Text file reader by blocks of lines for unix like systems in freepascal/lazarus

gistfile1.txt

program fr;
// Simple file reader-viewer by blocks of lines
// programmer: Aleksandar Josifoski     about.me/josifsk
// reading text file by blocks of requested number of lines
// Usage is fr numoflines YourFile for example: fr 5 Love.txt
// j next, k previous, g first, G last line, q or ESC quit, s for saving current block of text
{$mode objfpc}{$H+}

uses Unix, keyboard, sysutils, Crt;

var
   f : Text;
   saved : Text;
   L, i, j : Longint;
   ar: array[1..100000] of String;
   si, sj, s, line : String;
   K : TKeyEvent;
   stcomm : String;
   numoflines : String;

begin
   // TextColor(Yellow);
   InitKeyBoard;
   numoflines:=ParamStr(1);
   writeln(numoflines);
   writeln(ParamStr(2));
   if  (ParamStr(2)='') or (FileExists(ParamStr(2))=false) then begin
                              writeln('Simple file reader-viewer by blocks of lines');
                              writeln('Usage is fr numoflines YourFile for example: fr 5 YourFile');
                              writeln('Shortcuts: j next, k previous, g first, G last line, q or ESC quit');
                              writeln('s for saving current block of text, which is saved in saved.txt');
                              DoneKeyBoard;
                              Halt;
                           end;
   stcomm:='tr -s ''\n'' <'+ParamStr(2)+' >frfile';
   L:= FpSystem(stcomm);
   L:= FpSystem('clear');
   AssignFile(f, 'frfile');
   reset(f);
   j:=0;
   while not(eof(f)) do begin
     j:=j+1; 
     s:='';
     for k:=1 to StrToInt(numoflines) do begin
         readln(f, line);
         s:= s + line + AnsiChar(#10);
     end;
     ar[j]:=s;
   end; // while
   CloseFile(f);
   AssignFile(saved, 'saved.txt');
   if FileExists('saved.txt') then Append(saved)
                              else rewrite(saved);
   if j=0 then begin
                 writeln('empty file, exit');
                 DoneKeyBoard;
                 Halt;
               end;  
   i:=1;
   str(j, sj);
   str(i, si);
   writeln(si+'/'+sj);
   writeln();
   writeln();
   write(ar[i]);
   repeat
    K:=GetKeyEvent;
    K:=TranslateKeyEvent(K);
            case GetKeyEventChar(K) of
             'j' : if i < j then begin
                      L:= FpSystem('clear');
                      i:=i+1;
                      str(i, si);
                      writeln(si+'/'+sj);
                      writeln();
                      writeln();
                      writeln(ar[i]);
                    end;
             'k' : if i > 1 then begin
                      L:= FpSystem('clear');
                      i:=i-1;
                      str(i, si);
                      writeln(si+'/'+sj);
                      writeln();
                      writeln();
                      writeln(ar[i]);                      
                    end;
             'g' :  begin 
                      L:= FpSystem('clear');
                      i:=1;
                      str(i, si);
                      writeln(si+'/'+sj);
                      writeln();
                      writeln();
                      writeln(ar[i]);                      
                    end;
             'G' :  begin 
                      L:= FpSystem('clear');
                      i:=j;
                      str(i, si);
                      writeln(si+'/'+sj);
                      writeln();
                      writeln();
                      writeln(ar[i]);                      
                    end;
             's' :  begin
                      str(i, si);
                      writeln(saved, ar[i]);
                    end;
             'q' : L:= FpSystem('clear');
             #27 : L:= FpSystem('clear');         
            end;
   until (GetKeyEventChar(K)='q') or (GetKeyEventChar(K)=#27); 
   DoneKeyBoard;
   CloseFile(saved);
end.