Linux Command Line Basics

When you first login to a Linux terminal, you are now using bash. Bourne-Again SHell (or BASH) is the default shell. A shell is the interpreter of anything you type on the terminal. In this post, I will briefly discuss the basics of linux command line.

Kinds of Shells

bash

  - Bourne Again SHell

  - based on earlier version of sh

  - commonly default shell in Linux

sh

  - Bourne shell

  - this is were BASH was based

  - not often used in Linux but usually a pointer to /bin/bash

tcsh

  - based on earlier C shell (csh)

  - not popular in Linux

csh

  - the original C Shell

  - not commonly used in Linux

  - very similar to tcsh

ksh

  - combination of features of csh and sh

zsh

  - Z Shell

  - evolution of ksh

How to start a shell?

There are several ways and here are the most common ways.

1. Once you logged in to a linux server (either by SSH or via console), you automatically starts your own shell

2. In xterm or GUI, you can right click on the desktop and open a terminal. In that way, you are also starting a shell

Parts of a shell

Understanding every part of the shell is very important specially for linux administrators because it is the one you interact with everytime you deal with Linux. As an example, when you logged as a regular, the shell will provide you a prompt where you can type your commands.

$ uname

Linux

$

The "$" represents something. In Linux by default, it means that you are logged in as regular user. You may customized it by changing the PS1 environment variable. A root's prompt starts with "#" so its important to clearly distinguished a regular user's prompt from a root's prompt. That can avoid accidentally running destructive command like "rm -fr *" on /. The command used in the example above is "uname" and its output is "Linux". Depending on the command you type, it may return an output or nor. Or it may even return an error.

Types of commands you can use

2 types of commands are "Internal" and "External" commands. Internal are the ones built-in in linux while External are the  ones came from outside sources (e.g came from installing a package). To see what type of command, execute the following:

$ type cd

cd is a shell builtin

$ type date

date is /bin/date

$

$ type -a pwd

pwd is a shell builtin

pwd is /bin/pwd

$

The first example tells us that "cd" is an Internal command while the second one indicates that "date" is an external command. There are some commands you may encounter that has both Internal and External versions. So to see it, add "-a" on type command as shown in the 3rd example above.

Getting help

Since we are dealing with command line, we often need more information on a particular command we are using. There are several ways on how to display command information.

1. Using "man" - perhaps the most important help utility in Linux

  man <command>  # displays man pages of a command

  man -k <string>  # searches the man database for commands (or description) matching the specified string

  man -K <string>  # similar as above but do a full search (takes significantly longer to complete)

  man <section number> <command/config file>  # prints information on a specific manual section (see discussion below)

Man contains sections which groups the type of information you can see on a particular command. Not all command have the same manual sections. Here are the example of manual sections:

Section Number

  1 -- user commands (contains traditional command usage and options)

  2 -- system calls provided by the kernel (not commonly used by sys ads)

  3 -- library calls

  4 -- device files

  5 -- file formats (these are config files like "man 5 fstab")

  6 -- games

  7 -- miscellaneous

  8 -- system administration commands (usually ones that must be run as root)

  9 -- kernel routines

On the 9 manual sections, only 1, 5, and 8 are the most commonly used by system administrators.

2. Using pinfo/info - not all commands support this

  info coreutils 'ls invocation'  # as an example, you can see this command under "SEE ALSO" of ls (1)

3. Exploring /usr/share/doc

  - some packages includes README files under this directory

  - you can see examples here

  - sometimes there are also sample configurations

Environment Variables

These are global values read by the shell and inherited by the user (which can also be overriden) during startup. To see all environment variables, type "env". Here are the common environment variables in Linux.

PS1 - this is your prompt (e.g [user@host] $)

HOME - contains path to your home directory

HOSTNAME - the hostname of current machine where you are logged in

LOGNAME - the username used to login to the machine

To display the values, use the following command:

echo $HOME