python中os.system()的返回值_python os.system

1、os.system(cmd)的返回值

         python中的os.system(cmd)的返回值与linux命令返回值（具体参见本文附加内容）的关系。
        大家都习惯用os.system()函数执行linux命令，该函数的返回值十进制数（分别对应一个16位的二进制数）。该函数的返回值与 linux命令返回值两者的转换关系为：该函数的返回值（十进制）转化成16二进制数，截取其高八位（如果低位数是0的情况下，有关操作系统的错误码共 131个，所以低位都是零），然后转乘十进制数即为 linux命令返回值0。
例如：
        os.system()返回值为0

对应：inux命令返回值也为0.

        os.system()返回值为256，十六位二进制数示为：00000001，00000000，高八位转乘十进制为 1

对应   linux命令返回值 1

        os.system()返回值为512，十六位二进制数示为：00000010，00000000，高八位转乘十进制为 2

对应   linux命令返回值 2

......其它同理
         os.system()返回值为32512，十六位二进制数示为：01111111，00000000，高八位转乘十进制为 127

 对应   linux命令返回值 127

........

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2. 问题

       /bin/xxx.py是一个返回码为1的程序。当python 程序使用os.system(”./bin/xxx.py”) 这样调用的时候， 成功运行后os.system 的返回值出现了问题，变成了256 ，也就是0×100。而不是正常应该返回的1。

3. 解决方法

      查阅了文档发现os.system()的返回为：
           On Unix, the return value is the exit status of the process encoded in the format specified for wait().
        而os.wait()的返回为：

a 16-bit number, whose low byte is the signal number that killed the process, and whose high byte is the exit status (if the signal number
is zero);

       os.system的返回值并不是执行程序的返回结果。而是一个16位的数，它的高位才是返回码。也就是说os.system()返回256即 0×0100，返回码应该是其高位0×01即1。所以要获取程序运行退出的值（比如C的main函数中的return 0），需要处理一下。

ret = os.system('./a.out')
ret >>= 8

       这样才能获取到正确的返回值。另外还要注意：python获取到的值是无符号整数，所以返回负值的时候，打印出来是很大的正值。比如返回-1，python 会获取到255，-2则254，以此类推。所以最好就判断是否为0就可以了，实在要判断自己写的c程序返回值，建议返回0,1,2,3等值，出错返回 -1。
        另外，我遇到一次明明处理好了返回值，c程序调试信息提示也该返回值0了，结果python获取到的是 -1，而且无论c程序返回多少，python都获取-1。后来排查c程序的问题，发现原来是因为我这个python程序本身是由另一个C程序调用的，而调 用它的那个C程序中将SIGCLD信号忽略了（这表明python是根据子进程退出时产生的信号来获取返回值的），我将那个C程序的SIGCLD绑定到函 数，即使那个函数什么也不做，python也能获取到正确的返回值了。

 

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linux命令执行后无论成功与否都有一个返回值：
如果为 0，则表示命令执行成功，其它值则表示错误，具体的错误码含义如下：

