Controlling SSH identities

SSH has a few strange undocumented “features”. One of which is the way it handles identities via agent and command line. It is possible to specify an identity file to use for ssh via the -i parameter (ssh -i identity_file $host). What the manpage doesn’t mention, is that the specified identity isn’t forced for the connection, it is just added to the list of possible identities.
To make matters worse, ssh tries the identities from the agent first. So if agent forwarding is enabled and valid for the destination the ssh command will never use the identity specified with -i. Why is this “bad”? Because the identity specified may be used for specific tasks with commands linked to them on the destination (e.g. automatic restarts, backups, …)

Sooo, as a solution I whipped up the following function as a workaround in my scripts, I add a function called “xssh”:

I know it looks ugly, if it finds a key in the agent it makes a ssh connection to the current host with agent forwarding deactivated and then executes the ssh parameters passed. If no key is found in the ssh agent it does everything as normal.

Script of the day – clean up stale .ssh/known_hosts

This little script takes an IP or hostname as a parameter, and if there is an offending key in the .ssh/known_hosts it removes it and replaces it with the current valid one useful if you are moving/reinstalling a large amount of servers …

bash: using the content of a variable as variable name

Since the implementation of Arrays in Bash is somewhat lacking compared to higher level programming languages (only one-dimensional), and hash lists require a bit of work to set up, you may run into a situation where you have a small list of key/value pairs that are both variable and you need to store.
There are various solutions for the problem, e.g. creating two arrays (one for the keys, one for the values, and combining them by using the same index values for the entries), or using the functions from the link above to build a hash list. For me the easiest way to solve the problem, if I only have a few variables and don’t want to bloat the code, is to (mis)use declare. declare is intended for setting the type of a variable (constant, array, integer,…), but has the nice side affect that you can use variables in the key name, and you can set the value of the variable.

usage:
declare ${Key}=${Value}

Example:
$File_Config is variable holding the name of a configfile, the content of the file could look like this:
foo:one
bar:three
foobar:bignumber

Snippet:

after the snippet has read the configfile, you can use $Configuration_foo, $Configuration_bar and $Configuration_foobar in your script. The keynames could also have came from a mysql query, array, command line args, …

Grid clicker script

I finished up my “grid clicker” script. it is intended to simplify testing of games where you have to click certain patterns (a grid). Typical facebook games like farmville or funfari are great examples of this. just download the file (here is the sourcecode), execute it, and an eye should appear in your task bar. Double clicking on the icon in your task bar will start the script, just follow the instructions and everything should be fine.

And while I am at it: here is a spreadsheet I started for plants/trees/animals in funfari: Google online spreadsheets

Farmville

Farmvill can be a fun way to pass some time, but if your fields get to be a bit big clicking on every single field can get to be a bit tiresome. I found this autohotkey script in the depths of the internet. It simplifies the process greatly 😉

bash scripting … switching into directory of the script

Sometimes it is useful to switch into the directory of the script e.g. when we need to call or include further files and don’t want to go through the hassle of searching for the script in the file system. Especially when symlinks are involved everything get a bit more interesting. This little snippet switches into the directory of the script, using readlink to dereference symlinks if the script is called via a symlink.

Bash scripting, traps and sleep

Today I ran into any old problem: you have a script that should do something when it recieves a signal (e.g. if someone sends it USR1 it should write to a log/syslog), but the script uses a long sleep because it normally only checks/calculates stuff every x min. If you send it a kill -USR1 $pid it will normally execute the trap AFTER the sleep is done, not so great. I figured of the following solution today: put the sleep in a while loop that checks if the full time was slept, and inside the loop a sleep that sleeps the X seconds remaing in the background followed by a wait.

If the script now recieves a USR1 it can kill the wait, execute the trap and will continue the remaining sleep on the next iteration of the loop.

Reboot Script for Linksys WAP200 access points

Since the Linksys WAP200 has a tendency to hang and not let any users connect anymore, this little script to reboot an accesspoint (webinterface still works fine). Replace user and password with correct values.

rrdtool tricks / tips

There are various different ways to generate rrdtool images. And most people will probably be generating elaborate ones out of perl or php scripts. The ones I am currently generating are built in a shell script (bash) and I was really having a problem with quotes and white space here for a while. If you generate the query and save it in a variable, the quotes go to hell and your white space (e.g. a space in a COMMENT) starts causing problems. my solution may not be the best, but it works without any trouble: Throw the whole command line into a temporary file and then use eval to create the graph. The following is not a complete statement, just a few lines as an example.