Now you know how to open a filehandle for output, overwriting any existing file with the same name. Suppose you wanted to make sure that there wasn't a file by that name (to keep you from accidentally blowing away your spreadsheet data or that important birthday calendar). If you were writing a shell script, you'd use something like -e
filename
to test if the file exists. Similarly, Perl uses -e
$filevar
to test for the existence of the file named by the scalar value in $filevar
. If this file exists, the result is true; otherwise it is false.[] For example:
$name = "index.html";
if (-e $name) {
print "I see you already have a file named $name\n";
} else {
print "Perhaps you'd like to make a file called $name\n";
}
The operand of the -e
operator is really just any scalar expression that evaluates to some string, including a string literal. Here's an example that checks to see whether both index.html and index.cgi exist in the current directory:
if (-e "index.html" && -e "index.cgi") {
print "You have both styles of index files here.\n";
}
Other operators are defined as well. For example, -r
$filevar
returns true if the file named in $filevar
exists and is readable. Similarly, -w
$filevar
tests whether it is writable. Here's an example that tests a user-specified filename for both readability and writability:
print "where? ";
$filename = <STDIN>;
chomp $filename; # toss pesky newline
if (-r $filename && -w $filename) {
# file exists, and I can read and write it
...
}
Many more file tests are available. Table 10.1 gives the complete list.
Table 10.1: File Tests and Their MeaningsFile Test | Meaning |
---|
-r | File or directory is readable |
-w | File or directory is writable |
-x | File or directory is executable |
-o | File or directory is owned by user |
-R | File or directory is readable by real user, not effective user (differs from -r for setuid programs) |
-W | File or directory is writable by real user, not effective user (differs from -w for setuid programs) |
-X | File or directory is executable by real user, not effective user (differs from -x for setuid programs) |
-O | File or directory is owned by real user, not effective user (differs from -o for setuid programs) |
-e | File or directory exists |
-z | File exists and has zero size (directories are never empty) |
-s | File or directory exists and has nonzero size (the value is the size in bytes) |
-f | Entry is a plain file |
-d | Entry is a directory |
-l | Entry is a symlink |
-S | Entry is a socket |
-p | Entry is a named pipe (a "fifo") |
-b | Entry is a block-special file (like a mountable disk) |
-c | Entry is a character-special file (like an I/O device) |
-u | File or directory is setuid |
-g | File or directory is setgid |
-k | File or directory has the sticky bit set |
-t | isatty() on the filehandle is true
|
-T | File is "text" |
-B | File is "binary" |
-M | Modification age in days |
-A | Access age in days |
-C | Inode-modification age in days |
Most of these tests return a simple true-false condition. A few don't, so let's talk about them.
The -s
operator does return true if the file is nonempty, but it's a particular kind of true. It's the length in bytes of the file, which evaluates as true for a nonzero number.
The age operators -M
, -A
, and -C
(yes, they're uppercase) return the number of days since the file was last modified, accessed, or had its inode changed.[] (The inode contains all of the information about the file except for its contents: see the stat
system call manpage for details.) This age value is fractional with a resolution of one second: 36 hours is returned as 1.5 days. If you compare the age with a whole number (say three), you'll get only the files that were changed exactly that many days ago, not one second more or less. This means you'll probably want a range comparison[] rather than an exact comparison to get files that are between three and four days old.
These operators can operate on filehandles as well as filenames. Giving a filehandle for the operand is all it takes. So to test whether the file opened as SOMEFILE
is executable, you can use:
if (-x SOMEFILE) {
# file open on SOMEFILE is executable
}
If you leave the filename or filehandle parameter off (that is, you have just -r
or -s
), the default operand is the file named in the $_
variable (there it is again!). So, to test a list of filenames to see which ones are readable, it's as simple as this:
foreach (@some_list_of_filenames) {
print "$_ is readable\n" if -r; # same as -r $_
}