shell运算

整数运算：expr，$(())，$[]

expr

使用expr进行运算，如果遇到特殊字符，则需要转义，如*

[[email?protected] ~]# num1=2
[[email?protected] ~]# num2=5
[[email?protected] ~]# expr $num1 + $num2
7
[[email?protected] ~]# expr $num1 - $num2
-3
[[email?protected] ~]# expr $num1 * $num2     #需要转移
10
[[email?protected] ~]# expr $num2 / $num1
2
[[email?protected] ~]# expr $num2 % $num1
1
[[email?protected] ~]#

$(()) $[]

? 不需要转义，都可以直接使用

计算字符串长度：

[[email?protected] opt]# a="abcd"
[[email?protected] opt]# echo $(($(echo $a|wc -c)-1))      # 比较麻烦
4
[[email?protected] opt]# echo ${#a}    # 高级用法
4
[[email?protected] opt]#

EOF多行输出：

# cat << EOF > test.sh
> #!/bin/bash
> #you Shell script writes here.
> EOF
 
结果：
引用# cat test.sh
#!/bin/bash
#you Shell script writes here.



[[email?protected] opt]# cat <<EOF
> hello world
> this is my girlfriend!
> what's wrong?
> Are you OK?
> EOF
hello world
this is my girlfriend!
what's wrong?
Are you OK?
[[email?protected] opt]#

if语句中的文件比较

条件成立，则执行&&后面的内容         # &&
条件不成立，则执行 || 后面的        # ||

数值比较