Linux common commands (4)

1. Check the specific location of the command
   which file
2. Check the environment variable
   env
3. Check the user-defined and system variable
   set
4. Bash saves the command history in the buffer or the default file ~/.bash_history in the
   historical command buffer. Save a lot of commands, the number of saved commands is defined by the environment variable HISTSIZE.
   You can use "!!" to re-execute the previous command, and you can use "! History number" to repeatedly execute the historical command with the specified ID (checked out by history)
5. View What are the options of
  set set -o
   open an option set -o option name
   close an option set +o option name