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.TH PERLFAQ5 1 "2006-01-07" "perl v5.8.8" "Perl Programmers Reference Guide"
perlfaq5 \- Files and Formats ($Revision: 1.42 $, $Date: 2005/12/31 00:54:37 $)
This section deals with I/O and the \*(L"f\*(R" issues: filehandles, flushing,
.Sh "How do I flush/unbuffer an output filehandle? Why must I do this?"
.IX Xref "flush buffer unbuffer autoflush"
.IX Subsection "How do I flush/unbuffer an output filehandle? Why must I do this?"
Perl does not support truly unbuffered output (except
insofar as you can \f(CW\*(C`syswrite(OUT, $char, 1)\*(C'\fR), although it
does support is \*(L"command buffering\*(R", in which a physical
write is performed after every output command.
The C standard I/O library (stdio) normally buffers
characters sent to devices so that there isn't a system call
for each byte. In most stdio implementations, the type of
output buffering and the size of the buffer varies according
to the type of device. Perl's \fIprint()\fR and \fIwrite()\fR functions
normally buffer output, while \fIsyswrite()\fR bypasses buffering
If you want your output to be sent immediately when you
execute \fIprint()\fR or \fIwrite()\fR (for instance, for some network
protocols), you must set the handle's autoflush flag. This
flag is the Perl variable $| and when it is set to a true
value, Perl will flush the handle's buffer after each
\&\fIprint()\fR or \fIwrite()\fR. Setting $| affects buffering only for
the currently selected default file handle. You choose this
handle with the one argument \fIselect()\fR call (see
"$|" in perlvar and \*(L"select\*(R" in perlfunc).
Use \fIselect()\fR to choose the desired handle, then set its
per-filehandle variables.
\& $old_fh = select(OUTPUT_HANDLE);
Some idioms can handle this in a single statement:
\& select((select(OUTPUT_HANDLE), $| = 1)[0]);
\& $| = 1, select $_ for select OUTPUT_HANDLE;
Some modules offer object-oriented access to handles and their
variables, although they may be overkill if this is the only
thing you do with them. You can use IO::Handle:
\& open(DEV, ">/dev/printer"); # but is this?
\& use IO::Socket; # this one is kinda a pipe?
\& my $sock = IO::Socket::INET->new( 'www.example.com:80' );
.Sh "How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?"
.IX Subsection "How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?"
Use the Tie::File module, which is included in the standard
distribution since Perl 5.8.0.
.Sh "How do I count the number of lines in a file?"
.IX Xref "file, counting lines lines line"
.IX Subsection "How do I count the number of lines in a file?"
One fairly efficient way is to count newlines in the file. The
following program uses a feature of tr///, as documented in perlop.
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.
\& open(FILE, $filename) or die "Can't open `$filename': $!";
\& while (sysread FILE, $buffer, 4096) {
\& $lines += ($buffer =~ tr/\en//);
This assumes no funny games with newline translations.
.ie n .Sh "How can I use Perl's ""\-i"" option from within a program?"
.el .Sh "How can I use Perl's \f(CW\-i\fP option from within a program?"
.IX Subsection "How can I use Perl's -i option from within a program?"
\&\f(CW\*(C`\-i\*(C'\fR sets the value of Perl's \f(CW$^I\fR variable, which in turn affects
the behavior of \f(CW\*(C`<>\*(C'\fR; see perlrun for more details. By
modifying the appropriate variables directly, you can get the same
behavior within a larger program. For example:
\& local($^I, @ARGV) = ('.orig', glob("*.c"));
\& print "This line should appear at the top of each file\en";
\& s/\eb(p)earl\eb/${1}erl/i; # Correct typos, preserving case
\& close ARGV if eof; # Reset $.
\& # $^I and @ARGV return to their old values here
This block modifies all the \f(CW\*(C`.c\*(C'\fR files in the current directory,
leaving a backup of the original data from each file in a new
\&\f(CW\*(C`.c.orig\*(C'\fR file.
.Sh "How can I copy a file?"
.IX Xref "copy file, copy"
.IX Subsection "How can I copy a file?"
(contributed by brian d foy)
Use the File::Copy module. It comes with Perl and can do a
true copy across file systems, and it does its magic in
\& copy( $original, $new_copy ) or die "Copy failed: $!";
If you can't use File::Copy, 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.
.Sh "How do I make a temporary file name?"
