MAN page from Old RedHat 5.X perl-5.004-4.i386.rpm
PERLFAQ5
Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index NAME
perlfaq5 - Files and Formats ($Revision: 1.22 $,
$Date: 1997/04/24 22:44:02 $)
DESCRIPTION
This section deals with I/O and the ``f'' issues: filehandles, flushing,formats, and footers.
How do I flush/unbuffer a filehandle? Why must I do this?
The C standard I/O library (stdio) normally buffers characters sent todevices. This is done for efficiency reasons, so that there isn't asystem call for each byte. Any time you use
print() or
write() inPerl, you go though this buffering.
syswrite() circumvents stdio andbuffering.
In most stdio implementations, the type of buffering and the size ofthe buffer varies according to the type of device. Disk files are blockbuffered, often with a buffer size of more than 2k. Pipes and socketsare often buffered with a buffer size between 1/2 and 2k. Serial devices(e.g. modems, terminals) are normally line-buffered, and stdio sendsthe entire line when it gets the newline.
Perl does not support truly unbuffered output (except insofar as you cansyswrite(OUT, $char, 1)). What it does instead support is ``commandbuffering'', in which a physical write is performed after every outputcommand. This isn't as hard on your system as unbuffering, but doesget the output where you want it when you want it.
If you expect characters to get to your device when you print them there,you'll want to autoflush its handle, as in the older:
use FileHandle; open(DEV, "<+/dev/tty"); # ceci n'est pas une pipe DEV->autoflush(1);
or the newer
IO::* modules:
use IO::Handle; open(DEV, ">/dev/printer"); # but is this? DEV->autoflush(1);
or even this:
use IO::Socket; # this one is kinda a pipe? $sock = IO::Socket::INET->new(PeerAddr => 'www.perl.com', PeerPort => 'http(80)', Proto => 'tcp'); die "$!" unless $sock;
$sock->autoflush(); $sock->print("GET /\015\012"); $document = join('', $sock->getlines()); print "DOC IS: $document\n";Note the hardcoded carriage return and newline in their octalequivalents. This is the
ONLY way (currently) to assure a properflush on all platforms, including Macintosh.
You can use select() and the $| variable to control autoflushing(see the section on $| in the perlvar manpage and the select entry in the perlfunc manpage):
$oldh = select(DEV); $| = 1; select($oldh);
You'll also see code that does this without a temporary variable, as in
select((select(DEV), $| = 1)[0]);
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?
Although humans have an easy time thinking of a text file as being asequence of lines that operates much like a stack of playing cards --or punch cards -- computers usually see the text file as a sequence ofbytes. In general, there's no direct way for Perl to seek to aparticular line of a file, insert text into a file, or remove textfrom a file.
(There are exceptions in special circumstances. Replacing a sequenceof bytes with another sequence of the same length is one. Another isusing the $DB_RECNO array bindings as documented in the DB_File manpage.Yet another is manipulating files with all lines the same length.)
The general solution is to create a temporary copy of the text file withthe changes you want, then copy that over the original.
$old = $file; $new = "$file.tmp.$$"; $bak = "$file.bak";
open(OLD, "< $old") or die "can't open $old: $!"; open(NEW, "> $new") or die "can't open $new: $!";
# Correct typos, preserving case while (<OLD>) { s/\b(p)earl\b/${1}erl/i; (print NEW $_) or die "can't write to $new: $!"; } close(OLD) or die "can't close $old: $!"; close(NEW) or die "can't close $new: $!";
rename($old, $bak) or die "can't rename $old to $bak: $!"; rename($new, $old) or die "can't rename $new to $old: $!";
Perl can do this sort of thing for you automatically with the
-icommand-line switch or the closely-related
$^I variable (seethe
perlrun manpage for more details). Note that
-i may require a suffix on some non-Unix systems; see theplatform-specific documentation that came with your port.
# Renumber a series of tests from the command line perl -pi -e 's/(^\s+test\s+)\d+/ $1 . ++$count /e' t/op/taint.t
# form a script local($^I, @ARGV) = ('.bak', 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 $. }If you need to seek to an arbitrary line of a file that changesinfrequently, you could build up an index of byte positions of wherethe line ends are in the file. If the file is large, an index ofevery tenth or hundredth line end would allow you to seek and readfairly efficiently. If the file is sorted, try the look.pl library(part of the standard perl distribution).
