WEEK 2: Shell Scripting

What is Shell Script?
Normally shells are interactive. It means shell accepts command from you (via keyboard) and execute them. But if you use command one by one (sequence of 'n' number of commands), then you can store this sequence of command to text file and have the shell execute this text file instead of entering the commands. This is known as shell script.

Shell script defined as:
"Shell Script is series of commands written in plain text file. Shell script is just like a batch file is MS-DOS but more powerful than the MS-DOS batch file."


Why to Write Shell Script?
    • Shell script can take input from the user, file and output them on to screen or a file.
    • Useful to create our own commands.
    • Saves lots of time.
    • To automate some tasks of day to day life.
    • System Administration tasks can be automated.


For our first shell script, we'll write a "Hello World" script.
Create a file first.sh (using any editor of your choice) as follows:
first.sh
#!/bin/sh
# This is a comment!
echo Hello World        # This is a comment, too!
The first line tells Unix that the file is to be executed by /bin/sh. This is the standard location of the Bourne shell on just about every Unix system. If you're using GNU/Linux, /bin/sh is normally a symbolic link to bash (or, more recently, dash).
The second line begins with a special symbol: #. This marks the line as a comment, and it is ignored completely by the shell.

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The only exception is when the very first line of the file starts with #! - as ours does. This is a special directive which Unix treats specially. It means that even if you are using csh, ksh, or anything else as your interactive shell, that what follows should be interpreted by the Bourne shell.
Similarly, a Perl script may start with the line #!/usr/bin/perl to tell your interactive shell that the program which follows should be executed by perl. For Bourne shell programming, we shall stick to #!/bin/sh.
The third line runs a command: echo, with two parameters, or arguments - the first is "Hello"; the second is "World".
Note that echo will automatically put a single space between its parameters.
The # symbol still marks a comment; the # and anything following it is ignored by the shell.
Now run chmod 755 first.sh to make the text file executable, and run ./first.sh.
Your screen should then look like this:
$ chmod 755 first.sh
$ ./first.sh
Hello World
$
You will probably have expected that! You could even just run at the shell prompt:
$ echo Hello World
Hello World
$



How to write shell script?

Following steps are required to write shell script:

(1) Use any editor like vi, gedit or mcedit to write shell script.

(2) After writing shell script set execute permission for your script as follows
syntax:
chmod permission your-script-name
Examples:
$ chmod +x your-script-name
$ chmod 755 your-script-name
Note: This will set read, write & execute permission (7 or binary 111) for owner
Read and execute permission only (5 or binary 101) for group and other.

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How to Execute your script?
syntax:
bash your-script-name
sh your-script-name
./your-script-name
Examples:
$ bash bar
$ sh bar
$ ./bar

NOTE In the last syntax  ./ means current directory, But only . (dot) means execute given command file in current shell without starting the new copy of shell, The syntax for . (dot) command is as follows
Syntax:
. command-name
Example:
$ . foo

Now you are ready to write a shell script that will print "Knowledge is Power" on screen. See the common vi command list , if you are new to vi.
$ vi second
#
# My first shell script
#
clear
echo "Knowledge is Power"
After saving the above script, you can run the script as follows:
$ ./second
This will not run script since we have not set execute permission for our script second; to do this type command
$ chmod 755 second
$ ./second
First screen will be clear, then Knowledge is Power is printed on screen.

How Shell Locates the file (My own bin directory to execute script)
Tip: For shell script file try to give file extension such as .sh, which can be easily identified by you as shell script.


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Exercise 1:
1) Write the following shell script, save it, execute it and note down the output.
# Script to print user information who currently login, current date & time
# Enter the following commands in a file
#
clear
echo "Hello $USER"
echo "Today is ";date
echo "Number of user login : " ; who | wc -l
echo "Calendar"
cal
exit 0
At the end, why statement exit 0 is used? What is the meaning of $?  See exit status for more information.



VARIABLES IN SHELL
In Linux (Shell), there are two types of variable:
(1) System variables - Created and maintained by Linux itself. This type of variable defined in CAPITAL LETTERS.
(2) User defined variables (UDV) - Created and maintained by user. This type of variable defined in lower letters. Some of the important System variables are:
System Variable
Meaning
BASH=/bin/bash
Our shell name
BASH_VERSION=1.14.7(1)
Our shell version name
COLUMNS=80
No. of columns for our screen
HOME=/home/ubuntu
Our home directory
LINES=25
No. of columns for our screen
LOGNAME=students
students Our logging name
OSTYPE=Linux
Our Os type
PATH=/usr/bin:/sbin:/bin:/usr/sbin
Our path settings
PS1=[\u@\h \W]\$
Our prompt settings
PWD=/home/ubuntu/Common
Our current working directory
SHELL=/bin/bash
Our shell name
USERNAME=ubuntu
User name who is currently login to this PC
NOTE that Some of the above settings can be different in your PC/Linux environment. You can print any of the above variables contains as follows:

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$ echo $USERNAME
$ echo $HOME

Exercise 2:
1) If you want to print your home directory location then you give command:
a) echo $HOME
OR
(b)echo HOME
Which of the above command is correct & why?  
Caution: Do not modify System variable, this can some time create problems.

How to define User defined variables (UDV)
To define UDV use following syntax
Syntax:
variable name=value
'value' is assigned to given 'variable name' and Value must be on right side = sign.

