Shell Scripting 0.0

• Published on Mar 23, 2019 by zphoenix

SHELL SCRIPTING 0.0

Shell Scripting is a series of command/s stored in a plain text file and execute the text file instead of typing each command it is Shell Script.

Shell scripts can take input from a user or file and output them to the screen. Whenever one do the same task over and over again, using of shell script automates the repetitive task. Shell acts as a interface between Kernel and User.

Shell Scripting [courtesy: Internet]

A script consists of

Shell Keyword - if..else, do..while.
Shell commands- cd, pwd, man, echo etc..
Linux binary commands - w, who, free etc..
Text processing utilities - grep, awk, cut.
Functions - frequently used commands together via functions. 
For  example, 
 /etc/init.d/functions is a file which contains functions which are used by most system shell 
 scripts in the directory. In simpler terms printf which is part of pre defined function.
Control flow- are statements which contains if..then..else or shell loops to perform repeated actions.

I. Introduction to Shell Scripting

1.Linux Shell

A Computer understand the language of 0’s and 1’s called binary language,it is difficult for all of us, to read and write in binary language. Wherein OS have a program called Shell or command line interpreter which accepts your instruction or commands written in high level language and translate it into computers binary language.
There are many types of shell like BASH, CSH, KSH, etc. Among them, BASH is very famous and it was developed by GNU. Bourne -Again SHell(BASH) is the default GNU/Linux shell and it does run on every version of Unix, ms-dos, and Windows platforms.
KSH - AT&T version of the Korn shell.
CSH - C like syntax.

Bash understands the below commands :

 such as if, else, for, while etc..
 (user defined functions such as genpasswd)
 such as cd, ls, pwd etc..
 such as /home/text or /bin/desktop/programs

2. Shell Script

Let’s use Nano in our case and to begin with script lets start with Hello World!.

nano hello.sh (editor opens and sh is the format)

#!/bin/bash
echo "Hello, World!" (echo works the same as print)
echo "Welcome to the Shell Scripting Tutorial !!."

bash hello.sh (changes the execution permission)

Hello, World!
Welcome to the Shell Scripting Tutorial !!.

Our script started with #! (shabang), it is the interpreter for execution in GNU/Linux or UNIX OS. Most of the shell script starts with #!/bin/bash.
And after shabang the path indicates the path Bash installed in OS and in GNU/Linux its under bin.

3. Commenting

Commenting can be done using # before line and for multiple line comment use «COMMENT keyword:

#!/bin/bash
#This is an Example of Single line Comment!
echo "Welcome !!"


#!/bin/bash
echo "Welcome!!"
<<COMMENT
Multiple line Comment : Usage
Commenting two or more lines of a script
Can be used to explain the written code
COMMENT
echo "End!!"   

II . VARIABLES & ENVIRONMENTS

Variables are used to store data and configuration options. There are two types of variable
1. System Variables.
2. User defined Variables.

1. System Variables

The variables are created and maintained by Bash Shell and it is is defined in CAPITAL LETTERS only. Aspects of shell can be configured by modifying system variables such as PATH, LANG, DISPLAY , HOSTNAME etc..

╔══════════════╦═════════════════════════════════════════════════╦═════════════════════════╗
║ System       ║                    Meaning                      ║ Variable Value Type     ║  
║ Variable     ║                                                 ║                         ║ 
║ Variable     ║                                                 ║                         ║ 
╠══════════════╬═════════════════════════════════════════════════╬═════════════════════════╣
║ BASH_VERSION ║ Holds the version of this instance of bash.     ║ echo $BASH_VERSION      ║
║ HOSTNAME     ║ The name of the your computer.                  ║ echo $HOSTNAME          ║
║ HOME         ║ The home directory of the current u             ║ echo $HOSTNAME          ║
║ HISTSIZE     ║ The number of commands to remember in the       ║ echo $HISTSIZE          ║
║              ║ command history.Default value is 500.           ║                         ║
║ HISTFILESIZE ║ The maximum lines contained in the history file.║ echo $HISTFILESIZE      ║
║ HISTFILE     ║ The file in which command history is saved.     ║ echo $HISTFILE          ║
║ CDPATH       ║ The search path for the cd command.             ║ echo $CDPATH            ║
║ IFS          ║ IFS is used for word splitting after expansion  ║ echo $IFS               ║
║ LANG         ║ Determine the locale category                   ║ echo $LANG              ║
║ PATH         ║ list of directories where shell search commands.║ echo $PATH              ║
║ PS1 	       ║ Your prompt settings. 	                      	 ║ echo $PS1               ║
║ TMOUT        ║ The default timeout for the read builtin command║ echo $TMOUT             ║
║              ║ The wait time for i/p after issuing the command ║                         ║
║ TERM         ║ Your login terminal type.                       ║ echo $TERM              ║
║              ║                                                 ║ export TERM=vt100       ║
║ SHELL        ║ Set path to login shell.                        ║ echo $SHELL             ║
║ DISPLAY      ║ Set X display name                              ║ echo $DISPLAY           ║
║              ║                                                 ║ export DISPLAY=:0.1     ║
║ EDITOR       ║ Set name of default text editor.                ║export EDITOR=/usr/bin/vi║
╚══════════════╩═════════════════════════════════════════════════╩═════════════════════════╝

