一、概述

shell中有两类字符：普通字符、元字符。

1. 普通字符

在Shell中除了本身的字面意思外没有其他特殊意义，即普通纯文本；

2. 元字符

是Shell的保留字符，在Shell中有着特殊的含义。

今天主要介绍一下shell中字符$的各种用法。

二、转义字符$

在linux shell脚本中经常用到字符$，下面是$的一些常见用法

名称 含义 $# 传给脚本的参数个数 $0 脚本本身的名字 $1 传递给该shell脚本的第一个参数 $2 传递给该shell脚本的第二个参数 $@ 传给脚本的所有参数的列表 $* 以一个单字符串显示所有向脚本传递的参数，与位置变量不同，参数可超过9个 $ 脚本运行的当前进程ID号 $? 显示最后命令的退出状态，0表示没有错误，其他表示有错误

实例1

建立脚本peng.sh如下：

#/bin/bash
total=$[ $1 * $2 + $3 ]
echo "$1 * $2 + $3 = $total"

运行如下：

./peng.sh 4 5 6

结果如下：

实例2

当把字符串输入给shell脚本的时候，注意是以空格作为分隔符，如果字符串本身就有空格的话，那么用双引号或者单引号，比如

#/bin/bash
echo "$1 is pretty nice!"

运行如下：

./peng.sh 一口Linux

结果如下：

实例3 #!/bin/sh
echo "参数个数:$#"
echo "脚本名字:$0"
echo "参数1:$1"
echo "参数2:$2"
echo "所有参数列表:$@"
echo "pid:$"
if [ $1 = 100 ]
then
echo "命令退出状态：$?"
exit 0 #参数正确，退出状态为0
else
echo "命令退出状态：$?"
exit 1 #参数错误，退出状态1
fi

执行结果：

实例4

使用for循环进行参数遍历

示例：

#!/bin/bash
number=65             #定义一个退出值
index=1               #定义一个计数器
if [ -z "$1" ];then   #对用户输入的参数做判断，如果未输入参数则返回脚本的用法并退出，退出值65
   echo "Usage:$0 + 参数"
   exit $number
fi
echo "listing args with \$*:"    #在屏幕输入，在$*中遍历参数
for arg in $*                                          
do
   echo "arg: $index = $arg"                 
   let index+=1
done
echo
index=1                         #将计数器重新设置为1
echo "listing args with \"\$@\":"    #在"$@"中遍历参数
for arg in "$@"
do
   echo "arg: $index = $arg"
   let index+=1
done

实例5

小技巧1：在"和*中遍历参数的区别

示例：

#!/bin/bash
number=11
if [ $# -eq 0 ];then
   echo "Usage: $0 + 参数"
   exit $number
fi
for i in $*               #在$*中遍历参数，此时每个参数都是独立的，会遍历$#次
do
   echo $i
done
echo
for i in "$*"            #在"$*"中遍历参数，此时"$*"被扩展为包含所有位置参数的单个字符串，只遍历一次
do
   echo $i
done

三、linux命令执行返回值**$?**说明

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 )。

1. 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：

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。

2. 我们可以试着 ls 一个不存在的文件：

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：

3. 将上次命令执行是否成功的返回值放到提示符里面去：export PS1="[\$?]${PS1}"

root@ubuntu:/home/peng/zhh# export PS1="[\$?]${PS1}"
[0]root@ubuntu:/home/peng/zhh# ls

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～

4. 用perror查看错误提示

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，我们可以运行下面命令：

$ 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～

该命令需要安装 apt-get install perror

5. 给出错误代码及其具体原因对照表：

"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"