I tend to use the system() function pretty often, and I want to make sure I detect any problems. So, I think this means I want to make sure that programs I run always have an exit status of 0.

My understanding of the system function is that it returns a 16-bit value, where the high 8 bits are the exit status, and the low 8 are the signal id. Is that correct?

It also seems that I don't actually have to save the exit status after every use of system(), since the value also gets stored in $?.

I don't expect the programs I'm running to be killed by any signals (unless I'm the one doing the sending) ... So, I think this means I'm only concerned that the high 8 bits are all off. Therefore, does this mean I should be carefully making my system calls like this:

system('some_shell_command') >> 8 and die "Failed!";

# or else (same thing),

system('some_program');
if ($? >> 8 != 0) {
    die "Failed!";
}

[download]

or would the following suffice? :

system('foobar') == 0 or die "Failed!";

# or equivalently

system('foobar');
if ($? != 0) {
    die "Failed!";
}

[download]

Also, I notice in the docs for system(), there's this:

if ($? == -1) {
    print "failed to execute: $!\n";
}
elsif ($? & 127) {
    printf "child died with signal %d, %s coredump\n",
      ($? & 127),  ($? & 128) ? 'with' : 'without';
}
else {
    printf "child exited with value %d\n", $? >> 8;
}

[download]

For the if expression, if the exit status is 16 bits, then the value goes from 0 to 655535. What does it mean to check if it's -1?

For the elsif expression, why are they bitwise anding it with 127? 127 in binary is 0b0111_1111. So, that's only giving me the bottom 7 bits of $?, not 8. Why throw away that top bit in the bottom byte?

In reply to return value from system call, exit status, shift right 8, bitwise and, $? by Anonymous Monk

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