In Perl, is it possible to determine if a script is being executed within another script (presumably via system or qx)?
$ cat foo.pl
print "foo";
print "\n" if not $in_qx; # or the like.
I realize this is not applicable if the script was being run via exec.
I know for certain that system runs the process as a fork and I know fork can return a value that is variable depending on whether you are in the parent or the child process. Not certain about qx.
Regardless, I'm not certain how to figure out if I'm in a forked process without actually performing a fork.
All processes are forked from another process (except init). You can sort of tell if the program was run from open, qx//, open2, or open3 by using the isatty function from POSIX, but there is no good way to determine if you are being run by system without looking at the process tree, and even then it can get murky (for instance system "nohup", "./foo.pl" will not have the calling perl process as its parent).
You could check "who's your daddy", using "getppid" (get parent id). Then check if your parent id is a perl script with pgrep or similar.
Do you control the caller? The simplest thing to do would be to pass an argument, e.g. --isforked.
Related
I have a perl script which calls another script. I am calling it using backticks and passing argument to that script and it works fine.
`CQPerl call_script.pl $agr1 $agr2 $arg3`;
But please suggest if there is another better way to do so. How can I check if the script errored out because of the calling script or the script that was called. How do I do that check from the calling script itself?
If you wan't to do error checking, backticks may be a wrong approach. You probably want to use the system function. See the documentation for all the details of error handling, examples included.
Perl has a number of possibilites to execute other scripts / commands:
backticks / qx{} When you want to read all the output at once after the program has terminated
exec When you wan't to continue your process as another program — never returns if succesfull
system When you are only interested in the success or failure of the command
open When you want to pipe information to or from the command
do and require Execute another Perl script here. Similar to C's #include
There are modules to do a three-way open so that you have access to STDIN, STDOUT and STDERR of the program you executed. See the apropriate parts of perlipc for advanced information.
And always use the multi-argument forms of these calls to avoid shell escaping (can be annoying and very insecure).
Check the value of the perl special variable $? to determine if there was an error.
I have a system call in Perl that looks like this:
system('/path/to/utility >> /redirect/to/log file');
But this waits for utility to complete. I just want to trigger this and let the Perl script finish irrespective of whether the utility call finished or not.
How can I do this?
I tried changing the line to
system('/path/to/utility >> /redirect/to/log file &');
but this syntax still waits for the call to finish on Windows. I need to make it work on Linux as well as Windows.
if ($^O eq 'MSWin32') {
system('start "" \\path\\to\\utility >> \\redirect\\to\\log_file');
} else {
system('/path/to/utility >> /redirect/to/log_file &');
}
or
if ($^O eq 'MSWin32') {
system(1, '\\path\\to\\utility >> \\redirect\\to\\log_file');
} else {
system('/path/to/utility >> /redirect/to/log_file &');
}
You could try looking at the fork keyword, and launch your system command from the forked process. See the perlipc manpage for examples.
This common task has been abstracted into the CPAN module Proc::Background
The easiest way on POSIX-like systems is in fact the one you already tried:
system('/path/to/utility >> /redirect/to/log_file &');
The easiest way to get a tool to execute in background on Windows is to use start.exe as a launch helper:
system('start /path/to/utility >> /redirect/to/log_file');
However, I don't know if it works to redirect the output to log file that way and I have no Windows system around this very moment for testing it.
Yes, that means you would need a code branch depending on current system, but other solutions may need one, too. If your Perl has fork() emulation, you can in fact use fork() on both systems (this is a bit more complicated, since you cannot redirect stdout to a logfile that easy, you first have to open it in Perl and make it stdout of the forked child before calling the new process). If your Windows Perl has no fork() emulation, you also need a code branch, since in that case you can use fork() only on UNIX and you'll need to use Win32::Process::Create with the DETACHED_PROCESS option on Windows.
But maybe you can first let us know if using start is already working for you. If it does not, maybe start.exe doesn't handle slashes. In that case you may have to use something like
C:\\path\\to\\utility
instead (double backslash is important! A single backslash has a special meaning in string literals; it's the escape character).
