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Full Discussion: Python Conditional Statements Based on Success of Last Command
Top Forums Programming Python Conditional Statements Based on Success of Last Command Post 302867715 by alister on Thursday 24th of October 2013 08:05:00 PM
Old 10-24-2013
Hi, wisecracker.

From the python os.system documentation:
Code:
On Unix, the return value is the exit status of the process encoded in the format specified for wait().

You should read the man page for the wait(2) system call. The integer returned, (32512 above) encodes more information than just the value passed to the exit system call. There are macros that are used to extract the actual value passed to exit(). Python's os module provides access to those macros. The two most relevant to your post:
Code:
os.WIFEXITED(status)

    Return True if the process exited using the exit(2) system call, otherwise return False.

    Availability: Unix.

os.WEXITSTATUS(status)

    If WIFEXITED(status) is true, return the integer parameter to the exit(2) system call.
    Otherwise, the return value is meaningless.

    Availability: Unix.

Those excerpts are from Python Standard Library - 16.1 - os.

To summarize, you cannot directly read the integer returned by wait(2) (which is the return value of os.system on UNIX). Also, you should not use bit shifting and/or masking to extract the relevant status bits because the encoding is allowed to differ between system implementations.

Regards,
Alister
Last edited by alister; 10-24-2013 at 09:15 PM..
This User Gave Thanks to alister For This Post:
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LEARN ABOUT XFREE86

wait.h

wait.h(3HEAD)							      Headers							     wait.h(3HEAD)

NAME

       wait.h, wait - wait status

SYNOPSIS

       #include <sys/wait.h>

DESCRIPTION

       When  a	process waits for status from its children using either the wait(3C) or waitpid(3C) function, the status returned can be evaluated
       with the following macros, defined in <sys/wait.h>. These macros evaluate to integral expressions. The stat argument to these macros is the
       integer value returned from wait() or waitpid().

       WCOREDUMP(stat)		       If the value of WIFSIGNALED (stat) is non-zero, this macro evaluates to a non-zero value if a core image of
				       the terminated child was created.

       WEXITSTATUS(stat)	       If the value of	WIFEXITED(stat) is non-zero, this macro evaluates to the exit code that the child  process
				       passed to  _exit() (see exit(2)) or exit(3C), or the value that the child process returned from main.

       WIFCONTINUED(stat)	       Evaluates to a non-zero value if status was returned for a child process that has continued.

       WIFEXITED(stat)		       Evaluates to a non-zero value if status was returned for a child process that terminated normally.

       WIFSIGNALED(stat)	       Evaluates to a non-zero value if status was returned for a child process that terminated due to the receipt
				       of a signal.

       WIFSTOPPED(stat) 	       Evaluates to a non-zero value if status was returned for a child process that is currently stopped.

       WSTOPSIG(stat)		       If the value of WIFSTOPPED(stat) is non-zero, this macro evaluates to the number of the signal that  caused
				       the child process to stop.

       WTERMSIG(stat)		       If  the	value  of   WIFSIGNALED(stat)  is  non-zero, this macro evaluates to the number of the signal that
				       caused the termination of the child process.

       The <sys/wait.h> header defines the symbolic constants listed below for use with waitpid(3C).

       WNOHANG	       Do not hang if no status is available; return immediately.

       WUNTRACED       Report status of stopped child process.

       The symbolic constants listed below are defined as possible values for the options argument to waitid(2).

       WEXITED	       Wait for processes that have exited.

       WSTOPPED        Status is returned for any child that has stopped upon receipt of a signal.

       WCONTINUED      Status is returned for any child that was stopped and has been continued.

       WNOHANG	       Return immediately if there are no children to wait for.

       WNOWAIT	       Keep the process whose status is returned in infop in a waitable state.

       The type idtype_t is defined as an enumeration type whose possible values include the following:

       P_ALL
       P_PID
       P_PGID

       The id_t and pid_t types are defined as described in <sys/types.h>.

       The siginfo_t type is defined as described in <signal.h>.

       The rusage structure is defined as described in <sys/resource.h>.

       Inclusion of the <sys/wait.h> header can also make visible all symbols from <signal.h> and <sys/resource.h>.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       exit(2), waitid(2), exit(3C), wait(3C), waitpid(3C),attributes(5), standards(5)

SunOS 5.10							    10 Sep 2004 						     wait.h(3HEAD)
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