2. Some commands about commands

The command type displays a command's type:
```
type type
```
```
type ls
```
```
type cp
```
The command cp is an executable program located on /usr/bin/cp.
The command which displays the location of an executable:
```
which ls
```
```
which cd
```
The command cd is not an executable but a shell builtin command.
```
type cd
```
The command help displays a help page for the shell builtin commands:
```
help cd
```
```
help mkdir
```
The command mkdir is not a shell builtin.
The option --help displays usage information:
```
mkdir --help
```
The command man displays the manual page of a program:
```
man ls
```
Manual pages are organized into different sections, where section 1 for example is about user commands, and section 5 is about file formats. So, these two commands will display different manual pages:
```
man passwd
```
```
man 5 passwd
```
The command info is another way to display manual pages:
```
info coreutils
```
```
info passwd
```
The command apropos displays appropriate commands:
```
apropos passwd
```
This is the same as:
```
man -k passwd
```
It makes a simple search on man pages for the term "passwd".
The command whatis displays a very brief description of a command:
```
whatis ls
```

The command alias is used to create new commands.

alias --help

alias

type alias

type ls

cd /usr; ls; cd -

type foo

alias foo="cd /usr; ls; cd -"

type foo

foo

unalias foo

type foo

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