Console Mode Pascal

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Console-Mode Pascal Programming

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.

Console mode programming without Lazarus

Even though many prefer to use the Lazarus IDE to write console mode programs, you can also write them with any text editor and compile them yourself by calling the FPC compiler, e.g. this for a program in example.pas:

fpc example.pas

Also, the text mode fp IDE (looks a bit like old Turbo Pascal) is available. In the next sections we will focus on programming with Lazarus.

Console mode programming with Lazarus

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 Console Application. 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.


Note: Under Windows a GUI application has no console and thus can not writeln or readln. You will get File not open errors. Disable under Project Options / Compiler Options / Linking / Target specific options / Win32 GUI application to create a console application. In Lazarus IDE 1.4, this is under Project(menu) / Project Options / Compiler Options / Config and Target / Target specific options / Win32 GUI application.


Note: Lazarus has some functionality for command line programs, such as copyfile. To use these, add a project requirement for LCLBase, which will not pull in the entire LCL. Then add the relevant unit to your uses clause.

Project type: Console Application

Selecting this project type in Lazarus creates a program with a new class derived from TCustomApplication. TCustomApplication provides a lot of the common things and makes programming command line utilities easy. For example checking command line options, writing help, checking environment variables and exception handling. All LCL programs use this automatically.

Project type: Program

As a demonstration, we will write a very minimalistic Pascal program. In the IDE, select the 'Program' project type and let the IDE helps you a bit. For example when you add another unit the IDE will automatically add the unitname to the program uses section. This behavior is defined in the project options. So you can go between 'Program' and 'Custom Program' at any time.

An example for complete beginners:

program Project1;
{$mode objfpc}{$H+}
  {$IFDEF UNIX}{$IFDEF UseCThreads}
  WriteLn('Hello World!');

Pascal scripts

Additionally, it is possible to write scripts that are dynamically compiled by InstantFPC, a cross-platform solution to run (small) Pascal programs as scripts. The shebang

#!/usr/bin/env instantfpc

permits to start the script like a stand-alone program.


Note: Though it is used to run Pascal scripts, InstantFPC uses regular Object Pascal, not the PascalScript script language.

Running a Program


When you have finished your program (or program fragment) you can compile and run it by selecting Run -> Build (or Quick Compile) 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??!!

If you have not yet saved the program, then the IDE put it into the temporary directory (e.g. /tmp under linux, C:\temp under windows, see Environment Options / Files / Directory for building test projects).

If you already saved the project, then the program was created in the same directory, where you saved the project1.lpi file.

Run in Console

You can execute the program by going to a console (terminal) window, then use cd to change to the directory and typing the name of the program. On Unix/Linux, if it is in the current directory you will have to start with ./ as the program probably won't be found in the PATH.

Example for Linux/Unix where a program is stored in /tmp:

cd /tmp 

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.

Run in IDE

In internal console: show in the terminal output window.

In redirect output.

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 saying:

"xterm: Can't execvp /usr/share/lazarus//tools/ 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.

Unfortunately, this method does not allow you to use the integrated debugger.

Run in IDE with redirected output

If you want to see what is written to stdout and want to use the integrated debugger as well, stdout can be redirected to a file with the following code:


  OutputFile: text;

  Assign(OutputFile, 'Output.txt');
  if FileExists(Filename) then begin
  else begin
    Rewrite(OutputFile);   { open file for writing, destroying contents, if any }

  ResultCode := fpdup(OutputFile, output);

  if ResultCode < 0 then begin
    raise Exception.CreateFmt('dup failed: %s', [ResultCode]);

Output.txt can then be viewed with 'tail -f output.txt' or viewed with an editor if 'tail' is not available on your os.

Alternatively, using Lazarus 0.9.31 or up (on Linux only): In the "View" menu, under "Debug Windows" there is an entry for a "console output" to view stdout

Unicode (UTF8) output

If you want your console mode program to show Unicode (UTF8) output on Windows Vista and higher (and perhaps on earlier versions, too), you can use the SetConsoleOutputCP command to set the console to the UTF8 character set.


Note: you will need to make sure the font of the console can show the letters you want to output (e.g. Greek, Cyrillic, Korean).


Note: you will need to include the Windows unit.

See LCL Unicode Support for more details on Unicode support in Lazarus and FPC.

Example program:

program uniconsole;

{$mode objfpc}{$H+}

    {$IFDEF UseCThreads}
  {Widestring manager needed for widestring support}
  Windows, {for setconsoleoutputcp}

UTF8TestString: string;

UTF8TestString:= 'rosé, водка and ούζο';
writeln ('plain: ' + UTF8TestString);
{Apparently we don't need UTF8ToConsole for this
UTF8ToConsole did not do anything for me in fact.}


You can use the Lazarus editor to try out all the examples in the standard Pascal textbooks, or you can write your own. Some of the most useful procedures are those for executing system commands or for running other programs (whether written in Pascal, C or Perl, or shell or batch scripts).

Execute shell command

Here is an example for a Custom Program or Program... that is Linux/Unix/OSX specific though:

program TryShell;
  Classes, Unix;
  S: LongInt;
  S := fpsystem('/bin/ls -la *.p*'); //lists .pp, .pas, .php, .png etc in current directory
  WriteLn('Program exited with status : ', S)

Example: update FPC and Lazarus

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

program LazUpdate;
  Classes, Unix;
  S: LongInt;
  S := fpsystem('cd /usr/local/src/fpc/devel/fpc ; make clean');
  S := fpsystem('cd /usr/local/src/fpc/devel/lazarus ; make clean');
  S := fpsystem('cd /usr/local/src/fpc/devel ; svn update fpc >& ~/cvscheckout.log');
  S := fpsystem('cd /usr/local/src/fpc/devel ; svn update lazarus >& ~/cvslaz.log');

Note that issuing the command

fpsystem('cd /somedirectory');

followed by

fpsystem ('do something in that subdirectory');

doesn't work, because after each fpsystem 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 buildfaq):

cd /usr/local/src/fpc/devel
cd fpc
make clean
cd ..
cd lazarus
make clean
cd ..
svn up fpc >& ~/cvscheckout.log
svn up lazarus >& ~/cvslaz.log

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

program LazUpdate1;
  Classes, Unix;
  S: LongInt; 
  S := fpsystem('')

See also