This is a Unix Tutorial where you can learn Unix OS Basics to get started with UNIX and Linux operating systems and covers Unix Directory structure , Unix Commads and Unix Shells

Unix Dir Structure

The table shows the Unix/Linux Directory structure and description of files it stores.

Directory	Description
/	Primary hierarchy root and root directory of the entire file system hierarchy.
/bin/	Executable binary command files.
/dev/	Has nodes for devices , hard disk , tape etc
/etc/	Host-specific configuration files such as /etc/inittab , /etc/hosts
/home/	Users’ home directories .
/lib/	Libraries needed by various Unix programs.
/mnt/	Temporary file system mount point .
/opt/	Optional application software packages installation directory .
/proc/	Virtual file system having kernel process files .
/sbin/	system binaries files such as init .
/tmp/	Temporary file system used to store temporary data of programs .
/usr/	user related command , programs , librar
/usr/bin/	Binary commands for all users .
/usr/include/	Include files required by programs..
/usr/lib/	Libraries for the binaries in /usr/bin/ and /usr/sbin/.
/usr/sbin/	system binaries for users .
/usr/share/	shared file system
/usr/src/	Source code files
/usr/local/	Optional user programs and packages are installed here .
/var/	Directory to store dynamic files such as logs . Most programs has /var as default location for writing log , lock and spool files .
/var/lock/	Lock files.
/var/log/	Log files.
/var/mail/	Users’ mailboxes.
/var/spool/	Spool for tasks waiting to be processed (e.g. print queues and unread mail).

Unix commands

Unix/Linux operating systems comes with online manual system which can be used to see the command details ,syntax options and examples on while working on a Unix system. Unix manual can be accessed using man command , syntax is man <command name>

Unix commands are listed in following sections based on command functionality and you can scroll down to see all the sections.

Unix Command Sections :
1. Wild card,
2. dir level Operation
3. File – Listing , moving
4. File Create , edit , View
5. File operation 6. Process
7. User Admin
8. System System Status
9. Environment Variable
10. Connectivity
11. Backup and recovery
12. Find files and directories You can also download a one page pdf version of the unix commands listed below.

Wild Card Characters

Single character and match all wild card characters which can be used in Unix commands to substitute one or more characters

* – The * wildcard character substitutes for one or more characters in a filename. For instance, to list all the files in your directory that end with .c, enter the command, ls *.c

? – ? (question mark) serves as wildcard character for any one character in a filename. For instance, if you have files named prog1, prog2, prog3, and prog3 in your directory, the Unix command:,

ls prog?

Directory level Operation

Commands to Change , make , move and remove the directory entries .

Change	cd dir Change to directory d
Make	mkdir dir Create new directory d
Move	mv dir1 dir2 Rename directory d1 as d2
Remove	rmdir dir Remove directory d

File – Listing , moving

Commands to list ,copy , move , rename , delete and type the files .

list , no details only names	ls filename , filename with wildcard character/s.
list , details	ls -1 filename , filename with wildcard character/s.
move to directory	mv filename dirname (wildcard character/s supported)
copy file to other/current directory	cp file directory/newfile or cp directory/oldfile .
Copy file file1 into file2	cp file1 file2
move (rename ) file	mv file1 file2 Rename file file1 as file2
Delete the file	rm file , rm -rf directory – Recursively remove files & directly without any warning.
Remove	rm file Delete (remove) file f
file	file filename , file command tries to determine the file type , text , executable etc after comparing the values in /etc/magic .

File Create , edit , View

Command/s to to create new files , edit & view existing file .

vi – vi full screen editor	vi filename , Opens a existing file or creates a new one does not exists.
ed – Line Text editor	ed filename
count – Line, word, & char	wc filename
Text content display – List contents of file at once	cat filename
Text content display by screen : List contents of file screen by screen	more filename
Concatenate – file1 & file2 into file3	cat file1 file2 >file3

File operation

Commands to change owner of file, compare files , sort a file , split large file , match a pattern in a file , list difference between two files .

Change read/write/execute mode of fil	chmod mode file
chown	chown [-R] [-h] owner[:group] file
Compare two files	cmp file1 file2
Sort Alphabetically	sort file
Sort Numerically	sort -n file
Split f into n-line pieces	split [-n] f
match pattern	grep pattern file Outputs lines that
Lists file differences	diff file1 file2
head f Output beginning of file	head file
Output end of file	tail file

Process

Commands to find the Status of a process , suspend the process , stop the process , interrupt a process & kill a process .

status process status stats	ps
Suspend current process	CTRL/z *
Interrupt processes	CTRL/c *
Stop screen scrolling	CTRL/s *
Resume screen scrolling	CTRL/q *
Sleep for n seconds	sleep n
Print list of jobs	jobs
Kill job n	kill %
Remove process n	kill -9 n
Resume background job n	bg [%n]
Resume foreground job n	fg [%n]
Exit from shell	exit

User Admin

Command to add user and change password of a user .

add a new user login to the system	# useradd -u 655 -g 20 -d /home/ttes testlogin loginname -u is userid , if not specified system takes highest available . -g group id should be existing in /etc/group , if not specified other or user is assigned. -d home directory , default is to use user as the directory name under the home directory. loginname – new login name to be created . #useradd testlogin will create a user by the name ‘testlogin’ with all default values .
password Change	passwd <user>
alias (csh/tcsh) – Create command	alias name1 name2
alias (ksh/bash) – Create alias command	alias name1=”name2″
alias – Remove alias	unalias name1[na2…]

System System Status

Command to find quota , date , users , logged in user , finger [username] and history of Unix commands issued .