When it comes to display the value of any given variable,

#!/bin/bash
VARIABLE=1029
printf "$VARIABLE\n"
printf "String %s\n" $VARIABLE
printf "Signed Decimal Number %d\n" $VARIABLE
printf "Floating Point Number %f\n" $VARIABLE

2. User Defined Variables

This type of variables are created and maintained by user. Variable defined here may use any valid variable name, but it is good practice to avoid all uppercase names as many are used by the shell.

III. DECISION MAKING

So far all the scripts we have followed is *sequential flow(line by line), is it possible to execute a command based on condition if it exist?*

If command is used for selective execution of certain commands of a script. A warning message can pop up and run a script more interactively using if command to execute the script based on any given condition.
A condition is an expression that evaluates to a boolean value (0/1). A condition is used in shell script loops and if statements.

echo $(( 7 + 4))    #Output : 11
echo $(( 5 < 2 ))   #Output : 0, Is 5 is less than 2? No
echo $(( 5 > 12 ))  #Output: 0, Is 5 greater than 12? No 
echo $(( 5 == 10 )) #Output: 0, Is 5 equal to 10? No
echo $(( 5 != 2 ))  #Output: 1, Is 5 is not equal to 2? Yes
echo $(( 1 < 2 ))   #Output: 1, Is 1 less than 2? Yes 
echo $(( 5 == 5 ))  #Output: 1, Is 5 equal to 5? Yes

1. Test Command

The test command is used to check file types and compare given values. Test is used in Decision Making statements too. It is used mainly for,
a. File attributes comparisons
b. Perform string comparisons
c. Basic arithmetic comparisons.

Note: Try man test command on terminal for complete list of conditions which are used for test command.

test <condition>
OR
test <condition> && <true-command>
OR
test <condition> || <false-command>
OR
test <condition> && <true-command> || <false-command>

test 5 -gt 2 && echo "Yes"
test 1 -lt 2 && echo "Yes"
test 5 -eq 9 || echo "No"
test 5 -gt 2 && echo "Yes" || echo "No"
test 5 -ne 10 && echo Yes || echo No

Yes
Yes
No
Yes
Yes

2. If Command Statements

a. if then
b. if else fi
c. Nested ifs
d. Multilevel if then else

a. if then

if condition     if test var == value     if test -f /file/exists
then             then                     then
  command1         command1                 command1 
  command2         command2                 command2
  ...              ...                      ...
  commandN         commandN                 commandN
fi               fi                       fi     

In a If then command, condition is checked and if its true then commands are executed, in false condition it exits. For Example,

#!/bin/bash
read -p "Enter a Password : " pass
if test "$pass" == "gnu"
then
  echo "Password verified."
fi

b. if else fi

 if condition       if test var -eq val    
 then               then                    
   command1           command1                 
   command2           command2                
   ...                ...                   
   commandN           commandN                 
 else               else                    
   command1           command1                
   command2           command2               
   ...                ...                    
   commandN           commandN               
 fi                 fi 

In a If else fi command, condition is checked and if its true then commands are executed, in false condition it else commands are executed. For Example,

#!/bin/bash
read -p "Enter a number : " num
if test $num -ge 0
then
	echo "$num is Positive number."
else
echo "$num number is Negative number."
fi

c. Nested If In a Nested I command, condition is checked and if its true then commands are executed interestingly we have another if condition as the then commands, so again we need to complete the inner if condition and once exited, we have false condition for the outer if command and else commands are executed.

if condition
then
	if condition
	then
		command1
		.....
		..
		commandN
	else
		command1
		....
		..
		commandN
	fi
else
	command1
	...
	.....
	commandN
fi

d. Multilevel if then elseIn a Multilevel If then else command, condition is checked and if its true then commands are executed, in false condition it moves to elif then condition1 is executed till we complete conditionN and then else commands of the condition are executed.

