查看上⼀条命令的返回值
Windows系统：
判断上个命令的返回值, if errorlevel == 命令执⾏返回码 command 语句.
例:
1@echo off
2dir
3rem 对命令成功执⾏,⽤这种⽐较字符串的⽅式来⽐较
4if %errorlevel% == 0 (echo dir命令执⾏成功) else (echo dir命令执⾏失败)
5pause
提⽰:
tip1: %errorlevel%是⼀个环境变量，它是⽤来存放上⼀条命令的执⾏后的返回码，如果命令是成功，则返回值为0，如果是失败，则返回0-255之间的值.
tip2: 想看看上⼀条命令返回了什么，直接echo %errorlevel% 就可以看到了
tip3: 执⾏net helpmsg %errorlevel%  就可以看到是什么报错信息例如: net helpmsg 2  显⽰：系统不到指定的⽂件.
Linux操作系统：
在Linux中不管是启动桌⾯程序还是在控制台执⾏命令都会有⼀个返回值，可以叫做错误号(perror)，在linux中执⾏命令后可通过命令：echo $?来查看上⼀条命令的返回值。要查看返回值的具体含义，可通过perror number查看。
Java中可通过 Runtime().exec(cmd) 执⾏命令，exec(prams)返回的是⼀个Process，可通过Process.waitFor()获取返回值。
以下是Linux中返回值对照表
"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"write的返回值
"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"

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