To find out if you are running on Windows or not, take a look at the variable $OSNAME or $^OS, it should say something like "MSWin32" on Windows.
I thought I remember reading somewhere about where perl can be configured to automatically load a certain .pm file on start up.
I know about PERL5OPT, but to my recollection, this was a specific file that would be loaded if it exists.
Is it a compile option that can be set (i.e. via Configure)?
Reading through perldoc perlrun it looks like you are looking for what is talked about in the -f option:
-f
Disable executing $Config{sitelib}/sitecustomize.pl at startup.
Perl can be built so that it by default will try to execute
$Config{sitelib}/sitecustomize.pl at startup (in a BEGIN block). This
is a hook that allows the sysadmin to customize how Perl behaves. It
can for instance be used to add entries to the #INC array to make Perl
find modules in non-standard locations.
Perl actually inserts the following code:
BEGIN {
do { local $!; -f "$Config{sitelib}/sitecustomize.pl"; }
&& do "$Config{sitelib}/sitecustomize.pl";
}
Since it is an actual do (not a require), sitecustomize.pl doesn't
need to return a true value. The code is run in package main , in its
own lexical scope. However, if the script dies, $# will not be set.
The value of $Config{sitelib} is also determined in C code and not
read from Config.pm , which is not loaded.
The code is executed very early. For example, any changes made to #INC
will show up in the output of perl -V. Of course, END blocks will be
likewise executed very late.
To determine at runtime if this capability has been compiled in your
perl, you can check the value of $Config{usesitecustomize} .
I've never done this, but it looks like if you put what you want in $Config{sitelib}/sitecustomize.pl you'll get what you are looking for.
See:
http://perldoc.perl.org/perlrun.html
http://www.nntp.perl.org/group/perl.perl5.porters/2007/10/msg129926.html
I'm confused by what you mean by "on start up". If you mean when a script / CGI / whatever is "started", then just use the module in the script:
use Data::Dumper;
Or do you mean something else?
#!/usr/bin/env perl
use warnings; use strict;
use 5.012;
use IPC::System::Simple qw(system);
system( 'xterm', '-geometry', '80x25-5-5', '-bg', 'green', '&' );
say "Hello";
say "World";
I tried this to run the xterm-command in the background, but it doesn't work:
No absolute path found for shell: &
What would be the right way to make it work?
Perl's system function has two modes:
taking a single string and passing it to the command shell to allow special characters to be processed
taking a list of strings, exec'ing the first and passing the remaining strings as arguments
In the first form you have to be careful to escape characters that might have a special meaning to the shell. The second form is generally safer since arguments are passed directly to the program being exec'd without the shell being involved.
In your case you seem to be mixing the two forms. The & character only has the meaning of "start this program in the background" if it is passed to the shell. In your program, the ampersand is being passed as the 5th argument to the xterm command.
As Jakob Kruse said the simple answer is to use the single string form of system. If any of the arguments came from an untrusted source you'd have to use quoting or escaping to make them safe.
If you prefer to use the multi-argument form then you'll need to call fork() and then probably use exec() rather than system().
Note that the list form of system is specifically there to not treat characters such as & as shell meta-characters.
From perlfaq8's answer to How do I start a process in the background?
(contributed by brian d foy)
There's not a single way to run code in the background so you don't have to wait for it to finish before your program moves on to other tasks. Process management depends on your particular operating system, and many of the techniques are in perlipc.
Several CPAN modules may be able to help, including IPC::Open2 or IPC::Open3, IPC::Run, Parallel::Jobs, Parallel::ForkManager, POE, Proc::Background, and Win32::Process. There are many other modules you might use, so check those namespaces for other options too.
If you are on a Unix-like system, you might be able to get away with a system call where you put an & on the end of the command:
system("cmd &")
You can also try using fork, as described in perlfunc (although this is the same thing that many of the modules will do for you).
