An Introduction to the Linux Commands for Beginners

An Introduction to the

Linux Command Shell

For Beginners

1.0 Introduction

The purpose of this document is to provide the reader with a fast and simple introduction to using

the Linux command shell and some of its basic utilities. It is assumed that the reader has zero or

very limited exposure to the Linux command prompt. This document is designed to accompany an

instructor-led tutorial on this subject, and therefore some details have been left out. Explanations,

practical examples, and references to DOS commands are made, where appropriate.

1.1 What is a command shell?

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A program that interprets commands

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in programs called

Allows a user to execute commands by typing them manually at a terminal, or automaticallyshell scripts.

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commands.

A shell is not an operating system. It is a way to interface with the operating system and run

1.2 What is BASH?

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BASH = Bourne Again SHell

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originally written by Steve Bourne for UNIX systems.

Bash is a shell written as a free replacement to the standard Bourne Shell (/bin/sh)

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program with and use from the command line.

It has all of the features of the original Bourne Shell, plus additions that make it easier to

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Since it is Free Software, it has been adopted as the default shell on most Linux systems.

1.3 How is BASH different from the DOS command prompt?

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that typing “

Case Sensitivity: In Linux/UNIX, commands and filenames are case sensitive, meaningEXIT” instead of the proper “exit” is a mistake.

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while the backslash “

“

about these special characters in a minute!

“\” vs. “/”: In DOS, the forward-slash “/” is the command argument delimiter,\” is a directory separator. In Linux/UNIX, the/” is the directory separator, and the “\” is an escape character. More

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that all files followed a format that allowed up to 8 characters in the

filename, followed by a period (“dot”), followed by an option extension,

up to 3 characters long (e.g.

no such thing as a file extension. Periods can be placed at any part of the

filename, and “extensions” may be interpreted differently by all

programs, or not at all.

Filenames: The DOS world uses the “eight dot three” filename convention, meaningFILENAME.TXT). In UNIX/Linux, there is

1.4 Special Characters

Before we continue to learn about Linux shell commands, it is important to know that there are

many symbols and characters that the shell interprets in special ways. This means that certain

typed characters: a) cannot be used in certain situations, b) may be used to perform special

operations, or, c) must be “escaped” if you want to use them in a normal way.

Character Description

\

with a backslash first.

Example:

Escape character. If you want to reference a special character, you must “escape” ittouch /tmp/filename\*

/

Example:

Directory separator, used to separate a string of directory names./usr/src/linux

.

Current directory. Can also “hide” files when it is the first character in a filename.

..

Parent directory

~

User's home directory

*

Example:

Represents 0 or more characters in a filename, or by itself, all files in a directory.pic*2002 can represent the files pic2002, picJanuary2002,

picFeb292002

, etc.

?

Example:

Represents a single character in a filename.hello?.txt can represent hello1.txt, helloz.txt, but not

hello22.txt

[ ]

Example:

Can be used to represent a range of values, e.g. [0-9], [A-Z], etc.hello[0-2].txt represents the names hello0.txt,

hello1.txt

, and hello2.txt

|

Example:

“Pipe”. Redirect the output of one command into another command.ls | more

>

Example:

Redirect output of a command into a new file. If the file already exists, over-write it.ls > myfiles.txt

>>

Example:

Redirect the output of a command onto the end of an existing file.echo .Mary 555-1234. >> phonenumbers.txt

<

Example:

Redirect a file as input to a program.more < phonenumbers.txt

;

Example:

Command separator. Allows you to execute multiple commands on a single line.cd /var/log ; less messages

&&

finished without errors.

Example:

Command separator as above, but only runs the second command if the first onecd /var/logs && less messages

&

Example:

Execute a command in the background, and immediately get your shell back.find / -name core > /tmp/corefiles.txt &

1.5 Executing Commands

The Command

PATH:

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type the name of the program to execute it.

Example: Typing “

Most common commands are located in your shell's “PATH”, meaning that you can justls” will execute the “ ls” command.

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Your shell's “PATH” variable includes the most common program locations, such as

/bin

, /usr/bin, /usr/X11R6/bin, and others.

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location of the command.