In the unique case of deleting lines at the end of a file, youcan use tell() and truncate(). The following code snippet deletesthe last line of a file without making a copy or reading thewhole file into memory:
open (FH, "+< $file"); while ( <FH> ) { $addr = tell(FH) unless eof(FH) } truncate(FH, $addr);Error checking is left as an exercise for the reader.
How do I count the number of lines in a file?
One fairly efficient way is to count newlines in the file. Thefollowing program uses a feature of tr///, as documented in the
perlop manpage.If your text file doesn't end with a newline, then it's not really aproper 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; How do I make a temporary file name?
Use the process
ID and/or the current time-value. If you need to havemany temporary files in one process, use a counter:
BEGIN { use IO::File; use Fcntl; my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMP} || $ENV{TEMP}; my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time()); sub temp_file { my $fh = undef; my $count = 0; until (defined($fh) || $count > 100) { $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e; $fh = IO::File->new($base_name, O_WRONLY|O_EXCL|O_CREAT, 0644) } if (defined($fh)) { return ($fh, $base_name); } else { return (); } } }Or you could simply use
IO::Handle::new_tmpfile.
How can I manipulate fixed-record-length files?
The most efficient way is using
pack() and
unpack(). This is fasterthan using
substr(). Here is a sample chunk of code to break up andput back together again some fixed-format input lines, in this casefrom the output of a normal, Berkeley-style ps:
# sample input line: # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what $PS_T = 'A6 A4 A7 A5 A*'; open(PS, "ps|"); $_ = <PS>; print; while (<PS>) { ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_); for $var (qw!pid tt stat time command!) { print "$var: <$$var>\n"; } print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command), "\n"; } 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?
You may have some success with typeglobs, as we always had to usein days of old:
local(*FH);
But while still supported, that isn't the best to go about gettinglocal filehandles. Typeglobs have their drawbacks. You may well wantto use the
FileHandle module, which creates new filehandles for you(see the
FileHandle manpage):
use FileHandle; sub findme { my $fh = FileHandle->new(); open($fh, "</etc/hosts") or die "no /etc/hosts: $!"; while (<$fh>) { print if /\b127\.(0\.0\.)?1\b/; } # $fh automatically closes/disappears here }Internally, Perl believes filehandles to be of class
IO::Handle. Youmay use that module directly if you'd like (see the
IO::Handle manpage), orone of its more specific derived classes.
Once you have IO::File or FileHandle objects, you can pass thembetween subroutines or store them in hashes as you would any otherscalar values:
use FileHandle;
# Storing filehandles in a hash and array foreach $filename (@names) { my $fh = new FileHandle($filename) or die; $file{$filename} = $fh; push(@files, $fh); } # Using the filehandles in the array foreach $file (@files) { print $file "Testing\n"; } # You have to do the { } ugliness when you're specifying the # filehandle by anything other than a simple scalar variable. print { $files[2] } "Testing\n"; # Passing filehandles to subroutines sub debug { my $filehandle = shift; printf $filehandle "DEBUG: ", @_; } debug($fh, "Testing\n");
How can I set up a footer format to be used with write()?
There's no builtin way to do this, but the
perlform manpage has a couple oftechniques to make it possible for the intrepid hacker.
How can I write() into a string?
See the
perlform manpage for an
swrite() function.
How can I output my numbers with commas added?
This one will do it for you:
sub commify { local $_ = shift; 1 while s/^(-?\d+)(\d{3})/$1,$2/; return $_; } $n = 23659019423.2331; print "GOT: ", commify($n), "\n";
GOT: 23,659,019,423.2331
You can't just:
s/^(-?\d+)(\d{3})/$1,$2/g;because you have to put the comma in and then recalculate yourposition.
