命令替換
語法格式
| 方法 | 語法格式 |
|---|---|
| 方法一 | command |
| 方法二 | $(command) |
小例子一：取出系統(tǒng)中所有的用戶并輸出
#!/bin/bash
index=1
users=`cat /etc/passwd | cut -d ":" -f 1`
for user in $users
do
echo "this is $index user:$user"
index=$((index+1))
done
小例子二： 根據(jù)系統(tǒng)時間計算今年或者明年
[wanghui@www example_1]$ echo "This is $(date +%Y) year"
[wanghui@www example_1]$ echo "This is $(($(date +%Y) +1 )) year" ##時間計算+1
[wanghui@www example_1]$ echo "$((20+30))" #運算
[wanghui@www example_1]$ num1=20
[wanghui@www example_1]$ num2=70
[wanghui@www example_1]$ echo "$(($num1 + $num2))" #這樣也可以echo "$((num1 + num2))"
小例子三：計算今年過去了多少星期，還剩多少星期
[wanghui@www example_1]$ vim example2.sh
#!/bin/bash
echo "This year has passed $(date +%j) days"
echo "This year has passed $(($(date +%j)/7)) week"
echo "There is $((365 - $(date +%j))) days before new year"
echo "There is $(((365 - $(date +%j))/7)) weeks before new year"
小例子四： 判定Nginx進程是否存在，不存在就自動拉起（https://segmentfault.com/a/1190000009723940）
#!/bin/bash
nginx_process_num=$(ps -ef | grep nginx | grep -v grep | wc -l)
if [ $nginx_process_num -eq 0 ];then
systemctl restart nginx
fi
變量類型
- declare=typeset
image.png
[wanghui@www shell_script]$ declare -r var2
[wanghui@www shell_script]$ var2="hello java"
---------聲明
[wanghui@www shell_script]$ num1=10
[wanghui@www shell_script]$ num2=$num1+10
[wanghui@www shell_script]$ echo $num2
10+10
[wanghui@www shell_script]$ declare -i num3
[wanghui@www shell_script]$ num3=$num1+10
[wanghui@www shell_script]$ echo $num3
[wanghui@www shell_script]$ declare -F #展示系統(tǒng)函數(shù)
[wanghui@www shell_script]$ declare -a array #聲明數(shù)組
[wanghui@www shell_script]$ array=("jones","mike","kobe","jordan") #定義數(shù)組
[wanghui@www shell_script]$ echo ${array[@]} #列出所有元素
聲明環(huán)境變量
[wanghui@www shell_script]$ num5=30
[wanghui@www shell_script]$ echo $num5
30
[wanghui@www shell_script]$ vim test1.sh
#!/bin/bash
echo $num5
[wanghui@www shell_script]$ echo $num5
30
[wanghui@www shell_script]$ bash test1.sh #不能引用num5
[wanghui@www shell_script]$ declare -x num5 #聲明即可
[wanghui@www shell_script]$ bash test1.sh
30
取消聲明的變量
declare +a
declare +r
declare +i
declare +x
expr計算
image.png
image.png
[wanghui@www shell_script]$ num1=30
[wanghui@www shell_script]$ num2=50
[wanghui@www shell_script]$ expr $num1 \> $num2
0
[wanghui@www shell_script]$ num1=17
[wanghui@www shell_script]$ num2=5
[wanghui@www shell_script]$ expr $num1 + $num2
22
[wanghui@www shell_script]$ num3=$(expr $num1 + $num2)
[wanghui@www shell_script]$ echo $num3
22
[wanghui@www shell_script]$ expr $num1 \* $num2
85
[wanghui@www shell_script]$ expr $num1 / $num2
3
[wanghui@www shell_script]$ expr $num1 % $num2
2
[wanghui@www shell_script]$ num3=$(($num1 + $num2))
[wanghui@www shell_script]$ echo $num3
22
[wanghui@www shell_script]$ num3=$(($num1 * $num2))
[wanghui@www shell_script]$ echo $num3
85
image.png
練習
提示用戶輸入一個正整數(shù)，然后計算1+2+3+4+....num,必須對num是否為正整數(shù)做出判斷，不符合應(yīng)當允許其再次輸入
[wanghui@www shell_script]$ vim sum.sh
#!/bin/bash
while true
do
read -p "Please input a positive number:" num
expr $num + 1 &> /dev/null
if [ $? -eq 0 ];then
if [ `expr $num \> 0` -eq 1 ];then
#echo "Yes,Positive Number"
for ((i=1;i<=$num;i++))
do
sum=`expr $sum + $i`
done
echo "1+2+3+...+$num=$sum"
exit
fi
fi
echo "error input unlegal.."
continue
done
bc計算
bc是bash內(nèi)建的運算器，支持浮點數(shù)運算
內(nèi)漸變量scale可以設(shè)置，默認是0
image.png
[wanghui@www shell_script]$ bc
bc 1.06.95
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
1+2
3
3-2
1
3*8
24
9/3
3
10/3
3
scale=2
10/3
3.33
[wanghui@www shell_script]$ echo "scale=9;2333.88/33" | bc #腳本中使用方法
70.723636363
浮點數(shù)運算
#!/bin/bash
read -p "input a number:" num1
read -p "input a number:" num2
echo "scale=2;$num1/$num2" | bc