.IX Xref "file, temporary"
.IX Subsection "How do I make a temporary file name?"
If you don't need to know the name of the file, you can use \f(CW\*(C`open()\*(C'\fR
with \f(CW\*(C`undef\*(C'\fR in place of the file name. The \f(CW\*(C`open()\*(C'\fR 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 \f(CW\*(C`new_tmpfile\*(C'\fR
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:
\& $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 \s-1ID\s0 and/or the current time\-value. If you need to have many
temporary files in one process, use a counter:
\& my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
\& my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
\& until (defined(fileno(FH)) || $count++ > 100) {
\& $base_name =~ s/-(\ed+)$/"-" . (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);
.Sh "How can I manipulate fixed-record-length files?"
.IX Xref "fixed-length file, fixed-length records"
.IX Subsection "How can I manipulate fixed-record-length files?"
The most efficient way is using \fIpack()\fR and
\&\fIunpack()\fR. This is faster than using
\&\fIsubstr()\fR when taking many, many strings. It is
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,
\& # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
\& my $PS_T = 'A6 A4 A7 A5 A*';
\& open my $ps, '-|', 'ps';
\& my @fields = qw( pid tt stat time command );
\& @process{@fields} = unpack($PS_T, $_);
\& for my $field ( @fields ) {
\& print "$field: <$process{$field}>\en";
\& print 'line=', pack($PS_T, @process{@fields} ), "\en";
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.
.Sh "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?"
.IX Xref "filehandle, local filehandle, passing filehandle, reference"
.IX Subsection "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?"
As of perl5.6, \fIopen()\fR 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!\en";
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( \e*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.
.Sh "How can I use a filehandle indirectly?"
.IX Xref "filehandle, indirect"
.IX Subsection "How can I use a filehandle indirectly?"
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 = \e*SOME_FH; # ref to typeglob (bless-able)
\& $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
Or, you can use the \f(CW\*(C`new\*(C'\fR 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
\& $fh = IO::Handle->new();
Then use any of those as you would a normal filehandle. Anywhere that
Perl is expecting a filehandle, an indirect filehandle may be used
instead. An indirect filehandle is just a scalar variable that contains
a filehandle. Functions like \f(CW\*(C`print\*(C'\fR, \f(CW\*(C`open\*(C'\fR, \f(CW\*(C`seek\*(C'\fR, or
the \f(CW\*(C`<FH>\*(C'\fR diamond operator will accept either a named filehandle
or a scalar variable containing one:
\& ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
\& print $ofh "Type it: ";
\& print $efh "What was that: $got";
If you're passing a filehandle to a function, you can write
the function in two ways:
\& print $fh "Sending to indirect filehandle\en";
Or it can localize a typeglob and use the filehandle directly:
\& print FH "Sending to localized filehandle\en";
Both styles work with either objects or typeglobs of real filehandles.
(They might also work with strings under some circumstances, but this
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 \f(CW\*(C`print\*(C'\fR, \f(CW\*(C`printf\*(C'\fR, 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 \f(CW\*(C`print\*(C'\fR and \f(CW\*(C`printf\*(C'\fR, you get around this by using a block and
an expression where you would place the filehandle:
\& print { $fd[1] } "funny stuff\en";
\& printf { $fd[1] } "Pity the poor %x.\en", 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:
\& print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\en";
\& print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\en";
This approach of treating \f(CW\*(C`print\*(C'\fR and \f(CW\*(C`printf\*(C'\fR 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 \f(CW\*(C`readline\*(C'\fR to read a record just
as \f(CW\*(C`<>\*(C'\fR does. Given the initialization shown above for \f(CW@fd\fR, this
would work, but only because \fIreadline()\fR 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.
.Sh "How can I set up a footer format to be used with \fIwrite()\fP?"
.IX Subsection "How can I set up a footer format to be used with write()?"
There's no builtin way to do this, but perlform has a couple of
techniques to make it possible for the intrepid hacker.
.Sh "How can I \fIwrite()\fP into a string?"
.IX Xref "write, into a string"
.IX Subsection "How can I write() into a string?"
See \*(L"Accessing Formatting Internals\*(R" in perlform for an \fIswrite()\fR function.
.Sh "How can I output my numbers with commas added?"
.IX Xref "number, commify"
.IX Subsection "How can I output my numbers with commas added?"
(contributed by brian d foy and Benjamin Goldberg)
You can use Number::Format to separate places in a number.
