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1=head1 NAME
2
3perlrun - how to execute the Perl interpreter
4
5=head1 SYNOPSIS
6
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7B<perl> [B<-acdhnpPsSTuUvw>] [B<-0[octal>]] [B<-D[number/list]>]
8 [B<-F regexp>] [B<-i[extension>]] [B<-I<lt>dir<gt>>]
9 [B<-l[octal]>] [B<-x[dir]>]
10 [programfile | B<-e command>] [argument ...]
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11
12=head1 DESCRIPTION
13
14Upon startup, Perl looks for your script in one of the following
15places:
16
17=over 4
18
19=item 1.
20
21Specified line by line via B<-e> switches on the command line.
22
23=item 2.
24
25Contained in the file specified by the first filename on the command line.
26(Note that systems supporting the #! notation invoke interpreters this way.)
27
28=item 3.
29
30Passed in implicitly via standard input. This only works if there are
31no filename arguments--to pass arguments to a STDIN script you
32must explicitly specify a "-" for the script name.
33
34=back
35
36With methods 2 and 3, Perl starts parsing the input file from the
37beginning, unless you've specified a B<-x> switch, in which case it
38scans for the first line starting with #! and containing the word
39"perl", and starts there instead. This is useful for running a script
40embedded in a larger message. (In this case you would indicate the end
41of the script using the __END__ token.)
42
43As of Perl 5, the #! line is always examined for switches as the line is
44being parsed. Thus, if you're on a machine that only allows one argument
45with the #! line, or worse, doesn't even recognize the #! line, you still
46can get consistent switch behavior regardless of how Perl was invoked,
47even if B<-x> was used to find the beginning of the script.
48
49Because many operating systems silently chop off kernel interpretation of
50the #! line after 32 characters, some switches may be passed in on the
51command line, and some may not; you could even get a "-" without its
52letter, if you're not careful. You probably want to make sure that all
53your switches fall either before or after that 32 character boundary.
54Most switches don't actually care if they're processed redundantly, but
55getting a - instead of a complete switch could cause Perl to try to
56execute standard input instead of your script. And a partial B<-I> switch
57could also cause odd results.
58
59Parsing of the #! switches starts wherever "perl" is mentioned in the line.
60The sequences "-*" and "- " are specifically ignored so that you could,
61if you were so inclined, say
62
63 #!/bin/sh -- # -*- perl -*- -p
64 eval 'exec perl $0 -S ${1+"$@"}'
65 if 0;
66
67to let Perl see the B<-p> switch.
68
69If the #! line does not contain the word "perl", the program named after
70the #! is executed instead of the Perl interpreter. This is slightly
71bizarre, but it helps people on machines that don't do #!, because they
72can tell a program that their SHELL is /usr/bin/perl, and Perl will then
73dispatch the program to the correct interpreter for them.
74
75After locating your script, Perl compiles the entire script to an
76internal form. If there are any compilation errors, execution of the
77script is not attempted. (This is unlike the typical shell script,
78which might run partway through before finding a syntax error.)
79
80If the script is syntactically correct, it is executed. If the script
81runs off the end without hitting an exit() or die() operator, an implicit
82C<exit(0)> is provided to indicate successful completion.
83
84=head2 Switches
85
86A single-character switch may be combined with the following switch, if
87any.
88
89 #!/usr/bin/perl -spi.bak # same as -s -p -i.bak
90
91Switches include:
92
93=over 5
94
95=item B<-0>I<digits>
96
97specifies the record separator (C<$/>) as an octal number. If there are
98no digits, the null character is the separator. Other switches may
99precede or follow the digits. For example, if you have a version of
100B<find> which can print filenames terminated by the null character, you
101can say this:
102
103 find . -name '*.bak' -print0 | perl -n0e unlink
104
105The special value 00 will cause Perl to slurp files in paragraph mode.
106The value 0777 will cause Perl to slurp files whole since there is no
107legal character with that value.
108
109=item B<-a>
110
111turns on autosplit mode when used with a B<-n> or B<-p>. An implicit
112split command to the @F array is done as the first thing inside the
113implicit while loop produced by the B<-n> or B<-p>.
114
115 perl -ane 'print pop(@F), "\n";'
116
117is equivalent to
118
119 while (<>) {
120 @F = split(' ');
121 print pop(@F), "\n";
122 }
123
124An alternate delimiter may be specified using B<-F>.
125
126=item B<-c>
127
128causes Perl to check the syntax of the script and then exit without
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129executing it. Actually, it I<will> execute C<BEGIN>, C<END>, and C<use> blocks,
130since these are considered as occurring outside the execution of
131your program.
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132
133=item B<-d>
134
135runs the script under the Perl debugger. See L<perldebug>.
136
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137=item B<-d:foo>
138
139runs the script under the control of a debugging or tracing module
a77489aa 140installed as Devel::foo. E.g., B<-d:DProf> executes the script using the
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141Devel::DProf profiler. See L<perldebug>.
