Shell特殊变量：$0、$n、$#、$*、$@、$?、$$

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一、特殊变量含义

变量	含义
$0	当前脚本的文件名
$n	传递给脚本或函数的参数。n是一个数字，表示第几个参数。例如，第一个参数是$1，第二个参数是$2，以此类推。
$#	传递给脚本或函数的参数个数
$*	传递给脚本或函数的所有参数
$@	传递给脚本或函数的所有参数，被双引号（" "）包含时，与 $* 稍有不同。$@ 会把所有位置参数当成单独的字段（相当于一个列表），下面会举例说明。
$？	上个命令的退出状态或函数的返回值
$$	当前shell进程ID。对于shell脚本，就是这些脚本所在的进程ID。

二、实例

1. 给脚本文件传递参数

代码如下（示例）：

编写下面的代码，并保存为test.sh

#!/bin/bash
echo "Process ID: $$"
echo "File Name: $0"
echo "First Parameter : $1"
echo "Second Parameter : $2"
echo "All parameters 1: $@"
echo "All parameters 2: $*"
echo "Total: $#"
echo "pid:$$"

运行test.sh，并附带参数，运行结果如下：

[wqf@b1i10 ~]$ . ./test.sh a b
Process ID: 27672
File Name: -bash
First Parameter : a
Second Parameter : b
All parameters 1: a b
All parameters 2: a b
Total: 2

2. 给函数传递参数

代码如下（示例）：

编写下面的代码，并保存为test.sh

#!/bin/bash

#定义函数
function func(){
    
    
	echo "Process ID: $$"
	echo "File Name: $0"
	echo "First Parameter : $1"
	echo "Second Parameter : $2"
	echo "All parameters 1: $@"
	echo "All parameters 2: $*"
	echo "Total: $#"
	echo "pid:$$"
}

#调用函数
func hello world!

运行test.sh，并附带参数，运行结果如下：

[wqf@b1i10 ~]$ . ./test.sh
Process ID: 27672
File Name: -bash
First Parameter : hello
Second Parameter : world!
All parameters 1: hello world!
All parameters 2: hello world!
Total: 2
pid:27672

3. $?的使用

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) 获取上个命令的退出状态的返回值

代码如下（示例1）：

在控制台随便执行一个命令，例如“ls”，然后echo $?，打印 $? 的值～

[wqf@b1i10 zxt_test]$ ls
drop_tables  make_data  push_data  python  rm_test  take_data
[wqf@b1i10 zxt_test]$ echo $?
0

代码如下（示例2）：

编写下面的代码，并保存为test.sh

#!/bin/sh

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

运行test.sh，并附带参数，运行结果如下：

[wqf@b1i10 ~]$ sh test.sh 100
命令退出状态：0

[wqf@b1i10 ~]$ sh test.sh 101
命令退出状态：1

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）获取函数的返回值

代码如下（示例）：

编写下面的代码，并保存为test.sh

#!/bin/bash
#得到两个数相加的和
function add(){
    
    
    return `expr $1 + $2`
}
add 23 50  #调用函数
echo $?  #获取函数返回值

运行test.sh，运行结果如下：

[wqf@b1i10 ~]$ sh test.sh
73

4. $*与$@对比

$* 和 $@ 都表示传递给函数或脚本的所有参数，不被双引号(" “)包含时，都以”$1" “$2” … “$n” 的形式输出所有参数。

但是当它们被双引号(" “)包含时，”$*" 会将所有的参数作为一个整体，以"$1 $2 … $n"的形式输出所有参数；“$@” 会将各个参数分开，以"$1" “$2” … “$n” 的形式输出所有参数，类似列表。

代码如下（示例1）：

编写下面的代码，并保存为test.sh

#!/bin/bash
#不被双引号(" ")包含
echo "\$*=" $*
echo "\$@=" $@

echo "print each param from $*"
for var in $*
do 
echo "$var"
done 

echo "print each param from $@"
for var in $@
do 
echo "$var"
done

运行 sh test.sh “a” “b” “c” “d”，运行结果如下：

$*= a b c d
$@= a b c d
print each param from a b c d
a
b
c
d
print each param from a b c d
a
b
c
d

代码如下（示例2）：

编写下面的代码，并保存为test.sh

#!/bin/bash
#被双引号(" ")包含
echo "\"\$*"=" "$*"
econ "\"\$@"=" "$@"

echo "print each param from \"\$*\""
for var in "$*"
do 
echo "$var"
done

echo "print each param from \"$@\""
for var in "$@"
do 
echo "$var"
done

运行 sh test.sh 1 2 3 4，运行结果如下：

"$*= 1 2 3 4
econ "$@= 1 2 3 4
print each param from "$*"
1 2 3 4
print each param from "1 2 3 4"
1
2
3
4

---

三、拓展（关于$?）

1、上次命令执行返回值不是0，可以用perror查看错误提示

代码如下（示例）：

[wqf@b1i10 ~]$ perror 2
bash: perror: command not found...

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”

2、应用：
将上次命令执行是否成功的返回值放到提示符里面去：export Pexport PS1=“[$?]${PS1}”
这个命令把 $? 的值放在的提示符的最前面，这样每次执行完命令，这个值都会自动更新，这下就一目了然了～S1=“[$?]${PS1}”

[wqf@b10 test]$ export Pexport PS1="[\$?]${
     
     PS1}" 

[0][wqf@b1i10 test]$ ls
drop_tables  make_data  push_data  python  rm_test  take_data
[0][wqf@b1i10 test]$ ls *.c
ls: cannot access *.c: No such file or directory
[2][wqf@b1i10 test]$