Is this a standard Perl language construction or a customization: open HANDLE, ">$fname"

user_13697561 0
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Not a Perl guru, working with an ancient script, ran into a construct I didn't recognize that yields results I don't expect. Curious whether this is the standard language, or a PM customization of sorts:

open FILE1, ">./$disk_file" or die "Can't open file: $disk_file: $?";

From the looks of this, file is to be opened for writing, but the log error says that file is not found. Perl's file i/o expects 3 parameters, not 2. Log doesn't have the die output, instead saying: "File not found"

Confused a bit here.

EDIT: Made it work using the answers below. Seemed like I was running a cashed version of the .pl for some time, instead of the newly-edited. Finally it caught up with a 2-param open, thanks y'all for your help!

error-handling file-io perl
3 Answers

That is the old 2-argument form of open. The second argument is a bit magical:

  • if it starts with '>' the remainder of the string is used as the name of a file to open for writing
  • if it starts with '<' the remainder of the string is used as the name of a file to open for reading (this is the default if '<' is omitted)
  • if it ends with '|' the string up to that point is interpreted as a command which is executed with its STDOUT connected to a pipe which your script will open for reading
  • if it starts with '|' the string after that point is interpreted as a command which is executed with its STDIN connected to a pipe which your script will open for writing This is a potentially security vulnerability because if your script accepts a filename as user input, the user can add a '|' at the beginning or end to trick your script into running a command.

The 3-argument form of open was added in (I think) version 5.8 so it has been a standard part of Perl for a very long time.

The FILE1 part is known as a bareword filehandle - which is a global. Modern style would be to use a lexical scalar like my $file1 instead.

user_2936520

See perldoc perlopen for details but, in brief...

Perl's open() will accept either two or three parameters (there's even a one-parameter version - which no-one ever uses). The two-parameter version is a slightly older style where the open mode and the filename are joined together in the second parameter.

So what you have is equivalent to:

open FILE1, '>', "./$disk_file" or die "Can't open file: $disk_file: $?";

A couple of other points.

  1. We prefer to use lexical variables as filehandles these days (so, open my $file1, ... instead of open FILE1, ...).
  2. I think you'll find that $! will be more useful in the error message than $?. $? contains the error from a child process, but there's no child process here.

Update: And none of this seems to be causing the problems that you're seeing. That seems to be caused by a file actually not being in the expected place. Can you please edit your question to add the exact error message that you're seeing.

user_72310

The other answers here are correct that's the two-argument syntax. They've done a good job covering why and how you should ideally change it, so I won't rehash here.

However they haven't tried to help you fix it, so let me try that...

This is a guess, but I suspect $disk_file contains a filename with a path (eg my_logs/somelog.log), and the directory part (my_logs in my entirely guessed example) doesn't exists, so is throwing an error. You could create that directory, or alter whatever sets that variable so it's writing to a location that does exist.

Bear in mind these paths will be relative to wherever you're running the script from - not relative to the script itself, so if there's a log directory (or whatever) in the same dir as the script you may want to cd to the script's dir first.

Dan_171210
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