linux如何查看环境变量

$ export TEST="Test..."       #增加一个环境变量TEST

$ readonly TEST                 #将环境变量TEST设为只读

$ unset TEST                       #会发现此变量不能被删除

-bash: unset: TEST: cannot unset: readonly variable

$ TEST="New"                    #会发现此也变量不能被修改

-bash: TEST: readonly variable