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1 | =head1 NAME |
2 | |
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3 | perlfaq8 - System Interaction ($Revision: 9681 $) |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | This section of the Perl FAQ covers questions involving operating |
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8 | system interaction. Topics include interprocess communication (IPC), |
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9 | control over the user-interface (keyboard, screen and pointing |
10 | devices), and most anything else not related to data manipulation. |
11 | |
12 | Read the FAQs and documentation specific to the port of perl to your |
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13 | operating system (eg, L<perlvms>, L<perlplan9>, ...). These should |
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14 | contain more detailed information on the vagaries of your perl. |
15 | |
16 | =head2 How do I find out which operating system I'm running under? |
17 | |
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18 | The $^O variable ($OSNAME if you use English) contains an indication of |
19 | the name of the operating system (not its release number) that your perl |
20 | binary was built for. |
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21 | |
22 | =head2 How come exec() doesn't return? |
23 | |
24 | Because that's what it does: it replaces your currently running |
25 | program with a different one. If you want to keep going (as is |
26 | probably the case if you're asking this question) use system() |
27 | instead. |
28 | |
29 | =head2 How do I do fancy stuff with the keyboard/screen/mouse? |
30 | |
31 | How you access/control keyboards, screens, and pointing devices |
32 | ("mice") is system-dependent. Try the following modules: |
33 | |
34 | =over 4 |
35 | |
36 | =item Keyboard |
37 | |
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38 | Term::Cap Standard perl distribution |
39 | Term::ReadKey CPAN |
40 | Term::ReadLine::Gnu CPAN |
41 | Term::ReadLine::Perl CPAN |
42 | Term::Screen CPAN |
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43 | |
44 | =item Screen |
45 | |
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46 | Term::Cap Standard perl distribution |
47 | Curses CPAN |
48 | Term::ANSIColor CPAN |
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49 | |
50 | =item Mouse |
51 | |
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52 | Tk CPAN |
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53 | |
54 | =back |
55 | |
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56 | Some of these specific cases are shown as examples in other answers |
57 | in this section of the perlfaq. |
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58 | |
59 | =head2 How do I print something out in color? |
60 | |
61 | In general, you don't, because you don't know whether |
62 | the recipient has a color-aware display device. If you |
63 | know that they have an ANSI terminal that understands |
64 | color, you can use the Term::ANSIColor module from CPAN: |
65 | |
ac9dac7f |
66 | use Term::ANSIColor; |
67 | print color("red"), "Stop!\n", color("reset"); |
68 | print color("green"), "Go!\n", color("reset"); |
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69 | |
70 | Or like this: |
71 | |
ac9dac7f |
72 | use Term::ANSIColor qw(:constants); |
73 | print RED, "Stop!\n", RESET; |
74 | print GREEN, "Go!\n", RESET; |
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75 | |
76 | =head2 How do I read just one key without waiting for a return key? |
77 | |
78 | Controlling input buffering is a remarkably system-dependent matter. |
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79 | On many systems, you can just use the B<stty> command as shown in |
c8db1d39 |
80 | L<perlfunc/getc>, but as you see, that's already getting you into |
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81 | portability snags. |
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82 | |
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83 | open(TTY, "+</dev/tty") or die "no tty: $!"; |
84 | system "stty cbreak </dev/tty >/dev/tty 2>&1"; |
85 | $key = getc(TTY); # perhaps this works |
86 | # OR ELSE |
87 | sysread(TTY, $key, 1); # probably this does |
88 | system "stty -cbreak </dev/tty >/dev/tty 2>&1"; |
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89 | |
90 | The Term::ReadKey module from CPAN offers an easy-to-use interface that |
91 | should be more efficient than shelling out to B<stty> for each key. |
92 | It even includes limited support for Windows. |
93 | |
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94 | use Term::ReadKey; |
95 | ReadMode('cbreak'); |
96 | $key = ReadKey(0); |
97 | ReadMode('normal'); |
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98 | |
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99 | However, using the code requires that you have a working C compiler |
100 | and can use it to build and install a CPAN module. Here's a solution |
101 | using the standard POSIX module, which is already on your systems |
102 | (assuming your system supports POSIX). |
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103 | |
ac9dac7f |
104 | use HotKey; |
105 | $key = readkey(); |
c8db1d39 |
106 | |
107 | And here's the HotKey module, which hides the somewhat mystifying calls |
108 | to manipulate the POSIX termios structures. |
109 | |
ac9dac7f |
110 | # HotKey.pm |
111 | package HotKey; |
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112 | |
ac9dac7f |
113 | @ISA = qw(Exporter); |
114 | @EXPORT = qw(cbreak cooked readkey); |
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115 | |
ac9dac7f |
116 | use strict; |
117 | use POSIX qw(:termios_h); |
118 | my ($term, $oterm, $echo, $noecho, $fd_stdin); |
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119 | |
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120 | $fd_stdin = fileno(STDIN); |
121 | $term = POSIX::Termios->new(); |
122 | $term->getattr($fd_stdin); |
123 | $oterm = $term->getlflag(); |
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124 | |
ac9dac7f |
125 | $echo = ECHO | ECHOK | ICANON; |
126 | $noecho = $oterm & ~$echo; |
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127 | |
ac9dac7f |
128 | sub cbreak { |
129 | $term->setlflag($noecho); # ok, so i don't want echo either |
130 | $term->setcc(VTIME, 1); |
131 | $term->setattr($fd_stdin, TCSANOW); |
132 | } |
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133 | |
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134 | sub cooked { |
135 | $term->setlflag($oterm); |
136 | $term->setcc(VTIME, 0); |
137 | $term->setattr($fd_stdin, TCSANOW); |
138 | } |
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139 | |
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140 | sub readkey { |
141 | my $key = ''; |
142 | cbreak(); |
143 | sysread(STDIN, $key, 1); |
144 | cooked(); |
145 | return $key; |
146 | } |
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147 | |
ac9dac7f |
148 | END { cooked() } |
c8db1d39 |
149 | |
ac9dac7f |
150 | 1; |
c8db1d39 |
151 | |
152 | =head2 How do I check whether input is ready on the keyboard? |
153 | |
154 | The easiest way to do this is to read a key in nonblocking mode with the |
155 | Term::ReadKey module from CPAN, passing it an argument of -1 to indicate |
156 | not to block: |
157 | |
ac9dac7f |
158 | use Term::ReadKey; |
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159 | |
ac9dac7f |
160 | ReadMode('cbreak'); |
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161 | |
ac9dac7f |
162 | if (defined ($char = ReadKey(-1)) ) { |
163 | # input was waiting and it was $char |
164 | } else { |
165 | # no input was waiting |
166 | } |
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167 | |
