linux grep、sed、awk 实操备忘

前提

# 声明 x,y,z，默认字符串连接

[cmd] x=1
[cmd] declare -p x
      declare -- x="1"
[cmd] y=1
[cmd] z=$x+$y && echo $z
      1+1
[cmd] declare -i z
[cmd] declare -p z
      declare -i z="1+1"
      
# 声明 i_z 整形，加法不再默认字符串连接
[cmd] declare -i i_z=$x+$y && echo $i_z
      2
[cmd] declare -p i_z
      declare -i i_z="2"

# 数组，不需要声明
[cmd] a_x1=1
[cmd] a_x2=2
[cmd] declare -p a_x
      declare -a a_x='([0]="1" [1]="2")'
      
# 声明只读，可以只有声明，不赋值，不可修改，不可 -p
[cmd] declare -r r_x
[cmd] declare -p r_x
      -bash: declare: r_x: not found
[cmd] declare -r x
[cmd] declare -p x
      declare -r x="1"
[cmd] declare -r i_z

# i_z 整形只读，只读设置不可撤销
[cmd] declare -p i_z
      declare -ir i_z="2"
[cmd] i_z=4
      -bash: i_z: readonly variable
[cmd] declare +r i_z
      -bash: declare: i_z: readonly variable