Wrapper Scripts in Linux

With kernel 2.2, you can associate not only apps but also define

actions for file-types by writing your own scripts

Last month we learnt that with kernel 2.2 in Red Hat, Linuxprovides

"miscellaneous binary format support", that is, you can use

file-type associations to load associated apps. You can add support for

countless binary formats by just telling the kernel which interpreter to

run for which binary. For example, Java runtime for Java class files.

Though this can be done directly, sometimes you may need to write a

wrapper or a shell script, to tweak things a bit before actually calling

the interpreter. In this article, we’ll write a small wrapper to

associate a file type with its base app. We’ll then take this concept of

wrappers further to associate an action with a file-type.

Wrappers for binary format support

Sometimes, you may want to pass arguments to the interpreter, or the

interpreter may require a filename in a form different from the real

filename (like in Java classes, for x.class the interpreter needs to be

called with x, not x.class). In that case, you’ll have to write a small

wrapper, and specify this wrapper as the interpreter while registering.

Suppose you want to register HTML as a format, with Netscape as the

associated program. The trouble is, if you specify Netscape as the

interpreter, you’ll have a new Netscape process for every HTML file that

you open. With something as big as Netscape, running more than two

instances at the same time can make your system crawl. Fortunately,

Netscape can be invoked with an option to open a file in an already

running Netscape process. But we’ll have to write a small wrapper for

that:

#!/bin/sh



# where is netscape


NETSCAPE=/usr/bin/netscape


echo "Opening that in Netscape..."


FILE=$1


$NETSCAPE-remote "openURL (file:$FILE)"


if < $? -ne 0 >; then


echo "Netscape is not already running. Starting it..."


$NETSCAPE $FILE


fi

Save that in, say, /usr/bin/netscape_ wrapper and make it executable.

What it does is that it first tries to see if Netscape is already running.

If it is, it opens the file in Netscape, otherwise starts a new Netscape

process. Now we can register HTML with the wrapper as the interpreter:

% cd /proc/sys/fs/binfmt_misc



% echo ":html:E::html::/usr/bin/netscape_wrapper:" > register

And you’re done!

You can enable or disable particular binary formats, or the entire

feature. To disable format foo, echo 0 to /proc/sys/fs/binfmt_misc/foo,

and to enable it again echo 1 to it.

To remove an entry, echo -1 to it. To remove all entries, echo -1 to

/proc/sys/fs/binfmt_misc/status.

Making it rip

To register all these file types every time at bootup, you’ll have to

put all these commands in the file /etc/rc.d/rc.local (this may be

different on non-Red Hat systems). This file runs at every bootup after

all other init scripts.

For experts and adventurous

It should be apparent by now, that this ability to associate programs

with files can be used for much more than just recognizing various binary

formats. It can be used to associate an action with every file type. And

that is a very powerful idea.

For example, what is it that you usually want to do with a LaTeX file?

Run latex on it to get a DVI file, and then view that in xdvi. What if we

write a wrapper to do these two steps and register LaTeX files to use this

wrapper? Two cumbersome steps are replaced by one short invocation, the

filename itself. It doesn’t get any simpler. At the same time, this

doesn’t tie the user in with those two steps. The file can always be

used normally, just like any non-recognized file.