142
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143=item B<-D>I<number>
144
145=item B<-D>I<list>
146
147sets debugging flags. To watch how it executes your script, use
148B<-D14>. (This only works if debugging is compiled into your
149Perl.) Another nice value is B<-D1024>, which lists your compiled
150syntax tree. And B<-D512> displays compiled regular expressions. As an
151alternative specify a list of letters instead of numbers (e.g. B<-D14> is
152equivalent to B<-Dtls>):
153
154 1 p Tokenizing and Parsing
155 2 s Stack Snapshots
156 4 l Label Stack Processing
157 8 t Trace Execution
158 16 o Operator Node Construction
159 32 c String/Numeric Conversions
160 64 P Print Preprocessor Command for -P
161 128 m Memory Allocation
162 256 f Format Processing
163 512 r Regular Expression Parsing
164 1024 x Syntax Tree Dump
165 2048 u Tainting Checks
166 4096 L Memory Leaks (not supported anymore)
167 8192 H Hash Dump -- usurps values()
168 16384 X Scratchpad Allocation
169 32768 D Cleaning Up
170
171=item B<-e> I<commandline>
172
173may be used to enter one line of script.
174If B<-e> is given, Perl
175will not look for a script filename in the argument list.
176Multiple B<-e> commands may
177be given to build up a multi-line script.
178Make sure to use semicolons where you would in a normal program.
179
180=item B<-F>I<regexp>
181
182specifies a regular expression to split on if B<-a> is also in effect.
183If regexp has C<//> around it, the slashes will be ignored.
184
185=item B<-i>I<extension>
186
187specifies that files processed by the C<E<lt>E<gt>> construct are to be edited
188in-place. It does this by renaming the input file, opening the output
189file by the original name, and selecting that output file as the default
190for print() statements. The extension, if supplied, is added to the name
191of the old file to make a backup copy. If no extension is supplied, no
192backup is made. From the shell, saying
193
194 $ perl -p -i.bak -e "s/foo/bar/; ... "
195
196is the same as using the script:
197
198 #!/usr/bin/perl -pi.bak
199 s/foo/bar/;
200
201which is equivalent to
202
203 #!/usr/bin/perl
204 while (<>) {
205 if ($ARGV ne $oldargv) {
206 rename($ARGV, $ARGV . '.bak');
207 open(ARGVOUT, ">$ARGV");
208 select(ARGVOUT);
209 $oldargv = $ARGV;
210 }
211 s/foo/bar/;
212 }
213 continue {
214 print; # this prints to original filename
215 }
216 select(STDOUT);
217
218except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
219know when the filename has changed. It does, however, use ARGVOUT for
220the selected filehandle. Note that STDOUT is restored as the
221default output filehandle after the loop.
222
223You can use C<eof> without parenthesis to locate the end of each input file,
224in case you want to append to each file, or reset line numbering (see
225example in L<perlfunc/eof>).
226
227=item B<-I>I<directory>
228
229may be used in conjunction with B<-P> to tell the C preprocessor where
230to look for include files. By default /usr/include and /usr/lib/perl
231are searched.
232
233=item B<-l>I<octnum>
234
235enables automatic line-ending processing. It has two effects: first,
236it automatically chomps the line terminator when used with B<-n> or
237B<-p>, and second, it assigns "C<$\>" to have the value of I<octnum> so that
238any print statements will have that line terminator added back on. If
239I<octnum> is omitted, sets "C<$\>" to the current value of "C<$/>". For
240instance, to trim lines to 80 columns:
241
242 perl -lpe 'substr($_, 80) = ""'
243
244Note that the assignment C<$\ = $/> is done when the switch is processed,
245so the input record separator can be different than the output record
246separator if the B<-l> switch is followed by a B<-0> switch:
247
248 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
249
250This sets $\ to newline and then sets $/ to the null character.
251
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252=item B<-m>I<module>
253
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254=item B<-M>I<module>
255
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256C<-m>I<module> executes C<use> I<module> C<();> before executing your
257script.
3c81428c 258
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259C<-M>I<module> executes C<use> I<module> C<;> before executing your
260script. You can use quotes to add extra code after the module name,
261e.g., C<-M'module qw(foo bar)'>.
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263If the first character after the C<-M> or C<-m> is a dash (C<->)
264then the 'use' is replaced with 'no'.
265
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266A little built-in syntactic sugar means you can also say
267C<-mmodule=foo> or C<-Mmodule=foo> as a shortcut for
268C<-M'module qw(foo)'>. Note that using the C<=> form
a77489aa 269removes the distinction between C<-m> and C<-M>.
3c81428c 270
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271To avoid the need to use quotes when importing more that one symbol
272with the C<=> form, the text following the C<=> is split into a list
273on commas (C<,>) rather than whitespace. The actual code generated
274by C<-Mmodule=foo,bar> is C<use module split(/,/,q{foo,bar})>.
275
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276=item B<-n>
277
278causes Perl to assume the following loop around your script, which
279makes it iterate over filename arguments somewhat like B<sed -n> or
280B<awk>:
281
282 while (<>) {
283 ... # your script goes here
284 }
285
286Note that the lines are not printed by default. See B<-p> to have
287lines printed. Here is an efficient way to delete all files older than
288a week:
289
290 find . -mtime +7 -print | perl -nle 'unlink;'
291
292This is faster than using the C<-exec> switch of B<find> because you don't
293have to start a process on every filename found.