Example:
$ no=10  # this is ok
$ 10=no # Error, NOT Ok, Value must be on right side of = sign.
To define variable called 'vehicle' having value Bus
$ vehicle=Bus
To define variable called n having value 10
$ n=10

To print or access UDV use following syntax
To print contains of variable 'vehicle' type
$ echo $vehicle
It will print 'Bus'
To print contains of variable 'n' type command as follows
$ echo $n
It will print ‘10’

echo Command
Use echo command to display text or value of variable.
echo [options] [string, variables...]
Displays text or variables value on screen.
Options
-n Do not output the trailing new line.


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-e Enable interpretation of the following backslash escaped characters in the strings:
\a alert (bell)
\b backspace
\c suppress trailing new line
\n new line
\r carriage return
\t horizontal tab
\\ backslash
For e.g. $ echo -e "An apple a day keeps away \a\t\tdoctor\n"
 How to display colourful text on screen with bold or blink effects, how to print text on any row, column on screen, click here for more!



Shell Arithmetic

Syntax:
expr op1 math-operator op2

Exercise 3:
What is the output of the following expressions?
$ expr 1 + 3
$ expr 2 - 1
$ expr 10 / 2
$ expr 20 % 3
$ expr 10 \* 3
$ echo `expr 6 + 3`

Exercise 4:
What is the meaning of Single quote (‘), Double quote (“) and Back quote (`) in shell?

















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5th Semester - UE17CS305 – Introduction to Operating Systems Laboratory



Wild cards (Filename Shorthand or meta Characters)

Wild card /Shorthand
Meaning
Examples
*
Matches any string or group of characters.
$ ls *
will show all files


$ ls a*
will show all files whose first name is starting with letter 'a'


$ ls *.c  
will show all files having extension .c


$ ls ut*.c
will show all files having extension .c but file name must begin with 'ut'.
?
Matches any single character.
$ ls ?  
will show all files whose names are 1 character long 


$ ls fo?
will show all files whose names are 3 character long and file name begin with fo
[...]
Matches any one of the enclosed characters
$ ls [abc]* 
will show all files beginning with letters a,b,c


Redirection of Standard output/input i.e. Input - Output redirection
It's possible to send output to a file or to read input from a file.
For e.g.
$ ls command gives output to screen; to send output to file of ls command use 
$ ls > filename
It means put output of ls command to filename.
There are three main redirection symbols >, >> and <

(1) > Redirection symbol
To output Linux-commands result (output of command or shell script) to file. Note that if file already exists, it will be overwritten, else new file is created. For e.g. To send output of ls command use
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$ ls > myfiles
Now if 'myfiles' file exist in your current directory it will be overwritten without any type of warning.
(2) >> Redirector Symbol
Syntax:
Linux-command >> filename
To output Linux-commands result (output of command or shell script) to END of file. Note that if file exists, it will be opened and new information/data will be written to END of file, without losing previous information/data. If file does not exist, then a new file is created. For e.g. To send output of date command to already exist file use command
$ date >> myfiles
(3) < Redirector Symbol
Syntax:
Linux-command < filename
To take input to Linux-command from file instead of key-board. For e.g. To take input for cat command use command
$ cat < myfiles

Exercise 5:
What does the following command do?
$ sort < myfile > sorted_file

Pipes
A pipe is a way to connect the output of one program to the input of another program without any temporary file.
"A pipe is nothing but a temporary storage place where the output of one command is stored and then passed as the input for the second command. Pipes are used to run more than two commands (Multiple commands) from the same command line."
Syntax:
command1 | command2



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Examples: Command using Pipes
Meaning or Use of Pipes
$ ls | more
Output of ls command is given as input to more command so that output is printed one screen (full page) at a time.
$ who | sort 
Output of who command is given as input to sort command so that it will print sorted list of users
$ who | sort > user_list
Same as above except output of sort is sent to (redirected) user_list file
$ who | wc -l 
Output of who command is given as input to wc command so that it will show number of user logged into the system
$ ls -l | wc  -l   
Output of ls command is given as input to wc command so that it will print number of files in current directory.
$ who | grep ubuntu
Output of who command is given as input to grep command so that it will print if particular user name is logged in, if not, nothing is printed 

If-else-fi construct
Create a shell script using if-else-fi statements and execute it

osch=0

echo "1. Unix (Sun OS)"
echo "2. Linux (Red Hat)"
echo -n "Select your os choice [1 or 2]? "
read osch

if [ $osch -eq 1 ] ; then

     echo "You selected Unix (Sun OS)"

else #### nested if i.e. if within if ######
            
       if [ $osch -eq 2 ] ; then
             echo "You selected Linux (Red Hat)"
       else
             echo "You don't like Unix/Linux OS."
       fi
fi

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Looping – while, until and for Loops

Create a shell script to print Welcome 5 times
for ((  i = 0 ;  i <= 4;  i++  ))
do
  echo "Welcome $i times"
done

Create a shell script to display numbers 0 to 9
#!/bin/sh

a=0

while [ $a -lt 10 ]
do
   echo $a
   a=`expr $a + 1`
done

Create a Shell script that uses the until loop to display the numbers 0 to 9
#!/bin/sh

a=0

until [ ! $a -lt 10 ]
do
   echo $a
   a=`expr $a + 1`
done

Exercise 6:
Create a shell script (using Bourne Shell or Bash) which converts all file names in the current directory to lowercase. You should execute the script and send a screenshot of the output.
Example:
If you have these files in the current directory:  ABC, foo1.c, Test, sample, foo2.TXT, myFile.Xa
Expected output after running the script: abc, foo1.c, test, sample, foo2.txt, myfile.xa




References: http://www.freeos.com/guides/lsst/
           https://linuxhint.com/30_bash_script_examples/
   ‘Man’ pages of individual commands on Linux system 

























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