ugh.book

Shell Programming 155

Hello, class. Today we are going to learn to program in “sh.” The
“sh” shell is a simple, versatile program, but we'll start with a basic
example:

Print the types of all the files in a directory.

(I heard that remark in the back! Those of you who are a little famil-
iar with the shell and bored with this can write “start an X11 client on
a remote machine” for extra credit. In the mean time, shh!)

While we're learning to sh, of course we also want the program we
are writing to be robust, portable, and elegant. I assume you've all
read the appropriate manual pages, so the following should be trivi-
ally obvious:

file *

Very nice, isn’t it? A simple solution for a simple problem; the
matches all the files in the directory. Well, not quite. Files beginning
with a dot are assumed to be uninteresting, and won’t match them.
There probably aren’t any, but since we do want to be robust, we’ll
use “ls” and pass a special flag:

for file in `ls -A`

do

file $file

done

There: elegant, robust... Oh dear, the “ls” on some systems doesn’t
take a “-A” flag. No problem, we'll pass -a instead and then weed out
the. and .. files:

for file in `ls -a`

do

if [ $file !=. -a $file != .. ]

then

file $file

fi

done

Not quite as elegant, but at least it’s robust and portable. What’s that?
“ls -a” doesn’t work everywhere either? No problem, we'll use “ls -f”
instead. It’s faster, anyway. I hope all this is obvious from reading
the manual pages.