Linux Shell Commands

In Linux, commands are ways or instructions through which you can instruct your system to do some action. Commands are executed in the command line.

Syntax:

command [option] [argument]

There are some commands which don't have any option or don't accept any argument such as 'clear' and 'pwd'.

clear

The 'clear' command clears out all the previous commands and outputs from terminal display.

pwd

The 'pwd' command stands for 'print working directory'. It doesn't accept any option or argument and displays the detail of current working directory.

Types of Commands

External or built-in commands

Built-in commands are internal commands that are built-in the shell. Built-in commands are called from the shell and executed directly within the shell itself. You can list all built-in commands with the help of 'help' and 'compgen -b' command. Some example of built-in commands are 'pwd', 'help', 'type', 'set', 'unset', etc.

External commands are other than built-in commands. These commands are programs which have their own binary and located in the filesystem. These are the commands that your system offer and are totally shell independent. Mostly these commands reside in /bin, /sbin, /usr/sbin.

type command

Linux 'type' command tell us whether a command given to the shell is a built-in or external command.

Syntax:

type <command>

Example:

type pwd
type cd
type man
type cat
type file

Look at above snapshot, commands like 'pwd' and 'cd' are built-in commands while commands 'man', 'cat', and 'file' are external commands.

Linux 'type' command also tells whether a command is aliased or not.

Example:

type ls  

Look at the above snapshot, 'type' command shows that 'ls' is an aliased command.

type -a

The 'type -a' option tells about all type of command whether it is built-in, external, or aliased. Some commands are both external and built-in commands. But built-in command will always takes priority until and unless path of external command is mentioned.

Syntax:

type -a <command>  

Example:

type -a echo  

Look at the above snapshot, 'echo' command is internal as well as external. To use external 'echo' command, path "/bin/echo" is mentioned.

which

Linux 'which' command locates the path of a command.

Syntax:

which <command1> <command2> <command3>....  

Example:

which ls pwd rmdir mkdir cp cd file man  

Look at the above snapshot, except 'cd' command, all other commands are external commands because bash has displayed their external path.