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168 | ReadMode('normal'); # restore normal tty settings |
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169 | |
170 | =head2 How do I clear the screen? |
171 | |
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172 | If you only have do so infrequently, use C<system>: |
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173 | |
ac9dac7f |
174 | system("clear"); |
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175 | |
176 | If you have to do this a lot, save the clear string |
177 | so you can print it 100 times without calling a program |
178 | 100 times: |
179 | |
ac9dac7f |
180 | $clear_string = `clear`; |
181 | print $clear_string; |
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182 | |
183 | If you're planning on doing other screen manipulations, like cursor |
184 | positions, etc, you might wish to use Term::Cap module: |
185 | |
ac9dac7f |
186 | use Term::Cap; |
187 | $terminal = Term::Cap->Tgetent( {OSPEED => 9600} ); |
188 | $clear_string = $terminal->Tputs('cl'); |
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189 | |
190 | =head2 How do I get the screen size? |
191 | |
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192 | If you have Term::ReadKey module installed from CPAN, |
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193 | you can use it to fetch the width and height in characters |
194 | and in pixels: |
195 | |
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196 | use Term::ReadKey; |
197 | ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize(); |
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198 | |
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199 | This is more portable than the raw C<ioctl>, but not as |
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200 | illustrative: |
201 | |
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202 | require 'sys/ioctl.ph'; |
203 | die "no TIOCGWINSZ " unless defined &TIOCGWINSZ; |
204 | open(TTY, "+</dev/tty") or die "No tty: $!"; |
205 | unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) { |
206 | die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ; |
207 | } |
208 | ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize); |
209 | print "(row,col) = ($row,$col)"; |
210 | print " (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel; |
211 | print "\n"; |
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212 | |
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213 | =head2 How do I ask the user for a password? |
214 | |
215 | (This question has nothing to do with the web. See a different |
216 | FAQ for that.) |
217 | |
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218 | There's an example of this in L<perlfunc/crypt>). First, you put the |
219 | terminal into "no echo" mode, then just read the password normally. |
220 | You may do this with an old-style ioctl() function, POSIX terminal |
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221 | control (see L<POSIX> or its documentation the Camel Book), or a call |
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222 | to the B<stty> program, with varying degrees of portability. |
223 | |
224 | You can also do this for most systems using the Term::ReadKey module |
225 | from CPAN, which is easier to use and in theory more portable. |
226 | |
ac9dac7f |
227 | use Term::ReadKey; |
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228 | |
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229 | ReadMode('noecho'); |
230 | $password = ReadLine(0); |
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231 | |
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232 | =head2 How do I read and write the serial port? |
233 | |
234 | This depends on which operating system your program is running on. In |
235 | the case of Unix, the serial ports will be accessible through files in |
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236 | /dev; on other systems, device names will doubtless differ. |
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237 | Several problem areas common to all device interaction are the |
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238 | following: |
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239 | |
240 | =over 4 |
241 | |
242 | =item lockfiles |
243 | |
244 | Your system may use lockfiles to control multiple access. Make sure |
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245 | you follow the correct protocol. Unpredictable behavior can result |
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246 | from multiple processes reading from one device. |
247 | |
248 | =item open mode |
249 | |
250 | If you expect to use both read and write operations on the device, |
251 | you'll have to open it for update (see L<perlfunc/"open"> for |
252 | details). You may wish to open it without running the risk of |
253 | blocking by using sysopen() and C<O_RDWR|O_NDELAY|O_NOCTTY> from the |
254 | Fcntl module (part of the standard perl distribution). See |
255 | L<perlfunc/"sysopen"> for more on this approach. |
256 | |
257 | =item end of line |
258 | |
259 | Some devices will be expecting a "\r" at the end of each line rather |
260 | than a "\n". In some ports of perl, "\r" and "\n" are different from |
261 | their usual (Unix) ASCII values of "\012" and "\015". You may have to |
262 | give the numeric values you want directly, using octal ("\015"), hex |
263 | ("0x0D"), or as a control-character specification ("\cM"). |
264 | |
ac9dac7f |
265 | print DEV "atv1\012"; # wrong, for some devices |
266 | print DEV "atv1\015"; # right, for some devices |
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267 | |
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268 | Even though with normal text files a "\n" will do the trick, there is |
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269 | still no unified scheme for terminating a line that is portable |
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270 | between Unix, DOS/Win, and Macintosh, except to terminate I<ALL> line |
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271 | ends with "\015\012", and strip what you don't need from the output. |
272 | This applies especially to socket I/O and autoflushing, discussed |
273 | next. |
274 | |
275 | =item flushing output |
276 | |
277 | If you expect characters to get to your device when you print() them, |
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278 | you'll want to autoflush that filehandle. You can use select() |
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279 | and the C<$|> variable to control autoflushing (see L<perlvar/$E<verbar>> |
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280 | and L<perlfunc/select>, or L<perlfaq5>, "How do I flush/unbuffer an |
281 | output filehandle? Why must I do this?"): |
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282 | |
ac9dac7f |
283 | $oldh = select(DEV); |
284 | $| = 1; |
285 | select($oldh); |
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286 | |
287 | You'll also see code that does this without a temporary variable, as in |
288 | |
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289 | select((select(DEV), $| = 1)[0]); |
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290 | |
c8db1d39 |
291 | Or if you don't mind pulling in a few thousand lines |
292 | of code just because you're afraid of a little $| variable: |
293 | |
ac9dac7f |
294 | use IO::Handle; |
295 | DEV->autoflush(1); |
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296 | |
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297 | As mentioned in the previous item, this still doesn't work when using |
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298 | socket I/O between Unix and Macintosh. You'll need to hard code your |