STDIN, STDOUT, and STDERR are shared
Both the main process and the backgrounded one (the "child" process) share the same STDIN, STDOUT and STDERR filehandles. If both try to access them at once, strange things can happen. You may want to close or reopen these for the child. You can get around this with opening a pipe (see open in perlfunc) but on some systems this means that the child process cannot outlive the parent.
Signals
You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too. SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is sent when you write to a filehandle whose child process has closed (an untrapped SIGPIPE can cause your program to silently die). This is not an issue with system("cmd&").
Zombies
You have to be prepared to "reap" the child process when it finishes.
$SIG{CHLD} = sub { wait };
$SIG{CHLD} = 'IGNORE';
You can also use a double fork. You immediately wait() for your first child, and the init daemon will wait() for your grandchild once it exits.
unless ($pid = fork) {
unless (fork) {
exec "what you really wanna do";
die "exec failed!";
}
exit 0;
}
waitpid($pid, 0);
See Signals in perlipc for other examples of code to do this. Zombies are not an issue with system("prog &").
Have you tried?
system('xterm -geometry 80x25-5-5 -bg green &');
http://www.rocketaware.com/perl/perlfaq8/How_do_I_start_a_process_in_the_.htm
This is not purely an explanation for Perl. The same problem is under C and other languages.
First understand what the system command does:
Forks
Under the child process call exec
The parent process is waiting for forked child process to finish
It does not matter if you pass multiple arguments or one argument. The difference is, with multiple arguments, the command is executed directly. With one argument, the command is wrapped by the shell, and finally executed as:
/bin/sh -c your_command_with_redirections_and_ambersand
When you pass a command as some_command par1 par2 &, then between the Perl interpreter and the command is the sh or bash process used as a wrapper, and it is waiting for some_command finishing. Your script is waiting for the shell interpreter, and no additional waitpid is needed, because Perl's function system does it for you.
When you want to implement this mechanism directly in your script, you should:
Use the fork function. See example: http://users.telenet.be/bartl/classicperl/fork/all.html
Under the child condition (if), use the exec function. Your user is similar to system, see the manual. Notice, exec causes the child process program/content/data cover by the executed command.
Under the parent condition (if, fork exits with non-zero), you use waitpid, using pid returned by the fork function.
This is why you can run the process in the background. I hope this is simple.
The simplest example:
if (my $pid = fork) { #exits 0 = false for child process, at this point is brain split
# parent ($pid is process id of child)
# Do something what you want, asynchronously with executed command
waitpid($pid); # Wait until child ends
# If you don't want to, don't wait. Your process ends, and then the child process will be relinked
# from your script to INIT process, and finally INIT will assume the child finishing.
# Alternatively, you can handle the SIGCHLD signal in your script
}
else {
# Child
exec('some_command arg1 arg2'); #or exec('some_command','arg1','arg2');
#exit is not needed, because exec completely overwrites the process content
}
I've got a Perl script that needs to execute another Perl script. This second script can be executed directly on the command line, but I need to execute it from within my first program. I'll need to pass it a few parameters that would normally be passed in when it's run standalone (the first script runs periodically, and executes the second script under a certain set of system conditions).
Preliminary Google searches suggest using backticks or a system() call. Are there any other ways to run it? (I'm guessing yes, since it's Perl we're talking about :P ) Which method is preferred if I need to capture output from the invoked program (and, if possible, pipe that output as it executes to stdout as though the second program were invoked directly)?
(Edit: oh, now SO suggests some related questions. This one is close, but not exactly the same as what I'm asking. The second program will likely take an hour or more to run (lots of I/O), so I'm not sure a one-off invocation is the right fit for this.)
You can just do it.
{
local #ARGV = qw<param1 param2 param3>;
do '/home/buddy/myscript.pl';
}
Prevents the overhead of loading in another copy of perl.
The location of your current perl interpreter can be found in the special variable $^X. This is important if perl is not in your path, or if you have multiple perl versions available but which to make sure you're using the same one across the board.