It handles locale information for those of you who want to insert
full stops instead (or anything else that they want to use,
This subroutine will add commas to your number:
\& 1 while s/^([-+]?\ed+)(\ed{3})/$1,$2/;
This regex from Benjamin Goldberg will add commas to numbers:
\& s/(^[-+]?\ed+?(?=(?>(?:\ed{3})+)(?!\ed))|\eG\ed{3}(?=\ed))/$1,/g;
It is easier to see with comments:
\& ^[-+]? # beginning of number.
\& \ed+? # first digits before first comma
\& (?= # followed by, (but not included in the match) :
\& (?>(?:\ed{3})+) # some positive multiple of three digits.
\& (?!\ed) # an *exact* multiple, not x * 3 + 1 or whatever.
\& \eG\ed{3} # after the last group, get three digits
\& (?=\ed) # but they have to have more digits after them.
.Sh "How can I translate tildes (~) in a filename?"
.IX Xref "tilde tilde expansion"
.IX Subsection "How can I translate tildes (~) in a filename?"
Use the <> (\fIglob()\fR) operator, documented in perlfunc. Older
versions of Perl require that you have a shell installed that groks
tildes. Recent perl versions have this feature built in. The
File::KGlob module (available from \s-1CPAN\s0) gives more portable glob
Within Perl, you may use this directly:
\& ^ ~ # find a leading tilde
\& [^/] # a non-slash character
\& * # repeated 0 or more times (0 means me)
\& : ( $ENV{HOME} || $ENV{LOGDIR} )
.Sh "How come when I open a file read-write it wipes it out?"
.IX Xref "clobber read-write clobbering truncate truncating"
.IX Subsection "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
\&\fIthen\fR gives you read-write access:
\& open(FH, "+> /path/name"); # WRONG (almost always)
Whoops. You should instead use this, which will fail if the file
\& open(FH, "+< /path/name"); # open for update
Using \*(L">\*(R" always clobbers or creates. Using \*(L"<\*(R" never does
either. The \*(L"+\*(R" doesn't change this.
Here are examples of many kinds of file opens. Those using \fIsysopen()\fR
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, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
\& or die "can't open /foo/somefile: $!":
Be warned that neither creation nor deletion of files is guaranteed to
be an atomic operation over \s-1NFS\s0. 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).
.ie n .Sh "Why do I sometimes get an ""Argument list too long"" when I use <*>?"
.el .Sh "Why do I sometimes get an ``Argument list too long'' when I use <*>?"
.IX Xref "argument list too long"
.IX Subsection "Why do I sometimes get an Argument list too long when I use <*>?"
The \f(CW\*(C`<>\*(C'\fR operator performs a globbing operation (see above).
In Perl versions earlier than v5.6.0, the internal \fIglob()\fR operator forks
\&\fIcsh\fR\|(1) to do the actual glob expansion, but
csh can't handle more than 127 items and so gives the error message
\&\f(CW\*(C`Argument list too long\*(C'\fR. 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 \fIreaddir()\fR and patterns, or use a module like File::KGlob,
one that doesn't use the shell to do globbing.
.Sh "Is there a leak/bug in \fIglob()\fP?"
.IX Subsection "Is there a leak/bug in glob()?"
Due to the current implementation on some operating systems, when you
use the \fIglob()\fR function or its angle-bracket alias in a scalar
context, you may cause a memory leak and/or unpredictable behavior. It's
best therefore to use \fIglob()\fR only in list context.
.ie n .Sh "How can I open a file with a leading "">"" or trailing blanks?"
.el .Sh "How can I open a file with a leading ``>'' or trailing blanks?"
.IX Xref "filename, special characters"
.IX Subsection "How can I open a file with a leading > or trailing blanks?"
(contributed by Brian McCauley)
The special two argument form of Perl's \fIopen()\fR function ignores
trailing blanks in filenames and infers the mode from certain leading
characters (or a trailing \*(L"|\*(R"). In older versions of Perl this was the
only version of \fIopen()\fR and so it is prevalent in old code and books.
Unless you have a particular reason to use the two argument form you
should use the three argument form of \fIopen()\fR which does not treat any
charcters in the filename as special.
\& open FILE, "<", " file "; # filename is " file "
\& open FILE, ">", ">file"; # filename is ">file"
.Sh "How can I reliably rename a file?"
.IX Xref "rename mv move file, rename ren"
.IX Subsection "How can I reliably rename a file?"
