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Introduction to Perl for Unix System Administrators

(Perl without excessive complexity)

by Dr Nikolai Bezroukov

4.2. Determining the Status of a File

• 4.2.1. File Predicates
• 4.2.2.  stat functiont

Files Predicates

As you have seen, when you open a file for writing, the existing contents of the file are destroyed. There are several dozens operators in Perl that can help to test  the status of the file. The most widely used are just file listed in the table below:

 File status operators.

First we will discuss probably the most popular test among listed above the -e test. If you want to open the file for writing if the file does not already exist, you can first test to see if a file exists using the -e operator.

All other file-test operators have the same syntax as the -e operator used below. This is a unary operator that accept string as its only operand. The value of the string should contain the name of the file to be tested. It can be fully qualified name or relative name.

unless (-e "/home/nnb/.profile") {
        die ("file /home/nnb/.profile does not exist");
}
open (SYSPROF, "/home/nnb/.profile");

If the file exists, the -e operator returns true; otherwise, it returns false. Similar tests exist to test other file conditions. Here is another example:

 unless (open(SYSIN, "c:/windows/win.ini" {

          if (-e "c:/windows/win.ini") {

                 die ("File  c:\\windows\\win.ini exists, but cannot be opened.\n"));

          } else {

                 die ("File c:\\windows\\win.ini does not exist.\n");

         }

If you use Windows and need specify full path using backslashes, please remember that they need to be doubles as backslash serves as a escape character in Perl literals. Actually you can use regular slash, but this is convenient solution only for people who get used to Unix.

You can even extend previous trick with | | operator, but it becomes somewhat less comprehensible and although this is a Perl idiom this is a bad Perl idiom -- I recommend against using it:

open(SYSIN, "infile") && !(-e "infile") || die("Cannot open infile\n");

In Unix and Windows NT (unless you are always using root and administrator IDs to login ;-) before you can open a file for reading, you must have permission to read the file. The -r file-test operator tests whether you have permission to read a file.

$fname="testfile";

unless (open(SYSIN, "$fname")) {

  if (!(-e "file1")) {die ("File $fname does not exist.\n");}

  unless (-r "file1")) {die ("You are not allowed to read $fname.\n");

  die ("File1 cannot be opened. Reason unknown\n");

}

 To check whether you have write permission on a file, use the -w file-test operator.

unless (-w "$fname") {

        print STDERR ("Can't write to $fname.\n");

}

The -x file-test operator is used mainly in Unix environment and it checks whether you have execute permission on the file (in other words, whether the system thinks this is an executable script, and whether you have permission to run it if it is), as illustrated here:

unless (-x "$fname") {

        print STDERR ("I can run $fname.\n");

}

The -s file-test operator, which returns the size of the file in bytes. This provides a more refined test for whether or not to open a file for writing: if the file exists but is empty, no information is lost if you overwrite the existing file.

$size = -s "outfile";

if ($size == 0) {

        print ("The file is empty.\n");

} else {

        print ("The file is $size bytes long.\n");

}

The file-test operators provide a way of retrieving information on a particular file. The most common file-test operators are

• -e, which tests whether a file exists
• -r, -w, and -x, which test whether a file has read, write, and execute permission, respectively
• -s, which returns the size of a file

Full list of file test operators contain more than a dozen entries as shown in the table below:

stat function

    ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
       $atime,$mtime,$ctime,$blksize,$blocks)
           = stat($filename);

Not all fields are supported on all filesystem types. Here are the meaning of the fields: 

  0 dev      device number of filesystem
  1 ino      inode number
  2 mode     file mode  (type and permissions). The result is in octal form and contains 

             five digits
  3 nlink    number of (hard) links to the file
  4 uid      numeric user ID of file's owner
  5 gid      numeric group ID of file's owner
  6 rdev     the device identifier (special files only)
  7 size     total size of file, in bytes
  8 atime    last access time since the epoch
  9 mtime    last modify time since the epoch
 10 ctime    inode change time (NOT creation time!) since the epoch
 11 blksize  preferred block size for file system I/O
 12 blocks   actual number of blocks allocated 

If stat is passed the special filehandle consisting of an underline, no stat is done, but the current contents of the stat structure from the last stat or filetest are returned. Example:

    if (-x $file && (($d) = stat(_)) && $d < 0) {
        print "$file is executable NFS file\n";
    }

(This works on machines only for which the device number is negative under NFS.)

In scalar context, stat() returns a boolean value indicating success or failure, and, if successful, sets the information associated with the special filehandle _.