- pack(TEMPLATE,LIST)
-
Takes an array or list of values and packs it into a binary structure,
returning the string containing the structure. The TEMPLATE is a
sequence of characters that give the order and type of values, as
follows:
A An ascii string, will be space padded.
a An ascii string, will be null padded.
c A signed char value.
C An unsigned char value.
s A signed short value.
S An unsigned short value.
i A signed integer value.
I An unsigned integer value.
l A signed long value.
L An unsigned long value.
n A short in `network' order.
N A long in `network' order.
f A single-precision float in the native format.
d A double-precision float in the native format.
p A pointer to a string.
v A short in `VAX' (little-endian) order.
V A long in `VAX' (little-endian) order.
x A null byte.
X Back up a byte.
@ Null fill to absolute position.
u A uuencoded string.
b A bit string (ascending bit order, like vec()).
B A bit string (descending bit order).
h A hex string (low nybble first).
H A hex string (high nybble first).
Each letter may optionally be followed by a number which gives a repeat
count. With all types except `a', `A', `b', `B',
`h', and `H', the pack function will gobble up that many
values from the LIST. A `*' for the repeat count means to
use however many items are left. The `a' and `A' types gobble
just one value, but pack it as a string of length count, padding with
nulls or spaces as necessary. (When unpacking, `A' strips trailing
spaces and nulls, but `a' does not.) Likewise, the `b' and
`B' fields pack a string that many bits long. The `h' and
`H' fields pack a string that many nybbles long. Real numbers
(floats and doubles) are in the native machine format only; due to the
multiplicity of floating formats around, and the lack of a standard
"network" representation, no facility for interchange has been made.
This means that packed floating point data written on one machine may
not be readable on another - even if both use IEEE floating point
arithmetic (as the endian-ness of the memory representation is not part
of the IEEE spec). Note that perl uses doubles internally for all
numeric calculation, and converting from double to float back to double
will lose precision (i.e. `unpack("f", pack("f", $foo))' will not
in general equal `$foo').
Examples:
$foo = pack("cccc",65,66,67,68);
# foo eq "ABCD"
$foo = pack("c4",65,66,67,68);
# same thing
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
The same template may generally also be used in the unpack
function.
- unpack(TEMPLATE,EXPR)
-
unpack
does the reverse of pack
: it takes a string
representing a structure and expands it out into an array value,
returning the array value. (In a scalar context, it merely returns the
first value produced.) The TEMPLATE has the same format as in the
pack
function. Here's a subroutine that does substring:
sub substr {
local($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ord { unpack("c",$_[0]); }
In addition, you may prefix a field with a `%<number>' to indicate
that you want a <number>-bit checksum of the items instead of the items
themselves. Default is a 16-bit checksum. For example, the following
computes the same number as the System V sum program:
while (<>) {
$checksum += unpack("%16C*", $_);
}
$checksum %= 65536;
- delete $ASSOC{KEY}
-
Deletes the specified value from the specified associative array.
Returns the deleted value, or the undefined value if nothing was
deleted. Deleting from $ENV{} modifies the environment.
Deleting from an array bound to a dbm file deletes the entry from the
dbm file.
The following deletes all the values of an associative array:
foreach $key (keys %ARRAY) {
delete $ARRAY{$key};
}
(But it would be faster to use the reset
command. Saying
`undef %ARRAY' is faster yet.)
- each(ASSOC_ARRAY)
-
- each ASSOC_ARRAY
-
Returns a 2 element array consisting of the key and value for the next
value of an associative array, so that you can iterate over it. Entries
are returned in an apparently random order. When the array is entirely
read, a null array is returned (which when assigned produces a FALSE (0)
value). The next call to
each()
after that will start iterating
again. The iterator can be reset only by reading all the elements from
the array. You must not modify the array while iterating over it.
There is a single iterator for each associative array, shared by all
each()
, keys()
and values()
function calls in the
program. The following prints out your environment like the
printenv
program, only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also keys()
and values()
.
- grep(EXPR,LIST)
-
Evaluates EXPR for each element of LIST (locally setting
`$_' to each element) and returns the array value consisting of
those elements for which the expression evaluated to true. In a scalar
context, returns the number of times the expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
Note that, since `$_' is a reference into the array value, it can
be used to modify the elements of the array. While this is useful and
supported, it can cause bizarre results if the LIST is not a named
array.
- join(EXPR,LIST)
-
- join(EXPR,ARRAY)
-
Joins the separate strings of LIST or ARRAY into a single
string with fields separated by the value of EXPR, and returns the
string. Example:
$_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
See split
function.
- keys(ASSOC_ARRAY)
-
- keys ASSOC_ARRAY
-
Returns a normal array consisting of all the keys of the named
associative array. The keys are returned in an apparently random order,
but it is the same order as either the
values()
or each()
function produces (given that the associative array has not been
modified). Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while ($#keys >= 0) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
- pop(ARRAY)
-
- pop ARRAY
-
Pops and returns the last value of the array, shortening the array by 1.
Has the same effect as:
$tmp = $ARRAY[$#ARRAY--];
If there are no elements in the array, returns the undefined value.
- push(ARRAY,LIST)
-
Treats ARRAY (`@' is optional) as a stack, and pushes the
values of LIST onto the end of ARRAY. The length of
ARRAY increases by the length of LIST. Has the same effect
as:
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient.
- reverse(LIST)
-
- reverse LIST
-
In an array context, returns an array value consisting of the elements
of LIST in the opposite order. In a scalar context, returns a
string value consisting of the bytes of the first element of LIST
in the opposite order.
- shift(ARRAY)
-
- shift ARRAY
-
- shift
-
Shifts the first value of the array off and returns it, shortening the
array by 1 and moving everything down. If there are no elements in the
array, returns the undefined value. If ARRAY is omitted, shifts
the `@ARGV' array in the main program, and the `@_' array in
subroutines. (This is determined lexically.) See also
unshift()
, push()
and pop()
. shift()
and
unshift()
do the same thing to the left end of an array that
push()
and pop()
do to the right end.
- sort(SUBROUTINE LIST)
-
- sort(LIST)
-
- sort SUBROUTINE LIST
-
- sort BLOCK LIST
-
- sort LIST
-
Sorts the LIST and returns the sorted array value. Nonexistent
values of arrays are stripped out. If SUBROUTINE or BLOCK
is omitted, sorts in standard string comparison order. If
SUBROUTINE is specified, gives the name of a subroutine that
returns an integer less than, equal to, or greater than 0, depending on
how the elements of the array are to be ordered. (The
<=>
and
cmp
operators are extremely useful in such routines.)