       if condition
       then
                   condition is true
                   execute all commands up to elif statement
       elif condition1 
       then
                   condition1 is true
                   execute all commands up to elif statement  
       elif condition2
       then
                   condition2 is true
                   execute all commands up to elif statement          
       elif conditionN
       then
                   conditionN is true
                   execute all commands up to else statement          
       else
                   None of the above conditions are true
                   execute all commands up to fi
       fi

3. Exit Status

Each Linux command executed have an exit status, it is nothing but an integer number. Zero(0) as a exit status means the command was successful in execution without any errors. And a non-zero (1–255) as a exit status means command was a failure.
Special shell variable $? can be used to get the exit status of the previously executed command and use echo command to print th exit status value.
For Example lets search username is found or not?

#!/bin/bash
FILE=/etc/passwd

read -p "Enter a user name : " username

#locate the username in the file passwd
if grep "^$username:" /etc/passwd >/dev/null
then
echo "User '$username' found in $FILE file."
else
echo "User '$username' not found in $FILE file."
fi

4. Boolean Operators

a. Logical AND - &&
b. Logical OR -

c. Logical Not - !

a. Logical AND - && It executes commands based on the exit status of another command where it runs second command only if first is successful.

command1 && command2

grep "^glugpesce" /etc/passwd && echo "Found in /etc/passwd"

b. Logical OR - && It executes commands based on the exit status of another command where it runs second command only if first is not successful.

command1 || command2

Checks whether we are root or not
test $(id -u) -eq 0  && echo "You are root" || echo "You are NOT root"

c. Logical Not - ! It checks whether the command is true or false (0/1).

! expression    [ ! expression ]    if test ! condition    if [ ! condition ]
                                    then                   then
                                      command1               command1
                                      ...                    ...
                                      commandN               commandN 
                                    fi                     fi

For Example: In the below case it executes if second command when first command is false i.e., it creates a new folder/ directory called shell (command: mkdir) if there is no folder or directory present in the machine.
Same is in the case of prog1.sh.

#!/bin/bash
[ ! -d shell ] && mkdir shell
[ ! -f prog1.sh ] && touch prog1.sh

5. Numerical Comparison

It can be done test command, the previous examples of program used these test command. The Numerical Comparisons commonly used are eq, ge, gt, lt, le, etc…

#Numerical Comparison
eq 	INTEGER1 -eq INTEGER2 	INTEGER1 is equal to INTEGER2 
ge 	INTEGER1 -ge INTEGER2 	INTEGER1 is greater than or equal to INTEGER2 
gt 	INTEGER1 -gt INTEGER2 	INTEGER1 is greater than INTEGER2 
le 	INTEGER1 -le INTEGER2 	INTEGER1 is less than or equal to INTEGER2
lt 	INTEGER1 -lt INTEGER2 	INTEGER1 is less than INTEGER2 
ne 	INTEGER1 -ne INTEGER2 	INTEGER1 is not equal to INTEGER2 

6. String Comparisons

It can also be done using test command.

STRING1 = STRING2

STRING1 != STRING2

-z STRING

The three condition can be checked in a shell script. For Example :

#!/bin/bash
read -s -p "Enter your password :  " pass
echo 
#checks whether the string is non zero
if test -z $pass 
then
    echo "No password was entered!!! Cannot verify an empty password!!!"
    exit 1
elif test "$pass" != "gnu"
then
    echo "Wrong Password !!"
else
    echo "Authentication of password Success!"
fi

7. File attributes comparisons

File attributes can be used making use of test commands or conditional execution or decision making statements.Commonly used among them are -f, -d, -a, -c, etc…

#File Attributes Comparison are

-a file : True if file exists. 
-b file : True if file exists and is a block special file. 
-c file : True if file exists and is a character special file. 
-d dir  : True if file exists and is a directory. 
-e file : True if file exists. 
-f file : True if file exists and is a regular file. 
-g file : True if file exists and is set-group-id.
-h file : True if file exists and is a symbolic link.
-k file : True if file exists and its ‘‘sticky’’ bit is set.
-p file : True if file exists and is a named pipe (FIFO).
-r file : True if file exists and is readable.
-s file : True if file exists and has a size greater than zero.
-t fd   : True if file descriptor fd is open and refers to a terminal.
-u file : True if file exists and its set-user-id bit is set.
-w file : True if file exists and is writable.
-x file : True if file exists and is executable.
-O file : True if file exists and is owned by the effective user id.
-G file : True if file exists and is owned by the effective group id.
-L file : True if file exists and is a symbolic link.
-S file : True if file exists and is a socket.
-N file : True if file exists and has been modified since it was last read. 

