Chapter 3. Essential skills


File IO with open() and close()

Perl provides many built in functions for dealing with files. They are commonly used so its worth learning how they work.

If built in functions are your weapon of choice, you will also want to know which CPAN module to use when things get more complicated (File::Spec, File::Temp, File::Copy, etc). Check out the Recommended Modules chapter to see the list of recommended modules for working with files.

Higher level modules like Path::Tiny and IO::All offer more safety, nicer syntax, and more over all convenience. Developers visiting from other languages will likely be more comfortable using one of these modules. See File IO with Path::Tiny (article available next week).

Opening files

To open a file, call open(). This will associate the file with a filehandle. The filehandle can be used to manipulate the file. open() returns false on failure so don't forget to check for errors. The error message can be found in the special global variable $!.

open(my $filehandle, $mode, $path)
    or die "Can't open $path: $!";

open(my $filehandle, "<", "/path/to/data.txt")
    or die "Can't open data.txt for reading: $!";

The mode indicates what level of access the filehandle will have. When you see mode values, think: unix shell redirection operators. Valid modes include:

See also in the official docs: opentut, open() and $!.

Writing to files

# Write to a filehandle
open(my $filehandle, '>', $path) or die "Can't open $path: $!";
print $filehandle "I eat danger for breakfast!\n";

# Write to stdout
print "I eat danger for breakfast!\n";
print STDOUT "I eat danger for breakfast, "\n"; # same as above

For your convenience, Perl sets up some special filehandles that are already open when your program starts: STDOUT, STDERR, and STDIN.

Notice there is some unusual syntax happening:

  1. There is no comma after the filehandle.
  2. The special filehandles STDOUT, STDERR, and STDIN don't use sigils (the $) at the beginning of the identifier.

Closing files

   || warn "close failed: $!";

Often you won't see developers closing filehandles explicitly. This is because a filehandle variable that goes out of scope is garbage collected and closed automatically. However this mechanism doesn't do any error checking.

Example 1: Read the whole file into memory

# Open the file read-only
open(my $filehandle, "<", "data.txt")
    or die "Can't open data.txt for reading: $!";

# Read the whole file into an array
my @lines = <$filehandle>; 

# Print the whole file to STDOUT
print @lines;


Example 2: Process a file line by line

# Open the file read-only
open(my $filehandle, "<", "data.txt")
    or die "Can't open data.txt for reading: $!";

# Read the file line by line
while (my $line = <$filehandle>) {
    chomp $line; # remove newline characater
    # Print each line to STDOUT
    print $line, "\n";


File test operators

File tests operators test a file for some condition (eg does the file exist?) and return true or false. The operators take a single argument which is a path or filehandle.

-r  File is readable by effective uid/gid.
-w  File is writable by effective uid/gid.
-x  File is executable by effective uid/gid.
-o  File is owned by effective uid.

-R  File is readable by real uid/gid.
-W  File is writable by real uid/gid.
-X  File is executable by real uid/gid.
-O  File is owned by real uid.

-e  File exists.
-z  File has zero size (is empty).
-s  File has nonzero size (returns size in bytes).

-f  File is a plain file.
-d  File is a directory.
-l  File is a symbolic link (false if symlinks aren't
    supported by the file system).
-p  File is a named pipe (FIFO), or Filehandle is a pipe.
-S  File is a socket.
-b  File is a block special file.
-c  File is a character special file.
-t  Filehandle is opened to a tty.

-u  File has setuid bit set.
-g  File has setgid bit set.
-k  File has sticky bit set.

-T  File is an ASCII or UTF-8 text file (heuristic guess).
-B  File is a "binary" file (opposite of -T).

-M  Script start time minus file modification time, in days.
-A  Same for access time.
-C  Same for inode change time (Unix, may differ for other

The list of operators above is quoted from the official Perl docs for file test operators. The docs also describe some more exotic details of these operators, but you probably don't need to know about them.


# print files which exist and are plain files
# ignore everything else (directories, sockets, etc)
for my $path (@paths) {
    next unless -e $path;  # ignore files that don't exist
    next unless -f $path;  # ignore specials
    print $path, "\n";