If your operating system supports a proper \fImv\fR\|(1) utility or its
functional equivalent, this works:
\& rename($old, $new) or system("mv", $old, $new);
It may be more portable to use the File::Copy module instead.
You just copy to the new file to the new name (checking return
values), then delete the old one. This isn't really the same
semantically as a \fIrename()\fR, which preserves meta-information like
permissions, timestamps, inode info, etc.
Newer versions of File::Copy export a \fImove()\fR function.
.Sh "How can I lock a file?"
.IX Xref "lock file, lock flock"
.IX Subsection "How can I lock a file?"
Perl's builtin \fIflock()\fR function (see perlfunc for details) will call
\&\fIflock\fR\|(2) if that exists, \fIfcntl\fR\|(2) if it doesn't (on perl version 5.004 and
later), and \fIlockf\fR\|(3) if neither of the two previous system calls exists.
On some systems, it may even use a different form of native locking.
Here are some gotchas with Perl's \fIflock()\fR:
Produces a fatal error if none of the three system calls (or their
close equivalent) exists.
\&\fIlockf\fR\|(3) does not provide shared locking, and requires that the
filehandle be open for writing (or appending, or read/writing).
Some versions of \fIflock()\fR can't lock files over a network (e.g. on \s-1NFS\s0 file
systems), so you'd need to force the use of \fIfcntl\fR\|(2) when you build Perl.
But even this is dubious at best. See the flock entry of perlfunc
and the \fI\s-1INSTALL\s0\fR file in the source distribution for information on
building Perl to do this.
Two potentially non-obvious but traditional flock semantics are that
it waits indefinitely until the lock is granted, and that its locks are
\&\fImerely advisory\fR. Such discretionary locks are more flexible, but
offer fewer guarantees. This means that files locked with \fIflock()\fR may
be modified by programs that do not also use \fIflock()\fR. Cars that stop
for red lights get on well with each other, but not with cars that don't
stop for red lights. See the perlport manpage, your port's specific
documentation, or your system-specific local manpages for details. It's
best to assume traditional behavior if you're writing portable programs.
(If you're not, you should as always feel perfectly free to write
for your own system's idiosyncrasies (sometimes called \*(L"features\*(R").
Slavish adherence to portability concerns shouldn't get in the way of
your getting your job done.)
For more information on file locking, see also
\&\*(L"File Locking\*(R" in perlopentut if you have it (new for 5.6).
.ie n .Sh "Why can't I just open(\s-1FH\s0, "">file.lock"")?"
.el .Sh "Why can't I just open(\s-1FH\s0, ``>file.lock'')?"
.IX Xref "lock, lockfile race condition"
.IX Subsection "Why can't I just open(FH, "">file.lock"")?"
A common bit of code \fB\s-1NOT\s0 \s-1TO\s0 \s-1USE\s0\fR is this:
\& sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
\& open(LCK, "> file.lock"); # THIS BROKEN CODE
This is a classic race condition: you take two steps to do something
which must be done in one. That's why computer hardware provides an
atomic test-and-set instruction. In theory, this \*(L"ought\*(R" to work:
\& sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
\& or die "can't open file.lock: $!";
except that lamentably, file creation (and deletion) is not atomic
over \s-1NFS\s0, so this won't work (at least, not every time) over the net.
Various schemes involving \fIlink()\fR have been suggested, but
these tend to involve busy\-wait, which is also subdesirable.
.Sh "I still don't get locking. I just want to increment the number in the file. How can I do this?"
.IX Xref "counter file, counter"
.IX Subsection "I still don't get locking. I just want to increment the number in the file. How can I do this?"
Didn't anyone ever tell you web-page hit counters were useless?
They don't count number of hits, they're a waste of time, and they serve
only to stroke the writer's vanity. It's better to pick a random number;
Anyway, this is what you can do if you can't help yourself.
\& use Fcntl qw(:DEFAULT :flock);
\& sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
\& flock(FH, LOCK_EX) or die "can't flock numfile: $!";
\& seek(FH, 0, 0) or die "can't rewind numfile: $!";
\& truncate(FH, 0) or die "can't truncate numfile: $!";
\& (print FH $num+1, "\en") or die "can't write numfile: $!";
\& close FH or die "can't close numfile: $!";
Here's a much better web-page hit counter:
\& $hits = int( (time() - 850_000_000) / rand(1_000) );
If the count doesn't impress your friends, then the code might. :\-)
.Sh "All I want to do is append a small amount of text to the end of a file. Do I still have to use locking?"