SUBROUTINE may be a scalar variable name, in which case the value
provides the name of the subroutine to use. In place of a
SUBROUTINE name, you can provide a BLOCK as an anonymous,
in-line sort subroutine.
In the interests of efficiency the normal calling code for subroutines
is bypassed, with the following effects: the subroutine may not be a
recursive subroutine, and the two elements to be compared are passed
into the subroutine not via `@_' but as `$a' and `$b'
(see example below). They are passed by reference so don't modify
`$a' and `$b'. Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort { $a cmp $b } @files;
# same thing in reversed order
@articles = sort { $b cmp $a } @files;
# sort numerically ascending
@articles = sort { $a <=> $b } @files;
# sort numerically descending
@articles = sort { $b <=> $a } @files;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming integers
}
@sortedclass = sort byage @class;
sub reverse { $b cmp $a; }
@harry = ('dog','cat','x','Cain','Abel');
@george = ('gone','chased','yz','Punished','Axed');
print sort @harry;
# prints AbelCaincatdogx
print sort reverse @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
- splice(ARRAY,OFFSET,LENGTH,LIST)
-
- splice(ARRAY,OFFSET,LENGTH)
-
- splice(ARRAY,OFFSET)
-
Removes the elements designated by OFFSET and LENGTH from an
array, and replaces them with the elements of LIST, if any.
Returns the elements removed from the array. The array grows or shrinks
as necessary. If LENGTH is omitted, removes everything from
OFFSET onward. The following equivalencies hold (assuming
$[ == 0
):
push(@a,$x,$y) splice(@a,$#x+1,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$x] = $y splice(@a,$x,1,$y);
Example, assuming array lengths are passed before arrays:
sub aeq { # compare two array values
local(@a) = splice(@_,0,shift);
local(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
- split(/PATTERN/,EXPR,LIMIT)
-
- split(/PATTERN/,EXPR)
-
- split(/PATTERN/)
-
- split
-
Splits a string into an array of strings, and returns it. (If not in an
array context, returns the number of fields found and splits into the
`@_' array. (In an array context, you can force the split into
`@_' by using `??' as the pattern delimiters, but it still
returns the array value.)) If EXPR is omitted, splits the
`$_' string. If PATTERN is also omitted, splits on
whitespace (`/[\t\n]+/'). Anything matching PATTERN is taken
to be a delimiter separating the fields. (Note that the delimiter may
be longer than one character.) If LIMIT is specified, splits into
no more than that many fields (though it may split into fewer). If
LIMIT is unspecified, trailing null fields are stripped (which
potential users of
pop()
would do well to remember). A pattern
matching the null string (not to be confused with a null pattern
`//', which is just one member of the set of patterns matching a
null string) will split the value of EXPR into separate characters
at each point it matches that way. For example:
print join(':', split(/ */, 'hi there'));
produces the output `h:i:t:h:e:r:e'.
The LIMIT parameter can be used to partially split a line
($login, $passwd, $remainder) = split(/:/, $_, 3);
(When assigning to a list, if LIMIT is omitted, perl supplies a
LIMIT one larger than the number of variables in the list, to
avoid unnecessary work. For the list above LIMIT would have been
4 by default. In time critical applications it behooves you not to
split into more fields than you really need.)
If the PATTERN contains parentheses, additional array elements are
created from each matching substring in the delimiter.
split(/([,-])/,"1-10,20");
produces the array value
(1,'-',10,',',20)
The pattern /PATTERN/ may be replaced with an expression to
specify patterns that vary at runtime. (To do runtime compilation only
once, use `/$variable/o'.) As a special case, specifying a space
(' ') will split on white space just as split with no arguments does,
but leading white space does NOT produce a null first field.
Thus, split(' ') can be used to emulate awk
's default behavior,
whereas `split(/ /)' will give you as many null initial fields as
there are leading spaces.
Example:
open(passwd, '/etc/passwd');
while (<passwd>) {
($login, $passwd, $uid, $gid, $gcos, $home, $shell)
= split(/:/);
...
}
(Note that `$shell' above will still have a newline on it. See
chop()
.) See also join
.
- unshift(ARRAY,LIST)
-
Does the opposite of a
shift
. Or the opposite of a push
,
depending on how you look at it. Prepends list to the front of the
array, and returns the number of elements in the new array.
unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
- values(ASSOC_ARRAY)
-
- values ASSOC_ARRAY
-
Returns a normal array consisting of all the values of the named
associative array. The values are returned in an apparently random
order, but it is the same order as either the
keys()
or
each()
function would produce on the same array. See also
keys()
and each()
.
- chmod(LIST)
-
- chmod LIST
-
Changes the permissions of a list of files. The first element of the
list must be the numerical mode. Returns the number of files
successfully changed.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
- chown(LIST)
-
- chown LIST
-
Changes the owner (and group) of a list of files. The first two
elements of the list must be the NUMERICAL uid and gid, in that
order. Returns the number of files successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up non-numeric uids in the passwd file:
print "User: ";
$user = <STDIN>;
chop($user);
print "Files: "
$pattern = <STDIN>;
chop($pattern);
open(pass, '/etc/passwd') || die "Can't open passwd: $!\n";
while (<pass>) {
($login,$pass,$uid,$gid) = split(/:/);
$uid{$login} = $uid;
$gid{$login} = $gid;
}
@ary = <${pattern}>; # get filenames
if ($uid{$user} eq '') {
die "$user not in passwd file";
}
else {
chown $uid{$user}, $gid{$user}, @ary;
}
- fcntl(FILEHANDLE,FUNCTION,SCALAR)
-
Implements the `fcntl(2)' function. You'll probably have to
say:
require "fcntl.ph"; # probably /usr/local/lib/perl/fcntl.ph
first to get the correct function definitions. If `fcntl.ph'
doesn't exist or doesn't have the correct definitions you'll have to
roll your own, based on your C header files such as
`<sys/fcntl.h>'. (There is a perl script called `h2ph' that
comes with the perl kit which may help you in this.) Argument
processing and value return works just like ioctl
below. Note
that fcntl
will produce a fatal error if used on a machine that
doesn't implement `fcntl(2)'.
