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8. A Bit of Programming

8.1 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 .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 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, because I don't like it and don't use it, so there. Suffice it here to say that:

A good beginner editor is joe: invoking it by typing jstar you'll get the same key bindings as the DOS 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."

8.2 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>.a; so -lm links /usr/lib/libm.a).

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'll want to use are ncurses, to handle textmode effects, and svgalib, to do graphics. If you feel brave enough to tackle X11 programming, there are libraries like the abovementioned XForms, Qt, Gtk and many others, which make writing X11 programs a breeze. Have a look at http://www.xnet.com/~blatura/linapp6.html .

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://sunsite.unc.edu:/pub/Linux/devel/debuggers/. It's a Borland IDE clone, and chances are that you'll like it.

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