Linux Server Configuration

1.2 Linux command line

When Linus Torvalds introduced Linux and for a long time thereafter, Linux did not have a graphical user interface (GUI): I ran on character-based terminals only. All the tools ran from a command line. Today the Linux GUI is important but many people—especially system administrators—run many command line programs. Command line utilities are often faster, more powerful, or more complete than their GUI counterparts. Sometimes there is no GUI counterpart to a textual utility; some people just prefer the hands-on feeling of the command line. When you work with a command line interface, you are working with a shell.

A shell provides an interface between the user and operating system kernel. It is a command interpreter that takes commands from users and executes it.

Linux’s most common command interpreter is called bash. Bash is the abbreviation of Bourne-Again Shell.

The shell is where commands are invoked. When started, the bash shell gives us a prompt and waits for a command to be entered. The command is typed at the shell prompt. The prompt usually ends in a dollar sign ($). After typing a command we need to press ENTER to invoke it. The shell will execute the command. Another prompt will then appear.

Shell commands consist of one or more words separated by spaces. The first word is the command to be run. Subsequent words are either options or arguments to the command. Options usually start with one or two hyphens.

Some examples of commands:

• 
  List all the files in the current directory:
  

$ ls

• 
  List the files in the ‘long format’ (giving more information):
  

$ ls –l

• 
  List full information about some specific files:
  

$ ls -l notes.txt report.txt

• 
  List full information about all the .txt files:
  

$ ls -l *.txt

• 
  List all files in long format, even the hidden ones:
  

$ ls -l -a

$ ls -la

The dollar ($) represents the prompt here. We need not type it.

Most command take parameters. Some commands require them. Parameters are also known as arguments. For example the command echo simply displays its arguments.

$ echo

$ echo hello there

hello there

The first echo command outputs a blank line and the second echo command outputs its arguments.

Commands are usually case sensitive. Most of the commands are in lower case.

$ echo whisper

whisper
$ ECHO shout
bash: ECHO: command not found

Often it is desired to repeat a previously executed command. The shell keeps a command history for this purpose.

• 
  We use UP and DOWN to scroll through the list of previously executed commands and then press ENTER to execute the desired command.
  
• 
  Commands can also be edited before being run. The LEFT and RIGHT cursor keys navigate across a command.
  
• 
  Extra characters can be typed at any point. BACKSPACE deletes characters to the left of the cursor. DEL and CTRL+D delete characters to the right.
  

Typically successful commands do not give any output. However, messages are displayed in the case of errors.