shell $相关总结

男娘i 2021-12-21 13:57 250阅读 0赞

# 1 $0, $1, $2，…, $n #

*  `$0`:这个特殊一点表示命令本身;
 *  `$1`: 表示第一个参数;
 *  `$2`:表示第二个参数；
 *  `$n`:表示第n个参数；

[root@localhost shell]# cat script.sh 
    echo $0
    echo $1
    echo $2
    [root@localhost shell]# ./script.sh Hello World
    ./script.sh
    Hello
    World

注意: 由于历史原因, $n只能针对10以内的数字，大于等于10需要用括号将数值包括起来

[jiangjian@localhost shell]$ cat echoArguments.sh 
    #! /bin/bash
    echo $1
    echo $9
    echo $10
    echo ${10}
    [jiangjian@localhost shell]$ ./echoArguments.sh 1 2 3 4 5 6 7 8 9 a b
    1
    9
    10
    a
    [jiangjian@localhost shell]$

# 2 $\# #

`$#`:表示实际的参数个数;

[root@localhost shell]# cat arguments-length.sh 
    echo "length is " $#
    [root@localhost shell]# ./arguments-length.sh 1 2 3 4 5 6
    length is  6

# 3 $$ #

`$$`:表示当前进程的pid;

[root@localhost shell]# cat output-pid.sh 
    echo "my pid is " $$
    [root@localhost shell]# ./output-pid.sh 
    my pid is  47282
    [root@localhost shell]# echo $$
    46946
    [root@localhost shell]#

# 4 $? #

`$?`:表示上一个命令执行的退出状态

[root@localhost shell]# cat error.sh 
    exit 66 
    [root@localhost shell]# ./error.sh 
    [root@localhost shell]# echo $?
    66
    [root@localhost shell]#

# 5 $! #

`$!`:表示最近一个后台执行程序的pid;

[root@localhost shell]# sleep 60 &
    [1] 47395
    [root@localhost shell]# sleep 70 &
    [2] 47396
    [root@localhost shell]# echo $!
    47396
    [root@localhost shell]#

# 6 $\* 和 $@的区别 #

`$*`: Stores all the arguments that were entered on the  
command line ($1 $2 …).

`"$@"`: Stores all the arguments that were entered  
on the command line, individually quoted ("$1" “$2” …).

[root@localhost shell]# cat arguments.sh 
    #!/bin/bash
    
    echo "Using \"\$*\":"
    for a in "$*"; do
        echo $a;
    done
    
    echo -e "\nUsing \$*:"
    for a in $*; do
        echo $a;
    done
    
    echo -e "\nUsing \"\$@\":"
    for a in "$@"; do
        echo $a;
    done
    
    echo -e "\nUsing \$@:"
    for a in $@; do
        echo $a;
    done  
    [root@localhost shell]# ./arguments.sh one two "three four"
    Using "$*":
    one two three four
    
    Using $*:
    one
    two
    three
    four
    
    Using "$@":
    one
    two
    three four
    
    Using $@:
    one
    two
    three
    four
    [root@localhost shell]#