3 perlrun - how to execute the Perl interpreter
7 B<perl> [switches] filename args
11 Upon startup, Perl looks for your script in one of the following
18 Specified line by line via B<-e> switches on the command line.
22 Contained in the file specified by the first filename on the command line.
23 (Note that systems supporting the #! notation invoke interpreters this way.)
27 Passed in implicitly via standard input. This only works if there are
28 no filename arguments--to pass arguments to a STDIN script you
29 must explicitly specify a "-" for the script name.
33 With methods 2 and 3, Perl starts parsing the input file from the
34 beginning, unless you've specified a B<-x> switch, in which case it
35 scans for the first line starting with #! and containing the word
36 "perl", and starts there instead. This is useful for running a script
37 embedded in a larger message. (In this case you would indicate the end
38 of the script using the __END__ token.)
40 As of Perl 5, the #! line is always examined for switches as the line is
41 being parsed. Thus, if you're on a machine that only allows one argument
42 with the #! line, or worse, doesn't even recognize the #! line, you still
43 can get consistent switch behavior regardless of how Perl was invoked,
44 even if B<-x> was used to find the beginning of the script.
46 Because many operating systems silently chop off kernel interpretation of
47 the #! line after 32 characters, some switches may be passed in on the
48 command line, and some may not; you could even get a "-" without its
49 letter, if you're not careful. You probably want to make sure that all
50 your switches fall either before or after that 32 character boundary.
51 Most switches don't actually care if they're processed redundantly, but
52 getting a - instead of a complete switch could cause Perl to try to
53 execute standard input instead of your script. And a partial B<-I> switch
54 could also cause odd results.
56 Parsing of the #! switches starts wherever "perl" is mentioned in the line.
57 The sequences "-*" and "- " are specifically ignored so that you could,
58 if you were so inclined, say
60 #!/bin/sh -- # -*- perl -*- -p
61 eval 'exec perl $0 -S ${1+"$@"}'
64 to let Perl see the B<-p> switch.
66 If the #! line does not contain the word "perl", the program named after
67 the #! is executed instead of the Perl interpreter. This is slightly
68 bizarre, but it helps people on machines that don't do #!, because they
69 can tell a program that their SHELL is /usr/bin/perl, and Perl will then
70 dispatch the program to the correct interpreter for them.
72 After locating your script, Perl compiles the entire script to an
73 internal form. If there are any compilation errors, execution of the
74 script is not attempted. (This is unlike the typical shell script,
75 which might run partway through before finding a syntax error.)
77 If the script is syntactically correct, it is executed. If the script
78 runs off the end without hitting an exit() or die() operator, an implicit
79 C<exit(0)> is provided to indicate successful completion.
83 A single-character switch may be combined with the following switch, if
86 #!/usr/bin/perl -spi.bak # same as -s -p -i.bak
94 specifies the record separator (C<$/>) as an octal number. If there are
95 no digits, the null character is the separator. Other switches may
96 precede or follow the digits. For example, if you have a version of
97 B<find> which can print filenames terminated by the null character, you
100 find . -name '*.bak' -print0 | perl -n0e unlink
102 The special value 00 will cause Perl to slurp files in paragraph mode.
103 The value 0777 will cause Perl to slurp files whole since there is no
104 legal character with that value.
108 turns on autosplit mode when used with a B<-n> or B<-p>. An implicit
109 split command to the @F array is done as the first thing inside the
110 implicit while loop produced by the B<-n> or B<-p>.
112 perl -ane 'print pop(@F), "\n";'
121 An alternate delimiter may be specified using B<-F>.
125 causes Perl to check the syntax of the script and then exit without
126 executing it. Actually, it will execute C<BEGIN> and C<use> blocks,
127 since these are considered part of the compilation.
131 runs the script under the Perl debugger. See L<perldebug>.
