1IPC::Open3(3pm) Perl Programmers Reference Guide IPC::Open3(3pm)
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6 IPC::Open3 - open a process for reading, writing, and error handling
7 using open3()
8
10 use Symbol 'gensym'; # vivify a separate handle for STDERR
11 my $pid = open3(my $chld_in, my $chld_out, my $chld_err = gensym,
12 'some', 'cmd', 'and', 'args');
13 # or pass the command through the shell
14 my $pid = open3(my $chld_in, my $chld_out, my $chld_err = gensym,
15 'some cmd and args');
16 # read from parent STDIN
18 # send STDOUT and STDERR to already open handle
19 open my $outfile, '>>', 'output.txt' or die "open failed: $!";
20 my $pid = open3('<&STDIN', $outfile, undef,
21 'some', 'cmd', 'and', 'args');
22 # write to parent STDOUT and STDERR
24 my $pid = open3(my $chld_in, '>&STDOUT', '>&STDERR',
25 'some', 'cmd', 'and', 'args');
26 # reap zombie and retrieve exit status
28 waitpid( $pid, 0 );
29 my $child_exit_status = $? >> 8;
30
32 Extremely similar to open2(), open3() spawns the given command and
33 connects $chld_out for reading from the child, $chld_in for writing to
34 the child, and $chld_err for errors. If $chld_err is false, or the
35 same file descriptor as $chld_out, then STDOUT and STDERR of the child
36 are on the same filehandle. This means that an autovivified lexical
37 cannot be used for the STDERR filehandle, but gensym from Symbol can be
38 used to vivify a new glob reference, see "SYNOPSIS". The $chld_in will
39 have autoflush turned on.
40 If $chld_in begins with "<&", then $chld_in will be closed in the
42 parent, and the child will read from it directly. If $chld_out or
43 $chld_err begins with ">&", then the child will send output directly to
44 that filehandle. In both cases, there will be a dup(2) instead of a
45 pipe(2) made.
46 If either reader or writer is the empty string or undefined, this will
48 be replaced by an autogenerated filehandle. If so, you must pass a
49 valid lvalue in the parameter slot so it can be overwritten in the
50 caller, or an exception will be raised.
51 The filehandles may also be integers, in which case they are understood
53 as file descriptors.
54 open3() returns the process ID of the child process. It doesn't return
56 on failure: it just raises an exception matching "/^open3:/". However,
57 "exec" failures in the child (such as no such file or permission
58 denied), are just reported to $chld_err under Windows and OS/2, as it
59 is not possible to trap them.
60 If the child process dies for any reason, the next write to $chld_in is
62 likely to generate a SIGPIPE in the parent, which is fatal by default.
63 So you may wish to handle this signal.
64 Note if you specify "-" as the command, in an analogous fashion to
66 "open(my $fh, "-|")" the child process will just be the forked Perl
67 process rather than an external command. This feature isn't yet
68 supported on Win32 platforms.
69 open3() does not wait for and reap the child process after it exits.
71 Except for short programs where it's acceptable to let the operating
72 system take care of this, you need to do this yourself. This is
73 normally as simple as calling "waitpid $pid, 0" when you're done with
74 the process. Failing to do this can result in an accumulation of
75 defunct or "zombie" processes. See "waitpid" in perlfunc for more
76 information.
77 If you try to read from the child's stdout writer and their stderr
79 writer, you'll have problems with blocking, which means you'll want to
80 use select() or IO::Select, which means you'd best use sysread()
81 instead of readline() for normal stuff.
82 This is very dangerous, as you may block forever. It assumes it's
84 going to talk to something like bc(1), both writing to it and reading
85 from it. This is presumably safe because you "know" that commands like
86 bc(1) will read a line at a time and output a line at a time. Programs
87 like sort(1) that read their entire input stream first, however, are
88 quite apt to cause deadlock.
89 The big problem with this approach is that if you don't have control
91 over source code being run in the child process, you can't control what
92 it does with pipe buffering. Thus you can't just open a pipe to "cat
93 -v" and continually read and write a line from it.
94
96 IPC::Open2
97 Like Open3 but without STDERR capture.
98 IPC::Run
100 This is a CPAN module that has better error handling and more
101 facilities than Open3.
102
104 The order of arguments differs from that of open2().
105perl v5.36.0 2022-08-30 IPC::Open3(3pm)