2020-08-20

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The Shell
COMP500
Spring 2019

Review from last week

● Types of Operating Systems

● Popularity of Linux

● Kernel Responsibilities and Versioning

● Common Uses of Linux

● Types of Licensing

Learning Objectives

● Describe the Linux Shell

● Identify Shell Commands

● Use Linux Help System

● Identify components of File System

● Use File Management Commands

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Using the Shell

The Shell
Program that allows a user to interact with the system using typed text
commands displayed in the screen

The most popular implementation is the Bash Shell (Bourne Again Shell). Other
examples are Dash, Fish and ZShell

When used properly, allows for a faster interaction with the machine so it is the
System Administrators’ tool of choice

Accessing the Shell

Provided by default on a basic Linux installation with no GUI installed (you will see
this shell during the lab sessions)

In a system with a GUI installed, run a terminal emulator program such as

Gnome-Terminal, X-Terminal, Terminator, etc.

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Dennis Ritchie

PDP-11

Ken Thompson

Teleprinter
(also called
Teletype or TTY)

https://en.wikipedia.org/wiki/Ken_Thompson#/media/File:Ken_Thompson_(sitting)_and_Dennis_Ritchie_at_PDP-11_(2876612463).jpg (CC BY-SA 2.0)

Terminal

https://i.stack.imgur.com/mTxcb.jpg

Shell Commands
A command is a program that is run by inputting its name in the shell

The command can be built-in, an alias or user-created

Examples of built-in are ls, top, mkdir

Alias are shorter versions of long commands: ll in place of ls -al

Users can create their own programs and define a text command for execution

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Shell Command Syntax

command [options...] [arguments...]

Example: rm -r /user

Command is the name of the program: “rm” is the command for remove

Options modify the behavior of the program: “-r” indicates recursive operation

Arguments are variables passed from the user to the program: “/user” is the
folder to be removed

Shell Command Syntax

IMPORTANT OBSERVATIONS

Commands are case sensitive:

Rm is not the same as rm

Commands, options and arguments are separated by spaces:

rm -r /user is not the same as rm-r/user

Shell Command Options

Short form uses a single dash and a character: ls -l

Short form options can be combined: ls -a -l becomes ls -al

Long form uses a double dash and a word: ls --long

Long and form options can be used in the same command

Options are all case-sensitive

BSD(UNIX) style options do not use hyphens: ps axu

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Shell Command Arguments

Arguments are commonly optional

Used to pass variables from the user to the command

Arguments are often file or directory names: rm -r /home

Arguments can also be values: sleep 120

Require single quotes if text contains special non-alphanumeric values

Command Autocompletion
If a user types cle and then hits <TAB> the shell autocompletes clear

Useful for saving time while typing long inputs

The text is auto completed only if characters typed so far are unique to the target

A second <TAB> gives a list of possible matches

Works with commands, file names and directory names

Command History
Can use ↑ and ↓ to cycle through previously used commands

Press the ↑ key to retrieve the last command from the command history

Press the ↓ key to go forward though the command history after using ↑ to go
back through the history

Command retrieved from history can be edited before being executed

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The Help System

The man Command

man is short for manual

Displays shell commands documentation, syntax, options, arguments and

examples

To display documentation type man and then the command (e.g. man ls)

● Move per line with Up/Down Arrows

● Scroll one screen at a time with Space (down) and b (up)
● Search the file for specific “term” with /term
● Exit with q key

Help page layout

NAME. A brief description of the command

SYNOPSIS. Syntax definition

DESCRIPTION. List and describes the command options

AUTHOR. The command developer

COPYRIGHT. License information

SEE ALSO. Other resources for additional information

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Man Page Sections

Man pages are classified in Sections (numbers in parentheses after command
name when viewing a man page)

1. Executable programs or shell commands

2. System calls
3. Library calls
4. Special files
5. File formats and conventions
6. Games
7. Miscellaneous
8. System administration
9. Kernel routines

