10. A Bit of Programming
10.1 Shell Scripts: .BAT Files on Steroids
If you used .BAT files to create shortcuts of long command lines (I did a
lot), this goal can be attained by inserting appropriate alias lines (see
example above) in profile or .bash_profile. But if your
.BATs were more complicated, then you'll love the scripting language made
available by the shell: it's as powerful as good ol' QBasic, if not more. It
has variables, structures like while, for, case, if... then... else, and
lots of other features: it can be a good alternative to a ``real''
programming language.
To write a script---the equivalent of a .BAT file under DOS---all you have
to do is write a standard ASCII file containing the instructions, save it,
then make it executable with the command chmod +x <scriptfile>.
To execute it, type its name.
A word of warning. The system editor is called vi, and in my
experience most new users find it very difficult to use. I'm not going to
explain how to use it; please consult Matt Welsh's book or search for a
tutorial on the net. Suffice it here to say that:
- to insert some text, type
ithen your text; - to delete characters, type <ESC> then
x; - to quit
viwhithout saving, type <ESC> then:q! - to save and quit, type <ESC> then
:wq.
A good beginner editor is joe: invoking it by typing jstar
you'll get the same key bindings as the DOS/Win editor. jed in
WordStar or IDE mode is even better. Please consult Section
Where to Find Applications to see where to get these
editors.
Writing scripts under bash is such a vast subject it would require
a book by itself, and I will not delve into the topic any further. I'll just
give you an example of shell script, from which you can extract some basic
rules:
#!/bin/sh
# sample.sh
# I am a comment
# don't change the first line, it must be there
echo "This system is: `uname -a`" # use the output of the command
echo "My name is $0" # built-in variables
echo "You gave me the following $# parameters: "$*
echo "The first parameter is: "$1
echo -n "What's your name? " ; read your_name
echo notice the difference: "hi $your_name" # quoting with "
echo notice the difference: 'hi $your_name' # quoting with '
DIRS=0 ; FILES=0
for file in `ls .` ; do
if [ -d ${file} ] ; then # if file is a directory
DIRS=`expr $DIRS + 1` # DIRS = DIRS + 1
elif [ -f ${file} ] ; then
FILES=`expr $FILES + 1`
fi
case ${file} in
*.gif|*jpg) echo "${file}: graphic file" ;;
*.txt|*.tex) echo "${file}: text file" ;;
*.c|*.f|*.for) echo "${file}: source file" ;;
*) echo "${file}: generic file" ;;
esac
done
echo "there are ${DIRS} directories and ${FILES} files"
ls | grep "ZxY--%%WKW"
if [ $? != 0 ] ; then # exit code of last command
echo "ZxY--%%WKW not found"
fi
echo "enough... type 'man bash' if you want more info."
10.2 C for Yourself
Under UNIX, the system language is C, love it or hate it. Scores of other languages (Java, FORTRAN, Pascal, Lisp, Basic, Perl, awk...) are also available.
Taken for granted that you know C, here are a couple of guidelines for those
of you who have been spoilt by Turbo C++ or one of its DOS kin. Linux's C
compiler is called gcc and lacks all the bells and whistles that
usually accompany its DOS counterparts: no IDE, on-line help, integrated
debugger, etc. It's just a rough command-line compiler, very powerful and
efficient. To compile your standard hello.c you'll do:
$ gcc hello.c
which will create an executable file called a.out. To give the
executable a different name, do
$ gcc -o hola hello.c
To link a library against a program, add the switch -l<libname>. For example, to link in the math library:
$ gcc -o mathprog mathprog.c -lm
(The -l<libname> switch forces gcc to link the
library /usr/lib/lib<libname>.so; so -lm links
/usr/lib/libm.so).
So far, so good. But when your prog is made of several source files, you'll
need to use the utility make. Let's suppose you have written an
expression parser: its source file is called parser.c and
#includes two header files, parser.h and xy.h. Then
you want to use the routines in parser.c in a program, say,
calc.c, which in turn #includes parser.h. What a mess!
What do you have to do to compile calc.c?
You'll have to write a so-called Makefile, which teaches the
compiler the dependencies between sources and objects files. In our example:
# This is Makefile, used to compile calc.c # Press the <TAB> key where indicated! calc: calc.o parser.o <TAB>gcc -o calc calc.o parser.o -lm # calc depends on two object files: calc.o and parser.o calc.o: calc.c parser.h <TAB>gcc -c calc.c # calc.o depends on two source files parser.o: parser.c parser.h xy.h <TAB>gcc -c parser.c # parser.o depends on three source files # end of Makefile.
Save this file as Makefile and type make to compile your
program; alternatively, save it as calc.mak and type make -f
calc.mak, and of course RMP. You can invoke some help about the C
functions, that are covered by man pages, section 3; for example,
$ man 3 printf
To debug your programs, use gdb. info gdb to learn how to
use it.
There are lots of libraries available; among the first you may want to use
are ncurses (textmode effects), and svgalib (console
graphics). Many editors can act as an IDE; emacs and jed,
for instance, also feature syntax highlighting, automatic indent, and so on.
Alternatively, get the package rhide from
ftp://metalab.unc.edu:/pub/Linux/devel/debuggers/. It's a Borland IDE
clone, and chances are that you'll like it.
10.3 X11 Programming
If you feel brave enough to tackle X11 programming (it's not that difficult), there are several libraries that make writing X11 programs a breeze. The main sites to visit are those of GTK+, http://www.gtk.org, and Qt, http://www.troll.no. Gtk+ is a C-based widget set originally written for the graphic package The GIMP ( http://www.gimp.org), and is used by the Gnome environment. Kdeveloper is based on C++-based Qt, used by KDE. Most likely, you'll use one of these.
Some of the best tools for visual programming are Kdevelop for Qt, http://www.kdevelop.org, and Glade for GTK+, http://glade.pn.org. This page has more information: http://www.free-soft.org/guitool/.
Multi-Platform Programming
Wouldn't it be nice if you could write code that compiled seamlessly under
Linux and Windows using gcc? As of this writing, there are
some widget sets that allow for more-or-less stable multi-platform
programming. As far as stability and completeness are concerned though, I
would say that the choice is narrowed down to only one: FLTK, the Fast Light
Tool Kit
http://www.fltk.org. It's amazingly small, quick, and
stable. It also has a semi-visual builder called Fluid.
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