How come when I open a file read-write it wipes it out?

Because you're using something like this, which truncates the file and then gives you read-write access:

    open(FH, "+> /path/name");		# WRONG (almost always)

Whoops. You should instead use this, which will fail if the file doesn't exist.

    open(FH, "+< /path/name");  	# open for update

Using ">" always clobbers or creates. Using "<" never does either. The "+" doesn't change this.

Here are examples of many kinds of file opens. Those using sysopen() all assume

    use Fcntl;

To open file for reading:

    open(FH, "< $path")                                 || die $!;
    sysopen(FH, $path, O_RDONLY)                        || die $!;

To open file for writing, create new file if needed or else truncate old file:

    open(FH, "> $path") || die $!;
    sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT)        || die $!;
    sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666)  || die $!;

To open file for writing, create new file, file must not exist:

    sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT)         || die $!;
    sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666)   || die $!;

To open file for appending, create if necessary:

    open(FH, ">> $path") || die $!;
    sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT)       || die $!;
    sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;

To open file for appending, file must exist:

    sysopen(FH, $path, O_WRONLY|O_APPEND)               || die $!;

To open file for update, file must exist:

    open(FH, "+< $path")                                || die $!;
    sysopen(FH, $path, O_RDWR)                          || die $!;

To open file for update, create file if necessary:

    sysopen(FH, $path, O_RDWR|O_CREAT)                  || die $!;
    sysopen(FH, $path, O_RDWR|O_CREAT, 0666)            || die $!;

To open file for update, file must not exist:

    sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT)           || die $!;
    sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666)     || die $!;

To open a file without blocking, creating if necessary:

    sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT)
	    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 successfully create or unlink the same file! Therefore O_EXCL isn't as exclusive as you might wish.

See also the new perlopentut if you have it (new for 5.6). Why do I sometimes get an "Argument list too long" when I use <*>?

The <> operator performs a globbing operation (see above). In Perl versions earlier than v5.6.0, the internal glob() operator forks csh(1) to do the actual glob expansion, but csh can't handle more than 127 items and so gives the error message Argument list too long. People who installed tcsh as csh won't have this problem, but their users may be surprised by it.

To get around this, either upgrade to Perl v5.6.0 or later, do the glob yourself with readdir() and patterns, or use a module like File::KGlob, one that doesn't use the shell to do globbing.

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