Alternatively, this commifies all numbers in a line regardless ofwhether they have decimal portions, are preceded by + or -, orwhatever:
# from Andrew Johnson <ajohnsonAATTgpu.srv.ualberta.ca> sub commify { my $input = shift; $input = reverse $input; $input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g; return reverse $input; } How can I translate tildes (~) in a filename?
Use the <> (
glob()) operator, documented in the
perlfunc manpage. Thisrequires that you have a shell installed that groks tildes, meaningcsh or tcsh or (some versions of) ksh, and thus may have portabilityproblems. The Glob::KGlob module (available from
CPAN) gives moreportable glob functionality.
Within Perl, you may use this directly:
$filename =~ s{ ^ ~ # find a leading tilde ( # save this in $1 [^/] # a non-slash character * # repeated 0 or more times (0 means me) ) }{ $1 ? (getpwnam($1))[7] : ( $ENV{HOME} || $ENV{LOGDIR} ) }ex; How come when I open the 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
Whoops. You should instead use this, which will fail if the filedoesn't exist.
open(FH, "+< /path/name"); # open for update
If this is an issue, try:
sysopen(FH, "/path/name", O_RDWR|O_CREAT, 0644);
Error checking is left as an exercise for the reader.
Why do I sometimes get an Argument list too long when I use <*>?
The
<> operator performs a globbing operation (see above).By default
glob() forks
csh(1) to do the actual glob expansion, butcsh can't handle more than 127 items and so gives the error message
Argument list too long. People who installed tcsh as csh won'thave this problem, but their users may be surprised by it.
To get around this, either do the glob yourself with Dirhandles andpatterns, or use a module like Glob::KGlob, one that doesn't use theshell to do globbing.
Is there a leak/bug in glob()?
Due to the current implementation on some operating systems, when youuse the
glob() function or its angle-bracket alias in a scalarcontext, you may cause a leak and/or unpredictable behavior. It'sbest therefore to use
glob() only in list context.
How can I open a file with a leading > or trailing blanks?
Normally perl ignores trailing blanks in filenames, and interpretscertain leading characters (or a trailing ``|") to mean somethingspecial. To avoid this, you might want to use a routine like this.It makes incomplete pathnames into explicit relative ones, and tacks atrailing null byte on the name to make perl leave it alone:
sub safe_filename { local $_ = shift; return m#^/# ? "$_\0" : "./$_\0"; } $fn = safe_filename("<<<something really wicked "); open(FH, "> $fn") or "couldn't open $fn: $!";You could also use the
sysopen() function (see the
sysopen entry in the
perlfunc manpage).
How can I reliably rename a file?
Well, usually you just use Perl's
rename() function. But that maynot work everywhere, in particular, renaming files across file systems.If your operating system supports a
mv(1) program or its moral equivalent,this works:
rename($old, $new) or system("mv", $old, $new);It may be more compelling to use the File::Copy module instead. Youjust copy to the new file to the new name (checking return values),then delete the old one. This isn't really the same semantics as areal
rename(), though, which preserves metainformation likepermissions, timestamps, inode info, etc.
How can I lock a file?
Perl's builtin
flock() function (see the
perlfunc manpage for details) will call
flock(2) if that exists,
fcntl(2) if it doesn't (on perl version 5.004 andlater), and
lockf(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
flock():
- 1
- Produces a fatal error if none of the three system calls (or theirclose equivalent) exists.
- 2
- lockf(3) does not provide shared locking, and requires that thefilehandle be open for writing (or appending, or read/writing).
- 3
- Some versions of flock() can't lock files over a network (e.g. on NFSfile systems), so you'd need to force the use of fcntl(2) when youbuild Perl. See the flock entry of the perlfunc manpage, and the INSTALLfile in the source distribution for information on building Perl to dothis.
The CPAN module File::Lock offers similar functionality and (if youhave dynamic loading) won't require you to rebuild perl if yourflock() can't lock network files.
What can't I just open(FH, >file.lock)?
A common bit of code
NOT TO USE 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 somethingwhich must be done in one. That's why computer hardware provides anatomic test-and-set instruction. In theory, this ``ought'' to work:
sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT, 0644) or die "can't open file.lock: $!":
except that lamentably, file creation (and deletion) is not atomicover
NFS, so this won't work (at least, not every time) over the net.Various schemes involving involving
link() have been suggested, butthese tend to involve busy-wait, which is also subdesirable.
