Changing parent process environment variable

[tcf22]

I don't even know if this is possible, but hopefully it is. This script needs to run on Win32 and Win64. I need to run a perl script from a shell and change the environment of the shell.

I tried

system("set VAR=5");

but that has no effect on the parent as I would suspect.

I have also tried using Win32::Process::Info to get the parent PID(hopefully there is an easier way).

use Win32::Process::Info;

my $pi = Win32::Process::Info->new ();
my %subs = $pi->Subprocesses();

my $ParentPID;
foreach(keys %subs)
{
    foreach my $pid(@{$subs{$_}})
    {
        if( $pid == $$ )
        {
            print "Parent PID is $_\n";
            $ParentPID = $_;
        }
    }
}

my @info = $pi->GetProcInfo($ParentPID);

use Data::Dumper;
print Dumper \@info;

but I don't know what to do with the parent PID to chenge the environment.

Thanks.

[BrowserUk]

You'd need to use Win32::API or Inline C to do this. See here where is says:

To programmatically add or modify system environment variables, add them to the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\Environment registry key, then broadcast a WM_SETTINGCHANGE message with lParam set to the string "Environment". This allows applications, such as the shell, to pick up your updates.

No guarentees that a pre-existing shell instance would see your changes though. You might be able to inject code into the parent process to do it, but that's nasty and awkward.

[cdarke]

It is not possible for a child process to directly alter the parent's environment block without using some shared memory magic - and that is true whatever the language used, compiled or not.

The environment block is set at process creation time and by default is a copy of the parent's. There is no concept of a "global" environment. All this is true on Windows and UNIX.

However on Windows, environment variables are also stored in the Registry, either in HKEY_CURRENT_USER\Environment (user variables) or HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\Environment (system variables). So you could use one of the (several) Win32 Perl modules to change the registry.

Now the tricky bit: for the parent to pick-up the change you should send a WM_SETTINGCHANGE message. I don't know how to do that from Perl, but there probably is a way. Look at the Win32:: namespace modules.

There are easier ways to pass a text string between programs ;-)

[cdarke]

If it helps, this is how I have changed the PATH env. var. using some other weird obscure language (OK, C++). Sorry it is not in Perl, I hope this is enough for you to get the gist:

#define ENVNAME "Path"
#define MAXLEN  1024

static void UpdatePath (char *szDir)
{
   char szPath [MAXLEN]={0};
   DWORD dwReturnValue;

   // Get previous value 
   if ( !GetEnvironmentVariable (ENVNAME, szPath, MAX_PATH) )
      ReportError ("GetEnvironmentVariable", GetLastError());

   // Update path 
   strcat (szPath, ";");
   strcat (szPath, szDir);

   // On W2K we need OSVERSIONINFOEX
   OSVERSIONINFO OsVer;
   OsVer.dwOSVersionInfoSize = sizeof (OsVer);
   GetVersionEx (&OsVer);

   if ( OsVer.dwPlatformId == VER_PLATFORM_WIN32_NT )
   {
      // Write to the user environment variables - 
      // only exists on WNT/W2K!

      HKEY hEnv;
      dwReturnValue = RegOpenKeyEx (HKEY_CURRENT_USER, 
                                    "Environment", 0, 
                                    KEY_SET_VALUE, 
                                    &hEnv) ;
      if ( dwReturnValue != ERROR_SUCCESS )
         ReportError ("RegOpenKeyEx", dwReturnValue);

      dwReturnValue = RegSetValueEx (hEnv, ENVNAME, 0, 
                                     REG_SZ, (BYTE*)szPath, 
                                     strlen(szPath)+1);

      if ( dwReturnValue != ERROR_SUCCESS )
         ReportError ("RegSetValueEx", dwReturnValue);

      RegFlushKey (hEnv);

      if ( hEnv ) RegCloseKey(hEnv);
   }
   
   // Tell the rest of the world (cmd line)

   if ( !SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, 0,
                     (LPARAM) "Environment", SMTO_ABORTIFHUNG,
                     5000, &dwReturnValue) )
   {
      ReportError ("SendMessageTimeout", GetLastError());
   }

   // Tell our children 

   if ( !SetEnvironmentVariable (ENVNAME, szPath) )
      ReportError ("SetEnvironmentVariable", GetLastError());

}  // UpdEnv

[ikegami]

Have the child output the set commands instead of executing it, and process the commands in the parent.

If the parent is a Perl script:

chomp( my @cmds = `child` );
foreach my $cmd (@cmds) {
   next unless /^set\s+(\S.*?)=(.*)/i;
   $ENV{$1} = $2;
}

If the parent is a batch file:

for /f "usebackq delims=" %%f in (`child`) do %%f

The child would output such as

echo set VAR=5

or

print("set VAR=5\n");

Update: Added second snippet.

[gamache]

I don't know that it is possible to change the parent shell's environment variables. Why are you trying to do this?

Perhaps another approach would be to exec a new shell with the environment you want?

[tcf22]

I'm not quite sure of the usefulness, but our IT department wants this for some sort of maintenance script. They say they have a compiled app that does it, but want to add more features than this app supports, so that is why I'm trying to do it.

[cdarke]

Since the posts above I have done some further digging.

First, the CPAN module Win32::Env has a BroadcastEnv method. However there are issues, as documented above and in the Win32::Env documentation.

Second, I have written a module that sets env.vars using DLL injection which I will put on CPAN. /msg me if you are interested.
I expect issues on Vista for this, and I have a fix, but I don't have access to Vista just yet. I will not upload onto CPAN until I have tested on Vista.

There is also the possibility of getting (as well as setting) another process's env.vars. - that is an extension I am currently working on.

I will keep this updated.

[cdarke]

I have just uploaded Win32::EnvProcess to PAUSE/CPAN.