-bash: ls: No such file or directory 产生的原因及修改方法

ubuntu出现如下错误：

{

Welcome to Ubuntu 16.04.5 LTS (GNU/Linux 4.15.0-42-generic x86_64)

?* Documentation: ?https://help.ubuntu.com
?* Management: ? ? https://landscape.canonical.com
?* Support: ? ? ? ?https://ubuntu.com/advantage

77 packages can be updated.
0 updates are security updates.

*** System restart required ***
Last login: Fri Dec ?7 13:59:33 2018 from 10.12.21.2
-bash: ls: No such file or directory
-bash: lesspipe: No such file or directory
-bash: dircolors: No such file or directory
-bash: ls: No such file or directory
[email?protected]:~$ sudo su -l
Command ‘sudo‘ is available in ‘/usr/bin/sudo‘
The command could not be located because ‘/usr/bin‘ is not included in the PATH environment variable.
sudo: command not found

}

?

原因是：

默认的环境变量错误，

#cat /etc/profile

export PATH=

?

修改错误:

#?export PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin?

# reboot