Perl 5 version 6.0 documentation

open

  • open FILEHANDLE,MODE,LIST
  • open FILEHANDLE,EXPR
  • open FILEHANDLE

    Opens the file whose filename is given by EXPR, and associates it with FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. (This is considered a symbolic reference, so use strict 'refs' should not be in effect.)

    If EXPR is omitted, the scalar variable of the same name as the FILEHANDLE contains the filename. (Note that lexical variables--those declared with my--will not work for this purpose; so if you're using my, specify EXPR in your call to open.) See perlopentut for a kinder, gentler explanation of opening files.

    If MODE is '<' or nothing, the file is opened for input. If MODE is '>' , the file is truncated and opened for output, being created if necessary. If MODE is '>>' , the file is opened for appending, again being created if necessary. You can put a '+' in front of the '>' or '<' to indicate that you want both read and write access to the file; thus '+<' is almost always preferred for read/write updates--the '+>' mode would clobber the file first. You can't usually use either read-write mode for updating textfiles, since they have variable length records. See the -i switch in perlrun for a better approach. The file is created with permissions of 0666 modified by the process' umask value.

    These various prefixes correspond to the fopen(3) modes of 'r' , 'r+' , 'w' , 'w+' , 'a' , and 'a+' .

    In the 2-arguments (and 1-argument) form of the call the mode and filename should be concatenated (in this order), possibly separated by spaces. It is possible to omit the mode if the mode is '<' .

    If the filename begins with '|' , the filename is interpreted as a command to which output is to be piped, and if the filename ends with a '|' , the filename is interpreted as a command which pipes output to us. See Using open() for IPC in perlipc for more examples of this. (You are not allowed to open to a command that pipes both in and out, but see IPC::Open2, IPC::Open3, and Bidirectional Communication with Another Process in perlipc for alternatives.)

    If MODE is '|-' , the filename is interpreted as a command to which output is to be piped, and if MODE is '-|' , the filename is interpreted as a command which pipes output to us. In the 2-arguments (and 1-argument) form one should replace dash ('-' ) with the command. See Using open() for IPC in perlipc for more examples of this. (You are not allowed to open to a command that pipes both in and out, but see IPC::Open2, IPC::Open3, and Bidirectional Communication in perlipc for alternatives.)

    In the 2-arguments (and 1-argument) form opening '-' opens STDIN and opening '>-' opens STDOUT.

    Open returns nonzero upon success, the undefined value otherwise. If the open involved a pipe, the return value happens to be the pid of the subprocess.

    If you're unfortunate enough to be running Perl on a system that distinguishes between text files and binary files (modern operating systems don't care), then you should check out binmode for tips for dealing with this. The key distinction between systems that need binmode and those that don't is their text file formats. Systems like Unix, MacOS, and Plan9, which delimit lines with a single character, and which encode that character in C as "\n" , do not need binmode. The rest need it.

    When opening a file, it's usually a bad idea to continue normal execution if the request failed, so open is frequently used in connection with die. Even if die won't do what you want (say, in a CGI script, where you want to make a nicely formatted error message (but there are modules that can help with that problem)) you should always check the return value from opening a file. The infrequent exception is when working with an unopened filehandle is actually what you want to do.

    Examples:

    1. $ARTICLE = 100;
    2. open ARTICLE or die "Can't find article $ARTICLE: $!\n";
    3. while (<ARTICLE>) {...
    4. open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
    5. # if the open fails, output is discarded
    6. open(DBASE, '+<', 'dbase.mine') # open for update
    7. or die "Can't open 'dbase.mine' for update: $!";
    8. open(DBASE, '+<dbase.mine') # ditto
    9. or die "Can't open 'dbase.mine' for update: $!";
    10. open(ARTICLE, '-|', "caesar <$article") # decrypt article
    11. or die "Can't start caesar: $!";
    12. open(ARTICLE, "caesar <$article |") # ditto
    13. or die "Can't start caesar: $!";
    14. open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
    15. or die "Can't start sort: $!";
    16. # process argument list of files along with any includes
    17. foreach $file (@ARGV) {
    18. process($file, 'fh00');
    19. }
    20. sub process {
    21. my($filename, $input) = @_;
    22. $input++; # this is a string increment
    23. unless (open($input, $filename)) {
    24. print STDERR "Can't open $filename: $!\n";
    25. return;
    26. }
    27. local $_;
    28. while (<$input>) { # note use of indirection
    29. if (/^#include "(.*)"/) {
    30. process($1, $input);
    31. next;
    32. }
    33. #... # whatever
    34. }
    35. }

    You may also, in the Bourne shell tradition, specify an EXPR beginning with '>&' , in which case the rest of the string is interpreted as the name of a filehandle (or file descriptor, if numeric) to be duped and opened. You may use & after > , >> , < , +> , +>> , and +< . The mode you specify should match the mode of the original filehandle. (Duping a filehandle does not take into account any existing contents of stdio buffers.) Duping file handles is not yet supported for 3-argument open().

