=head1 NAME perlfaq5 - Files and Formats =head1 DESCRIPTION This section deals with I/O and the "f" issues: filehandles, flushing, formats, and footers. =head2 How do I flush/unbuffer an output filehandle? Why must I do this? X X X X (contributed by brian d foy) You might like to read Mark Jason Dominus's "Suffering From Buffering" at http://perl.plover.com/FAQs/Buffering.html . Perl normally buffers output so it doesn't make a system call for every bit of output. By saving up output, it makes fewer expensive system calls. For instance, in this little bit of code, you want to print a dot to the screen for every line you process to watch the progress of your program. Instead of seeing a dot for every line, Perl buffers the output and you have a long wait before you see a row of 50 dots all at once: # long wait, then row of dots all at once while( <> ) { print "."; print "\n" unless ++$count % 50; #... expensive line processing operations } To get around this, you have to unbuffer the output filehandle, in this case, C. You can set the special variable C<$|> to a true value (mnemonic: making your filehandles "piping hot"): $|++; # dot shown immediately while( <> ) { print "."; print "\n" unless ++$count % 50; #... expensive line processing operations } The C<$|> is one of the per-filehandle special variables, so each filehandle has its own copy of its value. If you want to merge standard output and standard error for instance, you have to unbuffer each (although STDERR might be unbuffered by default): { my $previous_default = select(STDOUT); # save previous default $|++; # autoflush STDOUT select(STDERR); $|++; # autoflush STDERR, to be sure select($previous_default); # restore previous default } # now should alternate . and + while( 1 ) { sleep 1; print STDOUT "."; print STDERR "+"; print STDOUT "\n" unless ++$count % 25; } Besides the C<$|> special variable, you can use C to give your filehandle a C<:unix> layer, which is unbuffered: binmode( STDOUT, ":unix" ); while( 1 ) { sleep 1; print "."; print "\n" unless ++$count % 50; } For more information on output layers, see the entries for C and C in L, and the C module documentation. If you are using C or one of its subclasses, you can call the C method to change the settings of the filehandle: use IO::Handle; open my( $io_fh ), ">", "output.txt"; $io_fh->autoflush(1); The C objects also have a C method. You can flush the buffer any time you want without auto-buffering $io_fh->flush; =head2 How do I change, delete, or insert a line in a file, or append to the beginning of a file? X (contributed by brian d foy) The basic idea of inserting, changing, or deleting a line from a text file involves reading and printing the file to the point you want to make the change, making the change, then reading and printing the rest of the file. Perl doesn't provide random access to lines (especially since the record input separator, C<$/>, is mutable), although modules such as C can fake it. A Perl program to do these tasks takes the basic form of opening a file, printing its lines, then closing the file: open my $in, '<', $file or die "Can't read old file: $!"; open my $out, '>', "$file.new" or die "Can't write new file: $!"; while( <$in> ) { print $out $_; } close $out; Within that basic form, add the parts that you need to insert, change, or delete lines. To prepend lines to the beginning, print those lines before you enter the loop that prints the existing lines. open my $in, '<', $file or die "Can't read old file: $!"; open my $out, '>', "$file.new" or die "Can't write new file: $!"; print $out "# Add this line to the top\n"; # <--- HERE'S THE MAGIC while( <$in> ) { print $out $_; } close $out; To change existing lines, insert the code to modify the lines inside the C loop. In this case, the code finds all lowercased versions of "perl" and uppercases them. The happens for every line, so be sure that you're supposed to do that on every line! open my $in, '<', $file or die "Can't read old file: $!"; open my $out, '>', "$file.new" or die "Can't write new file: $!"; print $out "# Add this line to the top\n"; while( <$in> ) { s/\b(perl)\b/Perl/g; print $out $_; } close $out; To change only a particular line, the input line number, C<$.>, is useful. First read and print the lines up to the one you want to change. Next, read the single line you want to change, change it, and print it. After that, read the rest of the lines and print those: while( <$in> ) # print the lines before the change { print $out $_; last if $. == 4; # line number before change } my $line = <$in>; $line =~ s/\b(perl)\b/Perl/g; print $out $line; while( <$in> ) # print the rest of the lines { print $out $_; } To skip lines, use the looping controls. The C in this example skips comment lines, and the C stops all processing once it encounters either C<__END__> or C<__DATA__>. while( <$in> ) { next if /^\s+#/; # skip comment lines last if /^__(END|DATA)__$/; # stop at end of code marker print $out $_; } Do the same sort of thing to delete a particular line by using C to skip the lines you don't want to show up in the output. This example skips every fifth line: while( <$in> ) { next unless $. % 5; print $out $_; } If, for some odd reason, you really want to see the whole file at once rather than processing line by line, you can slurp it in (as long as you can fit the whole thing in memory!): open my $in, '<', $file or die "Can't read old file: $!" open my $out, '>', "$file.new" or die "Can't write new file: $!"; my @lines = do { local $/; <$in> }; # slurp! # do your magic here print $out @lines; Modules such as C and C can help with that too. If you can, however, avoid reading the entire file at once. Perl won't give that memory back to the operating system until the process finishes. You can also use Perl one-liners to modify a file in-place. The following changes all 'Fred' to 'Barney' in F, overwriting the file with the new contents. With the C<-p> switch, Perl wraps a C loop around the code you specify with C<-e>, and C<-i> turns on in-place editing. The current line is in C<$_>. With C<-p>, Perl automatically prints the value of C<$_> at the end of the loop. See L for more details. perl -pi -e 's/Fred/Barney/' inFile.txt To make a backup of C, give C<-i> a file extension to add: perl -pi.bak -e 's/Fred/Barney/' inFile.txt To change only the fifth line, you can add a test checking C<$.>, the input line number, then only perform the operation when the test passes: perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt To add lines before a certain line, you can add a line (or lines!) before Perl prints C<$_>: perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt You can even add a line to the beginning of a file, since the current line prints at the end of the loop: perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt To insert a line after one already in the file, use the C<-n> switch. It's just like C<-p> except that it doesn't print C<$_> at the end of the loop, so you have to do that yourself. In this case, print C<$_> first, then print the line that you want to add. perl -ni -e 'print; print "Put after fifth line\n" if $. == 5' inFile.txt To delete lines, only print the ones that you want. perl -ni -e 'print unless /d/' inFile.txt ... or ... perl -pi -e 'next unless /d/' inFile.txt =head2 How do I count the number of lines in a file? X X X One fairly efficient way is to count newlines in the file. The following program uses a feature of tr///, as documented in L. If your text file doesn't end with a newline, then it's not really a proper text file, so this may report one fewer line than you expect. $lines = 0; open(FILE, $filename) or die "Can't open `$filename': $!"; while (sysread FILE, $buffer, 4096) { $lines += ($buffer =~ tr/\n//); } close FILE; This assumes no funny games with newline translations. =head2 How can I use Perl's C<-i> option from within a program? X<-i> X C<-i> sets the value of Perl's C<$^I> variable, which in turn affects the behavior of C<< <> >>; see L for more details. By modifying the appropriate variables directly, you can get the same behavior within a larger program. For example: # ... { local($^I, @ARGV) = ('.orig', glob("*.c")); while (<>) { if ($. == 1) { print "This line should appear at the top of each file\n"; } s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case print; close ARGV if eof; # Reset $. } } # $^I and @ARGV return to their old values here This block modifies all the C<.c> files in the current directory, leaving a backup of the original data from each file in a new C<.c.orig> file. =head2 How can I copy a file? X X X (contributed by brian d foy) Use the C module. It comes with Perl and can do a true copy across file systems, and it does its magic in a portable fashion. use File::Copy; copy( $original, $new_copy ) or die "Copy failed: $!"; If you can't use C, you'll have to do the work yourself: open the original file, open the destination file, then print to the destination file as you read the original. You also have to remember to copy the permissions, owner, and group to the new file. =head2 How do I make a temporary file name? X If you don't need to know the name of the file, you can use C with C in place of the file name. In Perl 5.8 or later, the C function creates an anonymous temporary file: open my $tmp, '+>', undef or die $!; Otherwise, you can use the File::Temp module. use File::Temp qw/ tempfile tempdir /; $dir = tempdir( CLEANUP => 1 ); ($fh, $filename) = tempfile( DIR => $dir ); # or if you don't need to know the filename $fh = tempfile( DIR => $dir ); The File::Temp has been a standard module since Perl 5.6.1. If you don't have a modern enough Perl installed, use the C class method from the IO::File module to get a filehandle opened for reading and writing. Use it if you don't need to know the file's name: use IO::File; $fh = IO::File->new_tmpfile() or die "Unable to make new temporary file: $!"; If you're committed to creating a temporary file by hand, use the process ID and/or the current time-value. If you need to have many temporary files in one process, use a counter: BEGIN { use Fcntl; my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP}; my $base_name = sprintf "%s/%d-%d-0000", $temp_dir, $$, time; sub temp_file { local *FH; my $count = 0; until( defined(fileno(FH)) || $count++ > 100 ) { $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e; # O_EXCL is required for security reasons. sysopen FH, $base_name, O_WRONLY|O_EXCL|O_CREAT; } if( defined fileno(FH) ) { return (*FH, $base_name); } else { return (); } } } =head2 How can I manipulate fixed-record-length files? X X The most efficient way is using L and L. This is faster than using L when taking many, many strings. It is slower for just a few. Here is a sample chunk of code to break up and put back together again some fixed-format input lines, in this case from