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How do I create a file only if it doesn't exist?

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How do I create a file only if it doesn't exist?

You need to use the O_CREAT and O_EXCL flags from the Fcntl module in conjunction with sysopen():

    use Fcntl;
    sysopen(FH, "/tmp/somefile", O_WRONLY|O_EXCL|O_CREAT, 0644)
                or die "can't open /tmp/somefile: $!":

Be warned that neither creation nor deletion of files is guaranteed to be an atomic operation over NFS. That is, two processes might both successful create or unlink the same file!

Source: Perl FAQ: Files and Formats
Copyright: Copyright (c) 1997 Tom Christiansen and Nathan Torkington.

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