- fileno(FILEHANDLE)
-
- fileno FILEHANDLE
-
Returns the file descriptor for a filehandle. Useful for constructing
bitmaps for
select()
. If FILEHANDLE is an expression, the
value is taken as the name of the filehandle.
- flock(FILEHANDLE,OPERATION)
-
Calls `flock(2)' on FILEHANDLE. See manual page for
`flock(2)' for definition of OPERATION. Returns true for success,
false on failure. Will produce a fatal error if used on a machine that
doesn't implement `flock(2)'. Here's a mailbox appender for BSD
systems.
$LOCK_SH = 1;
$LOCK_EX = 2;
$LOCK_NB = 4;
$LOCK_UN = 8;
sub lock {
flock(MBOX,$LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
sub unlock {
flock(MBOX,$LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
|| die "Can't open mailbox: $!";
do lock();
print MBOX $msg,"\n\n";
do unlock();
- link(OLDFILE,NEWFILE)
-
Creates a new filename linked to the old filename. Returns 1 for
success, 0 otherwise.
- lstat(FILEHANDLE)
-
- lstat FILEHANDLE
-
- lstat(EXPR)
-
- lstat SCALARVARIABLE
-
Does the same thing as the
stat()
function, but stats a symbolic
link instead of the file the symbolic link points to. If symbolic links
are unimplemented on your system, a normal stat is done.
- readlink(EXPR)
-
- readlink EXPR
-
- readlink
-
Returns the value of a symbolic link, if symbolic links are implemented.
If not, gives a fatal error. If there is some system error, returns the
undefined value and sets `$!' (errno). If EXPR is omitted,
uses `$_'.
- rename(OLDNAME,NEWNAME)
-
Changes the name of a file. Returns 1 for success, 0 otherwise. Will
not work across filesystem boundaries.
- stat(FILEHANDLE)
-
- stat FILEHANDLE
-
- stat(EXPR)
-
- stat SCALARVARIABLE
-
Returns a 13-element array giving the statistics for a file, either the
file opened via FILEHANDLE, or named by EXPR.
Returns a null list if the
stat
fails.
Typically used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
If stat
is passed the special filehandle consisting of an
underline (`_'), no stat is done, but the current contents of the
stat structure from the last stat or filetest are returned.
Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This only works on machines for which the device number is negative
under NFS.)
- symlink(OLDFILE,NEWFILE)
-
Creates a new filename symbolically linked to the old filename. Returns
1 for success, 0 otherwise. On systems that don't support symbolic
links, produces a fatal error at run time. To check for that, use
eval:
$symlink_exists = (eval 'symlink("","");', $@ eq '');
- truncate(FILEHANDLE,LENGTH)
-
- truncate(EXPR,LENGTH)
-
Truncates the file opened on FILEHANDLE, or named by EXPR,
to the specified length. Produces a fatal error if
truncate
isn't implemented on your system.
- unlink(LIST)
-
- unlink LIST
-
- unlink
-
Deletes a list of files. If EXPR is not specified, deletes file
specified by `$_'. Returns the number of files successfully
deleted.
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink <*.bak>;
Note: unlink will not delete directories unless you are superuser and
the `-U' flag is supplied to perl. Even if these conditions
are met, be warned that unlinking a directory can inflict damage on your
filesystem. Use rmdir
instead.
- utime(LIST)
-
- utime LIST
-
Changes the access and modification times on each file of a list of
files. The first two elements of the list must be the NUMERICAL access
and modification times, in that order. Returns the number of files
successfully changed. The inode modification time of each file is set
to the current time. Example of a "touch" command:
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
- binmode(FILEHANDLE)
-
- binmode FILEHANDLE
-
Arranges for the file to be read in binary mode in operating
systems that distinguish between binary and text files. Files that are
not read in binary mode have CR LF sequences translated to LF on input
and LF translated to CR LF on output.
binmode
has no effect
under Unix. If FILEHANDLE is an expression, the value is taken as
the name of the filehandle.
- close(FILEHANDLE)
-
- close FILEHANDLE
-
Closes the file or pipe associated with the file handle. You don't have
to close FILEHANDLE if you are immediately going to do another
open on it, since open will close it for you. (See
open
.)
However, an explicit close on an input file resets the line counter
(`$.'), while the implicit close done by open
does not.
Also, closing a pipe will wait for the process executing on the pipe to
complete, in case you want to look at the output of the pipe afterwards.
Closing a pipe explicitly also puts the status value of the command into
`$?'. Example:
open(OUTPUT, '|sort >foo'); # pipe to sort
... # print stuff to output
close OUTPUT; # wait for sort to finish
open(INPUT, 'foo'); # get sort's results
FILEHANDLE may be an expression whose value gives the real
filehandle name.
- eof(FILEHANDLE)
-
- eof()
-
- eof
-
Returns 1 if the next read on FILEHANDLE will return end of file,
or if FILEHANDLE is not open. FILEHANDLE may be an
expression whose value gives the real filehandle name. (Note that this
function actually reads a character and the
ungetc
's it, so it is
not very useful in an interactive context.) An eof
without an
argument returns the eof status for the last file read. Empty
parentheses `()' may be used to indicate the pseudo file formed of
the files listed on the command line, i.e. `eof()' is reasonable to
use inside a `while (<>)' loop to detect the end of only the last
file. Use `eof(ARGV)' or eof
without the parentheses to
test EACH file in a `while (<>)' loop. Examples:
# insert dashes just before last line of last file
while (<>) {
if (eof()) {
print "--------------\n";
}
print;
}
# reset line numbering on each input file
while (<>) {
print "$.\t$_";
if (eof) { # Not eof().
close(ARGV);
}
}
- getc(FILEHANDLE)
-
- getc FILEHANDLE
-
- getc
-
Returns the next character from the input file attached to
FILEHANDLE, or a null string at EOF. If FILEHANDLE is
omitted, reads from STDIN.
- open(FILEHANDLE,EXPR)
-
- open(FILEHANDLE)
-
- 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. If EXPR is omitted,
the scalar variable of the same name as the FILEHANDLE contains the
filename. If the filename begins with `<' or nothing, the file is
opened for input. If the filename begins with `>', the file is opened
for output. If the filename begins with `>>', the file is opened for
appending. (You can put a `+' in front of the `>' or `<' to
indicate that you want both read and write access to the file.) 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 command which pipes input to us. (You may not
have a command that pipes both in and out.) Opening `-' opens
`STDIN' and opening `>-' opens `STDOUT'.
open
returns non-zero upon success, the undefined value otherwise. If the open
involved a pipe, the return value happens to be the pid of the subprocess.