294
295C<BEGIN> and C<END> blocks may be used to capture control before or after
296the implicit loop, just as in B<awk>.
297
298=item B<-p>
299
300causes Perl to assume the following loop around your script, which
301makes it iterate over filename arguments somewhat like B<sed>:
302
303
304 while (<>) {
305 ... # your script goes here
306 } continue {
307 print;
308 }
309
310Note that the lines are printed automatically. To suppress printing
311use the B<-n> switch. A B<-p> overrides a B<-n> switch.
312
313C<BEGIN> and C<END> blocks may be used to capture control before or after
314the implicit loop, just as in awk.
315
316=item B<-P>
317
318causes your script to be run through the C preprocessor before
319compilation by Perl. (Since both comments and cpp directives begin
320with the # character, you should avoid starting comments with any words
321recognized by the C preprocessor such as "if", "else" or "define".)
322
323=item B<-s>
324
325enables some rudimentary switch parsing for switches on the command
326line after the script name but before any filename arguments (or before
327a B<-->). Any switch found there is removed from @ARGV and sets the
328corresponding variable in the Perl script. The following script
329prints "true" if and only if the script is invoked with a B<-xyz> switch.
330
331 #!/usr/bin/perl -s
332 if ($xyz) { print "true\n"; }
333
334=item B<-S>
335
336makes Perl use the PATH environment variable to search for the
337script (unless the name of the script starts with a slash). Typically
338this is used to emulate #! startup on machines that don't support #!,
339in the following manner:
340
341 #!/usr/bin/perl
342 eval "exec /usr/bin/perl -S $0 $*"
343 if $running_under_some_shell;
344
345The system ignores the first line and feeds the script to /bin/sh,
346which proceeds to try to execute the Perl script as a shell script.
347The shell executes the second line as a normal shell command, and thus
348starts up the Perl interpreter. On some systems $0 doesn't always
349contain the full pathname, so the B<-S> tells Perl to search for the
350script if necessary. After Perl locates the script, it parses the
351lines and ignores them because the variable $running_under_some_shell
352is never true. A better construct than C<$*> would be C<${1+"$@"}>, which
353handles embedded spaces and such in the filenames, but doesn't work if
354the script is being interpreted by csh. In order to start up sh rather
355than csh, some systems may have to replace the #! line with a line
356containing just a colon, which will be politely ignored by Perl. Other
357systems can't control that, and need a totally devious construct that
358will work under any of csh, sh or Perl, such as the following:
359
360 eval '(exit $?0)' && eval 'exec /usr/bin/perl -S $0 ${1+"$@"}'
361 & eval 'exec /usr/bin/perl -S $0 $argv:q'
362 if 0;
363
364=item B<-T>
365
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366forces "taint" checks to be turned on so you can test them. Ordinarily these checks are
367done only when running setuid or setgid. It's a good idea to turn
368them on explicitly for programs run on another's behalf, such as CGI
369programs. See L<perlsec>.
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370
371=item B<-u>
372
373causes Perl to dump core after compiling your script. You can then
374take this core dump and turn it into an executable file by using the
375B<undump> program (not supplied). This speeds startup at the expense of
376some disk space (which you can minimize by stripping the executable).
377(Still, a "hello world" executable comes out to about 200K on my
378machine.) If you want to execute a portion of your script before dumping,
379use the dump() operator instead. Note: availability of B<undump> is
380platform specific and may not be available for a specific port of
381Perl.
382
383=item B<-U>
384
385allows Perl to do unsafe operations. Currently the only "unsafe"
386operations are the unlinking of directories while running as superuser,
387and running setuid programs with fatal taint checks turned into
388warnings.
389
390=item B<-v>
391
392prints the version and patchlevel of your Perl executable.
393
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394=item B<-V>
395
396prints summary of the major perl configuration values and the current
397value of @INC.
398
399=item B<-V:name>
400
401Prints to STDOUT the value of the named configuration variable.
402
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403=item B<-w>
404
405prints warnings about identifiers that are mentioned only once, and
406scalar variables that are used before being set. Also warns about
407redefined subroutines, and references to undefined filehandles or
408filehandles opened readonly that you are attempting to write on. Also
409warns you if you use values as a number that doesn't look like numbers, using
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410an array as though it were a scalar, if
411your subroutines recurse more than 100 deep, and innumerable other things.
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412See L<perldiag> and L<perltrap>.
413
414=item B<-x> I<directory>
415
416tells Perl that the script is embedded in a message. Leading
417garbage will be discarded until the first line that starts with #! and
418contains the string "perl". Any meaningful switches on that line will
419be applied (but only one group of switches, as with normal #!
420processing). If a directory name is specified, Perl will switch to
421that directory before running the script. The B<-x> switch only
422controls the the disposal of leading garbage. The script must be
423terminated with C<__END__> if there is trailing garbage to be ignored (the
424script can process any or all of the trailing garbage via the DATA
425filehandle if desired).
426
427
428=back