File Test Operators

Sometimes it's not necessary to actually open a file; we just want to check to see if a file exists, or if it's readable or writable. If you are creating a file, it's always a good idea to check to see if a similarly named file already exists, so you can decide whether to overwrite it or not. Perl provides a set of operators, originally found in shell programming languages, to make these kinds of tests very easy. Table  summarizes some of the more useful file test operators.

Table: File Test Operators

Op	Tests	Op	Tests	Op	Tests
-r	Readable	-w	Writable	-x	Executable
-l	Link	-e	Existence	-f	Plain File
-d	Directory	-T	Text	-B	Binary
-z	Zero size	-s	Returns size

As might be expected, if you call any of the operators in Table  without following the operator with a filename, a filename of $_ is assumed.

File test operators are useful to check on the existence or permissions of files before you try to open a filehandle, since you can often diagnose problems before a failed open might cause your program to terminate. For example, before trying to open a file for writing, you can check to see if the file exists, and you have the appropriate permissions:

if(-f $filename){ 
    if(not -w $filename){
        print("$filename exists, but is not writable\n");
    }
    else{
        open(FI,">$filename") || die "Couldn't open $filename";
    }
}
else{
    print("$filename doesn't exist\n");
}

File test operators are very handy if you wish to save a file without overwriting an existing file of the same name. The following code will create a filename consisting of a name followed by a period and a number - each time a like named file is created, it will get a higher number than the last file of the same name:

     $i = 1;
     $newname = $filename;
     while(-f $newname){
        $newname = $filename.$i;
        $i++;
     }

Another useful feature of the file test operators is that they ``remember'' the last file on which they were invoked and will reuse that information when they are called with the unusual argument of a single underscore(_). This allows you to use several different operators in succession, without the added overhead of repeating the file query multiple times. Suppose we wish to write a program that will list some properties of a file if it exists, or a message that the file doesn't exist otherwise. One approach is illustrated by the following code.

     if(-e $file){
        if(-w _){
           print "writable\n";
        }
        if(-x _){
           print "executable\n";
        }
     }
     else{
        print "$file doesn't exist\n";
     }

By using the underscore instead of repeating the filename, perl reuses the information it obtained when it made the first file query, without having to consult with the operating system again.