对于某些监测脚本和探测命令蛮有用的；
在 Linux 下，不管你是启动一个桌面程序也好，还是在控制台下运行命令，所有的程序在结束时，都会返回一个数字值，这个值叫做返回值，或者称为错误号 ( Error Number )。
在控制台下，有一个特殊的环境变量 $?，保存着前一个程序的返回值，我们可以试试：
$ ls *.png Diagram1.png eqn.png peazip.png eqn4.png Mandelbrot.png x2.png $ echo $? 0先随便执行个命令，比如像上面的 ls 某些文件，然后通过 echo $?，打印 $? 的值～
我们发现返回值是 0，这是什么意思呢？
只要返回值是 0，就代表程序执行成功了～
也就是说，如果 $? 变量的值不是 0 的话，就表示上一个命令在执行的过程中出错了。我们可以试着 ls 一个不存在的文件：
$ ls linuxgem ls: 无法访问 linuxgem: 没有那个文件或目录 $ echo $? 2这样，对于某些打印出一堆不熟悉的英文信息的命令，可以方便的看出它是否成功执行结束。
不过，每次都 echo $? 一下太累了，没关系，你可以把 $? 放在提示符里去：
galeki@localhost ~ $ export PS1="[\$?]${PS1}" [0]galeki@localhost ~ $export PS1="[\$?]${PS1}" 这个命令把 $? 的值放在的提示符的最前面，这样每次执行完命令，这个值都会自动更新，这下就一目了然了～
那么如果返回的值不是 0，我们要怎么知道是那里出错了呢？ 大多数的程序出错都会给出提示，如果没有提示的话，可以用 perror 这个程序来查看错误的信息，比如返回值是 2，我们可以运行：
$ perror 2 OS error code 2: No such file or directory这样就清楚了，原来是文件不存在～
错误对照表：
"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""MySQL error code 132: Old database file""MySQL error code 133: No record read before update""MySQL error code 134: Record was already deleted (or record file crashed)""MySQL error code 135: No more room in record file""MySQL error code 136: No more room in index file""MySQL error code 137: No more records (read after end of file)""MySQL error code 138: Unsupported extension used for table""MySQL error code 139: Too big row""MySQL error code 140: Wrong create options""MySQL error code 141: Duplicate unique key or constraint on write or update""MySQL error code 142: Unknown character set used""MySQL error code 143: Conflicting table definitions in sub-tables of MERGE table""MySQL error code 144: Table is crashed and last repair failed""MySQL error code 145: Table was marked as crashed and should be repaired""MySQL error code 146: Lock timed out; Retry transaction""MySQL error code 147: Lock table is full; Restart program with a larger locktable""MySQL error code 148: Updates are not allowed under a read only transactions""MySQL error code 149: Lock deadlock; Retry transaction""MySQL error code 150: Foreign key constraint is incorrectly formed""MySQL error code 151: Cannot add a child row""MySQL error code 152: Cannot delete a parent row"
转自：http://blog.csdn.net/wyabc1986/article/details/7876673转载于:https://www.cnblogs.com/commanderzhu/p/5327569.html
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