    Here is a script that saves, redirects, and restores STDOUT and STDERR:

    1. #!/usr/bin/perl
    2. open(OLDOUT, ">&STDOUT");
    3. open(OLDERR, ">&STDERR");
    4. open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
    5. open(STDERR, ">&STDOUT") || die "Can't dup stdout";
    6. select(STDERR); $| = 1; # make unbuffered
    7. select(STDOUT); $| = 1; # make unbuffered
    8. print STDOUT "stdout 1\n"; # this works for
    9. print STDERR "stderr 1\n"; # subprocesses too
    10. close(STDOUT);
    11. close(STDERR);
    12. open(STDOUT, ">&OLDOUT");
    13. open(STDERR, ">&OLDERR");
    14. print STDOUT "stdout 2\n";
    15. print STDERR "stderr 2\n";

    If you specify '<&=N' , where N is a number, then Perl will do an equivalent of C's fdopen of that file descriptor; this is more parsimonious of file descriptors. For example:

    1. open(FILEHANDLE, "<&=$fd")

    Note that this feature depends on the fdopen() C library function. On many UNIX systems, fdopen() is known to fail when file descriptors exceed a certain value, typically 255. If you need more file descriptors than that, consider rebuilding Perl to use the sfio library.

    If you open a pipe on the command '-' , i.e., either '|-' or '-|' with 2-arguments (or 1-argument) form of open(), then there is an implicit fork done, and the return value of open is the pid of the child within the parent process, and 0 within the child process. (Use defined($pid) to determine whether the open was successful.) The filehandle behaves normally for the parent, but i/o to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child process the filehandle isn't opened--i/o happens from/to the new STDOUT or STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe command gets executed, such as when you are running setuid, and don't want to have to scan shell commands for metacharacters. The following triples are more or less equivalent:

    1. open(FOO, "|tr '[a-z]' '[A-Z]'");
    2. open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
    3. open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
    4. open(FOO, "cat -n '$file'|");
    5. open(FOO, '-|', "cat -n '$file'");
    6. open(FOO, '-|') || exec 'cat', '-n', $file;

    See Safe Pipe Opens in perlipc for more examples of this.

    Beginning with v5.6.0, Perl will attempt to flush all files opened for output before any operation that may do a fork, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the autoflush() method of IO::Handle on any open handles.

    On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor as determined by the value of $^F. See $^F in perlvar.

    Closing any piped filehandle causes the parent process to wait for the child to finish, and returns the status value in $? .

    The filename passed to 2-argument (or 1-argument) form of open() will have leading and trailing whitespace deleted, and the normal redirection characters honored. This property, known as "magic open", can often be used to good effect. A user could specify a filename of "rsh cat file |", or you could change certain filenames as needed:

    1. $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
    2. open(FH, $filename) or die "Can't open $filename: $!";

    Use 3-argument form to open a file with arbitrary weird characters in it,

    1. open(FOO, '<', $file);

    otherwise it's necessary to protect any leading and trailing whitespace:

    1. $file =~ s#^(\s)#./$1#;
    2. open(FOO, "< $file\0");

    (this may not work on some bizzare filesystems). One should conscientiously choose between the the magic and 3-arguments form of open():

    1. open IN, $ARGV[0];

    will allow the user to specify an argument of the form "rsh cat file |" , but will not work on a filename which happens to have a trailing space, while

    1. open IN, '<', $ARGV[0];

    will have exactly the opposite restrictions.

    If you want a "real" C open (see open(2) on your system), then you should use the sysopen function, which involves no such magic (but may use subtly different filemodes than Perl open(), which is mapped to C fopen()). This is another way to protect your filenames from interpretation. For example:

    1. use IO::Handle;
    2. sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
    3. or die "sysopen $path: $!";
    4. $oldfh = select(HANDLE); $| = 1; select($oldfh);
    5. print HANDLE "stuff $$\n");
    6. seek(HANDLE, 0, 0);
    7. print "File contains: ", <HANDLE>;

    Using the constructor from the IO::Handle package (or one of its subclasses, such as IO::File or IO::Socket ), you can generate anonymous filehandles that have the scope of whatever variables hold references to them, and automatically close whenever and however you leave that scope:

    1. use IO::File;
    2. #...
    3. sub read_myfile_munged {
    4. my $ALL = shift;
    5. my $handle = new IO::File;
    6. open($handle, "myfile") or die "myfile: $!";
    7. $first = <$handle>
    8. or return (); # Automatically closed here.
    9. mung $first or die "mung failed"; # Or here.
    10. return $first, <$handle> if $ALL; # Or here.
    11. $first; # Or here.
    12. }

    See seek for some details about mixing reading and writing.