Examples:
$article = 100;
open article || die "Can't find article $article: $!\n";
while (<article>) {...
open(LOG, '>>/usr/spool/news/twitlog');
# (log is reserved)
open(article, "caesar <$article |");
# decrypt article
open(extract, "|sort >/tmp/Tmp$$");
# $$ is our process#
# process argument list of files along with any includes
foreach $file (@ARGV) {
do process($file, 'fh00'); # no pun intended
}
sub process {
local($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
while (<$input>) { # note the use of indirection
if (/^#include "(.*)"/) {
do process($1, $input);
next;
}
... # whatever
}
}
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)
which is to be duped and opened. You may use `&' after `>',
`>>', `<', `+>', `+>>' and `+<'. The mode you
specify should match the mode of the original filehandle. Here is a
script that saves, redirects, and restores `STDOUT' and
`STDERR':
#!/usr/bin/perl
open(SAVEOUT, ">&STDOUT");
open(SAVEERR, ">&STDERR");
open(STDOUT, ">foo.out") || die "Can't redirect stdout";
open(STDERR, ">&STDOUT") || die "Can't dup stdout";
select(STDERR); $| = 1; # make unbuffered
select(STDOUT); $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
close(STDOUT);
close(STDERR);
open(STDOUT, ">&SAVEOUT");
open(STDERR, ">&SAVEERR");
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you open a pipe on the command `-', i.e. either `|-' or
`-|', then there is an implicit fork done, and the return value of
open is the pid of the child within the parent process, and 0 within the
child process. (Use `defined($pid)' to determine if 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 pairs are equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, "cat -n '$file'|");
open(FOO, "-|") || exec 'cat', '-n', $file;
Explicitly closing any piped filehandle causes the parent process to
wait for the child to finish, and returns the status value in `$?'.
Note: on any operation which may do a fork, unflushed buffers remain
unflushed in both processes, which means you may need to set `$|'
to avoid duplicate output.
The filename that is passed to open will have leading and trailing
whitespace deleted. In order to open a file with arbitrary weird
characters in it, it's necessary to protect any leading and trailing
whitespace thusly:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
- pipe(READHANDLE,WRITEHANDLE)
-
Opens a pair of connected pipes like the corresponding system call.
Note that if you set up a loop of piped processes, deadlock can occur
unless you are very careful. In addition, note that perl's pipes use
stdio buffering, so you may need to set `$|' to flush your
WRITEHANDLE after each command, depending on the application.
[Requires version 3.0 patchlevel 9.]
- print(FILEHANDLE LIST)
-
- print(LIST)
-
- print FILEHANDLE LIST
-
- print LIST
-
- print
-
Prints a string or a comma-separated list of strings. Returns non-zero
if successful. FILEHANDLE may be a scalar variable name, in which
case the variable contains the name of the filehandle, thus introducing
one level of indirection. (NOTE: If FILEHANDLE is a variable and
the next token is a term, it may be misinterpreted as an operator unless
you interpose a `+' or put parens around the arguments.) If
FILEHANDLE is omitted, prints by default to standard output (or to
the last selected output channel--see
select()
).If LIST is also omitted, prints `$_' to `STDOUT'. To set
the default output channel to something other than `STDOUT' use the
select operation. Note that, because print
takes a LIST,
anything in the LIST is evaluated in an array context, and any
subroutine that you call will have one or more of its expressions evaluated
in an array context. Also be careful not to follow the print
keyword with a left parenthesis unless you want the corresponding right
parenthesis to terminate the arguments to the print
---interpose a
`+' or put parens around all the arguments.
- printf(FILEHANDLE LIST)
-
- printf(LIST)
-
- printf FILEHANDLE LIST
-
- printf LIST
-
- printf
-
Equivalent to a `print FILEHANDLE sprintf(LIST)'.
- read(FILEHANDLE,SCALAR,LENGTH,OFFSET)
-
- read(FILEHANDLE,SCALAR,LENGTH)
-
Attempts to read LENGTH bytes of data into variable SCALAR
from the specified FILEHANDLE. Returns the number of bytes
actually read, or
undef
if there was an error. SCALAR
will be grown or shrunk to the length actually read. An OFFSET
may be specified to place the read data at some other place than the
beginning of the string. This call is actually implemented in terms of
stdio's fread
call. To get a true read
system call,
see sysread
.
- select(RBITS,WBITS,EBITS,TIMEOUT)
-
This calls the select system call with the bitmasks specified, which can
be constructed using
fileno()
and vec()
, along these
lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles you might wish to write a
subroutine:
sub fhbits {
local(@fhlist) = split(' ',$_[0]);
local($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = &fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready:
$nfound = select($rout=$rin, $wout=$win,
$eout=$ein, undef);
Any of the bitmasks can also be undef
. The timeout, if
specified, is in seconds, which may be fractional. NOTE: not all
implementations are capable of returning the `$timeleft'. If not,
they always return `$timeleft' equal to the supplied
`$timeout'.
- seek(FILEHANDLE,POSITION,WHENCE)
-
Randomly positions the file pointer for FILEHANDLE, just like the
fseek()
call of stdio. FILEHANDLE may be an expression
whose value gives the name of the filehandle. Returns 1 upon success, 0
otherwise.
- select(FILEHANDLE)
-
- select
-
Returns the currently selected filehandle. Sets the current default
filehandle for output, if FILEHANDLE is supplied. This has two
effects: first, a
write
or a print
without a filehandle
will default to this FILEHANDLE. Second, references to variables
related to output will refer to this output channel. For example, if
you have to set the top of form format for more than one output channel,
you might do the following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives the name of the
actual filehandle. Thus:
$oldfh = select(STDERR); $| = 1; select($oldfh);
- tell(FILEHANDLE)
-
- tell FILEHANDLE
-
- tell
-
Returns the current file position for FILEHANDLE.
FILEHANDLE may be an expression whose value gives the name of the
actual filehandle. If FILEHANDLE is omitted, assumes the file
last read.
- write(FILEHANDLE)
-
- write(EXPR)
-
- write
-
Writes a formatted record (possibly multi-line) to the specified file,
using the format associated with that file. By default the format for a
file is the one having the same name is the filehandle, but the format
for the current output channel (see
select
) may be set explicitly
by assigning the name of the format to the `$~' variable.
