【转载】python中os.system()的返回值

最近遇到os.system()执行系统命令的情况,上网搜集了一下,整理如下,以备不时之需,同时也希望能帮到某些人。


一、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.systemv()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                      linux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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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。



解决: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"



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