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299 | line terminators, in that case. |
300 | |
301 | =item non-blocking input |
302 | |
303 | If you are doing a blocking read() or sysread(), you'll have to |
304 | arrange for an alarm handler to provide a timeout (see |
305 | L<perlfunc/alarm>). If you have a non-blocking open, you'll likely |
306 | have a non-blocking read, which means you may have to use a 4-arg |
307 | select() to determine whether I/O is ready on that device (see |
308 | L<perlfunc/"select">. |
309 | |
310 | =back |
311 | |
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312 | While trying to read from his caller-id box, the notorious Jamie Zawinski |
ac9dac7f |
313 | C<< <jwz@netscape.com> >>, after much gnashing of teeth and fighting with sysread, |
c8db1d39 |
314 | sysopen, POSIX's tcgetattr business, and various other functions that |
315 | go bump in the night, finally came up with this: |
316 | |
ac9dac7f |
317 | sub open_modem { |
318 | use IPC::Open2; |
319 | my $stty = `/bin/stty -g`; |
320 | open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1"); |
321 | # starting cu hoses /dev/tty's stty settings, even when it has |
322 | # been opened on a pipe... |
323 | system("/bin/stty $stty"); |
324 | $_ = <MODEM_IN>; |
325 | chomp; |
326 | if ( !m/^Connected/ ) { |
327 | print STDERR "$0: cu printed `$_' instead of `Connected'\n"; |
328 | } |
c8db1d39 |
329 | } |
c8db1d39 |
330 | |
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331 | =head2 How do I decode encrypted password files? |
332 | |
333 | You spend lots and lots of money on dedicated hardware, but this is |
334 | bound to get you talked about. |
335 | |
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336 | Seriously, you can't if they are Unix password files--the Unix |
ac003c96 |
337 | password system employs one-way encryption. It's more like hashing |
338 | than encryption. The best you can do is check whether something else |
339 | hashes to the same string. You can't turn a hash back into the |
340 | original string. Programs like Crack can forcibly (and intelligently) |
341 | try to guess passwords, but don't (can't) guarantee quick success. |
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342 | |
343 | If you're worried about users selecting bad passwords, you should |
344 | proactively check when they try to change their password (by modifying |
345 | passwd(1), for example). |
346 | |
347 | =head2 How do I start a process in the background? |
348 | |
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349 | Several modules can start other processes that do not block |
350 | your Perl program. You can use IPC::Open3, Parallel::Jobs, |
351 | IPC::Run, and some of the POE modules. See CPAN for more |
352 | details. |
353 | |
354 | You could also use |
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355 | |
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356 | system("cmd &") |
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357 | |
358 | or you could use fork as documented in L<perlfunc/"fork">, with |
359 | further examples in L<perlipc>. Some things to be aware of, if you're |
360 | on a Unix-like system: |
361 | |
362 | =over 4 |
363 | |
c8db1d39 |
364 | =item STDIN, STDOUT, and STDERR are shared |
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365 | |
366 | Both the main process and the backgrounded one (the "child" process) |
367 | share the same STDIN, STDOUT and STDERR filehandles. If both try to |
368 | access them at once, strange things can happen. You may want to close |
369 | or reopen these for the child. You can get around this with |
370 | C<open>ing a pipe (see L<perlfunc/"open">) but on some systems this |
371 | means that the child process cannot outlive the parent. |
372 | |
373 | =item Signals |
374 | |
375 | You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too. |
376 | SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is |
377 | sent when you write to a filehandle whose child process has closed (an |
378 | untrapped SIGPIPE can cause your program to silently die). This is |
379 | not an issue with C<system("cmd&")>. |
380 | |
381 | =item Zombies |
382 | |
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383 | You have to be prepared to "reap" the child process when it finishes. |
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384 | |
ac9dac7f |
385 | $SIG{CHLD} = sub { wait }; |
197aec24 |
386 | |
ac9dac7f |
387 | $SIG{CHLD} = 'IGNORE'; |
197aec24 |
388 | |
389 | You can also use a double fork. You immediately wait() for your |
390 | first child, and the init daemon will wait() for your grandchild once |
49d635f9 |
391 | it exits. |
392 | |
393 | unless ($pid = fork) { |
ac9dac7f |
394 | unless (fork) { |
395 | exec "what you really wanna do"; |
396 | die "exec failed!"; |
397 | } |
398 | exit 0; |
399 | } |
400 | waitpid($pid, 0); |
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401 | |
402 | See L<perlipc/"Signals"> for other examples of code to do this. |
403 | Zombies are not an issue with C<system("prog &")>. |
404 | |
405 | =back |
406 | |
407 | =head2 How do I trap control characters/signals? |
408 | |
c8db1d39 |
409 | You don't actually "trap" a control character. Instead, that character |
410 | generates a signal which is sent to your terminal's currently |
411 | foregrounded process group, which you then trap in your process. |
b73a15ae |
412 | Signals are documented in L<perlipc/"Signals"> and the |
b432a672 |
413 | section on "Signals" in the Camel. |
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414 | |
6670e5e7 |
415 | You can set the values of the %SIG hash to be the functions you want |
c98c5709 |
416 | to handle the signal. After perl catches the signal, it looks in %SIG |
417 | for a key with the same name as the signal, then calls the subroutine |
418 | value for that key. |
419 | |
420 | # as an anonymous subroutine |
6670e5e7 |
421 | |
c98c5709 |
422 | $SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) }; |
6670e5e7 |
423 | |
c98c5709 |
424 | # or a reference to a function |
6670e5e7 |
425 | |
c98c5709 |
426 | $SIG{INT} = \&ouch; |
6670e5e7 |
427 | |
c98c5709 |
428 | # or the name of the function as a string |
6670e5e7 |
429 | |
430 | $SIG{INT} = "ouch"; |
c98c5709 |
431 | |
432 | Perl versions before 5.8 had in its C source code signal handlers which |
433 | would catch the signal and possibly run a Perl function that you had set |
434 | in %SIG. This violated the rules of signal handling at that level |
435 | causing perl to dump core. Since version 5.8.0, perl looks at %SIG |
436 | *after* the signal has been caught, rather than while it is being caught. |
437 | Previous versions of this answer were incorrect. |
68dc0745 |
438 | |
68dc0745 |
439 | =head2 How do I modify the shadow password file on a Unix system? |
440 | |
a6dd486b |
441 | If perl was installed correctly and your shadow library was written |
c8db1d39 |
442 | properly, the getpw*() functions described in L<perlfunc> should in |
443 | theory provide (read-only) access to entries in the shadow password |
444 | file. To change the file, make a new shadow password file (the format |
197aec24 |
445 | varies from system to system--see L<passwd> for specifics) and use |
446 | pwd_mkdb(8) to install it (see L<pwd_mkdb> for more details). |
68dc0745 |
447 | |
448 | =head2 How do I set the time and date? |
449 | |
450 | Assuming you're running under sufficient permissions, you should be |
451 | able to set the system-wide date and time by running the date(1) |
452 | program. (There is no way to set the time and date on a per-process |
453 | basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT; |
454 | the VMS equivalent is C<set time>. |
455 | |
8305e449 |
456 | However, if all you want to do is change your time zone, you can |
