shell编程系列3--命令替换

shell编程系列3--命令替换

命令替换

命令替换总结
方法1 `command`
方法2 $(command)

例子1：
获取系统的所有用户并输出

for循环能以空格、换行、tab键作为分隔符

[root@localhost shell]# cat example_1.sh
#!/bin/bash
#

index=
for user in `cat /etc/passwd | cut -d ":" -f `
do
    echo "this is $index user: $user"
    index=$(($index + ))
done

[root@localhost shell]# sh example_1.sh
this is  user: root
this is  user: bin
this is  user: daemon
this is  user: adm
this is  user: lp
this is  user: sync
this is  user: shutdown
this is  user: halt
this is  user: mail
this is  user: operator
this is  user: games
this is  user: ftp
this is  user: nobody
this is  user: systemd-network
this is  user: dbus
this is  user: polkitd
this is  user: sshd
this is  user: postfix
this is  user: ajie
this is  user: chrony
this is  user: deploy

例子2：
根据系统时间计算今年或明年
echo "this is $(date +%Y) year"
echo "this is $(( $(date +%Y) + 1)) year"

[root@localhost shell]# echo "This is $(date +%Y) year"
This is  year
[root@localhost shell]# echo "This is $(($(date +%Y) + 1)) year"
This is  year
[root@localhost shell]# echo "$((20+30))"

例子3：
根据系统时间获取今年还剩下多少星期，已经过了多少星期
# 今天是今年的第多少天
date +j

# 今年已经过去了多少天
echo "this year have passed $(date +%j) days"
echo "this year have passed $(($(date +%j) / 7)) weeks"

# 今年还剩余多少天
echo "there is $((365 - $(date +%j))) days before new year"
echo "there is $(((365 - $(date +%j)) / 7 )) weeks before new year"

总结：
``和$()两者是等价的，但推荐初学者使用$()，易于掌握；缺点是极少数UNIX可能不支持，但``两者都支持
$(())主要用来进行整数运算，包括加减乘除，引用变量前面可以加$，也可以不加$

$(( ( + ) /  ))
num1=;num2=
((num++));
((num--));
$(( $num1 + $num2 * ))

语法不是很严格

是否加$都会计算
[root@localhost shell]# num1=
[root@localhost shell]# num2=
[root@localhost shell]# echo "$(($num1 + $num2))"

[root@localhost shell]# echo "$((num1 + num2))"

.例子4：
判断nginx进程是否存在，如果没有需求拉起这个进程

[root@localhost shell]# cat example_3.sh
#!/bin/bash
#

nginx_process_num=$(ps -ef|grep nginx|grep -v grep|wc -l)

if [ $nginx_process_num -eq  ];then
systemctl start nginx
fi