.com"
>
> I've updated teh script to your recommendations but I still get the
> local hosts hostname in teh output instead of the remote servers
> hostname. Any other thoughts?
>
> I now run the following:
>
> for i in server1 server2;do ssh root@$i "DNSNAME=$(basename
> $(hostname)$);echo $DNSNAME";done

You need to use single quotes instead of double quotes - see the rules
in the bash man page about quote expansion. A single quoted string is
not subject to any expansion by the shell on the client machine but a
double quoted string will be expanded on the client before the ssh
command is executed.

$ ssh abox 'DNSNAME=$(basename $(hostname));echo $DNSNAME'
abox.example.com

I still don't think that basename will do what you want here...

Regards,
Bryn.


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[Dan Track]

On Wed, Nov 4, 2009 at 2:29 PM, Bryn M. Reeves <bmr@...> wrote:

> On Wed, 2009-11-04 at 13:32 +0000, Dan Track wrote:
>> Hi Bryn,
>>
>> Many thanks. I tried hostname -s but I keep getting the following:
>>
>> hostname: Host name lookup failure
>
> Possibly your resolver on the servers is not configured to search its
> own local domain. Add a line like this to /etc/resolv.conf:
>
> search mylocaldomain.com
>
> Or, if you configure the resolver via dhcp add a directive on the server
> to pass this over to clients.
>
>> This may be because the hostname's are short already e.g just
>> "server1" instead of "server1.example.com"
>>
>> I've updated teh script to your recommendations but I still get the
>> local hosts hostname in teh output instead of the remote servers
>> hostname. Any other thoughts?
>>
>> I now run the following:
>>
>> for i in server1 server2;do ssh root@$i "DNSNAME=$(basename
>> $(hostname)$);echo $DNSNAME";done
>
> You need to use single quotes instead of double quotes - see the rules
> in the bash man page about quote expansion. A single quoted string is
> not subject to any expansion by the shell on the client machine but a
> double quoted string will be expanded on the client before the ssh
> command is executed.
>
> $ ssh abox 'DNSNAME=$(basename $(hostname));echo $DNSNAME'
> abox.example.com
>
> I still don't think that basename will do what you want here...
>
> Regards,
> Bryn.

Great that single quote worked. It's nwo returning the correct result.
Many thanks. The basename command works well.

Thanks
Dan

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[Todd Zullinger]

Dan Track wrote:
> The basename command works well.

At what?  Unless your hostname contains a /, I don't see how basename
would do anything the way you are using it.

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[Cameron Simpson]

On 04Nov2009 14:56, Dan Track <dan.track@...> wrote:
| On Wed, Nov 4, 2009 at 2:29 PM, Bryn M. Reeves <bmr@...> wrote:
| >> for i in server1 server2;do ssh root@$i "DNSNAME=$(basename
| >> $(hostname)$);echo $DNSNAME";done
| >
| > You need to use single quotes instead of double quotes - see the rules
| > in the bash man page about quote expansion. A single quoted string is
| > not subject to any expansion by the shell on the client machine but a
| > double quoted string will be expanded on the client before the ssh
| > command is executed.
| >
| > $ ssh abox 'DNSNAME=$(basename $(hostname));echo $DNSNAME'
| > abox.example.com
| >
| > I still don't think that basename will do what you want here...
|
| Great that single quote worked. It's nwo returning the correct result.
| Many thanks. The basename command works well.

I'm echoing from both Todd and Bryn's remarks about basename being useless
here (try replacing $(basename $(hostname)) with just $(hostname) and see the
script do exactly the sam thing).

However, I want to point out that because ssh hands its
arguments-joined-together-with-spaces to the shell at the far end,
you can do a lot of testing like this:

  sh -c 'some shell command...'

and when happy, run it remotely thus:

  ssh remote-host 'some shell command...'

Also, to see how it is looking while debugging, you can do this:

  ( set -x
    sh -c 'some shell command...'
  )

or on one line if you're command line editing:

  ( set -x; sh -c 'some shell command...' )

to see how the 'some shell command...' bit is prepared (since in your
real world example that's got double quotes and stuff), and you can also
go:

  sh -xc 'some shell command...'

to see exactly what the shell is doing with the string you're giving it.