.IX Xref "append file, append"
.IX Subsection "All I want to do is append a small amount of text to the end of a file. Do I still have to use locking?"
If you are on a system that correctly implements \fIflock()\fR and you use the
example appending code from \*(L"perldoc \-f flock\*(R" everything will be \s-1OK\s0
even if the \s-1OS\s0 you are on doesn't implement append mode correctly (if
such a system exists.) So if you are happy to restrict yourself to OSs
that implement \fIflock()\fR (and that's not really much of a restriction)
then that is what you should do.
If you know you are only going to use a system that does correctly
implement appending (i.e. not Win32) then you can omit the \fIseek()\fR from
If you know you are only writing code to run on an \s-1OS\s0 and filesystem that
does implement append mode correctly (a local filesystem on a modern
Unix for example), and you keep the file in block-buffered mode and you
write less than one buffer-full of output between each manual flushing
of the buffer then each bufferload is almost guaranteed to be written to
the end of the file in one chunk without getting intermingled with
anyone else's output. You can also use the \fIsyswrite()\fR function which is
simply a wrapper around your systems \fIwrite\fR\|(2) system call.
There is still a small theoretical chance that a signal will interrupt
the system level \fIwrite()\fR operation before completion. There is also a
possibility that some \s-1STDIO\s0 implementations may call multiple system
level \fIwrite()\fRs even if the buffer was empty to start. There may be some
systems where this probability is reduced to zero.
.Sh "How do I randomly update a binary file?"
.IX Xref "file, binary patch"
.IX Subsection "How do I randomly update a binary file?"
If you're just trying to patch a binary, in many cases something as
\& perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
However, if you have fixed sized records, then you might do something more
\& $RECSIZE = 220; # size of record, in bytes
\& $recno = 37; # which record to update
\& open(FH, "+<somewhere") || die "can't update somewhere: $!";
\& seek(FH, $recno * $RECSIZE, 0);
\& read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
\& seek(FH, -$RECSIZE, 1);
Locking and error checking are left as an exercise for the reader.
Don't forget them or you'll be quite sorry.
.Sh "How do I get a file's timestamp in perl?"
.IX Xref "timestamp file, timestamp"
.IX Subsection "How do I get a file's timestamp in perl?"
If you want to retrieve the time at which the file was last
read, written, or had its meta-data (owner, etc) changed,
you use the \fB\-A\fR, \fB\-M\fR, or \fB\-C\fR file test operations as
documented in perlfunc. These retrieve the age of the
file (measured against the start-time of your program) in
days as a floating point number. Some platforms may not have
all of these times. See perlport for details. To
retrieve the \*(L"raw\*(R" time in seconds since the epoch, you
would call the stat function, then use \fIlocaltime()\fR,
\&\fIgmtime()\fR, or \fIPOSIX::strftime()\fR to convert this into
\& $write_secs = (stat($file))[9];
\& printf "file %s updated at %s\en", $file,
\& scalar localtime($write_secs);
If you prefer something more legible, use the File::stat module
(part of the standard distribution in version 5.004 and later):
\& # error checking left as an exercise for reader.
\& $date_string = ctime(stat($file)->mtime);
\& print "file $file updated at $date_string\en";
The \fIPOSIX::strftime()\fR approach has the benefit of being,
in theory, independent of the current locale. See perllocale
.Sh "How do I set a file's timestamp in perl?"
.IX Xref "timestamp file, timestamp"
.IX Subsection "How do I set a file's timestamp in perl?"
You use the \fIutime()\fR function documented in \*(L"utime\*(R" in perlfunc.
By way of example, here's a little program that copies the
read and write times from its first argument to all the rest
\& die "usage: cptimes timestamp_file other_files ...\en";
\& ($atime, $mtime) = (stat($timestamp))[8,9];
\& utime $atime, $mtime, @ARGV;
Error checking is, as usual, left as an exercise for the reader.
The perldoc for utime also has an example that has the same
effect as \fItouch\fR\|(1) on files that \fIalready exist\fR.
Certain file systems have a limited ability to store the times
on a file at the expected level of precision. For example, the
\&\s-1FAT\s0 and \s-1HPFS\s0 filesystem are unable to create dates on files with
a finer granularity than two seconds. This is a limitation of
the filesystems, not of \fIutime()\fR.
