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I'm trying to run a set of commands on a remote machine, that includes an if statement. I'm using this to shutdown a list of kvm instances. The first line should be fine, but could be prettier, rest needs some help. 

ssh root@kvmsrv 'virsh shutdown $host 2> /dev/null; virsh destroy $host 2> /dev/null; sleep 2; virsh undefine $host 2> /dev/null' # Also needs to be run on remote machine # to disconnect iscsi iscsiadm -m session | grep $host if [ $? == 0 ]; then iscsiadm -m node -T $stserver.$host -u fi # then on local machine tid=`ssh root@storage1 'cat /proc/net/iet/volume' | grep $host | head -1 | awk '{print $1}' | awk -F: '{print $2}'` if [ $tid ]; then echo "Deleting tid:$tid from $stserver." ssh root@$stserver "ietadm --op delete --tid=$tid" fi

I guess I could do the same for the second portion, just separate it all with ; but if anyone goes to read my code later on they'd probably be scratching their heads...

Is there a way to format all this so its readable and still makes sense and have all the remote commands execute properly?

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4 Answers 4

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  1. Put your long and complex sequence of commands in its own shell script, let's call it virsh-shutdown-remote.sh

  2. Run ssh and use input redirection to run the script:

    ssh root@kvmsrv < virsh-shutdown-remote.sh

Putting the commands in a script is a good idea in any case.

As an extra tip, instead of this:

iscsiadm -m session | grep $host if [ $? == 0 ]; then iscsiadm -m node -T $stserver.$host -u fi

You can write the same thing simpler and shorter on a single line:

iscsiadm -m session | grep $host && iscsiadm -m node -T $stserver.$host -u
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You can use HEREDOCS, e.g:

ssh root@kvmsrv <<EOF virsh shutdown $host 2> /dev/null virsh destroy $host 2> /dev/null sleep 2 virsh undefine $host 2> /dev/null # to disconnect iscsi iscsiadm -m session | grep $host if [ \$? == 0 ] ; then iscsiadm -m node -T $stserver.$host -u fi EOF

Please be aware that I escaped $? as otherwise it would be evaluated by your local shell and not from your remote shell. As $host seems to be defined locally you won't have to escape it.

Just a small comment though you didn't ask for it. You can remotely run virsh commands, without the need to explicitly ssh into the host, e.g:

virsh -c qemu+ssh://root@kvmsrv/system destroy host

will work. You also don't need ssh as libvirt allows you to authenticate with ssl certs insteads.

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    You forgot to escape his other variables, such as \$host, \$stserver. In fact it's better to write the whole thing as ssh root@kvmsrv <<"EOF" in which case you don't need to escape the $-s inside, and just use his script verbatim. Commented Jun 17, 2012 at 8:12
  • @janos as i said $host seems to be defined locally - the script never defines/get it somewhere. The same goes for $stserver. But i agree "EOF" is often better than escaping. Commented Jun 17, 2012 at 15:55
  • Notice that he single-quoted his very first command, so at least there $host is defined on the remote side. In the end he uses $host again in a local context, but it would be wrong to assume they are the same. As per his second block of code, from the context I think it's intended to run remotely, using variables defined on the remote, but I agree this is not at all 100% clear. Commented Jun 18, 2012 at 4:56
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Maybe just add commentary to explain things that (should be) obvious? I do this regularly in scripts to remind my own ignorant butt what I was up to when I wrote the [censored] thing in the first place.

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Well, the simple answer is that it is actually possible to rewrite ssh root@kvmsrv 'cmd1; cmd2; cmd3' as:

ssh root@kvmsrv ' cmd1 cmd2 cmd3 '

The only thing to remember is that single quotes "inside" single quotes should be expressed as: '\''. Examples:

ssh localhost ' echo hi whoami ls -ld / var="'\''" echo "$var" if [ $? == 0 ]; then echo "escaping single quote worked" fi ' # get single quote from file and print it echo "'" > sqfile ssh localhost ' #set -xv echo hi cat sqfile var="$(cat sqfile)" echo "$var" ' # pass a single quote to the remote host and print it # while keeping the outer single quotes for the ssh command escsquote="'\''" squote="'" squote="'${squote//\'/${escsquote}}'" # bash ssh localhost 'sh -c '\'' #set -xv echo hi echo "$1" '\''' _ "$squote"
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