All of this before throwing over the net with ssh.

Cheers,
--
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http://www.cskk.ezoshosting.com/cs/

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[Konstantin Svist-2]

On 11/04/2009 05:10 AM, Dan Track wrote:

> Hi,
>
> I'm running a command like this:
>
> for i in server1 server2;do ssh root@$i "`hostname`";done.
>
> However the hostname command always outputs the hostname of the server
> that the above command is run from. I'd like to know how to run this
> hostname command so that it actually runs on server 1, server2 etc..
>
> Thanks
> Dan
>
>    

Content inside double quotes is evaluated by shell, so backtick
expansion happens on localhost.
Use single quotes, instead, like so:

for i in server1 server2;do ssh root@$i '`hostname`';done


HTH

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[Bill Davidsen]

Dan Track wrote:

> On Wed, Nov 4, 2009 at 1:33 PM, Bryn M. Reeves <bmr@...> wrote:
>> On Wed, 2009-11-04 at 13:13 +0000, Dan Track wrote:
>>> On Wed, Nov 4, 2009 at 1:10 PM, Dan Track <dan.track@...> wrote:
>>>> Hi,
>>>>
>>>> I'm running a command like this:
>>>>
>>>> for i in server1 server2;do ssh root@$i "`hostname`";done.
>>>>
>>>> However the hostname command always outputs the hostname of the server
>>>> that the above command is run from. I'd like to know how to run this
>>>> hostname command so that it actually runs on server 1, server2 etc..
>>>>
>>>> Thanks
>>>> Dan
>>>>
>>> Sorry just to add the actual script was like this:
>>>
>>> for i in server1 server2;do ssh root@$i "DNSNAME=\"basename
>>> \`hostname\`\";echo $DNSNAME";done
>> Not sure why you're setting a variable here but to have "basename" run
>> as a command and assign the output to DNSNAME you need to have basename
>> inside a pair of backticks too.
>>
>> You'll then hit another problem because you want to have nested
>> backticks (one pair for basename and another for hostname). Bash
>> supports '$()' as an alternative to backticks that does allow nesting -
>> writing $(hostname) is equivalent to `hostname` and allows you to write
>> $(basename $(hostname)).
>>
>> I'm not sure basename is going to do what you want here though - are you
>> looking for the short host name or the domain name? The basename command
>> separates components of a path based on the '/' (or whatever the system
>> defined path separator is). E.g.:
>>
>> $ DNS=$(basename $(hostname))
>> $ echo $DNS
>> breeves.fab.redhat.com
>>
>> If you just want the short hostname you can pass -s to hostname:
>>
>> $ ssh pe1950-1.gsslab hostname -s
>> pe1950-1
>>
>> Or the domain with -d:
>>
>> $ ssh pe1950-1.gsslab hostname -d
>> gsslab.fab.redhat.com
>>
>> Have a look at the man page for hostname for more options.
>>
>> Regards,
>> Bryn.
>
> Hi Bryn,
>
> Many thanks. I tried hostname -s but I keep getting the following:
>
> hostname: Host name lookup failure
>
> This may be because the hostname's are short already e.g just
> "server1" instead of "server1.example.com"
>
> I've updated teh script to your recommendations but I still get the
> local hosts hostname in teh output instead of the remote servers
> hostname. Any other thoughts?
>
> I now run the following:
>
> for i in server1 server2;do ssh root@$i "DNSNAME=$(basename
> $(hostname)$);echo $DNSNAME";done

              A        A
              |        |
        what is that?  |
                       |
           need a backslash before the $
>
> Thanks
> Dan
>


--
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   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

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[Rick Stevens-3]

Bryn M. Reeves wrote:

> On Wed, 2009-11-04 at 13:32 +0000, Dan Track wrote:
>> Hi Bryn,
>>
>> Many thanks. I tried hostname -s but I keep getting the following:
>>
>> hostname: Host name lookup failure
>
> Possibly your resolver on the servers is not configured to search its
> own local domain. Add a line like this to /etc/resolv.conf:
>
> search mylocaldomain.com
>
> Or, if you configure the resolver via dhcp add a directive on the server
> to pass this over to clients.
>
>> This may be because the hostname's are short already e.g just
>> "server1" instead of "server1.example.com"
>>
>> I've updated teh script to your recommendations but I still get the
>> local hosts hostname in teh output instead of the remote servers
>> hostname. Any other thoughts?
>>
>> I now run the following:
>>
>> for i in server1 server2;do ssh root@$i "DNSNAME=$(basename
>> $(hostname)$);echo $DNSNAME";done
>
> You need to use single quotes instead of double quotes - see the rules
> in the bash man page about quote expansion. A single quoted string is
> not subject to any expansion by the shell on the client machine but a
> double quoted string will be expanded on the client before the ssh
> command is executed.
>
> $ ssh abox 'DNSNAME=$(basename $(hostname));echo $DNSNAME'
> abox.example.com
>
> I still don't think that basename will do what you want here...

It's not quotes, it's the backtick.  Don't backtick the command, just

        for i in server1 server2; do ssh root@$i hostname; done

Example:

        [root@prophead ~]# for i in bigdog golem3; do ssh root@$i hostname; done
        bigdog.hci.com
        golem3.hci.com

In the OP's case, the backtick is being invoked and therefore the host
name is being passed as the command to execute on the remote machine:

        [root@prophead ~]# for i in bigdog golem3; do ssh root@$i "`hostname`";
done
        bash: prophead.hci.com: command not found
        bash: prophead.hci.com: command not found

See?
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[Bryn M. Reeves-2]

On 11/04/2009 11:39 PM, Rick Stevens wrote:

> Bryn M. Reeves wrote:
>> On Wed, 2009-11-04 at 13:32 +0000, Dan Track wrote:
>>> Hi Bryn,
>>>
>>> Many thanks. I tried hostname -s but I keep getting the following:
>>>
>>> hostname: Host name lookup failure
>>
>> Possibly your resolver on the servers is not configured to search its
>> own local domain. Add a line like this to /etc/resolv.conf:
>>
>> search mylocaldomain.com
>>
>> Or, if you configure the resolver via dhcp add a directive on the server
>> to pass this over to clients.
>>
>>> This may be because the hostname's are short already e.g just
>>> "server1" instead of "server1.example.com"
>>>
>>> I've updated teh script to your recommendations but I still get the
>>> local hosts hostname in teh output instead of the remote servers
>>> hostname. Any other thoughts?
>>>
>>> I now run the following:
>>>
>>> for i in server1 server2;do ssh root@$i "DNSNAME=$(basename
>>> $(hostname)$);echo $DNSNAME";done
>>
>> You need to use single quotes instead of double quotes - see the rules
>> in the bash man page about quote expansion. A single quoted string is
>> not subject to any expansion by the shell on the client machine but a
>> double quoted string will be expanded on the client before the ssh
>> command is executed.
>>
>> $ ssh abox 'DNSNAME=$(basename $(hostname));echo $DNSNAME'
>> abox.example.com
>>
>> I still don't think that basename will do what you want here...
>
> It's not quotes, it's the backtick. Don't backtick the command, just

Yes, they are quotes (in my example above). They are there to disable
shell expansion on the client machine and to ensure the whole command
line is sent to the remote system unchanged.

> for i in server1 server2; do ssh root@$i hostname; done

Read the whole thread :)

This was suggested way back but the OP wants to do the nested shell
expansion thing.

Regards,
Bryn.

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[Bryn M. Reeves-2]

On 11/04/2009 09:23 PM, Bill Davidsen wrote:
>> for i in server1 server2;do ssh root@$i "DNSNAME=$(basename
>> $(hostname)$);echo $DNSNAME";done
>              A      A
>              |      |
> what is that?       |

That's the closing paren for the nested shell expansion that begins with
basename on the line above:

$(basename $(hostname))
  ^                    ^

The uselessness of basename in this situation has been pointed out but
the syntax is just fine.

> need a backslash before the $

Not if you use single quotes as was already suggested.

Regards,
Bryn.

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