68dc0745 |
457 | probably get away with setting an environment variable: |
458 | |
ac9dac7f |
459 | $ENV{TZ} = "MST7MDT"; # unixish |
460 | $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms |
461 | system "trn comp.lang.perl.misc"; |
68dc0745 |
462 | |
463 | =head2 How can I sleep() or alarm() for under a second? |
464 | |
465 | If you want finer granularity than the 1 second that the sleep() |
466 | function provides, the easiest way is to use the select() function as |
0325b4c4 |
467 | documented in L<perlfunc/"select">. Try the Time::HiRes and |
83df6a1d |
468 | the BSD::Itimer modules (available from CPAN, and starting from |
469 | Perl 5.8 Time::HiRes is part of the standard distribution). |
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470 | |
471 | =head2 How can I measure time under a second? |
472 | |
65acb1b1 |
473 | In general, you may not be able to. The Time::HiRes module (available |
83df6a1d |
474 | from CPAN, and starting from Perl 5.8 part of the standard distribution) |
475 | provides this functionality for some systems. |
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476 | |
65acb1b1 |
477 | If your system supports both the syscall() function in Perl as well as |
478 | a system call like gettimeofday(2), then you may be able to do |
479 | something like this: |
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480 | |
ac9dac7f |
481 | require 'sys/syscall.ph'; |
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482 | |
ac9dac7f |
483 | $TIMEVAL_T = "LL"; |
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484 | |
ac9dac7f |
485 | $done = $start = pack($TIMEVAL_T, ()); |
68dc0745 |
486 | |
ac9dac7f |
487 | syscall(&SYS_gettimeofday, $start, 0) != -1 |
488 | or die "gettimeofday: $!"; |
68dc0745 |
489 | |
ac9dac7f |
490 | ########################## |
491 | # DO YOUR OPERATION HERE # |
492 | ########################## |
68dc0745 |
493 | |
ac9dac7f |
494 | syscall( &SYS_gettimeofday, $done, 0) != -1 |
495 | or die "gettimeofday: $!"; |
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496 | |
ac9dac7f |
497 | @start = unpack($TIMEVAL_T, $start); |
498 | @done = unpack($TIMEVAL_T, $done); |
68dc0745 |
499 | |
ac9dac7f |
500 | # fix microseconds |
501 | for ($done[1], $start[1]) { $_ /= 1_000_000 } |
68dc0745 |
502 | |
ac9dac7f |
503 | $delta_time = sprintf "%.4f", ($done[0] + $done[1] ) |
504 | - |
505 | ($start[0] + $start[1] ); |
68dc0745 |
506 | |
507 | =head2 How can I do an atexit() or setjmp()/longjmp()? (Exception handling) |
508 | |
509 | Release 5 of Perl added the END block, which can be used to simulate |
510 | atexit(). Each package's END block is called when the program or |
197aec24 |
511 | thread ends (see L<perlmod> manpage for more details). |
c8db1d39 |
512 | |
513 | For example, you can use this to make sure your filter program |
514 | managed to finish its output without filling up the disk: |
515 | |
ac9dac7f |
516 | END { |
517 | close(STDOUT) || die "stdout close failed: $!"; |
518 | } |
c8db1d39 |
519 | |
a6dd486b |
520 | The END block isn't called when untrapped signals kill the program, |
521 | though, so if you use END blocks you should also use |
68dc0745 |
522 | |
523 | use sigtrap qw(die normal-signals); |
524 | |
525 | Perl's exception-handling mechanism is its eval() operator. You can |
526 | use eval() as setjmp and die() as longjmp. For details of this, see |
46fc3d4c |
527 | the section on signals, especially the time-out handler for a blocking |
b432a672 |
528 | flock() in L<perlipc/"Signals"> or the section on "Signals" in |
b73a15ae |
529 | the Camel Book. |
68dc0745 |
530 | |
531 | If exception handling is all you're interested in, try the |
532 | exceptions.pl library (part of the standard perl distribution). |
533 | |
534 | If you want the atexit() syntax (and an rmexit() as well), try the |
535 | AtExit module available from CPAN. |
536 | |
a6dd486b |
537 | =head2 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean? |
68dc0745 |
538 | |
539 | Some Sys-V based systems, notably Solaris 2.X, redefined some of the |
540 | standard socket constants. Since these were constant across all |
541 | architectures, they were often hardwired into perl code. The proper |
542 | way to deal with this is to "use Socket" to get the correct values. |
543 | |
544 | Note that even though SunOS and Solaris are binary compatible, these |
545 | values are different. Go figure. |
546 | |
547 | =head2 How can I call my system's unique C functions from Perl? |
548 | |
a6dd486b |
549 | In most cases, you write an external module to do it--see the answer |
68dc0745 |
550 | to "Where can I learn about linking C with Perl? [h2xs, xsubpp]". |
551 | However, if the function is a system call, and your system supports |
552 | syscall(), you can use the syscall function (documented in |
553 | L<perlfunc>). |
554 | |
555 | Remember to check the modules that came with your distribution, and |
ac9dac7f |
556 | CPAN as well--someone may already have written a module to do it. On |
197aec24 |
557 | Windows, try Win32::API. On Macs, try Mac::Carbon. If no module |
558 | has an interface to the C function, you can inline a bit of C in your |
559 | Perl source with Inline::C. |
68dc0745 |
560 | |
561 | =head2 Where do I get the include files to do ioctl() or syscall()? |
562 | |
563 | Historically, these would be generated by the h2ph tool, part of the |
564 | standard perl distribution. This program converts cpp(1) directives |
565 | in C header files to files containing subroutine definitions, like |
566 | &SYS_getitimer, which you can use as arguments to your functions. |
567 | It doesn't work perfectly, but it usually gets most of the job done. |
568 | Simple files like F<errno.h>, F<syscall.h>, and F<socket.h> were fine, |
569 | but the hard ones like F<ioctl.h> nearly always need to hand-edited. |
570 | Here's how to install the *.ph files: |
571 | |
ac9dac7f |
572 | 1. become super-user |
573 | 2. cd /usr/include |
574 | 3. h2ph *.h */*.h |
68dc0745 |
575 | |
576 | If your system supports dynamic loading, for reasons of portability and |
577 | sanity you probably ought to use h2xs (also part of the standard perl |
578 | distribution). This tool converts C header files to Perl extensions. |
579 | See L<perlxstut> for how to get started with h2xs. |
580 | |
581 | If your system doesn't support dynamic loading, you still probably |
582 | ought to use h2xs. See L<perlxstut> and L<ExtUtils::MakeMaker> for |
583 | more information (in brief, just use B<make perl> instead of a plain |
584 | B<make> to rebuild perl with a new static extension). |
585 | |
586 | =head2 Why do setuid perl scripts complain about kernel problems? |
587 | |
588 | Some operating systems have bugs in the kernel that make setuid |
589 | scripts inherently insecure. Perl gives you a number of options |
590 | (described in L<perlsec>) to work around such systems. |
591 | |
592 | =head2 How can I open a pipe both to and from a command? |
593 | |
594 | The IPC::Open2 module (part of the standard perl distribution) is an |
c8db1d39 |
595 | easy-to-use approach that internally uses pipe(), fork(), and exec() to do |
596 | the job. Make sure you read the deadlock warnings in its documentation, |
197aec24 |
597 | though (see L<IPC::Open2>). See |
598 | L<perlipc/"Bidirectional Communication with Another Process"> and |
13a2d996 |
599 | L<perlipc/"Bidirectional Communication with Yourself"> |
c8db1d39 |
600 | |
601 | You may also use the IPC::Open3 module (part of the standard perl |
602 | distribution), but be warned that it has a different order of |
603 | arguments from IPC::Open2 (see L<IPC::Open3>). |
68dc0745 |
604 | |
3fe9a6f1 |
605 | =head2 Why can't I get the output of a command with system()? |
606 | |
46fc3d4c |
607 | You're confusing the purpose of system() and backticks (``). system() |