Examples:

./program

To execute commands that are not in your current PATH, you have to give the complete/home/bob/myprogram(Execute a program in the current directory)

~/bin/program

(Execute program from a personal bin directory)

Command Syntax

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different things. Typical command syntax can look something like this:

Commands can be run by themselves, or you can pass in additional arguments to make them do

command [-argument] [-argument] [--argument] [file]

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Examples: ls List files in current directory

ls -l

Lists files in “long” format

ls -l --color

As above, with colourized output

cat filename

Show contents of a file

cat -n filename

Show contents of a file, with line numbers

2.0 Getting Help

When you're stuck and need help with a Linux command, help is usually only a few keystrokes

away! Help on most Linux commands is typically built right into the commands themselves,

available through online help programs (“man pages” and “info pages”), and of course online.

2.1 Using a Command's Built-In Help

Many commands have simple “help” screens that can be invoked with special command flags.

These flags usually look like “

Example:

-h” or “--help”.grep --help

2.2 Online Manuals: “Man Pages”

The best source of information for most commands can be found in the online manual pages,

known as “man pages” for short. To read a command's man page, type “

Examples:

man command”.man ls Get help on the “ls” command.

man man

To search for a particular word within a man page, type “

type the “Q” key.

Sometimes, you might not remember the name of Linux command and you need to search for it.

For example, if you want to know how to change a file's permissions, you can search the man page

descriptions for the word “permission” like this:

A manual about how to use the manual!/word”. To quit from a man page, just

man -k permission

If you look at the output of this command, you will find a line that looks something like:

chmod (1) - change file access permissions

Now you know that “

show you the

chmod” is the command you were looking for. Typing “man chmod” willchmod command's manual page!

2.3 Info Pages

Some programs, particularly those released by the Free Software Foundation, use info pages as

their main source of online documentation. Info pages are similar to man page, but instead of

being displayed on one long scrolling screen, they are presented in shorter segments with links to

other pieces of information. Info pages are accessed with the “

Linux distributions, “

For example:

info” command, or on somepinfo” (a nicer info browser).info df Loads the “df” info page.

3.0 Navigating the Linux Filesystem

The Linux filesystem is a tree-like hierarchy hierarchy of directories and files. At the base of the

filesystem is the “

user). Unlike DOS or Windows filesystems that have multiple “roots”, one for each disk drive, the

Linux filesystem mounts all disks somewhere underneath the

describes many of the most common Linux directories.

/” directory, otherwise known as the “root” (not to be confused with the root/ filesystem. The following table

3.1 The Linux Directory Layout

Directory Description

The nameless base of the filesystem. All other directories, files, drives, and

devices are attached to this root. Commonly (but incorrectly) referred to as

the “slash” or “/” directory. The “/” is just a directory separator, not a

directory itself.

/bin

Essential command binaries (programs) are stored here (bash, ls, mount,

tar

, etc.)

/boot

Static files of the boot loader.

/dev

they are kept under this directory.

Device files. In Linux, hardware devices are acceessd just like other files, and

/etc

Host-specific system configuration files.

/home

Location of users' personal home directories (e.g. /home/susan).

/lib

Essential shared libraries and kernel modules.

/proc

Process information pseudo-filesystem. An interface to kernel data structures.

/root

The root (superuser) home directory.

/sbin

Essential system binaries (fdisk, fsck, init, etc).

/tmp

Temporary files. All users have permission to place temporary files here.

/usr

documentation, and much more).

The base directory for most shareable, read-only data (programs, libraries,

/usr/bin

Most user programs are kept here (cc, find, du, etc.).

/usr/include

Header files for compiling C programs.

/usr/lib

Libraries for most binary programs.

/usr/local

where files are stored on the network. Locally-installed files go in

“Locally” installed files. This directory only really matters in environments

/usr/local/bin

software packages installed from source, or software not officially shipped

with the distribution.

, /usr/local/lib, etc.). Also often used for

/usr/sbin

Non-vital system binaries (lpd, useradd, etc.)

/usr/share

man pages, etc.).

Architecture-independent data (icons, backgrounds, documentation, terminfo,

/usr/src

Program source code. E.g. The Linux Kernel, source RPMs, etc.

/usr/X11R6

The X Window System.