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?
Anyway, this is what to do:
use Fcntl; sysopen(FH, "numfile", O_RDWR|O_CREAT, 0644) or die "can't open numfile: $!"; flock(FH, 2) or die "can't flock numfile: $!"; $num = <FH> || 0; seek(FH, 0, 0) or die "can't rewind numfile: $!"; truncate(FH, 0) or die "can't truncate numfile: $!"; (print FH $num+1, "\n") or die "can't write numfile: $!"; # DO NOT UNLOCK THIS UNTIL YOU CLOSE 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. :-)
How do I randomly update a binary file?
If you're just trying to patch a binary, in many cases something assimple as this works:
perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacsHowever, if you have fixed sized records, then you might do something morelike this:
$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: $!"; # munge the record seek(FH, $recno * $RECSIZE, 0); print FH $record; close FH;
Locking and error checking are left as an exercise for the reader.Don't forget them, or you'll be quite sorry.
Don't forget to set binmode() under DOS-like platforms when operatingon files that have anything other than straight text in them. See thedocs on open() and on binmode() for more details.
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
-M,
-A, or
-C filetest operations as documented in the
perlfunc manpage. Theseretrieve the age of the file (measured against the start-time of yourprogram) in days as a floating point number. To retrieve the ``raw''time in seconds since the epoch, you would call the stat function,then use
localtime(),
gmtime(), or
POSIX::strftime() to convert thisinto human-readable form.
Here's an example:
$write_secs = (stat($file))[9]; print "file $file updated at ", scalar(localtime($file)), "\n";
If you prefer something more legible, use the File::stat module(part of the standard distribution in version 5.004 and later):
use File::stat; use Time::localtime; $date_string = ctime(stat($file)->mtime); print "file $file updated at $date_string\n";
Error checking is left as an exercise for the reader.
How do I set a file's timestamp in perl?
You use the
utime() function documented in the
utime entry in the
perlfunc manpage.By way of example, here's a little program that copies theread and write times from its first argument to all the restof them.
if (@ARGV < 2) { die "usage: cptimes timestamp_file other_files ...\n"; } $timestamp = shift; ($atime, $mtime) = (stat($timestamp))[8,9]; utime $atime, $mtime, @ARGV;Error checking is left as an exercise for the reader.
Note that utime() currently doesn't work correctly with Win95/NTports. A bug has been reported. Check it carefully before usingit on those platforms.
How do I print to more than one file at once?
If you only have to do this once, you can do this:
for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }To connect up to one filehandle to several output filehandles, it'seasiest to use the
tee(1) program if you have it, and let it take careof the multiplexing:
open (FH, "| tee file1 file2 file3");
Otherwise you'll have to write your own multiplexing print function --or your own tee program -- or use Tom Christiansen's, at
http://www.perl.com/CPAN/authors/id/
TOMC/scripts/tct.gz, which iswritten in Perl.
In theory a IO::Tee class could be written, but to date we haven'tseen such.
How can I read in a file by paragraphs?
Use the
$\ variable (see the
perlvar manpage for details). You can eitherset it to
"" to eliminate empty paragraphs (
"abc\n\n\n\ndef",for instance, gets treated as two paragraphs and not three), or
"\n\n" to accept empty paragraphs.
How can I read a single character from a file? From the keyboard?
You can use the builtin
getc() function for most filehandles, butit won't (easily) work on a terminal device. For
STDIN, either usethe Term::ReadKey module from
CPAN, or use the sample code inthe
getc entry in the
perlfunc manpage.
If your system supports POSIX, you can use the following code, whichyou'll note turns off echo processing as well.