Top of form processing is handled automatically: if there is
insufficient room on the current page for the formatted record, the page
is advanced by writing a form feed, a special top-of-page format is used
to format the new page header, and then the record is written. By
default the top-of-page format is the name of the filehandle with
_TOP
appended, but it may be dynamically set to the format of
your choice by assigning the name to the `$^' variable while the
filehandle is select
ed. The number of lines remaining on the
current page is in variable `$-', which can be set to 0 to force a
new page.
If FILEHANDLE is unspecified, output goes to the current default
output channel, which starts out as `STDOUT' but may be changed by
the select
operator. If the FILEHANDLE is an EXPR,
then the expression is evaluated and the resulting string is used to
look up the name of the FILEHANDLE at run time. See section Formats,
for more info.
Note that write
is NOT the opposite of read
.
- m/PATTERN/gio
-
- /PATTERN/gio
-
Searches a string for a pattern match, and returns true (1) or false
(''). If no string is specified via the `=~' or `!~'
operator, the `$_' string is searched. (The string specified with
`=~' need not be an lvalue--it may be the result of an expression
evaluation, but remember the `=~' binds rather tightly.)
See section Regular Expressions, for more info.
If `/' is the delimiter then the initial `m' is optional.
With the `m' you can use any pair of non-alphanumeric characters as
delimiters. This is particularly useful for matching Unix path names
that contain `/'. If the final delimiter is followed by the
optional letter `i', the matching is done in a case-insensitive
manner. PATTERN may contain references to scalar variables, which
will be interpolated (and the pattern recompiled) every time the pattern
search is evaluated. (Note that `$)' and `$|' may not be
interpolated because they look like end-of-string tests.) If you want
such a pattern to be compiled only once, add an `o' after the
trailing delimiter. This avoids expensive run-time recompilations, and
is useful when the value you are interpolating won't change over the
life of the script. If the PATTERN evaluates to a null string,
the most recent successful regular expression is used instead.
If used in a context that requires an array value, a pattern match returns
an array consisting of the subexpressions matched by the parentheses in
the pattern, i.e. `($1, $2, $3...)'. It does NOT
actually set `$1', `$2', etc. in this case, nor does it set
`$+', `$`', `$&' or `$''. If the match fails, a null
array is returned. If the match succeeds, but there were no parentheses,
an array value of (1) is returned.
Examples:
open(tty, '/dev/tty');
<tty> =~ /^y/i && do foo(); # do foo if desired
if (/Version: *([0-9.]*)/) { $version = $1; }
next if m#^/usr/spool/uucp#;
# poor man's grep
$arg = shift;
while (<>) {
print if /$arg/o; # compile only once
}
if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
This last example splits `$foo' into the first two words and the
remainder of the line, and assigns those three fields to `$F1',
`$F2' and `$Etc'. The conditional is true if any variables
were assigned, i.e. if the pattern matched.
The `g' modifier specifies global pattern matching--that is,
matching as many times as possible within the string. How it behaves
depends on the context. In an array context, it returns a list of
all the substrings matched by all the parentheses in the regular expression.
If there are no parentheses, it returns a list of all the matched strings,
as if there were parentheses around the whole pattern. In a scalar context,
it iterates through the string, returning TRUE each time it matches, and
FALSE when it eventually runs out of matches. (In other words, it remembers
where it left off last time and restarts the search at that point.) It
presumes that you have not modified the string since the last match.
Modifying the string between matches may result in undefined behavior.
(You can actually get away with in-place modifications via substr()
that do not change the length of the entire string. In general, however,
you should be using s///g
for such modifications.) Examples:
# array context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
$/ = ""; $* = 1;
while ($paragraph = <>) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
print "$sentences\n";
- ?PATTERN?
-
This is just like the
/pattern/
search, except that it matches only
once between calls to the reset
operator. This is a useful
optimization when you only want to see the first occurrence of something in
each file of a set of files, for instance. Only `??' patterns local
to the current package are reset.
- s/PATTERN/REPLACEMENT/gieo
-
Searches a string for a pattern, and if found, replaces that pattern
with the replacement text and returns the number of substitutions made.
Otherwise it returns false (0). The `g' is optional, and if
present, indicates that all occurrences of the pattern are to be
replaced. The `i' is also optional, and if present, indicates that
matching is to be done in a case-insensitive manner. The `e' is
likewise optional, and if present, indicates that the replacement string
is to be evaluated as an expression rather than just as a double-quoted
string. Any non-alphanumeric delimiter may replace the slashes; if
single quotes are used, no interpretation is done on the replacement
string (the `e' modifier overrides this, however); if backquotes
are used, the replacement string is a command to execute whose output
will be used as the actual replacement text.
If the PATTERN is delimited by bracketing quotes, the
REPLACEMENT has its own pair of quotes, which may or may not be
bracketing quotes, e.g.
s(foo)(bar)
or s<foo>/bar/
.
If no string is specified
via the `=~' or `!~' operator, the `$_' string is
searched and modified. (The string specified with `=~' must be a
scalar variable, an array element, or an assignment to one of those,
i.e. an lvalue.) If the pattern contains a `$' that looks like a
variable rather than an end-of-string test, the variable will be
interpolated into the pattern at run-time. If you only want the pattern
compiled once the first time the variable is interpolated, add an
`o' at the end. If the PATTERN evaluates to a null string,
the most recent successful regular expression is used instead.
See section Regular Expressions, for more info. Examples:
s/\bgreen\b/mauve/g; # don't change wintergreen
$path =~ s|/usr/bin|/usr/local/bin|;
s/Login: $foo/Login: $bar/; # run-time pattern
($foo = $bar) =~ s/bar/foo/;
$_ = 'abc123xyz';
s/\d+/$&*2/e; # yields `abc246xyz'
s/\d+/sprintf("%5d",$&)/e; # yields `abc 246xyz'
s/\w/$& x 2/eg; # yields `aabbcc 224466xxyyzz'
s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
(Note the use of `$' instead of `\' in the last example.
See section Regular Expressions.)