137 sets debugging flags. To watch how it executes your script, use
138 B<-D14>. (This only works if debugging is compiled into your
139 Perl.) Another nice value is B<-D1024>, which lists your compiled
140 syntax tree. And B<-D512> displays compiled regular expressions. As an
141 alternative specify a list of letters instead of numbers (e.g. B<-D14> is
142 equivalent to B<-Dtls>):
144 1 p Tokenizing and Parsing
146 4 l Label Stack Processing
148 16 o Operator Node Construction
149 32 c String/Numeric Conversions
150 64 P Print Preprocessor Command for -P
151 128 m Memory Allocation
152 256 f Format Processing
153 512 r Regular Expression Parsing
154 1024 x Syntax Tree Dump
155 2048 u Tainting Checks
156 4096 L Memory Leaks (not supported anymore)
157 8192 H Hash Dump -- usurps values()
158 16384 X Scratchpad Allocation
161 =item B<-e> I<commandline>
163 may be used to enter one line of script.
164 If B<-e> is given, Perl
165 will not look for a script filename in the argument list.
166 Multiple B<-e> commands may
167 be given to build up a multi-line script.
168 Make sure to use semicolons where you would in a normal program.
172 specifies a regular expression to split on if B<-a> is also in effect.
173 If regexp has C<//> around it, the slashes will be ignored.
175 =item B<-i>I<extension>
177 specifies that files processed by the C<E<lt>E<gt>> construct are to be edited
178 in-place. It does this by renaming the input file, opening the output
179 file by the original name, and selecting that output file as the default
180 for print() statements. The extension, if supplied, is added to the name
181 of the old file to make a backup copy. If no extension is supplied, no
182 backup is made. From the shell, saying
184 $ perl -p -i.bak -e "s/foo/bar/; ... "
186 is the same as using the script:
188 #!/usr/bin/perl -pi.bak
191 which is equivalent to
195 if ($ARGV ne $oldargv) {
196 rename($ARGV, $ARGV . '.bak');
197 open(ARGVOUT, ">$ARGV");
204 print; # this prints to original filename
208 except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
209 know when the filename has changed. It does, however, use ARGVOUT for
210 the selected filehandle. Note that STDOUT is restored as the
211 default output filehandle after the loop.
213 You can use C<eof> without parenthesis to locate the end of each input file,
214 in case you want to append to each file, or reset line numbering (see
215 example in L<perlfunc/eof>).
217 =item B<-I>I<directory>
219 may be used in conjunction with B<-P> to tell the C preprocessor where
220 to look for include files. By default /usr/include and /usr/lib/perl
225 enables automatic line-ending processing. It has two effects: first,
226 it automatically chomps the line terminator when used with B<-n> or
227 B<-p>, and second, it assigns "C<$\>" to have the value of I<octnum> so that
228 any print statements will have that line terminator added back on. If
229 I<octnum> is omitted, sets "C<$\>" to the current value of "C<$/>". For
230 instance, to trim lines to 80 columns:
232 perl -lpe 'substr($_, 80) = ""'
234 Note that the assignment C<$\ = $/> is done when the switch is processed,
235 so the input record separator can be different than the output record
236 separator if the B<-l> switch is followed by a B<-0> switch:
238 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
240 This sets $\ to newline and then sets $/ to the null character.
244 causes Perl to assume the following loop around your script, which
245 makes it iterate over filename arguments somewhat like B<sed -n> or
249 ... # your script goes here
252 Note that the lines are not printed by default. See B<-p> to have
253 lines printed. Here is an efficient way to delete all files older than
256 find . -mtime +7 -print | perl -nle 'unlink;'
258 This is faster than using the C<-exec> switch of B<find> because you don't
259 have to start a process on every filename found.
261 C<BEGIN> and C<END> blocks may be used to capture control before or after
262 the implicit loop, just as in B<awk>.
266 causes Perl to assume the following loop around your script, which
267 makes it iterate over filename arguments somewhat like B<sed>:
271 ... # your script goes here
276 Note that the lines are printed automatically. To suppress printing
277 use the B<-n> switch. A B<-p> overrides a B<-n> switch.
