The preferred distinguish built-in commands and external commands:
Built-in commands: shell program comes with command, a system kernel boot command can be used
External commands: command in system PATH variable path
How to View command is a built-in command or external command:
type -a cd
Clear cache hash -r list
Command execution flow summary: When we execute a command when the process is as follows
1: Check whether the command to use absolute paths of execution
2: Check whether there is a command alias alias
3: Check the command is a command internal or external command
4: If it is an internal Bash commands executed directly, if it is an external command to check whether the HASH list, there is a direct call
5: If the HASH cache does not exist, by PASH path to find the command
6: If the path is not the path of the command is found, an error