When executing external commands, including other Perl programs, determining if they actually ran can be quite difficult. Inspecting $? can leave lasting mental scars, so I prefer to use IPC::System::Simple (available from the CPAN):
use strict;
use warnings;
use IPC::System::Simple qw(system capture);
# Run a command, wait until it finishes, and make sure it works.
# Output from this program goes directly to STDOUT, and it can take input
# from your STDIN if required.
system($^X, "yourscript.pl", #ARGS);
# Run a command, wait until it finishes, and make sure it works.
# The output of this command is captured into $results.
my $results = capture($^X, "yourscript.pl", #ARGS);
In both of the above examples any arguments you wish to pass to your external program go into #ARGS. The shell is also avoided in both of the above examples, which gives you a small speed advantage, and avoids any unwanted interactions involving shell meta-characters. The above code also expects your second program to return a zero exit value to indicate success; if that's not the case, you can specify an additional first argument of allowable exit values:
# Both of these commands allow an exit value of 0, 1 or 2 to be considered
# a successful execution of the command.
system( [0,1,2], $^X, "yourscript.pl", #ARGS );
# OR
capture( [0,1,2, $^X, "yourscript.pl", #ARGS );
If you have a long-running process and you want to process its data while it's being generated, then you're probably going to need a piped open, or one of the more heavyweight IPC modules from the CPAN.
Having said all that, any time you need to be calling another Perl program from Perl, you may wish to consider if using a module would be a better choice. Starting another program carries quite a few overheads, both in terms of start-up costs, and I/O costs for moving data between processes. It also significantly increases the difficulty of error handling. If you can turn your external program into a module, you may find it simplifies your overall design.
All the best,
Paul
I can think of a few ways to do this. You already mentioned the first two, so I won't go into detail on them.
backticks: $retVal = `perl somePerlScript.pl`;
system() call
eval
The eval can be accomplished by slurping the other file into a string (or a list of strings), then 'eval'ing the strings. Heres a sample:
#!/usr/bin/perl
open PERLFILE, "<somePerlScript.pl";
undef $/; # this allows me to slurp the file, ignoring newlines
my $program = <PERLFILE>;
eval $program;
4 . do: do 'somePerlScript.pl'
You already got good answers to your question, but there's always the posibility to take a different point of view: maybe you should consider refactoring the script that you want to run from the first script. Turn the functionality into a module. Use the module from the first and from the second script.
If you need to asynchronously call your external script -you just want to launch it and not wait for it to finish-, then :
# On Unix systems, either of these will execute and just carry-on
# You can't collect output that way
`myscript.pl &`;
system ('myscript.pl &');
# On Windows systems the equivalent would be
`start myscript.pl`;
system ('start myscript.pl');
# If you just want to execute another script and terminate the current one
exec ('myscript.pl');
Use backticks if you need to capture the output of the command.
Use system if you do not need to capture the output of the command.
TMTOWTDI: so there are other ways too, but those are the two easiest and most likely.
See the perlipc documentation for several options for interprocess communication.
If your first script merely sets up the environment for the second script, you may be looking for exec.
#!/usr/bin/perl
use strict;
open(OUTPUT, "date|") or die "Failed to create process: $!\n";
while (<OUTPUT>)
{
print;
}
close(OUTPUT);
print "Process exited with value " . ($? >> 8) . "\n";
This will start the process date and pipe the output of the command to the OUTPUT filehandle which you can process a line at a time. When the command is finished you can close the output filehandle and retrieve the return value of the process. Replace date with whatever you want.
I wanted to do something like this to offload non-subroutines into an external file to make editing easier. I actually made this into a subroutine. The advantage of this way is that those "my" variables in the external file get declared in the main namespace. If you use 'do' they apparently don't migrate to the main namespace. Note the presentation below doesn't include error handling
sub getcode($) {
my #list;
my $filename = shift;
open (INFILE, "< $filename");
#list = <INFILE>;
close (INFILE);
return \#list;
}
# and to use it:
my $codelist = [];
$codelist = getcode('sourcefile.pl');
eval join ("", #$codelist);