.Sh "How do I print to more than one file at once?"
.IX Xref "print, to multiple files"
.IX Subsection "How do I print to more than one file at once?"
To connect one filehandle to several output filehandles,
you can use the IO::Tee or Tie::FileHandle::Multiplex modules.
If you only have to do this once, you can print individually
\& for $fh (FH1, FH2, FH3) { print $fh "whatever\en" }
.Sh "How can I read in an entire file all at once?"
.IX Xref "slurp file, slurping"
.IX Subsection "How can I read in an entire file all at once?"
You can use the File::Slurp module to do it in one step.
\& $all_of_it = read_file($filename); # entire file in scalar
\& @all_lines = read_file($filename); # one line perl element
The customary Perl approach for processing all the lines in a file is to
do so one line at a time:
\& open (INPUT, $file) || die "can't open $file: $!";
\& # do something with $_
\& close(INPUT) || die "can't close $file: $!";
This is tremendously more efficient than reading the entire file into
memory as an array of lines and then processing it one element at a time,
which is often\*(--if not almost always\*(--the wrong approach. Whenever
you should think long and hard about why you need everything loaded at
once. It's just not a scalable solution. You might also find it more
fun to use the standard Tie::File module, or the DB_File module's
\&\f(CW$DB_RECNO\fR bindings, which allow you to tie an array to a file so that
accessing an element the array actually accesses the corresponding
You can read the entire filehandle contents into a scalar.
\& open (INPUT, $file) || die "can't open $file: $!";
That temporarily undefs your record separator, and will automatically
close the file at block exit. If the file is already open, just use this:
\& $var = do { local $/; <INPUT> };
For ordinary files you can also use the read function.
\& read( INPUT, $var, -s INPUT );
The third argument tests the byte size of the data on the \s-1INPUT\s0 filehandle
and reads that many bytes into the buffer \f(CW$var\fR.
.Sh "How can I read in a file by paragraphs?"
.IX Xref "file, reading by paragraphs"
.IX Subsection "How can I read in a file by paragraphs?"
Use the \f(CW$/\fR variable (see perlvar for details). You can either
set it to \f(CW""\fR to eliminate empty paragraphs (\f(CW"abc\en\en\en\endef"\fR,
for instance, gets treated as two paragraphs and not three), or
\&\f(CW"\en\en"\fR to accept empty paragraphs.
Note that a blank line must have no blanks in it. Thus
\&\f(CW"fred\en\ \enstuff\en\en"\fR is one paragraph, but \f(CW"fred\en\enstuff\en\en"\fR is two.
.Sh "How can I read a single character from a file? From the keyboard?"
.IX Xref "getc file, reading one character at a time"
.IX Subsection "How can I read a single character from a file? From the keyboard?"
You can use the builtin \f(CW\*(C`getc()\*(C'\fR function for most filehandles, but
it won't (easily) work on a terminal device. For \s-1STDIN\s0, either use
the Term::ReadKey module from \s-1CPAN\s0 or use the sample code in
\&\*(L"getc\*(R" in perlfunc.
If your system supports the portable operating system programming
interface (\s-1POSIX\s0), you can use the following code, which you'll note
turns off echo processing as well.
\& use POSIX qw(:termios_h);
\& my ($term, $oterm, $echo, $noecho, $fd_stdin);
\& $fd_stdin = fileno(STDIN);
\& $term = POSIX::Termios->new();
\& $term->getattr($fd_stdin);
\& $oterm = $term->getlflag();
\& $echo = ECHO | ECHOK | ICANON;
\& $noecho = $oterm & ~$echo;
\& $term->setlflag($noecho);
\& $term->setcc(VTIME, 1);
\& $term->setattr($fd_stdin, TCSANOW);
\& $term->setlflag($oterm);
\& $term->setcc(VTIME, 0);
\& $term->setattr($fd_stdin, TCSANOW);
\& sysread(STDIN, $key, 1);
The Term::ReadKey module from \s-1CPAN\s0 may be easier to use. Recent versions
include also support for non-portable systems as well.
\& open(TTY, "</dev/tty");
\& print "Gimme a char: ";
\& $key = ReadKey 0, *TTY;
\& printf "\enYou said %s, char number %03d\en",
.Sh "How can I tell whether there's a character waiting on a filehandle?"
.IX Subsection "How can I tell whether there's a character waiting on a filehandle?"
