perl.1

早期freebsd实现

	#!/usr/bin/perl
	eval "exec /usr/bin/perl \-S $0 $*"
		if $running_under_some_shell;

.fi
The system ignores the first line and feeds the script to /bin/sh,
which proceeds to try to execute the
.I perl
script as a shell script.
The shell executes the second line as a normal shell command, and thus
starts up the
.I perl
interpreter.
On some systems $0 doesn't always contain the full pathname,
so the
.B \-S
tells
.I perl
to search for the script if necessary.
After
.I perl
locates the script, it parses the lines and ignores them because
the variable $running_under_some_shell is never true.
A better construct than $* would be ${1+"$@"}, which handles embedded spaces
and such in the filenames, but doesn't work if the script is being interpreted
by csh.
In order to start up sh rather than csh, some systems may have to replace the
#! line with a line containing just
a colon, which will be politely ignored by perl.
Other systems can't control that, and need a totally devious construct that
will work under any of csh, sh or perl, such as the following:
.nf

.ne 3
	eval '(exit $?0)' && eval 'exec /usr/bin/perl -S $0 ${1+"$@"}'
	& eval 'exec /usr/bin/perl -S $0 $argv:q'
		if 0;

.fi
.TP 5
.B \-u
causes
.I perl
to dump core after compiling your script.
You can then take this core dump and turn it into an executable file
by using the undump program (not supplied).
This speeds startup at the expense of some disk space (which you can
minimize by stripping the executable).
(Still, a "hello world" executable comes out to about 200K on my machine.)
If you are going to run your executable as a set-id program then you
should probably compile it using taintperl rather than normal perl.
If you want to execute a portion of your script before dumping, use the
dump operator instead.
Note: availability of undump is platform specific and may not be available
for a specific port of perl.
.TP 5
.B \-U
allows
.I perl
to do unsafe operations.
Currently the only \*(L"unsafe\*(R" operations are the unlinking of directories while
running as superuser, and running setuid programs with fatal taint checks
turned into warnings.
.TP 5
.B \-v
prints the version and patchlevel of your
.I perl
executable.
.TP 5
.B \-w
prints warnings about identifiers that are mentioned only once, and scalar
variables that are used before being set.
Also warns about redefined subroutines, and references to undefined
filehandles or filehandles opened readonly that you are attempting to
write on.
Also warns you if you use == on values that don't look like numbers, and if
your subroutines recurse more than 100 deep.
.TP 5
.BI \-x directory
tells
.I perl
that the script is embedded in a message.
Leading garbage will be discarded until the first line that starts
with #! and contains the string "perl".
Any meaningful switches on that line will be applied (but only one
group of switches, as with normal #! processing).
If a directory name is specified, Perl will switch to that directory
before running the script.
The
.B \-x
switch only controls the the disposal of leading garbage.
The script must be terminated with _\|_END_\|_ if there is trailing garbage
to be ignored (the script can process any or all of the trailing garbage
via the DATA filehandle if desired).
.SH ENVIRONMENT
.Ip HOME 12 4
Used if chdir has no argument.
.Ip LOGDIR 12 4
Used if chdir has no argument and HOME is not set.
.Ip PATH 12 4
Used in executing subprocesses, and in finding the script if \-S
is used.
.Ip PERLLIB 12 4
A colon-separated list of directories in which to look for Perl library
files before looking in the standard library and the current directory.
.Ip PERLDB 12 4
The command used to get the debugger code.  If unset, uses
.br

	require 'perldb.pl'

.PP
Apart from these,
.I perl
uses no other environment variables, except to make them available
to the script being executed, and to child processes.
However, scripts running setuid would do well to execute the following lines
before doing anything else, just to keep people honest:
.nf

.ne 3
    $ENV{\'PATH\'} = \'/bin:/usr/bin\';    # or whatever you need
    $ENV{\'SHELL\'} = \'/bin/sh\' if $ENV{\'SHELL\'} ne \'\';
    $ENV{\'IFS\'} = \'\' if $ENV{\'IFS\'} ne \'\';

