Input/Output

Files can be opened for input or output using the open() function. As in languages like C, the open() statement associates a filehandle with a filename and a mode for the file, namely input, output or append.

Here are some examples. In these examples the function die exits after printing a diagnostic. It is separated by an or whose semantics is that the second part of the or is executed only if the first part fails.

   open(INFILE,  "input.txt")   or die "Can't open input.txt";
     # Open in read mode -- could also write open(INFILE,"<input.txt")
   open(OUTFILE, ">output.txt") or die "Can't open output.txt";
     # > indicates open in (over)write mode
   open(LOGFILE, ">>my.log")    or die "Can't open logfile";
     # >> indicates open in append mode

Reading a file is achieved using the <> operator on a filehandle. In scalar context it reads a single line from the filehandle, while in list context it reads the whole file in, assigning each line to an element of the list:

   $line  = <INFILE>;  # Read one line from INFILE into $line
   @lines = <INFILE>;  # Read all lines from INFILE into list @lines

The <> operator is most often seen in a while loop:

   while ($line = <INFILE>) {     # assigns each line in turn to $line
     print "Just read in this line:", $line;
   }

The example above also introduces the print statement. The simplest form of the print statement takes a list of string arguments separated by commas. print can also take an optional first argument specifying which filehandle to print to:

   print "Hello, world";
   print "Hello ", $name,"\n";
   print OUTFILE $record;
   print LOGFILE $logmessage;

To close a file, we call the function close with the corresponding filehandle.