Other Sources of Help

Command info displays help information about a command: info ls

Option --help displays help information about a command: ls --help

Not all commands have --help option

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Utility of the Help System

Provides a quick source of information about shell commands

Provides description about usage, syntax and examples

Packages often feature extra help content that can be found in /usr/doc or
/usr/share/doc

No reason for user for stopping workflow

Find an Actual Program File

The command whereis displays the location in disk of an executable program. It
also shows the source code and documentation if available

Also possible to use locate command which can find any file provided the correct
permission

The command updatedb updates information for locate command. It is scheduled

for execution daily but the root user can run anytime using the command
updatedb

Basic Shell Commands

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uname
Prints system information

Linux Kernel name uname -s

Linux Kernel release uname -r

Processor type uname -p

Operating System uname -o

All the information uname -a

sudo
Executes a command as the root user (or another user)

Short for "super user do"

One of the most used commands for its implications

The user must have the proper permissions in order to use sudo

Install applications: sudo apt install php

Access files outside of user’s home directory: sudo vim /etc/passwd

https://linuxacademy.com/blog/linux/linux-commands-for-beginners-sudo/

exec
Executes another command that replaces the current shell

Receives another command as argument: exec zsh

It does not return upon successful completion

Cannot be used inside a pipeline

https://www.computerhope.com/unix/bash/exec.htm

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history
Search, delete and manipulate the command history

Display a list of commands entered since the start of the session: history

Re-run the 3rd command on the history list: !3

Search commands containing ‘foo’: history | grep foo

Clear all history items: history -c

https://opensource.com/article/18/6/history-command

echo

Prints a line of text that is passed as an argument

Mostly used in shell scripts and batch files to output status text to STDOUT

Output text can be redirected to a file

For example, “echo hello”

Managing Inputs and Outputs

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STDIN, STDOUT and STDERR

STDIN. Standard input, by default originated in the keyboard

STDOUT. Standard output, by default sent to the screen

STDERR. Standard error, by default sent to the screen

All have a system-defined variable used in the shell. STDIN is $0, STDOUT is $1
and STDERR is $2

Piping
A command expects some input in order to produce an output

The pipeline symbol ‘|’ allows to apply the output of a command

as the input of another. Thus, joining two or more commands
processing an initial input to produce a final output

The example below applies the output of cat command (text) as

the input of grep command which will look for occurences of the
word “hello”

cat myfile | grep “hello”

Redirection
Output redirection “>” STDOUT or STDERR are redirected to another output or file

cat myfile > newfile saves the contents of myfile to newfile instead of
displaying it

cat myfile >> newfile appends instead of overwrite

Input redirection “<” STDIN is redirected from another input

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The Filesystem

Files and Directories in Linux

Everything is a file in Linux (text files, programs, binary files, devices, etc)

Allowed characters are letters, digits, hyphens, underscores and dots

All file names are case sensitive

Directories are entities that group files

Paths to Directories and Files

Any file or directory has a path

Paths referred to root directory ‘/’ are ‘absolute’ paths

/home/user1/file.txt

Paths referred to the current directory are ‘relative’ paths

file.txt (current location is already /home/user1/)

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The home Directory

Each user of a Linux system is assigned a folder inside /home/

The user has all permissions (read, write and execute) inside his/hers home folder

Denoted by the “~” character (i.e. ~/testfile is actually /home/user/testfile)

Only root user has access to all files and directories

Use sudo to access other directories outside of your home directory

Present Working Directory

The command ‘pwd’ means present working directory

Provides the absolute path to current location of a user

Sometimes is displayed in the prompt:

The Root Directory ‘/’

Top level directory with no parent directory

Denoted with the symbol ‘/’

All other directories fork from it

https://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard
https://www.howtogeek.com/117435/htg-explains-the-linux-directory-structure-explained/

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/bin
Essential binaries (programs) for default commands are stored in this directory