.fi
.SH FILES
/tmp/perl\-eXXXXXX	temporary file for
.B \-e
commands.
.SH SEE ALSO
The complete perl documentation can be found in the
UNIX System manager's Manual (SMM:19).
.br
a2p	awk to perl translator
.br
s2p	sed to perl translator
.SH DIAGNOSTICS
Compilation errors will tell you the line number of the error, with an
indication of the next token or token type that was to be examined.
(In the case of a script passed to
.I perl
via
.B \-e
switches, each
.B \-e
is counted as one line.)
.PP
Setuid scripts have additional constraints that can produce error messages
such as \*(L"Insecure dependency\*(R".
See the section on setuid scripts.
.SH TRAPS
Accustomed
.IR awk
users should take special note of the following:
.Ip * 4 2
Semicolons are required after all simple statements in
.I perl
(except at the end of a block).
Newline is not a statement delimiter.
.Ip * 4 2
Curly brackets are required on ifs and whiles.
.Ip * 4 2
Variables begin with $ or @ in
.IR perl .
.Ip * 4 2
Arrays index from 0 unless you set $[.
Likewise string positions in substr() and index().
.Ip * 4 2
You have to decide whether your array has numeric or string indices.
.Ip * 4 2
Associative array values do not spring into existence upon mere reference.
.Ip * 4 2
You have to decide whether you want to use string or numeric comparisons.
.Ip * 4 2
Reading an input line does not split it for you.  You get to split it yourself
to an array.
And the
.I split
operator has different arguments.
.Ip * 4 2
The current input line is normally in $_, not $0.
It generally does not have the newline stripped.
($0 is the name of the program executed.)
.Ip * 4 2
$<digit> does not refer to fields\(emit refers to substrings matched by the last
match pattern.
.Ip * 4 2
The
.I print
statement does not add field and record separators unless you set
$, and $\e.
.Ip * 4 2
You must open your files before you print to them.
.Ip * 4 2
The range operator is \*(L".\|.\*(R", not comma.
(The comma operator works as in C.)
.Ip * 4 2
The match operator is \*(L"=~\*(R", not \*(L"~\*(R".
(\*(L"~\*(R" is the one's complement operator, as in C.)
.Ip * 4 2
The exponentiation operator is \*(L"**\*(R", not \*(L"^\*(R".
(\*(L"^\*(R" is the XOR operator, as in C.)
.Ip * 4 2
The concatenation operator is \*(L".\*(R", not the null string.
(Using the null string would render \*(L"/pat/ /pat/\*(R" unparsable,
since the third slash would be interpreted as a division operator\(emthe
tokener is in fact slightly context sensitive for operators like /, ?, and <.
And in fact, . itself can be the beginning of a number.)
.Ip * 4 2
.IR Next ,
.I exit
and
.I continue
work differently.
.Ip * 4 2
The following variables work differently
.nf

	  Awk	\h'|2.5i'Perl
	  ARGC	\h'|2.5i'$#ARGV
	  ARGV[0]	\h'|2.5i'$0
	  FILENAME\h'|2.5i'$ARGV
	  FNR	\h'|2.5i'$. \- something
	  FS	\h'|2.5i'(whatever you like)
	  NF	\h'|2.5i'$#Fld, or some such
	  NR	\h'|2.5i'$.
	  OFMT	\h'|2.5i'$#
	  OFS	\h'|2.5i'$,
	  ORS	\h'|2.5i'$\e
	  RLENGTH	\h'|2.5i'length($&)
	  RS	\h'|2.5i'$/
	  RSTART	\h'|2.5i'length($\`)
	  SUBSEP	\h'|2.5i'$;

.fi
.Ip * 4 2
When in doubt, run the
.I awk
construct through a2p and see what it gives you.
.PP
Cerebral C programmers should take note of the following:
.Ip * 4 2
Curly brackets are required on ifs and whiles.
.Ip * 4 2
You should use \*(L"elsif\*(R" rather than \*(L"else if\*(R"
.Ip * 4 2
.I Break
and
.I continue
become
.I last
and
.IR next ,
respectively.
.Ip * 4 2
There's no switch statement.
.Ip * 4 2
Variables begin with $ or @ in
.IR perl .
.Ip * 4 2
Printf does not implement *.
.Ip * 4 2
Comments begin with #, not /*.
.Ip * 4 2
You can't take the address of anything.
.Ip * 4 2
ARGV must be capitalized.
.Ip * 4 2
The \*(L"system\*(R" calls link, unlink, rename, etc. return nonzero for success, not 0.
.Ip * 4 2
Signal handlers deal with signal names, not numbers.
.PP
Seasoned
.I sed
programmers should take note of the following:
.Ip * 4 2
Backreferences in substitutions use $ rather than \e.
.Ip * 4 2
The pattern matching metacharacters (, ), and | do not have backslashes in front.
.Ip * 4 2
The range operator is .\|. rather than comma.
.PP
Sharp shell programmers should take note of the following:
.Ip * 4 2
The backtick operator does variable interpretation without regard to the
presence of single quotes in the command.
.Ip * 4 2
The backtick operator does no translation of the return value, unlike csh.
.Ip * 4 2
Shells (especially csh) do several levels of substitution on each command line.
.I Perl
does substitution only in certain constructs such as double quotes,
backticks, angle brackets and search patterns.
.Ip * 4 2
Shells interpret scripts a little bit at a time.
.I Perl
compiles the whole program before executing it.
.Ip * 4 2
The arguments are available via @ARGV, not $1, $2, etc.
.Ip * 4 2
The environment is not automatically made available as variables.
.SH BUGS
.PP
.I Perl
is at the mercy of your machine's definitions of various operations
such as type casting, atof() and sprintf().
.PP
If your stdio requires a seek or eof between reads and writes on a particular
stream, so does
.IR perl .
(This doesn't apply to sysread() and syswrite().)
.PP
While none of the built-in data types have any arbitrary size limits (apart
from memory size), there are still a few arbitrary limits:
a given identifier may not be longer than 255 characters,
and no component of your PATH may be longer than 255 if you use \-S.
A regular expression may not compile to more than 32767 bytes internally.
.PP
.I Perl
actually stands for Pathologically Eclectic Rubbish Lister, but don't tell
anyone I said that.
.SH AUTHOR
Larry Wall <[e-mail]>
.br
MS-DOS port by Diomidis Spinellis <[e-mail]>