the output of a normal, Berkeley-style ps: # sample input line: # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what my $PS_T = 'A6 A4 A7 A5 A*'; open my $ps, '-|', 'ps'; print scalar <$ps>; my @fields = qw( pid tt stat time command ); while (<$ps>) { my %process; @process{@fields} = unpack($PS_T, $_); for my $field ( @fields ) { print "$field: <$process{$field}>\n"; } print 'line=', pack($PS_T, @process{@fields} ), "\n"; } We've used a hash slice in order to easily handle the fields of each row. Storing the keys in an array means it's easy to operate on them as a group or loop over them with for. It also avoids polluting the program with global variables and using symbolic references. =head2 How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles? X X X As of perl5.6, open() autovivifies file and directory handles as references if you pass it an uninitialized scalar variable. You can then pass these references just like any other scalar, and use them in the place of named handles. open my $fh, $file_name; open local $fh, $file_name; print $fh "Hello World!\n"; process_file( $fh ); If you like, you can store these filehandles in an array or a hash. If you access them directly, they aren't simple scalars and you need to give C a little help by placing the filehandle reference in braces. Perl can only figure it out on its own when the filehandle reference is a simple scalar. my @fhs = ( $fh1, $fh2, $fh3 ); for( $i = 0; $i <= $#fhs; $i++ ) { print {$fhs[$i]} "just another Perl answer, \n"; } Before perl5.6, you had to deal with various typeglob idioms which you may see in older code. open FILE, "> $filename"; process_typeglob( *FILE ); process_reference( \*FILE ); sub process_typeglob { local *FH = shift; print FH "Typeglob!" } sub process_reference { local $fh = shift; print $fh "Reference!" } If you want to create many anonymous handles, you should check out the Symbol or IO::Handle modules. =head2 How can I use a filehandle indirectly? X An indirect filehandle is using something other than a symbol in a place that a filehandle is expected. Here are ways to get indirect filehandles: $fh = SOME_FH; # bareword is strict-subs hostile $fh = "SOME_FH"; # strict-refs hostile; same package only $fh = *SOME_FH; # typeglob $fh = \*SOME_FH; # ref to typeglob (bless-able) $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob Or, you can use the C method from one of the IO::* modules to create an anonymous filehandle, store that in a scalar variable, and use it as though it were a normal filehandle. use IO::Handle; # 5.004 or higher $fh = IO::Handle->new(); Then use any of those as you would a normal filehandle. Anywhere that Perl is expecting a filehandle, an indirect filehandle may be used instead. An indirect filehandle is just a scalar variable that contains a filehandle. Functions like C, C, C, or the C<< >> diamond operator will accept either a named filehandle or a scalar variable containing one: ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR); print $ofh "Type it: "; $got = <$ifh> print $efh "What was that: $got"; If you're passing a filehandle to a function, you can write the function in two ways: sub accept_fh { my $fh = shift; print $fh "Sending to indirect filehandle\n"; } Or it can localize a typeglob and use the filehandle directly: sub accept_fh { local *FH = shift; print FH "Sending to localized filehandle\n"; } Both styles work with either objects or typeglobs of real filehandles. (They might also work with strings under some circumstances, but this is risky.) accept_fh(*STDOUT); accept_fh($handle); In the examples above, we assigned the filehandle to a scalar variable before using it. That is because only simple scalar variables, not expressions or subscripts of hashes or arrays, can be used with built-ins like C, C, or the diamond operator. Using something other than a simple scalar variable as a filehandle is illegal and won't even compile: @fd = (*STDIN, *STDOUT, *STDERR); print $fd[1] "Type it: "; # WRONG $got = <$fd[0]> # WRONG print $fd[2] "What was that: $got"; # WRONG With C and C, you get around this by using a block and an expression where you would place the filehandle: print { $fd[1] } "funny stuff\n"; printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559; # Pity the poor deadbeef. That block is a proper block like any other, so you can put more complicated code there. This sends the message out to one of two places: $ok = -x "/bin/cat"; print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n"; print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n"; This approach of treating C and C like object methods calls doesn't work for the diamond operator. That's because it's a real operator, not just a function with a comma-less argument. Assuming you've been storing typeglobs in your structure as we did above, you can use the built-in function named C to read a record just as C<< <> >> does. Given the initialization shown above for @fd, this would work, but only because readline() requires a typeglob. It doesn't work with objects or strings, which might be a bug we haven't fixed yet. $got = readline($fd[0]); Let it be noted that the flakiness of indirect filehandles is not related to whether they're strings, typeglobs, objects, or anything else. It's the syntax of the fundamental operators. Playing the object game doesn't help you at all here. =head2 How can I set up a footer format to be used with write()? X