608 | runs a command and returns exit status information (as a 16 bit value: |
c8db1d39 |
609 | the low 7 bits are the signal the process died from, if any, and |
46fc3d4c |
610 | the high 8 bits are the actual exit value). Backticks (``) run a |
3fe9a6f1 |
611 | command and return what it sent to STDOUT. |
612 | |
ac9dac7f |
613 | $exit_status = system("mail-users"); |
614 | $output_string = `ls`; |
3fe9a6f1 |
615 | |
68dc0745 |
616 | =head2 How can I capture STDERR from an external command? |
617 | |
618 | There are three basic ways of running external commands: |
619 | |
ac9dac7f |
620 | system $cmd; # using system() |
621 | $output = `$cmd`; # using backticks (``) |
622 | open (PIPE, "cmd |"); # using open() |
68dc0745 |
623 | |
624 | With system(), both STDOUT and STDERR will go the same place as the |
a6dd486b |
625 | script's STDOUT and STDERR, unless the system() command redirects them. |
68dc0745 |
626 | Backticks and open() read B<only> the STDOUT of your command. |
627 | |
49d635f9 |
628 | You can also use the open3() function from IPC::Open3. Benjamin |
629 | Goldberg provides some sample code: |
630 | |
631 | To capture a program's STDOUT, but discard its STDERR: |
632 | |
ac9dac7f |
633 | use IPC::Open3; |
634 | use File::Spec; |
635 | use Symbol qw(gensym); |
636 | open(NULL, ">", File::Spec->devnull); |
637 | my $pid = open3(gensym, \*PH, ">&NULL", "cmd"); |
638 | while( <PH> ) { } |
639 | waitpid($pid, 0); |
49d635f9 |
640 | |
641 | To capture a program's STDERR, but discard its STDOUT: |
642 | |
ac9dac7f |
643 | use IPC::Open3; |
644 | use File::Spec; |
645 | use Symbol qw(gensym); |
646 | open(NULL, ">", File::Spec->devnull); |
647 | my $pid = open3(gensym, ">&NULL", \*PH, "cmd"); |
648 | while( <PH> ) { } |
649 | waitpid($pid, 0); |
49d635f9 |
650 | |
651 | To capture a program's STDERR, and let its STDOUT go to our own STDERR: |
652 | |
ac9dac7f |
653 | use IPC::Open3; |
654 | use Symbol qw(gensym); |
655 | my $pid = open3(gensym, ">&STDERR", \*PH, "cmd"); |
656 | while( <PH> ) { } |
657 | waitpid($pid, 0); |
49d635f9 |
658 | |
659 | To read both a command's STDOUT and its STDERR separately, you can |
660 | redirect them to temp files, let the command run, then read the temp |
661 | files: |
662 | |
ac9dac7f |
663 | use IPC::Open3; |
664 | use Symbol qw(gensym); |
665 | use IO::File; |
666 | local *CATCHOUT = IO::File->new_tmpfile; |
667 | local *CATCHERR = IO::File->new_tmpfile; |
668 | my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd"); |
669 | waitpid($pid, 0); |
670 | seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR; |
671 | while( <CATCHOUT> ) {} |
672 | while( <CATCHERR> ) {} |
49d635f9 |
673 | |
674 | But there's no real need for *both* to be tempfiles... the following |
675 | should work just as well, without deadlocking: |
676 | |
ac9dac7f |
677 | use IPC::Open3; |
678 | use Symbol qw(gensym); |
679 | use IO::File; |
680 | local *CATCHERR = IO::File->new_tmpfile; |
681 | my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd"); |
682 | while( <CATCHOUT> ) {} |
683 | waitpid($pid, 0); |
684 | seek CATCHERR, 0, 0; |
685 | while( <CATCHERR> ) {} |
49d635f9 |
686 | |
687 | And it'll be faster, too, since we can begin processing the program's |
688 | stdout immediately, rather than waiting for the program to finish. |
689 | |
68dc0745 |
690 | With any of these, you can change file descriptors before the call: |
691 | |
ac9dac7f |
692 | open(STDOUT, ">logfile"); |
693 | system("ls"); |
68dc0745 |
694 | |
695 | or you can use Bourne shell file-descriptor redirection: |
696 | |
ac9dac7f |
697 | $output = `$cmd 2>some_file`; |
698 | open (PIPE, "cmd 2>some_file |"); |
68dc0745 |
699 | |
700 | You can also use file-descriptor redirection to make STDERR a |
701 | duplicate of STDOUT: |
702 | |
ac9dac7f |
703 | $output = `$cmd 2>&1`; |
704 | open (PIPE, "cmd 2>&1 |"); |
68dc0745 |
705 | |
706 | Note that you I<cannot> simply open STDERR to be a dup of STDOUT |
707 | in your Perl program and avoid calling the shell to do the redirection. |
708 | This doesn't work: |
709 | |
ac9dac7f |
710 | open(STDERR, ">&STDOUT"); |
711 | $alloutput = `cmd args`; # stderr still escapes |
68dc0745 |
712 | |
713 | This fails because the open() makes STDERR go to where STDOUT was |
714 | going at the time of the open(). The backticks then make STDOUT go to |
715 | a string, but don't change STDERR (which still goes to the old |
716 | STDOUT). |
717 | |
718 | Note that you I<must> use Bourne shell (sh(1)) redirection syntax in |
719 | backticks, not csh(1)! Details on why Perl's system() and backtick |
06a5f41f |
720 | and pipe opens all use the Bourne shell are in the |
721 | F<versus/csh.whynot> article in the "Far More Than You Ever Wanted To |
49d635f9 |
722 | Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . To |
06a5f41f |
723 | capture a command's STDERR and STDOUT together: |
68dc0745 |
724 | |
ac9dac7f |
725 | $output = `cmd 2>&1`; # either with backticks |
726 | $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe |
727 | while (<PH>) { } # plus a read |
c8db1d39 |
728 | |
729 | To capture a command's STDOUT but discard its STDERR: |
730 | |
ac9dac7f |
731 | $output = `cmd 2>/dev/null`; # either with backticks |
732 | $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe |
733 | while (<PH>) { } # plus a read |
c8db1d39 |
734 | |
735 | To capture a command's STDERR but discard its STDOUT: |
736 | |
ac9dac7f |
737 | $output = `cmd 2>&1 1>/dev/null`; # either with backticks |
738 | $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe |
739 | while (<PH>) { } # plus a read |
c8db1d39 |
740 | |
741 | To exchange a command's STDOUT and STDERR in order to capture the STDERR |
742 | but leave its STDOUT to come out our old STDERR: |
743 | |
ac9dac7f |
744 | $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks |
745 | $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe |
746 | while (<PH>) { } # plus a read |
c8db1d39 |
747 | |
748 | To read both a command's STDOUT and its STDERR separately, it's easiest |
2359510d |
749 | to redirect them separately to files, and then read from those files |
750 | when the program is done: |
c8db1d39 |
751 | |
ac9dac7f |
752 | system("program args 1>program.stdout 2>program.stderr"); |
c8db1d39 |
753 | |
754 | Ordering is important in all these examples. That's because the shell |
755 | processes file descriptor redirections in strictly left to right order. |
756 | |
ac9dac7f |
757 | system("prog args 1>tmpfile 2>&1"); |
758 | system("prog args 2>&1 1>tmpfile"); |
c8db1d39 |
759 | |
760 | The first command sends both standard out and standard error to the |
761 | temporary file. The second command sends only the old standard output |
762 | there, and the old standard error shows up on the old standard out. |
68dc0745 |
763 | |
764 | =head2 Why doesn't open() return an error when a pipe open fails? |
765 | |
49d635f9 |
766 | If the second argument to a piped open() contains shell |
dfdf0730 |
767 | metacharacters, perl fork()s, then exec()s a shell to decode the |
768 | metacharacters and eventually run the desired program. If the program |
769 | couldn't be run, it's the shell that gets the message, not Perl. All |
770 | your Perl program can find out is whether the shell itself could be |
771 | successfully started. You can still capture the shell's STDERR and |
772 | check it for error messages. See L<"How can I capture STDERR from an |
773 | external command?"> elsewhere in this document, or use the |
49d635f9 |
774 | IPC::Open3 module. |
dfdf0730 |
775 | |
49d635f9 |
776 | If there are no shell metacharacters in the argument of open(), Perl |
dfdf0730 |
777 | runs the command directly, without using the shell, and can correctly |
778 | report whether the command started. |
68dc0745 |
779 | |
780 | =head2 What's wrong with using backticks in a void context? |