#!/usr/bin/perl -w use strict; $| = 1; for (1..4) { my $got; print "gimme: "; $got = getone(); print "--> $got\n"; } exit; BEGIN { use POSIX qw(:termios_h); my ($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;
sub cbreak { $term->setlflag($noecho); $term->setcc(VTIME, 1); $term->setattr($fd_stdin, TCSANOW); } sub cooked { $term->setlflag($oterm); $term->setcc(VTIME, 0); $term->setattr($fd_stdin, TCSANOW); } sub getone { my $key = ''; cbreak(); sysread(STDIN, $key, 1); cooked(); return $key; } }
END { cooked() }The Term::ReadKey module from
CPAN may be easier to use:
use Term::ReadKey; open(TTY, "</dev/tty"); print "Gimme a char: "; ReadMode "raw"; $key = ReadKey 0, *TTY; ReadMode "normal"; printf "\nYou said %s, char number %03d\n", $key, ord $key;
For
DOS systems, Dan Carson <dbcAATTtc.fluke.
COM> reports the following:
To put the PC in ``raw'' mode, use ioctl with some magic numbers gleanedfrom msdos.c (Perl source file) and Ralf Brown's interrupt list (comesacross the net every so often):
$old_ioctl = ioctl(STDIN,0,0); # Gets device info $old_ioctl &= 0xff; ioctl(STDIN,1,$old_ioctl | 32); # Writes it back, setting bit 5
Then to read a single character:
sysread(STDIN,$c,1); # Read a single character
And to put the
PC back to ``cooked'' mode:
ioctl(STDIN,1,$old_ioctl); # Sets it back to cooked mode.
So now you have
$c. If
ord($c) == 0, you have a two byte code, whichmeans you hit a special key. Read another byte with
sysread(STDIN,$c,1),and that value tells you what combination it was according to thistable:
# PC 2-byte keycodes = ^@ + the following:
# HEX KEYS # --- ---- # 0F SHF TAB # 10-19 ALT QWERTYUIOP # 1E-26 ALT ASDFGHJKL # 2C-32 ALT ZXCVBNM # 3B-44 F1-F10 # 47-49 HOME,UP,PgUp # 4B LEFT # 4D RIGHT # 4F-53 END,DOWN,PgDn,Ins,Del # 54-5D SHF F1-F10 # 5E-67 CTR F1-F10 # 68-71 ALT F1-F10 # 73-77 CTR LEFT,RIGHT,END,PgDn,HOME # 78-83 ALT 1234567890-= # 84 CTR PgUp
This is all trial and error I did a long time ago, I hope I'm reading thefile that worked.
How can I tell if there's a character waiting on a filehandle?
You should check out the Frequently Asked Questions list incomp.unix.* for things like this: the answer is essentially the same.It's very system dependent. Here's one solution that works on
BSDsystems:
sub key_ready { my($rin, $nfd); vec($rin, fileno(STDIN), 1) = 1; return $nfd = select($rin,undef,undef,0); }You should look into getting the Term::ReadKey extension from
CPAN.
How do I open a file without blocking?
You need to use the O_NDELAY or O_NONBLOCK flag from the Fcntl modulein conjunction with
sysopen():
use Fcntl; sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644) or die "can't open /tmp/somefile: $!":
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 inconjunction with
sysopen():
use Fcntl; sysopen(FH, "/tmp/somefile", O_WRONLY|O_EXCL|O_CREAT, 0644) or die "can't open /tmp/somefile: $!":
Be warned that neither creation nor deletion of files is guaranteed tobe an atomic operation over
NFS. That is, two processes might bothsuccessful create or unlink the same file!
How do I do a tail -f in perl?
First try
seek(GWFILE, 0, 1);
The statement
seek(GWFILE, 0, 1) doesn't change the current position,but it does clear the end-of-file condition on the handle, so that thenext <
GWFILE> makes Perl try again to read something.
If that doesn't work (it relies on features of your stdio implementation),then you need something more like this:
for (;;) { for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) { # search for some stuff and put it into files } # sleep for a while seek(GWFILE, $curpos, 0); # seek to where we had been }If this still doesn't work, look into the
POSIX module.
POSIX definesthe
clearerr() method, which can remove the end of file condition on afilehandle. The method: read until end of file,
clearerr(), read somemore. Lather, rinse, repeat.
How do I dup() a filehandle in Perl?
