run-mongo: Only use pgrep on OS X

The fix to meteor#3999 didn't work on newer Linux systems whose pgrep contains
the backwards-incompatible change
https://gitorious.org/procps/procps/commit/f12277c74d591245767d77badb6bb6af91335656
to not include arguments.

Since the inspiration to use pgrep instead of 'ps ax' was to work around
an OS X bug, just avoid pgrep on Linux in the first place.

Fixes meteor#4115.

tools/run-mongo.js

@@ -135,15 +135,26 @@ if (process.platform === 'win32') {
     // However, 'pgrep' only started shipping with OS X 10.8 (and may be less
     // common on Linux too), so we check to see if it exists and fall back to
     // 'ps' if we can't find it.
-    child_process.exec(
-      'if type pgrep >/dev/null 2>&1; then ' +
+    //
+    // We avoid using pgrep on Linux, because some versions of Linux pgrep
+    // require you to pass -a/--list-full to include the arguments in the
+    // output, and other versions fail if you pass that option. We have not
+    // observed the Unicode corruption on Linux, so using ps ax there is fine.
+    var psScript = 'ps ax';
+    if (process.platform === 'darwin') {
+      psScript =
+        'if type pgrep >/dev/null 2>&1; then ' +
         // -lf means to display and match against full argument lists.
         // pgrep exits 1 if no processes match the argument; we're OK
         // considering this as a success, but we don't want other errors
         // to be ignored.  Note that this is sh not bash, so we can't use
         // [[.
         'pgrep -lf mongod; test "$?" -eq 0 -o "$?" -eq 1;' +
-        'else ps ax; fi',
+        'else ps ax; fi';
+    }
+
+    child_process.exec(
+      psScript,
       // we don't want this to randomly fail just because you're running lots of
       // processes. 10MB should be more than ps ax will ever spit out; the default
       // is 200K, which at least one person hit (#2158).