The very first thing you should do is look into getting the Term::ReadKey
extension from \s-1CPAN\s0. As we mentioned earlier, it now even has limited
support for non-portable (read: not open systems, closed, proprietary,
not \s-1POSIX\s0, not Unix, etc) systems.
You should also check out the Frequently Asked Questions list in
comp.unix.* for things like this: the answer is essentially the same.
It's very system dependent. Here's one solution that works on \s-1BSD\s0
\& vec($rin, fileno(STDIN), 1) = 1;
\& return $nfd = select($rin,undef,undef,0);
If you want to find out how many characters are waiting, there's
also the \s-1FIONREAD\s0 ioctl call to be looked at. The \fIh2ph\fR tool that
comes with Perl tries to convert C include files to Perl code, which
can be \f(CW\*(C`require\*(C'\fRd. \s-1FIONREAD\s0 ends up defined as a function in the
\&\fIsys/ioctl.ph\fR file:
\& require 'sys/ioctl.ph';
\& ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\en";
\& $size = unpack("L", $size);
If \fIh2ph\fR wasn't installed or doesn't work for you, you can
\&\fIgrep\fR the include files by hand:
\& % grep FIONREAD /usr/include/*/*
\& /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
Or write a small C program using the editor of champions:
\& #include <sys/ioctl.h>
\& printf("%#08x\en", FIONREAD);
\& % cc -o fionread fionread.c
And then hard code it, leaving porting as an exercise to your successor.
\& $FIONREAD = 0x4004667f; # XXX: opsys dependent
\& ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\en";
\& $size = unpack("L", $size);
\&\s-1FIONREAD\s0 requires a filehandle connected to a stream, meaning that sockets,
pipes, and tty devices work, but \fInot\fR files.
.ie n .Sh "How do I do a ""tail \-f"" in perl?"
.el .Sh "How do I do a \f(CWtail \-f\fP in perl?"
.IX Subsection "How do I do a tail -f in perl?"
The statement \f(CW\*(C`seek(GWFILE, 0, 1)\*(C'\fR doesn't change the current position,
but it does clear the end-of-file condition on the handle, so that the
next <\s-1GWFILE\s0> makes Perl try again to read something.
If that doesn't work (it relies on features of your stdio implementation),
then you need something more like this:
\& for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
\& # search for some stuff and put it into files
\& seek(GWFILE, $curpos, 0); # seek to where we had been
If this still doesn't work, look into the \s-1POSIX\s0 module. \s-1POSIX\s0 defines
the \fIclearerr()\fR method, which can remove the end of file condition on a
filehandle. The method: read until end of file, \fIclearerr()\fR, read some
more. Lather, rinse, repeat.
There's also a File::Tail module from \s-1CPAN\s0.
.Sh "How do I \fIdup()\fP a filehandle in Perl?"
.IX Subsection "How do I dup() a filehandle in Perl?"
If you check \*(L"open\*(R" in perlfunc, you'll see that several of the ways
to call \fIopen()\fR should do the trick. For example:
\& open(LOG, ">>/foo/logfile");
\& open(STDERR, ">&LOG");
Or even with a literal numeric descriptor:
\& $fd = $ENV{MHCONTEXTFD};
\& open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
Note that \*(L"<&STDIN\*(R" makes a copy, but \*(L"<&=STDIN\*(R" make
an alias. That means if you close an aliased handle, all
aliases become inaccessible. This is not true with
Error checking, as always, has been left as an exercise for the reader.
.Sh "How do I close a file descriptor by number?"
.IX Xref "file, closing file descriptors"
.IX Subsection "How do I close a file descriptor by number?"
This should rarely be necessary, as the Perl \fIclose()\fR function is to be
used for things that Perl opened itself, even if it was a dup of a
numeric descriptor as with \s-1MHCONTEXT\s0 above. But if you really have
to, you may be able to do this:
\& require 'sys/syscall.ph';
\& $rc = syscall(&SYS_close, $fd + 0); # must force numeric
\& die "can't sysclose $fd: $!" unless $rc == -1;
Or, just use the fdopen(3S) feature of \fIopen()\fR:
\& open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
.ie n .Sh "Why can't I use ""C:\etemp\efoo"" in \s-1DOS\s0 paths? Why doesn't `C:\etemp\efoo.exe` work?"