781 | |
782 | Strictly speaking, nothing. Stylistically speaking, it's not a good |
dfdf0730 |
783 | way to write maintainable code. Perl has several operators for |
784 | running external commands. Backticks are one; they collect the output |
785 | from the command for use in your program. The C<system> function is |
197aec24 |
786 | another; it doesn't do this. |
68dc0745 |
787 | |
dfdf0730 |
788 | Writing backticks in your program sends a clear message to the readers |
789 | of your code that you wanted to collect the output of the command. |
790 | Why send a clear message that isn't true? |
68dc0745 |
791 | |
792 | Consider this line: |
793 | |
ac9dac7f |
794 | `cat /etc/termcap`; |
68dc0745 |
795 | |
dfdf0730 |
796 | You forgot to check C<$?> to see whether the program even ran |
797 | correctly. Even if you wrote |
68dc0745 |
798 | |
ac9dac7f |
799 | print `cat /etc/termcap`; |
68dc0745 |
800 | |
a6dd486b |
801 | this code could and probably should be written as |
68dc0745 |
802 | |
ac9dac7f |
803 | system("cat /etc/termcap") == 0 |
68dc0745 |
804 | or die "cat program failed!"; |
805 | |
ee891a00 |
806 | which will echo the cat command's output as it is generated, instead |
807 | of waiting until the program has completed to print it out. It also |
808 | checks the return value. |
68dc0745 |
809 | |
ac9dac7f |
810 | C<system> also provides direct control over whether shell wildcard |
68dc0745 |
811 | processing may take place, whereas backticks do not. |
812 | |
813 | =head2 How can I call backticks without shell processing? |
814 | |
49d635f9 |
815 | This is a bit tricky. You can't simply write the command |
816 | like this: |
68dc0745 |
817 | |
ac9dac7f |
818 | @ok = `grep @opts '$search_string' @filenames`; |
68dc0745 |
819 | |
ac003c96 |
820 | As of Perl 5.8.0, you can use C<open()> with multiple arguments. |
821 | Just like the list forms of C<system()> and C<exec()>, no shell |
49d635f9 |
822 | escapes happen. |
823 | |
ac9dac7f |
824 | open( GREP, "-|", 'grep', @opts, $search_string, @filenames ); |
825 | chomp(@ok = <GREP>); |
826 | close GREP; |
49d635f9 |
827 | |
828 | You can also: |
68dc0745 |
829 | |
ac9dac7f |
830 | my @ok = (); |
831 | if (open(GREP, "-|")) { |
832 | while (<GREP>) { |
833 | chomp; |
834 | push(@ok, $_); |
ac003c96 |
835 | } |
836 | close GREP; |
ac9dac7f |
837 | } else { |
838 | exec 'grep', @opts, $search_string, @filenames; |
839 | } |
68dc0745 |
840 | |
ac003c96 |
841 | Just as with C<system()>, no shell escapes happen when you C<exec()> a |
842 | list. Further examples of this can be found in L<perlipc/"Safe Pipe |
843 | Opens">. |
68dc0745 |
844 | |
ac003c96 |
845 | Note that if you're using Windows, no solution to this vexing issue is |
846 | even possible. Even if Perl were to emulate C<fork()>, you'd still be |
847 | stuck, because Windows does not have an argc/argv-style API. |
c8db1d39 |
848 | |
54310121 |
849 | =head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)? |
68dc0745 |
850 | |
a6dd486b |
851 | Some stdio's set error and eof flags that need clearing. The |
68dc0745 |
852 | POSIX module defines clearerr() that you can use. That is the |
853 | technically correct way to do it. Here are some less reliable |
854 | workarounds: |
855 | |
856 | =over 4 |
857 | |
858 | =item 1 |
859 | |
860 | Try keeping around the seekpointer and go there, like this: |
861 | |
ac9dac7f |
862 | $where = tell(LOG); |
863 | seek(LOG, $where, 0); |
68dc0745 |
864 | |
865 | =item 2 |
866 | |
867 | If that doesn't work, try seeking to a different part of the file and |
868 | then back. |
869 | |
870 | =item 3 |
871 | |
872 | If that doesn't work, try seeking to a different part of |
873 | the file, reading something, and then seeking back. |
874 | |
875 | =item 4 |
876 | |
877 | If that doesn't work, give up on your stdio package and use sysread. |
878 | |
879 | =back |
880 | |
881 | =head2 How can I convert my shell script to perl? |
882 | |
883 | Learn Perl and rewrite it. Seriously, there's no simple converter. |
884 | Things that are awkward to do in the shell are easy to do in Perl, and |
885 | this very awkwardness is what would make a shell->perl converter |
886 | nigh-on impossible to write. By rewriting it, you'll think about what |
887 | you're really trying to do, and hopefully will escape the shell's |
46fc3d4c |
888 | pipeline datastream paradigm, which while convenient for some matters, |
68dc0745 |
889 | causes many inefficiencies. |
890 | |
891 | =head2 Can I use perl to run a telnet or ftp session? |
892 | |
46fc3d4c |
893 | Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from |
a93751fa |
894 | CPAN). http://www.cpan.org/scripts/netstuff/telnet.emul.shar |
46fc3d4c |
895 | will also help for emulating the telnet protocol, but Net::Telnet is |
896 | quite probably easier to use.. |
897 | |
898 | If all you want to do is pretend to be telnet but don't need |
899 | the initial telnet handshaking, then the standard dual-process |
900 | approach will suffice: |
901 | |
ac9dac7f |
902 | use IO::Socket; # new in 5.004 |
903 | $handle = IO::Socket::INET->new('www.perl.com:80') |
904 | or die "can't connect to port 80 on www.perl.com: $!"; |
905 | $handle->autoflush(1); |
906 | if (fork()) { # XXX: undef means failure |
907 | select($handle); |
908 | print while <STDIN>; # everything from stdin to socket |
909 | } else { |
910 | print while <$handle>; # everything from socket to stdout |
911 | } |
912 | close $handle; |
913 | exit; |
68dc0745 |
914 | |
915 | =head2 How can I write expect in Perl? |
916 | |
917 | Once upon a time, there was a library called chat2.pl (part of the |
c8db1d39 |
918 | standard perl distribution), which never really got finished. If you |
919 | find it somewhere, I<don't use it>. These days, your best bet is to |
920 | look at the Expect module available from CPAN, which also requires two |
921 | other modules from CPAN, IO::Pty and IO::Stty. |
68dc0745 |
922 | |
923 | =head2 Is there a way to hide perl's command line from programs such as "ps"? |
924 | |
925 | First of all note that if you're doing this for security reasons (to |
926 | avoid people seeing passwords, for example) then you should rewrite |
927 | your program so that critical information is never given as an |
928 | argument. Hiding the arguments won't make your program completely |
929 | secure. |
930 | |
931 | To actually alter the visible command line, you can assign to the |
932 | variable $0 as documented in L<perlvar>. This won't work on all |
933 | operating systems, though. Daemon programs like sendmail place their |
934 | state there, as in: |
935 | |
ac9dac7f |
936 | $0 = "orcus [accepting connections]"; |
68dc0745 |
937 | |
938 | =head2 I {changed directory, modified my environment} in a perl script. How come the change disappeared when I exited the script? How do I get my changes to be visible? |
939 | |
940 | =over 4 |
941 | |
942 | =item Unix |
943 | |
a6dd486b |
944 | In the strictest sense, it can't be done--the script executes as a |
68dc0745 |
945 | different process from the shell it was started from. Changes to a |
a6dd486b |
946 | process are not reflected in its parent--only in any children |
68dc0745 |
947 | created after the change. There is shell magic that may allow you to |
948 | fake it by eval()ing the script's output in your shell; check out the |
197aec24 |
949 | comp.unix.questions FAQ for details. |
68dc0745 |
950 | |
68dc0745 |
951 | =back |
952 | |
953 | =head2 How do I close a process's filehandle without waiting for it to complete? |
954 | |
955 | Assuming your system supports such things, just send an appropriate signal |
a6dd486b |
956 | to the process (see L<perlfunc/"kill">). It's common to first send a TERM |
68dc0745 |
957 | signal, wait a little bit, and then send a KILL signal to finish it off. |
