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Learning Perl, 5th Edition by Tom Phoenix, Randal L. Schwartz, brian d foy

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Chapter 12. File Tests

Earlier, we showed how to open a filehandle for output. Normally, that will create a new file, wiping out any existing file with the same name. Perhaps you want to check that there isn’t a file by that name. Perhaps you need to know how old a given file is. Or perhaps you want to go through a list of files to find which ones are larger than a certain number of bytes and not accessed for a certain amount of time. Perl has a complete set of tests you can use to find out information about files.

File Test Operators

Before we start a program that creates a new file, let’s make sure that the file doesn’t already exist so that we don’t accidentally overwrite a vital spreadsheet datafile or that important birthday calendar. For this, we use the -e file test, testing a filename for existence:

die "Oops! A file called '$filename' already exists.\n"
  if -e $filename;

Notice that we don’t include $! in this die message, since we’re not reporting that the system refused a request in this case. Here’s an example of checking whether a file is being kept up-to-date. In this case, we’re testing an already opened filehandle, instead of a string filename. Let’s say that our program’s configuration file should be updated every week or two. (Maybe it’s checking for computer viruses, say.) If the file hasn’t been modified in the past 28 days, then something is wrong:

warn "Config file is looking pretty old!\n"
  if -M CONFIG > 28;

The third example is more complex. Here, let’s say that ...

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