Console Mode Pascal

Console-Mode Pascal Programming

by User:Kirkpatc

Many of us were writing Pascal programs long before Graphic User Interfaces (GUIs) and Integrated Development Environments (IDEs) became fashionable. Many others are beginners at Pascal programming and need to be able to try out the basic tools of the language. Still others need to write console- or text-mode applications in order to perform complex system-control tasks.

Lazarus provides an ideal environment for learning Pascal, and for developing text-mode programs. All the functionality of the Integrated Development Environment can be used, including the Source Editor with its syntax highlighting, access to the libraries, complex searching and code completion tools, and syntax checking. If you don't want a Form with its visual components, you don't need one, but the Lazarus Source Editor is still a great environment for program development. You can compile and run your program during development, without ever leaving the Editor.

To start a console-mode program, go to the Main Menu and select Project -> New Project and then select either Program or Custom Program. The IDE will not generate all the extra files associated with a full graphic application, and will not open an Object Inspector box, but will open the Source Editor with a skeleton program structure and await your programming input.

When you have finished your program (or program fragment) you can compile and run it by selecting Run -> Run from the Main Menu or clicking on the Green (Run) triangle symbol in the Button Bar. Any compiler messages (warnings, progress reports or error messages) will appear in the Message Box, and hopefully there will eventually be a message to say

'Project "Project1" successfully built.:)'.

But where is the program??!!

You can execute the program by going to a console (terminal) window and typing the name of the program (in Unix/Linux, if it is in the current directory you will probably have to type

./Project1

as it won't be found in the standard PATH). However, it can be very inconvenient to keep skipping out of the Lazarus Editor and into a terminal window and back again. Fortunately, there is a mechanism that allows a terminal window to be opened from within the Lazarus environment.

From the Main Menu, select Run -> Run Parameters, then check the box for "Use launching application". The first time you do this and try the Compile/Run sequence, you will probably get a rude message to say

"xterm: Can't execvp /usr/share/lazarus//tools/runwait.sh: Permission denied".  

If this happens, you need to change the permissions on the appropriate file (for example using chmod +x filename, or using the Windows utility for changing permissions); you might have to do this as root. After this, each time you launch your program, a console box will appear and all your text i/o (readln, writeln etc) will appear in it.

After your program has finished execution, a message "Press enter" appears on the screen. Thus any output your program generated will remain on the screen until you have had a chance to read it; after you press 'enter' the console window closes.

You can use the Lazarus editor to try out all the examples in the standard Pascal text-books, or you can write your own. Some of the most useful procedures are those for executing system commands or for running other programs (which may have been written in Pascal, C or Perl, or may be shell or Batch scripts).

Here is an example:

Program TryShell;
uses classes, unix;
var S: longint;
begin
  S := shell ('/bin/ls -la *.p*'); //lists .pp, .pas, .php, .png etc in current directory
  writeln ('Program exited with status : ', S)
end.

Rather more complex commands can be executed. For example, if you have already checked out the CVS repositories for FPC and Lazarus (see fpc buildfaq) you could keep your FPC and Lazarus source files up-to-date by retrieval from the CVS repository with the following sequence of calls:

Program LazUpdate;
uses classes, unix;
var s : longint;
begin 
  S := shell ('cd /usr/local/src/fpc/devel/fpc ; make clean');
  S := shell ('cd /usr/local/src/fpc/devel/lazarus ; make clean');
  S := shell ('cd /usr/local/src/fpc/devel ; cvs -z 3 update -Pd fpc >& ~/cvscheckout.log');
  S := shell ('cd /usr/local/src/fpc/devel ; cvs -z 3 update -Pd lazarus >& ~/cvslaz.log');
end.

Note that issuing the command

shell ('cd /somedirectory')

followed by

shell ('do something in that subdirectory')

doesn't work, because after each shell function call the execution of the program returns to the directory it started in; so we need to include multiple statements per line within our calls to shell.

Of course, you don't have to enter every command as a separate line of Pascal; you could create a script file like this (from fpc buildfaq):

#!/bin/sh
cd /usr/local/src/fpc/devel
cd fpc
make clean
cd ..
cd lazarus
make clean
cd ..
cvs -z 3 update -Pd fpc >& ~/cvscheckout.log
cvs -z 3 update -Pd lazarus >& ~/cvslaz.log 

Name it updatelaz.sh, and then call it from a Pascal program thus:

Program LazUpdate1;
uses classes, unix;
var S : longint; 
begin
  S := shell ('updatelaz.sh')
end.

Orginal contributors

This page has been converted from the epikwiki version. Original content by User:Kirkpatc.