- study(SCALAR)
-
- study SCALAR
-
- study
-
Takes extra time to study SCALAR (`$_' if unspecified) in
anticipation of doing many pattern matches on the string before it is
next modified. This may or may not save time, depending on the nature
and number of patterns you are searching on, and on the distribution of
character frequencies in the string to be searched--you probably want
to compare runtimes with and without it to see which runs faster. Those
loops which scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most. You may have only
one study active at a time--if you study a different scalar the first
is "unstudied". (The way study works is this: a linked list of every
character in the string to be searched is made, so we know, for example,
where all the `k' characters are. From each search string, the
rarest character is selected, based on some static frequency tables
constructed from some C programs and English text. Only those places
that contain this "rarest" character are examined.)
For example, here is a loop which inserts index producing entries before
any line containing a certain pattern:
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
...
print;
}
In searching for `/\bfoo\b/', only those locations in `$_'
that contain `f' will be looked at, because `f' is rarer than
`o'. In general, this is a big win except in pathological cases.
The only question is whether it saves you more time than it took to
build the linked list in the first place.
Note that if you have to look for strings that you don't know till
runtime, you can build an entire loop as a string and eval that to avoid
recompiling all your patterns all the time. Together with undefining
`$/' to input entire files as one record, this can be very fast,
often faster than specialized programs like `fgrep'. The following
scans a list of files (`@files') for a list of words
(`@words'), and prints out the names of those files that contain a
match:
$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/
eval $search; # this screams
$/ = "\n"; # put back to normal input delim
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
- tr/SEARCHLIST/REPLACEMENTLIST/cds
-
- y/SEARCHLIST/REPLACEMENTLIST/cds
-
Translates all occurrences of the characters found in the search list
with the corresponding character in the replacement list. It returns
the number of characters replaced or deleted. If no string is specified
via the `=~' or `!~' operator, the `$_' string is
translated. (The string specified with `=~' must be a scalar
variable, an array element, or an assignment to one of those, i.e. an
lvalue.) For
sed
devotees, y
is provided as a synonym for
tr
. If the SEARCHLIST is delimited by bracketing quotes,
the REPLACEMENTLIST has its own pair of quotes, which may or may
not be bracketing quotes, e.g. tr[A-Z][a-z]
or
tr(+-*/)/ABCD/
.
If the `c' modifier is specified, the SEARCHLIST character
set is complemented. If the `d' modifier is specified, any
characters specified by SEARCHLIST that are not found in
REPLACEMENTLIST are deleted. (Note that this is slightly more
flexible than the behavior of some tr
programs, which delete
anything they find in the SEARCHLIST, period.) If the `s'
modifier is specified, sequences of characters that were translated to
the same character are squashed down to 1 instance of the
character.
If the `d' modifier was used, the REPLACEMENTLIST is always
interpreted exactly as specified. Otherwise, if the
REPLACEMENTLIST is shorter than the SEARCHLIST, the final
character is replicated till it is long enough. If the
REPLACEMENTLIST is null, the SEARCHLIST is replicated. The
latter is useful for counting characters in a class, or for squashing
character sequences in a class.
Examples:
$ARGV[1] =~ y/A-Z/a-z/; # canonicalize to lower case
$cnt = tr/*/*/; # count the stars in $_
$cnt = tr/0-9//; # count the digits in $_
tr/a-zA-Z//s; # bookkeeper -> bokeper
($HOST = $host) =~ tr/a-z/A-Z/;
y/\001-@[-_{-\177/ /; # change non-alphas to space
# (before the c & s modifiers)
y/a-zA-Z/ /cs; # change non-alphas to single space
# (version 3.0 patchlevel 40+)
tr/\200-\377/\0-\177/; # delete 8th bit
- alarm(SECONDS)
-
- alarm SECONDS
-
Arranges to have a `SIGALRM' delivered to this process after the
specified number of seconds (minus 1, actually) have elapsed. Thus,
alarm(15)
will cause a `SIGALRM' at some point more than 14
seconds in the future. Only one timer may be counting at once. Each
call disables the previous timer, and an argument of 0 may be supplied
to cancel the previous timer without starting a new one. The returned
value is the amount of time remaining on the previous timer.
- chroot(FILENAME)
-
- chroot FILENAME
-
- chroot
-
Does the same as the system call of that name. If you don't know what
it does, don't worry about it. If FILENAME is omitted, does
chroot
to `$_'.
- die(LIST)
-
- die LIST
-
- die
-
Outside of an
eval
, prints the value of LIST to
`STDERR' and exits with the current value of `$!' (errno). As
of version 3.0 patchlevel 27, die
without LIST specified is
equivalent to
die 'Died';
If `$!' is 0, exits with the value of `($? >> 8)' (`command`
status). If `($? >> 8)' is 0, exits with 255. Inside an
eval
, the error message is stuffed into `$@' and the
eval
is terminated with the undefined value.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' || die "Can't cd to spool: $!\n"
If the value of EXPR does not end in a newline, the current script
line number and input line number (if any) are also printed, and a
newline is supplied. Hint: sometimes appending ", stopped" to your
message will cause it to make better sense when the string "at foo line
123" is appended. Suppose you are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also exit
.
- exec(LIST)
-
- exec LIST
-
If there is more than one argument in LIST, or if LIST is an
array with more than one value, calls
execvp()
with the arguments
in LIST. If there is only one scalar argument, the argument is
checked for shell metacharacters. If there are any, the entire argument
is passed to `/bin/sh -c' for parsing. If there are none, the
argument is split into words and passed directly to execvp()
,
which is more efficient. Note: exec
(and system
) do not
flush your output buffer, so you may need to set `$|' to avoid lost
output. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argument, but want to lie
to the program you are executing about its own name, you can specify the
program you actually want to run by assigning that to a variable and
putting the name of the variable in front of the LIST without a
comma. (This always forces interpretation of the LIST as a
multi-valued list, even if there is only a single scalar in the list.)
Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
- exit(EXPR)
-
- exit EXPR
-
Evaluates EXPR and exits immediately with that value. Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also die
. If EXPR is omitted, exits with 0 status.
- fork
-
Does a
fork()
call. Returns the child pid to the parent process
and 0 to the child process. Note: unflushed buffers remain unflushed in
both processes, which means you may need to set `$|' to avoid
duplicate output.