958 | |
959 | =head2 How do I fork a daemon process? |
960 | |
961 | If by daemon process you mean one that's detached (disassociated from |
962 | its tty), then the following process is reported to work on most |
963 | Unixish systems. Non-Unix users should check their Your_OS::Process |
964 | module for other solutions. |
965 | |
966 | =over 4 |
967 | |
968 | =item * |
969 | |
197aec24 |
970 | Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty> |
c8db1d39 |
971 | for details. Or better yet, you can just use the POSIX::setsid() |
972 | function, so you don't have to worry about process groups. |
68dc0745 |
973 | |
974 | =item * |
975 | |
976 | Change directory to / |
977 | |
978 | =item * |
979 | |
980 | Reopen STDIN, STDOUT, and STDERR so they're not connected to the old |
981 | tty. |
982 | |
983 | =item * |
984 | |
985 | Background yourself like this: |
986 | |
ac9dac7f |
987 | fork && exit; |
68dc0745 |
988 | |
989 | =back |
990 | |
1a91aff4 |
991 | The Proc::Daemon module, available from CPAN, provides a function to |
992 | perform these actions for you. |
993 | |
68dc0745 |
994 | =head2 How do I find out if I'm running interactively or not? |
995 | |
ac9dac7f |
996 | Good question. Sometimes C<-t STDIN> and C<-t STDOUT> can give clues, |
68dc0745 |
997 | sometimes not. |
998 | |
ac9dac7f |
999 | if (-t STDIN && -t STDOUT) { |
1000 | print "Now what? "; |
1001 | } |
68dc0745 |
1002 | |
1003 | On POSIX systems, you can test whether your own process group matches |
1004 | the current process group of your controlling terminal as follows: |
1005 | |
ac9dac7f |
1006 | use POSIX qw/getpgrp tcgetpgrp/; |
1007 | |
1008 | # Some POSIX systems, such as Linux, can be |
1009 | # without a /dev/tty at boot time. |
1010 | if (!open(TTY, "/dev/tty")) { |
1011 | print "no tty\n"; |
1012 | } else { |
1013 | $tpgrp = tcgetpgrp(fileno(*TTY)); |
1014 | $pgrp = getpgrp(); |
1015 | if ($tpgrp == $pgrp) { |
1016 | print "foreground\n"; |
1017 | } else { |
1018 | print "background\n"; |
1019 | } |
1020 | } |
68dc0745 |
1021 | |
1022 | =head2 How do I timeout a slow event? |
1023 | |
1024 | Use the alarm() function, probably in conjunction with a signal |
b73a15ae |
1025 | handler, as documented in L<perlipc/"Signals"> and the section on |
b432a672 |
1026 | "Signals" in the Camel. You may instead use the more flexible |
b73a15ae |
1027 | Sys::AlarmCall module available from CPAN. |
68dc0745 |
1028 | |
49d635f9 |
1029 | The alarm() function is not implemented on all versions of Windows. |
1030 | Check the documentation for your specific version of Perl. |
1031 | |
68dc0745 |
1032 | =head2 How do I set CPU limits? |
1033 | |
1034 | Use the BSD::Resource module from CPAN. |
1035 | |
1036 | =head2 How do I avoid zombies on a Unix system? |
1037 | |
1038 | Use the reaper code from L<perlipc/"Signals"> to call wait() when a |
1039 | SIGCHLD is received, or else use the double-fork technique described |
49d635f9 |
1040 | in L<perlfaq8/"How do I start a process in the background?">. |
68dc0745 |
1041 | |
1042 | =head2 How do I use an SQL database? |
1043 | |
04d666b1 |
1044 | The DBI module provides an abstract interface to most database |
1045 | servers and types, including Oracle, DB2, Sybase, mysql, Postgresql, |
1046 | ODBC, and flat files. The DBI module accesses each database type |
1047 | through a database driver, or DBD. You can see a complete list of |
1048 | available drivers on CPAN: http://www.cpan.org/modules/by-module/DBD/ . |
1049 | You can read more about DBI on http://dbi.perl.org . |
1050 | |
1051 | Other modules provide more specific access: Win32::ODBC, Alzabo, iodbc, |
1052 | and others found on CPAN Search: http://search.cpan.org . |
68dc0745 |
1053 | |
1054 | =head2 How do I make a system() exit on control-C? |
1055 | |
1056 | You can't. You need to imitate the system() call (see L<perlipc> for |
1057 | sample code) and then have a signal handler for the INT signal that |
c8db1d39 |
1058 | passes the signal on to the subprocess. Or you can check for it: |
1059 | |
ac9dac7f |
1060 | $rc = system($cmd); |
1061 | if ($rc & 127) { die "signal death" } |
68dc0745 |
1062 | |
1063 | =head2 How do I open a file without blocking? |
1064 | |
1065 | If you're lucky enough to be using a system that supports |
1066 | non-blocking reads (most Unixish systems do), you need only to use the |
1067 | O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with |
1068 | sysopen(): |
1069 | |
ac9dac7f |
1070 | use Fcntl; |
1071 | sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644) |
1072 | or die "can't open /foo/somefile: $!": |
68dc0745 |
1073 | |
c98c5709 |
1074 | =head2 How do I tell the difference between errors from the shell and perl? |
1075 | |
ac003c96 |
1076 | (answer contributed by brian d foy) |
c98c5709 |
1077 | |
1078 | When you run a Perl script, something else is running the script for you, |
1079 | and that something else may output error messages. The script might |
1080 | emit its own warnings and error messages. Most of the time you cannot |
1081 | tell who said what. |
1082 | |
1083 | You probably cannot fix the thing that runs perl, but you can change how |
1084 | perl outputs its warnings by defining a custom warning and die functions. |
1085 | |
1086 | Consider this script, which has an error you may not notice immediately. |
1087 | |
1088 | #!/usr/locl/bin/perl |
1089 | |
1090 | print "Hello World\n"; |
1091 | |
1092 | I get an error when I run this from my shell (which happens to be |
1093 | bash). That may look like perl forgot it has a print() function, |
1094 | but my shebang line is not the path to perl, so the shell runs the |
1095 | script, and I get the error. |
1096 | |
1097 | $ ./test |
1098 | ./test: line 3: print: command not found |
1099 | |
1100 | A quick and dirty fix involves a little bit of code, but this may be all |
1101 | you need to figure out the problem. |
1102 | |
1103 | #!/usr/bin/perl -w |
6670e5e7 |
1104 | |
c98c5709 |
1105 | BEGIN { |
1106 | $SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; }; |
1107 | $SIG{__DIE__} = sub{ print STDERR "Perl: ", @_; exit 1}; |
1108 | } |
6670e5e7 |
1109 | |
c98c5709 |
1110 | $a = 1 + undef; |
1111 | $x / 0; |
1112 | __END__ |
1113 | |
1114 | The perl message comes out with "Perl" in front. The BEGIN block |
1115 | works at compile time so all of the compilation errors and warnings |
1116 | get the "Perl:" prefix too. |
1117 | |
1118 | Perl: Useless use of division (/) in void context at ./test line 9. |
1119 | Perl: Name "main::a" used only once: possible typo at ./test line 8. |
1120 | Perl: Name "main::x" used only once: possible typo at ./test line 9. |
1121 | Perl: Use of uninitialized value in addition (+) at ./test line 8. |
1122 | Perl: Use of uninitialized value in division (/) at ./test line 9. |
1123 | Perl: Illegal division by zero at ./test line 9. |
1124 | Perl: Illegal division by zero at -e line 3. |
1125 | |
1126 | If I don't see that "Perl:", it's not from perl. |
1127 | |
1128 | You could also just know all the perl errors, and although there are |
1129 | some people who may know all of them, you probably don't. However, they |
1130 | all should be in the perldiag manpage. If you don't find the error in |
1131 | there, it probably isn't a perl error. |
1132 | |
1133 | Looking up every message is not the easiest way, so let perl to do it |
1134 | for you. Use the diagnostics pragma with turns perl's normal messages |
1135 | into longer discussions on the topic. |
1136 | |
1137 | use diagnostics; |
1138 | |
1139 | If you don't get a paragraph or two of expanded discussion, it |
1140 | might not be perl's message. |
1141 | |
d92eb7b0 |
1142 | =head2 How do I install a module from CPAN? |
1143 | |
1144 | The easiest way is to have a module also named CPAN do it for you. |
197aec24 |
1145 | This module comes with perl version 5.004 and later. |
