=head1 NAME
-perlfaq8 - System Interaction ($Revision: 1.2 $, $Date: 2001/10/16 13:27:22 $)
+perlfaq8 - System Interaction ($Revision: 1.8 $, $Date: 2002/05/16 12:41:42 $)
=head1 DESCRIPTION
Note that you I<must> use Bourne shell (sh(1)) redirection syntax in
backticks, not csh(1)! Details on why Perl's system() and backtick
-and pipe opens all use the Bourne shell are in
-http://www.cpan.org/doc/FMTEYEWTK/versus/csh.whynot .
-To capture a command's STDERR and STDOUT together:
+and pipe opens all use the Bourne shell are in the
+F<versus/csh.whynot> article in the "Far More Than You Ever Wanted To
+Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz . To
+capture a command's STDERR and STDOUT together:
$output = `cmd 2>&1`; # either with backticks
$pid = open(PH, "cmd 2>&1 |"); # or with an open pipe
=head2 Why doesn't open() return an error when a pipe open fails?
-Because the pipe open takes place in two steps: first Perl calls
-fork() to start a new process, then this new process calls exec() to
-run the program you really wanted to open. The first step reports
-success or failure to your process, so open() can only tell you
-whether the fork() succeeded or not.
+If the second argument to a piped C<open> contains shell
+metacharacters, perl fork()s, then exec()s a shell to decode the
+metacharacters and eventually run the desired program. If the program
+couldn't be run, it's the shell that gets the message, not Perl. All
+your Perl program can find out is whether the shell itself could be
+successfully started. You can still capture the shell's STDERR and
+check it for error messages. See L<"How can I capture STDERR from an
+external command?"> elsewhere in this document, or use the
+L<IPC::Open3> module.
-To find out if the exec() step succeeded, you have to catch SIGCHLD
-and wait() to get the exit status. You should also catch SIGPIPE if
-you're writing to the child--you may not have found out the exec()
-failed by the time you write. This is documented in L<perlipc>.
-
-In some cases, even this won't work. If the second argument to a
-piped open() contains shell metacharacters, perl fork()s, then exec()s
-a shell to decode the metacharacters and eventually run the desired
-program. Now when you call wait(), you only learn whether or not the
-I<shell> could be successfully started...it's best to avoid shell
-metacharacters.
-
-On systems that follow the spawn() paradigm, open() I<might> do what
-you expect--unless perl uses a shell to start your command. In this
-case the fork()/exec() description still applies.
+If there are no shell metacharacters in the argument of C<open>, Perl
+runs the command directly, without using the shell, and can correctly
+report whether the command started.
=head2 What's wrong with using backticks in a void context?
Strictly speaking, nothing. Stylistically speaking, it's not a good
-way to write maintainable code because backticks have a (potentially
-humongous) return value, and you're ignoring it. It's may also not be very
-efficient, because you have to read in all the lines of output, allocate
-memory for them, and then throw it away. Too often people are lulled
-to writing:
+way to write maintainable code. Perl has several operators for
+running external commands. Backticks are one; they collect the output
+from the command for use in your program. The C<system> function is
+another; it doesn't do this.
- `cp file file.bak`;
-
-And now they think "Hey, I'll just always use backticks to run programs."
-Bad idea: backticks are for capturing a program's output; the system()
-function is for running programs.
+Writing backticks in your program sends a clear message to the readers
+of your code that you wanted to collect the output of the command.
+Why send a clear message that isn't true?
Consider this line:
`cat /etc/termcap`;
-You haven't assigned the output anywhere, so it just wastes memory
-(for a little while). You forgot to check C<$?> to see whether
-the program even ran correctly, too. Even if you wrote
+You forgot to check C<$?> to see whether the program even ran
+correctly. Even if you wrote
print `cat /etc/termcap`;
There are a number of excellent interfaces to SQL databases. See the
DBD::* modules available from http://www.cpan.org/modules/DBD .
-A lot of information on this can be found at
-http://www.symbolstone.org/technology/perl/DBI/
+A lot of information on this can be found at http://dbi.perl.org/
=head2 How do I make a system() exit on control-C?
=head2 How do I install a module from CPAN?
The easiest way is to have a module also named CPAN do it for you.
-This module comes with perl version 5.004 and later. To manually install
-the CPAN module, or any well-behaved CPAN module for that matter, follow
-these steps:
+This module comes with perl version 5.004 and later.
+
+ $ perl -MCPAN -e shell
+
+ cpan shell -- CPAN exploration and modules installation (v1.59_54)
+ ReadLine support enabled
+
+ cpan> install Some::Module
+
+To manually install the CPAN module, or any well-behaved CPAN module
+for that matter, follow these steps:
=over 4
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
+Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
All rights reserved.
This documentation is free; you can redistribute it and/or modify it
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