Linux命令的返回值


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 instal
cf98
led"

"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"