Shell special variables: Shell $0, $#, $*, $@, $?, $$ and command line arguments

As mentioned earlier, variable names can only contain numbers, letters and underscores, because some variables containing other characters have special meanings, and such variables are called special variables.

For example, $ represents the ID of the current Shell process, that is, pid, see the following code:

$echo$$

operation result

list of special variables
variable	meaning
$0	filename of the current script
$n	Arguments passed to scripts or functions. n is a number indicating the number of parameters. For example, the first argument is $1 and the second argument is $2.
$#	The number of arguments passed to the script or function.
$*	All arguments passed to the script or function.
$@	All arguments passed to the script or function. When enclosed in double quotes (" "), it is slightly different from $*, which will be described below.
$?	The exit status of the last command, or the return value of a function.
$$	Current shell process ID. For shell scripts, this is the process ID in which the scripts reside.

command line arguments

The arguments passed to the script when it is run are called command line arguments. Command-line arguments are represented by $n, eg, $1 for the first argument, $2 for the second argument, and so on.

See the script below:

#!/bin/bash
echo"File Name: $0"
echo"First Parameter : $1"
echo"First Parameter : $2"
echo"Quoted Values: $@"
echo"Quoted Values: $*"
echo"Total Number of Parameters : $#"

operation result:

$. / test.sh Zara Ali
File Name : ./test.sh
First Parameter : Zara
Second Parameter : Ali
Quoted Values: Zara Ali
Quoted Values: Zara Ali
Total Number of Parameters : 2

Difference between $* and $@

Both $* and $@ represent all arguments passed to a function or script, and when not enclosed in double quotes (" "), all arguments are output in the form of "$1" "$2" … "$n".

But when they are enclosed in double quotes (" "), "$*" will output all parameters as a whole in the form of "$1 $2 ... $n"; "$@" will separate the parameters, Output all arguments as "$1" "$2" … "$n".

The following example can clearly see the difference between $* and $@:

#!/bin/bash
echo"\$*="$*
echo"\"\$*\"=""$*"
echo"\$@="$@
echo"\"\$@\"=""$@"
echo"print each param from\$*"
for varin$*
do
echo"$var"
done
echo"print each param from\$@"
for varin$@
do
echo"$var"
done
echo"print each param from\"\$*\""
for varin"$*"
do
echo"$var"
done
echo"print each param from\"\$@\""
for varin"$@"
do
echo"$var"
done

执行 ./test.sh "a" "b" "c" "d"，看到下面的结果：

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

退出状态

$? 可以获取上一个命令的退出状态。所谓退出状态，就是上一个命令执行后的返回结果。

退出状态是一个数字，一般情况下，大部分命令执行成功会返回 0，失败返回 1。

不过，也有一些命令返回其他值，表示不同类型的错误。

下面例子中，命令成功执行：

$./test.sh Zara Ali
File Name : ./test.sh
First Parameter : Zara
Second Parameter : Ali
Quoted Values: Zara Ali
Quoted Values: Zara Ali
Total Number of Parameters : 2
$echo $?
0
$

前面已经讲到，变量名只能包含数字、字母和下划线，因为某些包含其他字符的变量有特殊含义，这样的变量被称为特殊变量。

例如，$ 表示当前Shell进程的ID，即pid，看下面的代码：

$echo $$

运行结果

特殊变量列表
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。
$#	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，下面将会讲到。
$?	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。

命令行参数

运行脚本时传递给脚本的参数称为命令行参数。命令行参数用 $n 表示，例如，$1 表示第一个参数，$2 表示第二个参数，依次类推。

请看下面的脚本：

#!/bin/bash
echo "File Name: $0"
echo "First Parameter : $1"
echo "First Parameter : $2"
echo "Quoted Values: $@"
echo "Quoted Values: $*"
echo "Total Number of Parameters : $#"

运行结果：

$./test.sh Zara Ali
File Name : ./test.sh
First Parameter : Zara
Second Parameter : Ali
Quoted Values: Zara Ali
Quoted Values: Zara Ali
Total Number of Parameters : 2

$* 和 $@ 的区别

$* 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。

下面的例子可以清楚的看到 $* 和 $@ 的区别：

#!/bin/bash
echo "\$*=" $*
echo "\"\$*\"=" "$*"
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
echo "print each param from \"\$*\""
for var in "$*"
do
echo "$var"
done
echo "print each param from \"\$@\""
for var in "$@"
do
echo "$var"
done

执行 ./test.sh "a" "b" "c" "d"，看到下面的结果：

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

退出状态

$./test.sh Zara Ali
File Name : ./test.sh
First Parameter : Zara
Second Parameter : Ali
Quoted Values: Zara Ali
Quoted Values: Zara Ali
Total Number of Parameters : 2
$echo $?
0
$

转自 https://www.cnblogs.com/wangcp-2014/p/6427689.html