279 C<BEGIN> and C<END> blocks may be used to capture control before or after
280 the implicit loop, just as in awk.
284 causes your script to be run through the C preprocessor before
285 compilation by Perl. (Since both comments and cpp directives begin
286 with the # character, you should avoid starting comments with any words
287 recognized by the C preprocessor such as "if", "else" or "define".)
291 enables some rudimentary switch parsing for switches on the command
292 line after the script name but before any filename arguments (or before
293 a B<-->). Any switch found there is removed from @ARGV and sets the
294 corresponding variable in the Perl script. The following script
295 prints "true" if and only if the script is invoked with a B<-xyz> switch.
298 if ($xyz) { print "true\n"; }
302 makes Perl use the PATH environment variable to search for the
303 script (unless the name of the script starts with a slash). Typically
304 this is used to emulate #! startup on machines that don't support #!,
305 in the following manner:
308 eval "exec /usr/bin/perl -S $0 $*"
309 if $running_under_some_shell;
311 The system ignores the first line and feeds the script to /bin/sh,
312 which proceeds to try to execute the Perl script as a shell script.
313 The shell executes the second line as a normal shell command, and thus
314 starts up the Perl interpreter. On some systems $0 doesn't always
315 contain the full pathname, so the B<-S> tells Perl to search for the
316 script if necessary. After Perl locates the script, it parses the
317 lines and ignores them because the variable $running_under_some_shell
318 is never true. A better construct than C<$*> would be C<${1+"$@"}>, which
319 handles embedded spaces and such in the filenames, but doesn't work if
320 the script is being interpreted by csh. In order to start up sh rather
321 than csh, some systems may have to replace the #! line with a line
322 containing just a colon, which will be politely ignored by Perl. Other
323 systems can't control that, and need a totally devious construct that
324 will work under any of csh, sh or Perl, such as the following:
326 eval '(exit $?0)' && eval 'exec /usr/bin/perl -S $0 ${1+"$@"}'
327 & eval 'exec /usr/bin/perl -S $0 $argv:q'
332 forces "taint" checks to be turned on. Ordinarily these checks are
333 done only when running setuid or setgid. See L<perlsec>.
337 causes Perl to dump core after compiling your script. You can then
338 take this core dump and turn it into an executable file by using the
339 B<undump> program (not supplied). This speeds startup at the expense of
340 some disk space (which you can minimize by stripping the executable).
341 (Still, a "hello world" executable comes out to about 200K on my
342 machine.) If you want to execute a portion of your script before dumping,
343 use the dump() operator instead. Note: availability of B<undump> is
344 platform specific and may not be available for a specific port of
349 allows Perl to do unsafe operations. Currently the only "unsafe"
350 operations are the unlinking of directories while running as superuser,
351 and running setuid programs with fatal taint checks turned into
356 prints the version and patchlevel of your Perl executable.
360 prints warnings about identifiers that are mentioned only once, and
361 scalar variables that are used before being set. Also warns about
362 redefined subroutines, and references to undefined filehandles or
363 filehandles opened readonly that you are attempting to write on. Also
364 warns you if you use values as a number that doesn't look like numbers, using
365 an array as though it were a scalar, if
366 your subroutines recurse more than 100 deep, and innumerable other things.
367 See L<perldiag> and L<perltrap>.
369 =item B<-x> I<directory>
371 tells Perl that the script is embedded in a message. Leading
372 garbage will be discarded until the first line that starts with #! and
373 contains the string "perl". Any meaningful switches on that line will
374 be applied (but only one group of switches, as with normal #!
375 processing). If a directory name is specified, Perl will switch to
376 that directory before running the script. The B<-x> switch only
377 controls the the disposal of leading garbage. The script must be
378 terminated with C<__END__> if there is trailing garbage to be ignored (the
379 script can process any or all of the trailing garbage via the DATA
380 filehandle if desired).