unset: unset [-f] [-v] [-n] [name ...]
    Unset values and attributes of shell variables and functions.

    For each NAME, remove the corresponding variable or function.

    Options:
      -f    treat each NAME as a shell function
      -v    treat each NAME as a shell variable
      -n    treat each NAME as a name reference and unset the variable itself
        rather than the variable it references

    Without options, unset first tries to unset a variable, and if that fails,
    tries to unset a function.

    Some variables cannot be unset; also see `readonly'.

    Exit Status:
    Returns success unless an invalid option is given or a NAME is read-only.

删除自定义变量或者函数

-f 删除一个函数

-n 删除一个别名和所指示的变量本身

-v 删除一个shell变量

没有任何选项的时候,会默认先去删除一个变量,若果失败了,再去删除一个函数.

有些变了无法被删除,可以去查看只读readonly属性.