8. Command line argument

A command line argument is nothing but an argument sent to a program being called.

cd shell

shell: No such file or directory.

Command line Arguments are listed below

$* : holds all command line parameters or arguments.
$# : holds the number of positional parameters.
$- : holds flags supplied to the shell.
$? : holds the return value set by the previously executed command.
$$ : holds the process number of the shell (current shell).
$! : hold the process number of the last background command.
$@ : holds all command line parameters or arguments.

In a command line argument special shell variable are used and they are
$1, $2, $3, $4, $5, …,$9.
Few shell variables such as
a. $@
b. $*

$@ are expanded as “$1” “$2” “$3” … “$n”.
$* are expanded as “$1y$2y$3y…$n”, where y is the value of $IFS which acts as an separator.

Program

#!/bin/bash
IFS="- "
echo "The script name : $0"
echo "The first argument to the script : $1"
echo "The second argument to the script : $2"
echo "The third argument to the script : $3"
echo "The fourth argument to the script : $4"
echo "The number of arguments passed to the script : $#"
echo "* Display all free Softwares names usinng (\$@ version) : $@"
echo 
echo "* Display all Free Softwares names using (\$* version) : $*"

Execute

bash cal_shell.sh VLC GIMP Firefox Inkspace

Output

The script name : cal1.sh
The first argument to the script : VLC
The second argument to the script : GIMP
The third argument to the script : Firefox
The fourth argument to the script : Inkspace
The number of arguments passed to the script : 4
* Display all free Softwares names usinng ($@ version) : VLC GIMP Firefox Inkspace

* Display all Free Softwares names using ($* version) : VLC-GIMP-Firefox-Inkspace

Exit CommandIn order to exit the BASH shell we can use exit command in out script.
Try to use *exit 0 at the end of previous program and echo it using echo $? and check the exit status?*

9. The Case statement

The case statement is easier to use than multilevel if then else, matching several values against one variable is a lot easier in the case statement. It is easier to read and write.

      case  $variable-name  in            case  $variable-name  in
            pattern1)                           pattern1|pattern2|pattern3)   
 		            command1                            command1                           
                ...                                 ...     
                commandN                            commandN
                ;;                                  ;;
            pattern2)                           pattern4|pattern5|pattern6)   
 		            command1                            command1 
                ...                                 ... 
                commandN                            commandN
                ;;                                  ;;
            patternN)                           pattern7|pattern8|patternN)
 		            command1                            command1 
                ...                                 ... 
                commandN                            commandN
                ;;                                  ;;
            *)                                  *)   
      esac                                 esac 

The case statement allows to check conditions in a easier manner and if a command-line is needed when the condition evaluates to true, it can be done. Here the $variable-name is compared against the conditions until a match is found. *) acts as default case when no match is found it is executed. Including ;; at the end of each commandN is a must. The shell executes all the statements up to the two semicolons. And the esac is always required inorder to indicate end of case statement.
For Example,

Program

#!/bin/bash
NOW=$(date +"%a")
case $NOW in
	    Mon)	
	echo "Full backup", $NOW;;
    Tue|Wed|Thu|Fri)
	echo "Partial backup", $NOW;;
    Sat|Sun)	
	echo "No backup", $NOW;;
    *) ;;
esac

Program

file 1: prog.sh
#!/bin/bash
var=$1
case $var in
        maths)
                echo "In Engineering we have MATHS in 1st to 4th Sem..."
                ;;
        phy)
                echo "Luckily or Sadly we have we have PHYSICS only for 1st or 2nd Sem..."
                ;;
        ece|cse|ise|ae|ipe|ce|eee)
                echo "Now its time to ask Seniors..."
                ;;
        *)
                echo "PSECE"
                echo "Welcome to $var Department !!!"
                echo "Have a nice time !!"
                echo "	$var : Departmental Issues!!  talk to HOD"	;;
esac

file2 : execute.sh
chmod +x prog.sh
./prog.sh maths
./prog.sh phy
./prog.sh ec

Output

In Engineering we have MATHS in 1st to 4th Sem...
Luckily or Sadly we have we have PHYSICS only for 1st or 2nd Sem...
PSECE
Welcome to ec Department !!!
Have a nice time !!
 ec : Departmental Issues!!  talk to HOD

We will continue other topics in next blog session . Thank you