The shell command execution in two ways:
Internal command (built by the shell)
Help: internal list of commands
enable cmd: Enable internal command
enable -n cmd: Disable internal command
all internal command to view the enabled: enable -a
External commands: external executable files
which cmd: command to view the file path, the path listed in the first match of
which -a cmd: print all matching path
which cmd --skip-alias cmd: remove the print command path alias
whereis: Find File or directory
View command types: type command
Cache Hash Table
The initial system Hash table is empty, if the system restart will be cleared when the external command is executed will find the command to the PATH, and then recorded in the Hash years, when then perform it to take Hash years, it will speed up execution.
Hash common usage
hash hash displays cache
hash hash -l display buffer, comprising hash aliases
hash -p path name to the path from the alias name
hash -t name Print name of the path
Clear name cache hash -d name
hash -r clear all caches
Order shell command search path: Alias -> internal command -> Hash storage path -> hard disk path