I was reading up on perlvar when i came across this -
The status returned by the last pipe close, backtick (`` ) command, successful call to wait() or waitpid(), or from the system() operator. This is just the 16-bit status word returned by the traditional Unix wait() system call (or else is made up to look like it). Thus, the exit value of the subprocess is really ($?>> 8 ), and $? & 127 gives which signal
What is a 16-bit status word? what does the operation '$?>> 8' signify? and how does a 16-bit word like '512' get converted to '2' after i do '$?>> 8' on it?
A 16-bit word is merely an amount of memory 16-bits in size. The word "word" implies the CPU can read it from memory with one instruction. (e.g. I worked on a machine that had 64K bytes of memory, but the CPU could only access it as 32K 16-bit words.)
Interpreted as an unsigned integer, an 16-bit word would look like a number between 0 and 216-1 = 65,535, but it's not necessarily an unsigned integer. In the case of $?, it's used to stored three unsigned integers.
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 15| 14| 13| 12| 11| 10| 9| 8| 7| 6| 5| 4| 3| 2| 1| 0|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
\-----------------------------/ \-/ \-------------------------/
Exit code core Signal that killed
(0..255) dumped (0..127)
(0..1)
If the OS wants to return "Exited with error code 2", it sets $? to (2 << 8) | (0 << 7) | (0 << 0).
+---+---+---+---+---+---+---+---+
| 2 | << 8
+---+---+---+---+---+---+---+---+
+---+
| 0 | << 7
+---+
+---+---+---+---+---+---+---+
| 0 | << 0
+---+---+---+---+---+---+---+
=================================================================
+---+---+---+---+---+---+---+---+
| 2 |
+---+---+---+---+---+---+---+---+
+---+
| 0 |
+---+
+---+---+---+---+---+---+---+
| 0 |
+---+---+---+---+---+---+---+
=================================================================
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 2 | 0 | 0 |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
If the OS wants to return "killed by signal 5; core dumped", it sets $? to (0 << 8) | (1 << 7) | (5 << 0).
+---+---+---+---+---+---+---+---+
| 0 | << 8
+---+---+---+---+---+---+---+---+
+---+
| 1 | << 7
+---+
+---+---+---+---+---+---+---+
| 5 | << 0
+---+---+---+---+---+---+---+
=================================================================
+---+---+---+---+---+---+---+---+
| 0 |
+---+---+---+---+---+---+---+---+
+---+
| 1 |
+---+
+---+---+---+---+---+---+---+
| 5 |
+---+---+---+---+---+---+---+
=================================================================
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 0 | 1 | 5 |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
$? >> 8 is simply doing the reverse operation.
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 2 | 0 | 0 |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
>> 8
=================================================================
+---+---+---+---+---+---+---+---+
| 2 |
+---+---+---+---+---+---+---+---+
It returns the number stored in bits 8 and up.