Display disk quota	quota
Print date & time	date
List logged in users	who
Display current user	whoami
Output user information	finger [username]
Display recent commands	history

Environment Variable

Command to set and export system environment variables .

set	set command alone displays the environment variables, it is used to set options in ksh like set -o vi
export	export variable , export makes variable visible in sub shells.
Set environment variable (csh/tcsh) to value v	sentenv name v
Set environment variable (ksh/bash) to value v	export name=v example : export TERM=vt100

Connectivity

Commands for connecting to other systems using telnet ,ssh , ftp , sftp , ping

Connecting to a remote host	$telnet hostname/ip address or $telnet Telnet brings up the login prompt of remote host and expects you to enter your user name & password .Without argument it enters command mode (telnet>) and accepts command listed by ? at telnet> prompt. Communication is not encrypted between two hosts.
Securely connecting to a remote host	ssh username@hostname or ssh -l username hostname Depending on ssh setting for your account you may or may not be asked a password to login. Your login/passwd will be same login password as you would use with telnet connection. Communication is encrypted between two hosts so if someone intercepts your communication he will not be able to use it.
Copy files from/to remote host	ftp hostname ftp expects you to enter your username/passwd or if it is ftp only account it will require ftp account password . put , mput (multipleput) command is used to transfer files to remote host. get , mget (multipleput) command is used to transfer files from remote host. ftp allows some limited number of commands to be executed at ftp> prompt & summary of ftp command can be found by using ? at ftp> prompt
Securely copy files from/to remote host	sftp username@hostname:remotefile localfile Communication is encrypted between two hosts.
Test the tcp/ip connectivity between two hosts	ping hostname If you can ping a host the host is reachable from the machine that you are using. Router/firewall configuration may prevent ping to succeed .

Unix Shells
Shell in UNIX acts as a command interpreter between user and Unix kernel as well as provides a strong scripting language .

Following are the different types of Unix shells ,

B shell , /bin/sh – This is the default Unix shell for many Unix operating systems.

Bourne shell was written by S. R. Bourne and its more emphasis is to use it as a scripting language rather than an interactive shell .
Some of the features are :

• Provided support for environment variables using parameters and exportable variables.
  Redirection of program output and error .
• Command substitution using back quotes: `command`.
  embed a file/commands using input redirector <<
  “for ~ do ~ done” loops
  “case ~ in ~ esac” for selecting and responding to a data value .

C-shell /bin/csh was designed to provide the interactive features lacking in b shell such as job control and aliasing .

K shell /bin/ksh – was created by David Korn and has features of both B shell and C shell along with some additional features .

Bash – the Bourne again shell was developed by GNU project .It is based on B shell language and has features of C and K shells.

tcsh is the default shell of FreeBSD and its descendants. Essentially it is C shell with programmable command line completion, command-line editing, and a few other features.

Zsh is a shell designed for interactive use and it has many of the useful features of bash, ksh, and tcsh along with many new features.

Unix Shells configuration files :

b shell

shell prompt : $
executable file : /bin/sh

Read on interactive/non interactive login to bash
/etc/profile
~/.profile

bash shell

shell prompt : $
executable file : /bin/bash

Read on interactive/non interactive login to bash
/etc/profile
~/.profile
~/.bash_profile
~/.bash_login

Always read on invoking bash
~/.bashrc

/etc/profile login login login
~/.profile login login

csh shell

shell prompt : %
executable file : /bin/csh

Read on csh shell invocation .
/etc/csh.cshrc
~/.cshrc

Read on interactive/non interactive login to tcsh shell
/etc/.login
~/.login
~/.logout
/etc/csh.login

ksh

shell prompt : $
executable file : /bin/ksh

Read on interactive/non interactive login to bash
/etc/profile
~/.profile

tcsh shell

shell prompt : &
executable file : /bin/tcsh

Read on tcsh shell invocation .
~/.tcshrc
/etc/csh.cshrc
~/.cshrc

Read on interactive/non interactive login to tcsh shell
/etc/.login
~/.login
~/.logout
/etc/csh.login

zsh

shell prompt : $
executable file : zsh

Configuration files :

Always read on invoking zsh
~/.zshenv always
/etc/zshenv always

only read on interactive login to zsh.
~/.zshrc
/etc/zshrc

Read on interactive/non interactive login to zsh
/etc/zprofile login
/etc/zlogin login
/etc/zlogout login
/.zprofile login
~/.zlogin login
~/.zlogout login