/var

Variable data: mail and printer spools, log files, lock files, etc.

3.2 Commands for Navigating the Linux Filesystems

The first thing you usually want to do when learning about the Linux filesystem is take some time

to look around and see what's there! These next few commands will: a) Tell you where you are,

b) take you somewhere else, and c) show you what's there. The following table describes the basic

operation of the

you might already be familiar with.

pwd, cd, and ls commands, and compares them to certain DOS commands that

Linux Command DOS Command Description

pwd

location in the directory tree.

cd “Print Working Directory”. Shows the current

cd

returns you to your home directory.

cd, chdir “Change Directory”. When typed all by itself, it

cd directory

Example:

cd directory Change into the specified directory name.cd /usr/src/linux

cd ~

used as a shortcut to your “home”, or other

directories relative to your home.

“~” is an alias for your home directory. It can be

cd ..

cd.. Move up one directory. For example, if you are in

/home/vic

up in

and you type “cd ..”, you will end/home.

cd -

back to your previous location!

Return to previous directory. An easy way to get

ls

format.

dir /w List all files in the current directory, in column

ls directory

Example:

dir directory List the files in the specified directory.ls /var/log

ls -l

also shows you additional info about the file, such

as ownership, permissions, date, and size.

dir List files in “long” format, one file per line. This

ls -a

are those files that begin with a “.”, e.g. The

dir /a List all files, including “hidden” files. Hidden files

.bash_history

file in your home directory.

ls -ld

directory

A “long” list of “directory”, but instead of showing

the directory contents, show the directory's detailed

information. For example, compare the output of

the following two commands:

ls -l /usr/bin

ls -ld /usr/bin

ls /usr/bin/d*

in the

dir d*.* List all files whose names begin with the letter “d”/usr/bin directory.

4.0 Piping and Re-Direction

Before we move on to learning even more commands, let's side-track to the topics of piping and

re-direction. The basic UNIX philosophy, therefore by extension the Linux philosophy, is to have

many small programs and utilities that do a particular job very well. It is the responsibility of the

programmer or user to combine these utilities to make more useful command sequences.

4.1 Piping Commands Together

The pipe character, “

command is “piped” into the next program, and if there is a second pipe, the output is sent to the

third program, etc. For example:

|”, is used to chain two or more commands together. The output of the first

ls -la /usr/bin | less

In this example, we run the command “

of the files in

output to a program called “l

ls -la /usr/bin”, which gives us a long listing of all/usr/bin. Because the output of this command is typically very long, we pipe theess”, which displays the output for us one screen at a time.

4.2 Redirecting Program Output to Files

There are times when it is useful to save the output of a command to a file, instead of displaying it

to the screen. For example, if we want to create a file that lists all of the MP3 files in a directory,

we can do something like this, using the “

>” redirection character:

ls -l /home/vic/MP3/*.mp3 > mp3files.txt

A similar command can be written so that instead of creating a new file called

we can append to the end of the original file:

mp3files.txt,

ls -l /home/vic/extraMP3s/*.mp3 >> mp3files.txt

5.0 Other Linux Commands

The following sections describe many other commands that you will find on most Linux systems.

I can't possibly cover the details of all of these commands in this document, so don't forget that you

can check the “man pages” for additional information. Not all of the listed commands will be

available on all Linux or UNIX distributions.

5.1 Working With Files and Directories

These commands can be used to: find out information about files, display files, and manipulate

them in other ways (copy, move, delete).

Linux

Command

DOS

Command

Description

file

For example, “

executable file.

Find out what kind of file it is.file /bin/ls” tells us that it is a Linux

cat

example:

created in the previous section.

type Display the contents of a text file on the screen. Forcat mp3files.txt would display the file we

head

Example:

Display the first few lines of a text file.head /etc/services

tail

Example:

Display the last few lines of a text file.tail /etc/services

tail -f

appended data as the file grows (very useful for following

log files!).

Example:

Display the last few lines of a text file, and then outputtail -f /var/log/messages

cp

Example:

copy Copies a file from one location to another.cp mp3files.txt /tmp

(copies the mp3files.txt file to the /tmp directory)

mv

ren, move

rename,

Moves a file to a new location, or renames it.