 "OS error code   1:  Operation not permitted"
 "OS error code   2:  No such file or directory"
 "OS error code   3:  No such process"
 "OS error code   4:  Interrupted system call"
 "OS error code   5:  Input/output error"
 "OS error code   6:  No such device or address"
 "OS error code   7:  Argument list too long"
 "OS error code   8:  Exec format error"
 "OS error code   9:  Bad file descriptor"
 "OS error code  10:  No child processes"
 "OS error code  11:  Resource temporarily unavailable"
 "OS error code  12:  Cannot allocate memory"
 "OS error code  13:  Permission denied"
 "OS error code  14:  Bad address"
 "OS error code  15:  Block device required"
 "OS error code  16:  Device or resource busy"
 "OS error code  17:  File exists"
 "OS error code  18:  Invalid cross-device link"
 "OS error code  19:  No such device"
 "OS error code  20:  Not a directory"
 "OS error code  21:  Is a directory"
 "OS error code  22:  Invalid argument"
 "OS error code  23:  Too many open files in system"
 "OS error code  24:  Too many open files"
 "OS error code  25:  Inappropriate ioctl for device"
 "OS error code  26:  Text file busy"
 "OS error code  27:  File too large"
 "OS error code  28:  No space left on device"
 "OS error code  29:  Illegal seek"
 "OS error code  30:  Read-only file system"
 "OS error code  31:  Too many links"
 "OS error code  32:  Broken pipe"
 "OS error code  33:  Numerical argument out of domain"
 "OS error code  34:  Numerical result out of range"
 "OS error code  35:  Resource deadlock avoided"
 "OS error code  36:  File name too long"
 "OS error code  37:  No locks available"
 "OS error code  38:  Function not implemented"
 "OS error code  39:  Directory not empty"
 "OS error code  40:  Too many levels of symbolic links"
 "OS error code  42:  No message of desired type"
 "OS error code  43:  Identifier removed"
 "OS error code  44:  Channel number out of range"
 "OS error code  45:  Level 2 not synchronized"
 "OS error code  46:  Level 3 halted"
 "OS error code  47:  Level 3 reset"
 "OS error code  48:  Link number out of range"
 "OS error code  49:  Protocol driver not attached"
 "OS error code  50:  No CSI structure available"
 "OS error code  51:  Level 2 halted"
 "OS error code  52:  Invalid exchange"
 "OS error code  53:  Invalid request descriptor"
 "OS error code  54:  Exchange full"
 "OS error code  55:  No anode"
 "OS error code  56:  Invalid request code"
 "OS error code  57:  Invalid slot"
 "OS error code  59:  Bad font file format"
 "OS error code  60:  Device not a stream"
 "OS error code  61:  No data available"
 "OS error code  62:  Timer expired"
 "OS error code  63:  Out of streams resources"
 "OS error code  64:  Machine is not on the network"
 "OS error code  65:  Package not installed"
 "OS error code  66:  Object is remote"
 "OS error code  67:  Link has been severed"
 "OS error code  68:  Advertise error"
 "OS error code  69:  Srmount error"
 "OS error code  70:  Communication error on send"
 "OS error code  71:  Protocol error"
 "OS error code  72:  Multihop attempted"
 "OS error code  73:  RFS specific error"
 "OS error code  74:  Bad message"
 "OS error code  75:  Value too large for defined data type"
 "OS error code  76:  Name not unique on network"
 "OS error code  77:  File descriptor in bad state"
 "OS error code  78:  Remote address changed"
 "OS error code  79:  Can not access a needed shared library"
 "OS error code  80:  Accessing a corrupted shared library"
 "OS error code  81:  .lib section in a.out corrupted"
 "OS error code  82:  Attempting to link in too many shared libraries"
 "OS error code  83:  Cannot exec a shared library directly"
 "OS error code  84:  Invalid or incomplete multibyte or wide character"
 "OS error code  85:  Interrupted system call should be restarted"
 "OS error code  86:  Streams pipe error"
 "OS error code  87:  Too many users"
 "OS error code  88:  Socket operation on non-socket"
 "OS error code  89:  Destination address required"
 "OS error code  90:  Message too long"
 "OS error code  91:  Protocol wrong type for socket"
 "OS error code  92:  Protocol not available"
 "OS error code  93:  Protocol not supported"
 "OS error code  94:  Socket type not supported"
 "OS error code  95:  Operation not supported"
 "OS error code  96:  Protocol family not supported"
 "OS error code  97:  Address family not supported by protocol"
 "OS error code  98:  Address already in use"
 "OS error code  99:  Cannot assign requested address"
 "OS error code 100:  Network is down"
 "OS error code 101:  Network is unreachable"
 "OS error code 102:  Network dropped connection on reset"
 "OS error code 103:  Software caused connection abort"
 "OS error code 104:  Connection reset by peer"
 "OS error code 105:  No buffer space available"
 "OS error code 106:  Transport endpoint is already connected"
 "OS error code 107:  Transport endpoint is not connected"
 "OS error code 108:  Cannot send after transport endpoint shutdown"
 "OS error code 109:  Too many references: cannot splice"
 "OS error code 110:  Connection timed out"
 "OS error code 111:  Connection refused"
 "OS error code 112:  Host is down"
 "OS error code 113:  No route to host"
 "OS error code 114:  Operation already in progress"
 "OS error code 115:  Operation now in progress"
 "OS error code 116:  Stale NFS file handle"
 "OS error code 117:  Structure needs cleaning"
 "OS error code 118:  Not a XENIX named type file"
 "OS error code 119:  No XENIX semaphores available"
 "OS error code 120:  Is a named type file"
 "OS error code 121:  Remote I/O error"
 "OS error code 122:  Disk quota exceeded"
 "OS error code 123:  No medium found"
 "OS error code 124:  Wrong medium type"
 "OS error code 125:  Operation canceled"
 "OS error code 126:  Required key not available"
 "OS error code 127:  Key has expired"
 "OS error code 128:  Key has been revoked"
 "OS error code 129:  Key was rejected by service"
 "OS error code 130:  Owner died"
 "OS error code 131:  State not recoverable"