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Many programs that you run from a DOS prompt allow you to specify extra information that the programs can use. This short page illustrates how you could get filenames the way copy and del do.

This raises two issues: How does a program obtain information from the Command Line? How does the user know how to use the program?

Sample code of a program which combines both methods of getting information from the user is provided.

The mechanism for accessing the arguments is quite simple. Consider the following Command Line which could be used to get the program copyfile to read a text file called orig.txt and create a copy called copy.txt.

convtxt orig.txt copy.txt

Two built-in functions will be utilized:

• ParamCount counts all items on the Command Line except the program name (in this case for a total of two) and returns this value.
• ParamStr requires an index into the parameter list as its parameter and returns the requested Command-Line parameter as a string, e.g., ParamStr(2) would return the string copy.txt.

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Since the user receives no explicit prompts, the program must make decisions given the information provided from the command line. Consider the following if structure:

   {Get SRC and DEST file names}
   if ParamCount = 2 then
     begin
       {Get from the Command Line}
       InName := ParamStr(1);
       OutName := ParamStr(2)
     end
   else
     begin
       {Inform user of correct usage and ask for file names}
       writeln('Usage: ', ParamStr(0), sourcefile destinationfile');
       write ('What is the source file? ');
       readln(InName);
       write ('What is the destination file? ');
       readln(OutName)
     end;

If the program name is followed by two parameters, the parameters are assigned to the string variables InName and OutName. Otherwise, the user is informed of Command-Line Usage and is then prompted for the file names.

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Abbreviated Sample Code to Copy a File

The following complete program demonstrates how to access and process the command line information. The purpose of the program is to create a copy of a file. N.B. There is no bulletproofing included for a nonexistent source file.

Program CopyFile;
{ Copy a file }
{ Usage: copyfile sourcefile destinationfile 
  OR: copyfile }
var
   NextCh : char;
   Infile, Outfile: file of char;
   InName, OutName: string;
begin
   {Get SRC and DEST file names}
   if ParamCount = 2 then
     begin
       {Get from the Command Line}
       InName := ParamStr(1);
       OutName := ParamStr(2)
     end
   else
     begin
       {Inform user of correct usage and ask for file names}
       writeln('Usage: ', ParamStr(0), sourcefile destinationfile');
       write ('What is the source file? ');
       readln(InName);
       write ('What is the destination file? ');
       readln(OutName)
     end;
   {Open files for access}
   assign(Infile, InName);
   assign(Outfile, OutName);
   reset(Infile);
   rewrite(Outfile);
   {Convert file}
   while not EOF(Infile) do
      begin
         read(Infile, NextCh);
         write(Outfile, NextCh)
      end;
   {Close files}
   close(Infile);
   close(Outfile);
   write(ParamStr(0), ' has successfully copied ');
   writeln('file ', Inname, ' to file ', Outname, '.')
end.

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