- getpwnam(NAME)
-
- getgrnam(NAME)
-
- gethostbyname(NAME)
-
- getnetbyname(NAME)
-
- getprotobyname(NAME)
-
- getpwuid(UID)
-
- getgrgid(GID)
-
- getservbyname(NAME,PROTO)
-
- gethostbyaddr(ADDR,ADDRTYPE)
-
- getnetbyaddr(ADDR,ADDRTYPE)
-
- getprotobynumber(NUMBER)
-
- getservbyport(PORT,PROTO)
-
- getpwent
-
- getgrent
-
- gethostent
-
- getnetent
-
- getprotoent
-
- getservent
-
- setpwent
-
- setgrent
-
- sethostent(STAYOPEN)
-
- setnetent(STAYOPEN)
-
- setprotoent(STAYOPEN)
-
- setservent(STAYOPEN)
-
- endpwent
-
- endgrent
-
- endhostent
-
- endnetent
-
- endprotoent
-
- endservent
-
These routines perform the same functions as their counterparts in the
system library. With an array context, the return values from the various
get routines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell) = getpw...
($name,$passwd,$gid,$members) = getgr...
($name,$aliases,$addrtype,$length,@addrs) = gethost...
($name,$aliases,$addrtype,$net) = getnet...
($name,$aliases,$proto) = getproto...
($name,$aliases,$port,$proto) = getserv...
(If the entry doesn't exist you get a null list.)
Within a scalar context, you get the name, unless the function was a
lookup by name, in which case you get the other thing, whatever it is.
(If the entry doesn't exist you get the undefined value.)
For example:
$uid = getpwnam
$name = getpwuid
$name = getpwent
$gid = getgrnam
$name = getgrgid
$name = getgrent
etc.
The `$members' value returned by getgr...
is a space
separated list of the login names of the members of the group.
For the gethost...
functions, if the h_errno
variable
is supported in C, it will be returned to you via `$?' if the function
call fails.
The `@addrs' value returned by a successful call
is a list of the raw addresses returned by the corresponding system
library call. In the Internet domain, each address is four bytes long and
you can unpack it by saying something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
- getlogin
-
Returns the current login from `/etc/utmp', if any. If null, use
getpwuid
.
$login = getlogin || (getpwuid($<))[0] || "Somebody";
- getpgrp(PID)
-
- getpgrp PID
-
- getpgrp
-
Returns the current process group for the specified PID, 0 for the
current process. Will produce a fatal error if used on a machine that
doesn't implement
getpgrp(2)
. If PID is omitted, returns process
group of current process. PID can be an expression.
- getppid
-
Returns the process id of the parent process.
- getpriority(WHICH,WHO)
-
Returns the current priority for a process, a process group, or a user.
(See the
getpriority(2)
man page.) Will produce a fatal error if
used on a machine that doesn't implement getpriority(2)
.
- ioctl(FILEHANDLE,FUNCTION,SCALAR)
-
Implements the
ioctl(2)
function.
You'll probably have to say
require "ioctl.ph"; # probably `/usr/local/lib/perl/ioctl.ph'
first to get the correct function definitions. If `ioctl.ph'
doesn't exist or doesn't have the correct definitions you'll have to
roll your own, based on your C header files such as
`<sys/ioctl.h>'. (There is a perl script called h2ph
that comes with the perl kit which may help you in this.) SCALAR
will be read and/or written depending on the FUNCTION---a pointer
to the string value of SCALAR will be passed as the third argument
of the actual ioctl
call. (If SCALAR has no string value
but does have a numeric value, that value will be passed rather than a
pointer to the string value. To guarantee this to be true, add a 0 to
the scalar before using it.) The pack()
and unpack()
functions are useful for manipulating the values of structures used by
ioctl()
.
The following example sets the erase character to DEL.
require 'ioctl.ph';
$sgttyb_t = "ccccs"; # 4 chars and a short
if (ioctl(STDIN,$TIOCGETP,$sgttyb)) {
@ary = unpack($sgttyb_t,$sgttyb);
$ary[2] = 127;
$sgttyb = pack($sgttyb_t,@ary);
ioctl(STDIN,$TIOCSETP,$sgttyb)
|| die "Can't ioctl: $!";
}
The return value of ioctl
(and fcntl
) is as follows:
if OS returns: perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus perl returns true on success and false on failure, yet you can still
easily determine the actual value returned by the operating system:
($retval = ioctl(...)) || ($retval = -1);
printf "System returned %d\n", $retval;
- kill(LIST)
-
- kill LIST
-
Sends a signal to a list of processes. The first element of the list
must be the signal to send. Returns the number of processes
successfully signaled.
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
If the signal is negative, kills process groups instead of processes.
(On System V, a negative process number will also kill process
groups, but that's not portable.) You may use a signal name in
quotes.
- setpgrp(PID,PGRP)
-
Sets the current process group for the specified PID, 0 for the
current process. Will produce a fatal error if used on a machine that
doesn't implement
setpgrp(2)
.
- setpriority(WHICH,WHO,PRIORITY)
-
Sets the current priority for a process, a process group, or a user.
(See the
setpriority(2)
man page.) Will produce a fatal error if
used on a machine that doesn't implement setpriority(2)
.
- sleep(EXPR)
-
- sleep EXPR
-
- sleep
-
Causes the script to sleep for EXPR seconds, or forever if no
EXPR. May be interrupted by sending the process a `SIGALRM'.
Returns the number of seconds actually slept. You probably cannot mix
alarm()
and sleep()
calls, since sleep()
is often
implemented using alarm()
.
- syscall(LIST)
-
- syscall LIST
-
Calls the system call specified as the first element of the list,
passing the remaining elements as arguments to the system call. If
unimplemented, produces a fatal error. The arguments are interpreted as
follows: if a given argument is numeric, the argument is passed as an
int. If not, the pointer to the string value is passed. You are
responsible to make sure a string is pre-extended long enough to receive
any result that might be written into a string. If your integer
arguments are not literals and have never been interpreted in a numeric
context, you may need to add 0 to them to force them to look like
numbers.
require 'syscall.ph'; # may need to run h2ph
syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
- sysread(FILEHANDLE,SCALAR,LENGTH,OFFSET)
-
- sysread(FILEHANDLE,SCALAR,LENGTH)
-
Attempts to read LENGTH bytes of data into variable SCALAR
from the specified FILEHANDLE, using the system call
read(2)
. It bypasses stdio, so mixing this with other kinds of
reads may cause confusion. Returns the number of bytes actually read,
or undef
if there was an error. SCALAR will be grown or
shrunk to the length actually read. An OFFSET may be specified to
place the read data at some other place than the beginning of the
string.