For example:

mv mp3files.txt /tmp

(copy the file to /tmp, and delete it from the original

location)

rm

del Delete a file. Example: rm /tmp/mp3files.txt

mkdir

md Make Directory. Example: mkdir /tmp/myfiles/

rmdir

rd, rmdir Remove Directory. Example: rmdir /tmp/myfiles/

5.2 Finding Things

The following commands are used to find files. “

approximately where they are, but sometimes you need more powerful tools such as these:

ls” is good for finding files if you already know

Linux

Command

Description

which

you want to know exactly where the “grep” command is located on the

filesystem, you can type “

like:

Shows the full path of shell commands found in your path. For example, ifwhich grep”. The output should be something/bin/grep

whereis

information is available). For example, to find out where “

page are, type: “

Locates the program, source code, and manual page for a command (if allls” and its manwhereis ls” The output will look something like:

ls: /bin/ls /usr/share/man/man1/ls.1.gz

locate

can find all files and directories that contain the name “mozilla” by typing:

A quick way to search for files anywhere on the filesystem. For example, you

locate mozilla

find

search for files matching certain patterns, as well as many other types of

searches. A simple example is:

A very powerful command, but sometimes tricky to use. It can be used to

find . -name \*mp3

This example starts searching in the current directory “.” and all subdirectories,

looking for files with “mp3” at the end of their names.

5.3 Informational Commands

The following commands are used to find out some information about the user or the system.

Linux Command Explanation

ps

Lists currently running process (programs).

w

Show who is logged on and what they are doing.

id

Print your user-id and group id's

df

Report filesystem disk space usage (“Disk Free” is how I remember it)

du

for the current directory.

Disk Usage in a particular directory. “du -s” provides a summary

top

activity on your computer in real-time. Type “Q” to quit.

Displays CPU processes in a full-screen GUI. A great way to see the

free

Displays amount of free and used memory in the system.

cat /proc/cpuinfo

Displays information about your CPU.

cat /proc/meminfo

Display lots of information about current memory usage.

uname -a

etc.)

Prints system information to the screen (kernel version, machine type,

5.4 Other Utilities

Here are some other commands that are useful to know.

Linux Command Description

clear

Clear the screen

echo

example: echo “Hello World”

Display text on the screen. Mostly useful when writing shell scripts. For

more

Display a file, or program output one page at a time. Examples:

more mp3files.txt

ls -la | more

less

backwards as well as forwards.

An improved replacement for the “more” command. Allows you to scroll

grep

which TCP network port is used by the “nfs” service, you can do this:

Search for a pattern in a file or program output. For example, to find out

grep .nfs. /etc/services

This looks for any line that contains the string “nfs” in the file “/etc/services”

and displays only those lines.

lpr

Print a file or program output. Examples:

lpr mp3files.txt

- Print the mp3files.txt file

ls -la | lpr

- Print the output of the “ls -la” command.

sort

Sort a file or program output. Example: sort mp3files.txt

su

The default account to switch to is the root/superuser account. Examples:

“Switch User”. Allows you to switch to another user's account temporarily.

su

- Switch the root account

su -

- Switch to root, and log in with root's environment

su larry

- Switch to Larry's account

5.5 Shortcuts to Make it all Easier!

When you start using the Bash shell more often, you will appreciate these shortcuts that can save

you very much typing time.

Shortcut Description

Up/Down Arrow Keys Scroll through your most recent commands. You can

scroll back to an old command, hit

command without having to re-type it.

“

ENTER, and execute thehistory” command Show your complete command history.

T

recognizes, you can have it automatically completed for

you if you press the

characters of your favourite Linux command, then hit

AB Completion If you type a partial command or filename that the shellTAB key. Try typing the first fewTAB

a couple of times to see what happens.

Complete recent commands with “!” Try this: Type “

of a recent command and press

!” followed by the first couple of lettersENTER! For example, type:

find /usr/bin -type f -name m\*

...and now type:

!fi

Search your command history with

CTRL-R

Press

command. It will search the commands for you, and once

you find the command you want, just press

Scrolling the screen with

PageUp

CTRL-R and then type any portion of a recentENTER.Shift-and Page Down

Scroll back and forward through your terminal.