Introduction to Shell Scripting

what is shell script: In the simplest terms, a shell script is a file containing a series of commands.The shell reads this file and carries out the commands as through they have been entered directly on the command line.

- Normally the shell script has the file extension.sh

- To create shell script you use a text editor.There are many text editor available for your linux system,both for the command line enviroment and the gui enviroment.Here is the list of common ones.

1) vi or vim is the command line interface text editor

2) gedit is the gui text editor

Following steps are required to write shell script

step 1) Use ant editor like vi or gedit to write shell script.

syntax: vi <script file name>

example: vi hello.sh

step2) After writing shell script set execute permission for your script as follows

syntax: chmod <permission> <script file name>

example: chmod <permission> <script file name>

chmod 777 <script file name>

- chmod 777

- 7 ------> myself(root)

- 7 ------> mygroup

- 7 ------> everyone

- let's look at how 7 is divided to set a file permisson:

- 4 ---->Read, 2----> Write, 1-----> Execute

- Shell Scripting:

#! /bin/bash

echo "Hello world"

echo "Our shell name is $BASH"

echo "our shell version $BASH_VERSION"

echo "our home directory is $HOME"

echo "our current working directory is $PWD"

# echo command

echo "Hello world"

# VARIABLE# uppercase by convention# Letters, Number, Underscore

NAME="Brad"

echo "My name is $NAME"

# user input

read -p "Enter your name: " NAME

echo "Hello $NAME, nice to meet you"

- If condition:

NAME="Brad"

if [ "$NAME" == "Brad" ]

then echo "Your name is Brad"

fi

Output: Your name is Brad

- If-else condition:

NAME="jack"

if [ "$NAME" == "Brad" ]

then

echo "Your name is Brad"

else

echo "Your name is not Brad"

fi

output: Your name is not Brad

else-if (elif):

NAME="jack"

if [ "$NAME" == "Brad" ]

then

echo "Your name is Brad"

elif [ "$NAME" == "jack" ]

then

echo "Your name is jack"

else

echo "Your name is not Brad or Jack"

fi

output:Your name is jack

#COMPARISON

# val1 -eq val2 Returns true if the value are equal

# val1 -ne val2 Returns true if the value are not equal

# val1 -gt val2 Returns true if the val1 is greater than val2

# val1 -ge val2 Returns true if the val1 is greater than or equal to val2

# val1 -lt val2 Returns true if val1 is less than val2

# val1 -le val2 Returns true if val1 is less than or equal to val2

NUM1=3

NUM2=5

if [ "$NUM1" -gt "$NUM2" ]

then

echo "$NUM1 is greater than $NUM2"

else

echo "$NUM1 is less than $NUM2"

fi

output:$NUM1 is less than $NUM2"

# file condition

# -d file True if the file is directory

# -e file True if the file exist

# -r file True if the file is readable

# -w file True if the file is writable

# -x file true if the file is executable

FILE="test.txt"

if [ -f "$FILE" ]

then

echo $FILE is a file

else

echo $FILE is not a file

fi

output:$FILE is a file

#CASE STATEMENT
read -p "Are you 21 or over? Y/N " ANSWEAR

case "$ANSWEAR" in

[yY] | [yY][eE][sS])

echo "you can have a beer :)"

;;

[nN] | [nN][oO])

echo "sorry no drinking"

;;

*)

echo "please enter y/yes or n/no" ;;

esac

# Simple for loop:
NAMES="Brad Kevin alice mark"

for NAME in $NAMES

do

echo Hello $NAME

done

- Renaming the file:

FILES=$(ls *.txt)

NEW="new"

for FILE in $FILES

do

echo "Remaining $FILE to new-$FILE"

mv $FILE to $NEW-$FILE

done

- While loop:

LINE=1

while read -r CURRENT_LINE

do

echo "$LINE: $CURRENT_LINE"

((LINE++))

done < "./new1.txt"

#function

function sayHello() {

echo "Hello world"

}

sayHello

output: Hello world

- function with parameter

function greet() {

echo "Hello i am $1 and i am $2"

}

greet "Brad" "36"

output: Hello i am Brad and i am 36