Commands used by all the users of the system are located here

Some common binaries are: bash, cat, date, echo, kill, mv, cp

/boot
Contains files needed to boot the system

Kernel initrd, vmlinux, grub files are located under /boot

i.e. initrd.img-4.15.0-24-generic, vmlinuz-4.15.0-24-generic

/dev
Contains device files (everything is a file in Linux, even hardware)

These include a file for each terminal, usb, or any device attached to the system

For example: /dev/tty1, /dev/usbmon0

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/etc
Contains configuration files required by all programs

This also contains startup and shutdown shell scripts used to start/stop individual
programs

Examples: /etc/hostname, /etc/resolv.conf

/home
Stores home directories for all non-root users

user1 will have a default home directory located at /home/user1

Home directory is denoted by ‘~’ symbol for the current user

/lib
Contains essential library files that support the binaries located under /bin and
/sbin

Library file names are either ld* or lib*.so.*

For example: ld-2.11.1.so, libncurses.so.5.7

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/media and /mnt

/media is a temporary directory for removable devices automatically assigned

In contrast, /mnt is a temporary directory for manual mount by sysadmins

For example, /media/cdrom for CD-ROM, /media/floppy for floppy drives,

/mnt/boot for a new Linux installation

/opt
opt stands for optional

Contains add-on applications from third party vendors

Add-on applications should be installed under either /opt/ or /opt/ sub-directory

/proc
Contains special files that represent system and process information

The directory /proc/{pid} contains information about the process with that
particular pid

Also contains information about system resources (i.e. /proc/uptime)

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/root
home directory of the root user

Instead of being located at /home/root, it’s located at /root

/sbin
Similar to /bin, /sbin contains binary executables

Binaries stored here are used for system maintenance purpose

Commands such as: iptables, reboot, fdisk, ifconfig, swapon

/srv
Contains data for services provided by the system

For example:
● Website files could be stored in /srv/www
● CVS files could be stored in /srv/cvs

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/usr
Contains binaries, libraries, documentation, and source-code for second level
programs

/usr/bin contains binary files for user programs. For example: at, awk, cc, less, scp

/usr/sbin contains binary files for system administrators. For example: atd, cron,
sshd, useradd, userdel

/usr/lib contains libraries for /usr/bin and /usr/sbin

/usr/local contains users programs that you install from source. For example,
when you install apache from source, it goes under /usr/local/apache2

/var
Content of files that are expected to change can be found under this directory

● system log files (/var/log)

● packages and database files (/var/lib)
● emails (/var/mail)
● print queues (/var/spool)
● lock files (/var/lock)
● temp files needed across reboots (/var/tmp)

File Management

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Change Directory
To change directories use the cd command: cd ~/Downloads

ls command with no arguments takes the user to home folder (~)

To stay in the current directory: cd .

To go to the parent directory: cd ..

Listing Files
List files and directories in a directory with ls command

Receives the location to list as argument: ls /home

Without arguments, lists the contents in the current directory: ls

Common options are:

● -a for all files including hidden files

● -l for long list format
● -R for recursive listing

Creating Files
Using the ‘touch’ command to create an empty file: touch myfile

Using the ‘echo’ command to redirect STDOUT to a file: echo “Hello” > myfile

Using the ‘cat’ command cat > myfile any text entered after this line will be sent to
the file ‘myfile’. End with Ctrl-D

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Displaying File Contents

One of the simplest methods is using ‘cat’ command: cat myfile

The text inside the file ‘myfile’ is displayed on the screen

Summary
● Describe the Linux Shell

● Identify Shell Commands

● Use the Linux Help System

● Identify Structure of the File System

● Use File Management Commands

Lab this Week – Lab 2

● Installing a LAMP Server using CLI

● Set of open-source software that can be used to build web applications

○ Linux operating system
○ Apache HTTP Server
○ MySQL relational database
○ PHP programming language

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Lecture Next Week

● Linux Kernel

● Kernel Provisioning

References
Nemeth, E., & Snyder, G., Hein, T., Whaley, B., Mackin, D. (2018). UNIX and
Linux System Administration Handbook. Toronto: Addison-Wesley.

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