76817d6d |
1146 | |
ac9dac7f |
1147 | $ perl -MCPAN -e shell |
76817d6d |
1148 | |
ac9dac7f |
1149 | cpan shell -- CPAN exploration and modules installation (v1.59_54) |
1150 | ReadLine support enabled |
76817d6d |
1151 | |
ac9dac7f |
1152 | cpan> install Some::Module |
76817d6d |
1153 | |
197aec24 |
1154 | To manually install the CPAN module, or any well-behaved CPAN module |
76817d6d |
1155 | for that matter, follow these steps: |
68dc0745 |
1156 | |
1157 | =over 4 |
1158 | |
1159 | =item 1 |
1160 | |
1161 | Unpack the source into a temporary area. |
1162 | |
1163 | =item 2 |
1164 | |
ac9dac7f |
1165 | perl Makefile.PL |
68dc0745 |
1166 | |
1167 | =item 3 |
1168 | |
ac9dac7f |
1169 | make |
68dc0745 |
1170 | |
1171 | =item 4 |
1172 | |
ac9dac7f |
1173 | make test |
68dc0745 |
1174 | |
1175 | =item 5 |
1176 | |
ac9dac7f |
1177 | make install |
68dc0745 |
1178 | |
1179 | =back |
1180 | |
1181 | If your version of perl is compiled without dynamic loading, then you |
1182 | just need to replace step 3 (B<make>) with B<make perl> and you will |
1183 | get a new F<perl> binary with your extension linked in. |
1184 | |
c8db1d39 |
1185 | See L<ExtUtils::MakeMaker> for more details on building extensions. |
b432a672 |
1186 | See also the next question, "What's the difference between require |
1187 | and use?". |
c8db1d39 |
1188 | |
1189 | =head2 What's the difference between require and use? |
1190 | |
1191 | Perl offers several different ways to include code from one file into |
1192 | another. Here are the deltas between the various inclusion constructs: |
1193 | |
ac9dac7f |
1194 | 1) do $file is like eval `cat $file`, except the former |
5e3006a4 |
1195 | 1.1: searches @INC and updates %INC. |
c8db1d39 |
1196 | 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code. |
1197 | |
ac9dac7f |
1198 | 2) require $file is like do $file, except the former |
c8db1d39 |
1199 | 2.1: checks for redundant loading, skipping already loaded files. |
1200 | 2.2: raises an exception on failure to find, compile, or execute $file. |
1201 | |
ac9dac7f |
1202 | 3) require Module is like require "Module.pm", except the former |
c8db1d39 |
1203 | 3.1: translates each "::" into your system's directory separator. |
1204 | 3.2: primes the parser to disambiguate class Module as an indirect object. |
1205 | |
ac9dac7f |
1206 | 4) use Module is like require Module, except the former |
c8db1d39 |
1207 | 4.1: loads the module at compile time, not run-time. |
1208 | 4.2: imports symbols and semantics from that package to the current one. |
1209 | |
1210 | In general, you usually want C<use> and a proper Perl module. |
46fc3d4c |
1211 | |
1212 | =head2 How do I keep my own module/library directory? |
1213 | |
ac003c96 |
1214 | When you build modules, tell Perl where to install the modules. |
1215 | |
1216 | For C<Makefile.PL>-based distributions, use the PREFIX and LIB options |
1217 | when generating Makefiles: |
46fc3d4c |
1218 | |
ac9dac7f |
1219 | perl Makefile.PL PREFIX=/mydir/perl LIB=/mydir/perl/lib |
46fc3d4c |
1220 | |
ac003c96 |
1221 | You can set this in your CPAN.pm configuration so modules automatically install |
1222 | in your private library directory when you use the CPAN.pm shell: |
46fc3d4c |
1223 | |
ac003c96 |
1224 | % cpan |
1225 | cpan> o conf makepl_arg PREFIX=/mydir/perl,LIB=/mydir/perl/lib |
1226 | cpan> o conf commit |
46fc3d4c |
1227 | |
ac003c96 |
1228 | For C<Build.PL>-based distributions, use the --install_base option: |
65acb1b1 |
1229 | |
ac003c96 |
1230 | perl Build.PL --install_base /mydir/perl |
1231 | |
1232 | You can configure CPAN.pm to automatically use this option too: |
65acb1b1 |
1233 | |
ac003c96 |
1234 | % cpan |
1235 | cpan> o conf mbuild_arg --install_base /mydir/perl |
1236 | cpan> o conf commit |
46fc3d4c |
1237 | |
1238 | =head2 How do I add the directory my program lives in to the module/library search path? |
1239 | |
ac003c96 |
1240 | (contributed by brian d foy) |
1241 | |
1242 | If you know the directory already, you can add it to C<@INC> as you would |
1243 | for any other directory. You might <use lib> if you know the directory |
1244 | at compile time: |
1245 | |
1246 | use lib $directory; |
1247 | |
1248 | The trick in this task is to find the directory. Before your script does |
1249 | anything else (such as a C<chdir>), you can get the current working |
1250 | directory with the C<Cwd> module, which comes with Perl: |
1251 | |
1252 | BEGIN { |
1253 | use Cwd; |
1254 | our $directory = cwd; |
1255 | } |
1256 | |
1257 | use lib $directory; |
1258 | |
1259 | You can do a similar thing with the value of C<$0>, which holds the |
1260 | script name. That might hold a relative path, but C<rel2abs> can turn |
1261 | it into an absolute path. Once you have the |
1262 | |
1263 | BEGIN { |
1264 | use File::Spec::Functions qw(rel2abs); |
1265 | use File::Basename qw(dirname); |
1266 | |
1267 | my $path = rel2abs( $0 ); |
1268 | our $directory = dirname( $path ); |
1269 | } |
1270 | |
1271 | use lib $directory; |
1272 | |
1273 | The C<FindBin> module, which comes with Perl, might work. It searches |
1274 | through C<$ENV{PATH}> (so your script has to be in one of those |
1275 | directories). You can then use that directory (in C<$FindBin::Bin>) |
1276 | to locate nearby directories you want to add: |
1277 | |
ac9dac7f |
1278 | use FindBin; |
ac003c96 |
1279 | use lib "$FindBin::Bin/../lib"; |
46fc3d4c |
1280 | |
f0d19b68 |
1281 | =head2 How do I add a directory to my include path (@INC) at runtime? |
46fc3d4c |
1282 | |
ac003c96 |
1283 | Here are the suggested ways of modifying your include path, including |
1284 | environment variables, run-time switches, and in-code statements: |
1285 | |
1286 | =over 4 |
1287 | |
1288 | =item the PERLLIB environment variable |
1289 | |
1290 | $ export PERLLIB=/path/to/my/dir |
1291 | $ perl program.pl |
1292 | |
1293 | =item the PERL5LIB environment variable |
46fc3d4c |
1294 | |
ac003c96 |
1295 | $ export PERL5LIB=/path/to/my/dir |
1296 | $ perl program.pl |
1297 | |
1298 | =item the perl -Idir command line flag |
1299 | |
1300 | $ perl -I/path/to/my/dir program.pl |
1301 | |
1302 | =item the use lib pragma: |
1303 | |
1304 | use lib "$ENV{HOME}/myown_perllib"; |
1305 | |
1306 | =back |
46fc3d4c |
1307 | |
ac003c96 |
1308 | The last is particularly useful because it knows about machine |
46fc3d4c |
1309 | dependent architectures. The lib.pm pragmatic module was first |
1310 | included with the 5.002 release of Perl. |
68dc0745 |
1311 | |
65acb1b1 |
1312 | =head2 What is socket.ph and where do I get it? |
1313 | |
e9d185f8 |
1314 | It's a Perl 4 style file defining values for system networking |
65acb1b1 |
1315 | constants. Sometimes it is built using h2ph when Perl is installed, |
1316 | but other times it is not. Modern programs C<use Socket;> instead. |
1317 | |
500071f4 |
1318 | =head1 REVISION |
1319 | |
e9d185f8 |
1320 | Revision: $Revision: 9681 $ |
500071f4 |
1321 | |
e9d185f8 |
1322 | Date: $Date: 2007-06-26 01:36:56 +0200 (Tue, 26 Jun 2007) $ |
500071f4 |
1323 | |
1324 | See L<perlfaq> for source control details and availability. |
1325 | |
fc36a67e |
1326 | =head1 AUTHOR AND COPYRIGHT |
1327 | |
ee891a00 |
1328 | Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and |
7678cced |
1329 | other authors as noted. All rights reserved. |
5a964f20 |
1330 | |
5a7beb56 |
1331 | This documentation is free; you can redistribute it and/or modify it |
1332 | under the same terms as Perl itself. |
5a964f20 |
1333 | |
1334 | Irrespective of its distribution, all code examples in this file |
1335 | are hereby placed into the public domain. You are permitted and |
1336 | encouraged to use this code in your own programs for fun |
1337 | or for profit as you see fit. A simple comment in the code giving |
1338 | credit would be courteous but is not required. |