shell特殊变量

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：

1

$ echo $$

运行结果

1

29949

特殊变量列表

命令行参数

1
2
3
4
5
6
7

echo "File Name: $0"
echo "First Parameter : $1"
echo "First Parameter : $2"
echo "Quoted Values: [email protected]"
echo "Quoted Values: $*"
echo "Total Number of Parameters : $#"

1
2
3
4
5
6
7

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

$* 和 [email protected] 的区别

$* 和 [email protected] 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；”[email protected]“ 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 [email protected] 的区别：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32

echo "$*=" $*
echo ""$*"=" "$*"
echo "[email protected]=" [email protected]
echo ""[email protected]"=" "[email protected]"
echo "print each param from $*"

for var in $*
do
    echo "$var"
done

echo "print each param from [email protected]"

for var in [email protected]
do
    echo "$var"
done

echo "print each param from "$*""

for var in "$*"
do
    echo "$var"
done

echo "print each param from "[email protected]""

for var in "[email protected]"
do
    echo "$var"
done

保存, 命名为test.sh.

执行 ./test.sh “a” “b” “c” “d”，看到下面的结果：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22

$*=  a b c d
"$*"= a b c d
[email protected]=  a b c d
"[email protected]"= a b c d

print each param from $*
a
b
c
d
print each param from [email protected]
a
b
c
d
print each param from "$*"
a b c d
print each param from "[email protected]"
a
b
c
d

退出状态

1
2
3
4
5
6
7
8
9
10

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