.el .Sh "Why can't I use ``C:\etemp\efoo'' in \s-1DOS\s0 paths? Why doesn't `C:\etemp\efoo.exe` work?"
.IX Xref "filename, DOS issues"
.IX Subsection "Why can't I use C:tempfoo in DOS paths? Why doesn't `C:tempfoo.exe` work?"
Whoops! You just put a tab and a formfeed into that filename!
Remember that within double quoted strings (\*(L"like\ethis\*(R"), the
backslash is an escape character. The full list of these is in
\&\*(L"Quote and Quote-like Operators\*(R" in perlop. Unsurprisingly, you don't
have a file called \*(L"c:(tab)emp(formfeed)oo\*(R" or
\&\*(L"c:(tab)emp(formfeed)oo.exe\*(R" on your legacy \s-1DOS\s0 filesystem.
Either single-quote your strings, or (preferably) use forward slashes.
Since all \s-1DOS\s0 and Windows versions since something like MS-DOS 2.0 or so
have treated \f(CW\*(C`/\*(C'\fR and \f(CW\*(C`\e\*(C'\fR the same in a path, you might as well use the
one that doesn't clash with Perl\*(--or the \s-1POSIX\s0 shell, \s-1ANSI\s0 C and \*(C+,
awk, Tcl, Java, or Python, just to mention a few. \s-1POSIX\s0 paths
.ie n .Sh "Why doesn't glob(""*.*"") get all the files?"
.el .Sh "Why doesn't glob(``*.*'') get all the files?"
.IX Subsection "Why doesn't glob(*.*) get all the files?"
Because even on non-Unix ports, Perl's glob function follows standard
Unix globbing semantics. You'll need \f(CW\*(C`glob("*")\*(C'\fR to get all (non\-hidden)
files. This makes \fIglob()\fR portable even to legacy systems. Your
port may include proprietary globbing functions as well. Check its
documentation for details.
.ie n .Sh "Why does Perl let me delete read-only files? Why does ""\-i"" clobber protected files? Isn't this a bug in Perl?"
.el .Sh "Why does Perl let me delete read-only files? Why does \f(CW\-i\fP clobber protected files? Isn't this a bug in Perl?"
.IX Subsection "Why does Perl let me delete read-only files? Why does -i clobber protected files? Isn't this a bug in Perl?"
This is elaborately and painstakingly described in the
\&\fIfile-dir-perms\fR article in the \*(L"Far More Than You Ever Wanted To
Know\*(R" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .
The executive summary: learn how your filesystem works. The
permissions on a file say what can happen to the data in that file.
The permissions on a directory say what can happen to the list of
files in that directory. If you delete a file, you're removing its
name from the directory (so the operation depends on the permissions
of the directory, not of the file). If you try to write to the file,
the permissions of the file govern whether you're allowed to.
.Sh "How do I select a random line from a file?"
.IX Xref "file, selecting a random line"
.IX Subsection "How do I select a random line from a file?"
Here's an algorithm from the Camel Book:
\& rand($.) < 1 && ($line = $_) while <>;
This has a significant advantage in space over reading the whole file
in. You can find a proof of this method in \fIThe Art of Computer
Programming\fR, Volume 2, Section 3.4.2, by Donald E. Knuth.
You can use the File::Random module which provides a function
\& use File::Random qw/random_line/;
\& my $line = random_line($filename);
Another way is to use the Tie::File module, which treats the entire
file as an array. Simply access a random array element.
.Sh "Why do I get weird spaces when I print an array of lines?"
.IX Subsection "Why do I get weird spaces when I print an array of lines?"
joins together the elements of \f(CW@lines\fR with a space between them.
If \f(CW@lines\fR were \f(CW\*(C`("little", "fluffy", "clouds")\*(C'\fR then the above
but if each element of \f(CW@lines\fR was a line of text, ending a newline
character \f(CW\*(C`("little\en", "fluffy\en", "clouds\en")\*(C'\fR then it would print:
If your array contains lines, just print them:
.SH "AUTHOR AND COPYRIGHT"
.IX Header "AUTHOR AND COPYRIGHT"
Copyright (c) 1997\-2006 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
Irrespective of its distribution, all code examples here are in the public
domain. You are permitted and encouraged to use this code and any
derivatives thereof in your own programs for fun or for profit as you
see fit. A simple comment in the code giving credit to the \s-1FAQ\s0 would
be courteous but is not required.
projects / OpenSPARC-T2-SAM / blob