- syswrite(FILEHANDLE,SCALAR,LENGTH,OFFSET)
-
- syswrite(FILEHANDLE,SCALAR,LENGTH)
-
Attempts to write LENGTH bytes of data from variable SCALAR
to the specified FILEHANDLE, using the system call
write(2)
. It bypasses stdio, so mixing this with prints may
cause confusion. Returns the number of bytes actually written, or
undef
if there was an error. An OFFSET may be specified
to place the read data at some other place than the beginning of the
string.
- system(LIST)
-
- system LIST
-
Does exactly the same thing as `exec LIST' except that a fork
is done first, and the parent process waits for the child process to
complete. Note that argument processing varies depending on the number
of arguments. The return value is the exit status of the program as
returned by the
wait()
call. To get the actual exit value divide
by 256. See also exec
.
- times
-
Returns a four-element array giving the user and system times, in
seconds, for this process and the children of this process.
($user,$system,$cuser,$csystem) = times;
- umask(EXPR)
-
- umask EXPR
-
- umask
-
Sets the umask for the process and returns the old one. If EXPR
is omitted, merely returns current umask.
- wait
-
Waits for a child process to terminate and returns the pid of the
deceased process, or -1 if there are no child processes. The status is
returned in `$?'.
- waitpid(PID,FLAGS)
-
Waits for a particular child process to terminate and returns the pid of
the deceased process or -1 if there are no such child process. The
status is returns in `$?'. If you say
require "sys/wait.ph";
...
waitpid(-1,&WNOHANG);
then you can do a non-blocking wait for any process. Non-blocking wait
is only available on machines supporting either the waitpid(2)
or
wait4(2)
system calls. However, waiting for a particular pid
with FLAGS of 0 is implemented everywhere. (Perl emulates
the system call by remembering the status values of processes that have
exited but have not been harvested by the Perl script yet.)
- warn(LIST)
-
- warn LIST
-
Produces a message on `STDERR' just like
die
, but doesn't
exit.
- dump LABEL
-
- dump
-
This causes an immediate core dump. Primarily this is so that you can use
the `undump' program to turn your core dump into an executable binary
after having initialized all your variables at the beginning of the
program. When the new binary is executed it will begin by executing a
`goto LABEL' (with all the restrictions that
goto
suffers). Think of it as a goto
with an intervening core dump and
reincarnation. If LABEL is omitted, restarts the program from the
top. WARNING: any files opened at the time of the dump will
NOT be open any more when the program is reincarnated, with
possible resulting confusion on the part of perl. See also
`-u'.
Example:
#!/usr/bin/perl
require 'getopt.pl';
require 'stat.pl';
%days = (
'Sun',1,
'Mon',2,
'Tue',3,
'Wed',4,
'Thu',5,
'Fri',6,
'Sat',7);
dump QUICKSTART if $ARGV[0] eq '-d';
QUICKSTART:
do Getopt('f');
...
- eval(EXPR)
-
- eval EXPR
-
- eval BLOCK
-
- eval
-
EXPR is parsed and executed as if it were a little perl
program. It is executed in the context of the current perl
program, so that any variable settings, subroutine or format definitions
remain afterwards. The value returned is the value of the last
expression evaluated, just as with subroutines. If there is a syntax
error or runtime error, or a
die
statement is executed, an
undefined value is returned by eval
, and `$@' is set to the
error message. If there was no error, `$@' is guaranteed to be a
null string. If EXPR is omitted, evaluates `$_'. The final
semicolon, if any, may be omitted from the expression.
Note that, since eval
traps otherwise-fatal errors, it is useful
for determining whether a particular feature (such as dbmopen
or
symlink
) is implemented. If is also Perl's exception
trapping mechanism, where the die
operator is used to raise
exceptions.
If the code to be executed doesn't vary, you may use the eval
BLOCK
form to trap run-time errors without incurring the penalty
of recompiling each time. The error, if any, is still returned in
`$@'. Evaluating a single-quoted string (as EXPR) has the
same effect, except that the eval EXPR
form reports syntax
errors at run time via `$@', whereas the eval BLOCK
form reports syntax errors at compile time. The eval EXPR
form is optimized to eval BLOCK
the first time it succeeds.
(Since the replacement side of a substitution is considered a
single-quoted string when you use the `e' modifier, the same
optimization occurs there.) Examples:
# make divide-by-zero non-fatal
eval { $answer = $a / $b; }; warn $@ if $@;
# optimized to same thing after first use
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = };
# a run-time error
eval '$answer ='; # sets $@
- ord(EXPR)
-
- ord EXPR
-
- ord
-
Returns the numeric ascii value of the first character of EXPR.
If EXPR is omitted, uses `$_'.
- q/STRING/
-
- qq/STRING/
-
- qx/STRING/
-
These are not really functions, but simply syntactic sugar to let you
avoid putting too many backslashes into quoted strings. The
q
operator is a generalized single quote, and the qq
operator a
generalized double quote. The qx
operator is a generalized
backquote. Any non-alphanumeric delimiter can be used in place of
`/', including newline. If the delimiter is an opening bracket or
parenthesis, the final delimiter will be the corresponding closing
bracket or parenthesis. (Embedded occurrences of the closing bracket
need to be backslashed as usual.) Examples:
$foo = q!I said, "You said, 'She said it.'"!;
$bar = q('This is it.');
$today = qx{ date };
$_ .= qq
*** The previous line contains the naughty word "$&".\n
if /(ibm|apple|awk)/; # :-)
- rand(EXPR)
-
- rand EXPR
-
- rand
-
Returns a random fractional number between 0 and the value of
EXPR. (EXPR should be positive.) If EXPR is omitted,
returns a value between 0 and 1. See also
srand()
.
- srand(EXPR)
-
- srand EXPR
-
- srand
-
Sets the random number seed for the
rand
operator. If EXPR
is omitted, does srand(time)
.
- sprintf(FORMAT,LIST)
-
Returns a string formatted by the usual
printf
conventions. The
`*' character is not supported.
- vec(EXPR,OFFSET,BITS)
-
Treats a string as a vector of unsigned integers, and returns the value
of the bitfield specified. May also be assigned to. BITS must be
a power of two from 1 to 32.
Vectors created with
vec()
can also be manipulated with the
logical operators `|', `&' and `^', which will assume a
bit vector operation is desired when both operands are strings. This
interpretation is not enabled unless there is at least one vec()
in your program, to protect older programs.
To transform a bit vector into a string or array of 0's and 1's, use
these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in place of the *.