If you check the
open entry in the
perlfunc manpage, you'll see that several of the waysto call
open() should do the trick. For example:
open(LOG, ">>/tmp/logfile"); open(STDERR, ">&LOG");
Or even with a literal numeric descriptor:
$fd = $ENV{MHCONTEXTFD}; open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)Error checking has been left as an exercise for the reader.
How do I close a file descriptor by number?
This should rarely be necessary, as the Perl
close() function is to beused for things that Perl opened itself, even if it was a dup of anumeric descriptor, as with
MHCONTEXT above. But if you really haveto, 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 $rc == -1;
Why can't I use C:\temp\foo in DOS paths? What doesn't `C:\temp\foo.exe` work?
Whoops! You just put a tab and a formfeed into that filename!Remember that within double quoted strings ("like\this"), thebackslash is an escape character. The full list of these is inthe section on
Quote and Quote-like Operators in the
perlop manpage. Unsurprisingly, you don'thave a file called ``
c:(tab)
emp(formfeed)oo'' or``
c:(tab)
emp(formfeed)oo.exe'' on your
DOS filesystem.
Either single-quote your strings, or (preferably) use forward slashes.Since all DOS and Windows versions since something like MS-DOS 2.0 or sohave treated / and \ the same in a path, you might as well use theone that doesn't clash with Perl -- or the POSIX shell, ANSI C and C++,awk, Tcl, Java, or Python, just to mention a few.
Why doesn't glob(*.*) get all the files?
Because even on non-Unix ports, Perl's glob function follows standardUnix globbing semantics. You'll need
glob("*") to get all (non-hidden)files.
Why does Perl let me delete read-only files? Why does -i clobber protected files? Isn't this a bug in Perl?
This is elaborately and painstakingly described in the ``Far More ThanYou Every Wanted To Know'' in
http://www.perl.com/CPAN/doc/
FMTEYEWTK/file-dir-perms .
The executive summary: learn how your filesystem works. Thepermissions on a file say what can happen to the data in that file.The permissions on a directory say what can happen to the list offiles in that directory. If you delete a file, you're removing itsname from the directory (so the operation depends on the permissionsof the directory, not of the file). If you try to write to the file,the permissions of the file govern whether you're allowed to.
How do I select a random line from a file?
Here's an algorithm from the Camel Book:
srand; rand($.) < 1 && ($line = $_) while <>;
This has a significant advantage in space over reading the wholefile in.
AUTHOR AND COPYRIGHT
Copyright (c) 1997 Tom Christiansen and Nathan Torkington.All rights reserved. See the
perlfaq manpage for distribution information.
Index
- NAME
- DESCRIPTION
- How do I flush/unbuffer a filehandle? Why must I do this?
- 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?
- How do I count the number of lines in a file?
- How do I make a temporary file name?
- How can I manipulate fixed-record-length files?
- 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?
- How can I set up a footer format to be used with write()?
- How can I write() into a string?
- How can I output my numbers with commas added?
- How can I translate tildes (~) in a filename?
- How come when I open the file read-write it wipes it out?
- Why do I sometimes get an Argument list too long when I use <*>?
- Is there a leak/bug in glob()?
- How can I open a file with a leading > or trailing blanks?
- How can I reliably rename a file?
- How can I lock a file?
- What can't I just open(FH, >file.lock)?
- I still don't get locking. I just want to increment the number in the file. How can I do this?
- How do I randomly update a binary file?
- How do I get a file's timestamp in perl?
- How do I set a file's timestamp in perl?
- How do I print to more than one file at once?
- How can I read in a file by paragraphs?
- How can I read a single character from a file? From the keyboard?
- How can I tell if there's a character waiting on a filehandle?
- How do I open a file without blocking?
- How do I create a file only if it doesn't exist?
- How do I do a tail -f in perl?
- How do I dup() a filehandle in Perl?
- How do I close a file descriptor by number?
- Why can't I use C:\temp\foo in DOS paths? What doesn't `C:\temp\foo.exe` work?
- Why doesn't glob(*.*) get all the files?
- Why does Perl let me delete read-only files? Why does -i clobber protected files? Isn't this a bug in Perl?
- How do I select a random line from a file?
- AUTHOR AND COPYRIGHT
This document was created byman2html,using the manual pages.
