== A little script: _sshup_

(It's been a while since [[the last little script LittleScriptsVI]].)

One of the things I do a fair bit around [[here
http://www.cs.toronto.edu/]] is reboot machines. Well, to be more
specific, I reboot machines and then wait for them to come back up so
that I can continue my testing, do more work on them, or verify that
everything is fine. Because I am not crazy I do not do this in the
machine room; I do it from my desk.

Waiting for machines to come up and checking periodically to see if they
have is tedious, repetitive, and boring. Like any lazy sysadmin, I long
ago automated this in a handy little script that I call _sshup_. Since
what I care about is when I can ssh in to newly-rebooted machines, the
script tests to see if a machine is up by checking to see if it can
connect to the machine's ssh port.

(You can do this with netcat; the version of the script that I actually
use is written in rc and uses a different netcat-like utility program
for reasons that don't fit in this margin.)

I generally run _sshup_ in an _xterm_ with [[zIconBeep set
XtermZiconbeep]]; start a new _xterm_, run _sshup_, iconify it, and do
something else until either the iconified _xterm_ notifies me that the
machine is up (because _sshup_ printed something) or I realize that too
much time has passed and go look into what's wrong. It's turned out to
be quite handy.

Here is a version of _sshup_ in Bourne shell:

 #!/bin/sh
 # usage: sshup host
 reachable() { nc -z $1 ssh; }
 while ! reachable $1; do
     sleep 15
 done
 echo $1 UP

(A real version would have some error checking and maybe not hard-code
the sleep interval.)