🐧Bash

Bash ScriptingLinux Handbook

Creating a Script

#! /bin/bash

echo "Hello World!"

Variables

Name="rohan"

echo $Name

echo ${Name} #same as previous one

User Input

NAME #variable NAME is declared

read  -p "Enter your name: " NAME #Taking the user input in the NAME variable

echo "Your name is $NAME" #Displaying the user inputed value

If Statement

#! /bin/bash

NAME="Rohan"

if [ "$NAME" == "Rohan" ]
then
   echo "Your name is $NAME"
fi

If-Else Statement

#! /bin/bash

NAME="Rohan"

if [ "$NAME" == "Rasdf" ]
then
        echo "Your name is $NAME"
else
        echo "Your name is not $NAME"
fi

Else-If (elif)

#! /bin/bash

NAME="Rohan"

if [ "$NAME" == "Rasdf" ]
then
        echo "Your name is $NAME"
        
elif [ "$NAME" == "Jack" ]
then
        echo "Your name is Jack"

else
        echo "Your name is not $NAME"
fi

Conditional Operators

Operator	Description
-eq	Returns true if two numbers are equivalent
-lt	Returns true if a number is less than another number
-gt	Returns true if a number is greater than another number
==	Returns true if two strings are equivalent
!=	Returns true if two strings are not equivalent
!	Returns true if the expression is false
-d	Check the existence of a directory
-e	Check the existence of a file
-r	Check the existence of a file and read permission
-w	Check the existence of a file and write permission
-x	Check the existence of a file and execute permission

#!/bin/bash
echo "Enter a number"
read n
if [ $n -lt 100 ]; then
printf "$n is less than 100\n"
fi

File Operations

• -e: Returns true value if a file exists.

• -f: Return true value if a file exists and regular file.

• -r: Return true value if a file exists and is readable.

• -w: Return true value if a file exists and is writable.

• -x: Return true value if a file exists and is executable.

• -d: Return true value if a file exists and is a directory.

#!/bin/bash

FILE="test.txt"

if [ -f "$FILE" ]
then
        echo "$FILE is a file"
else
        echo "$FILE is not a file"
fi

CASE Statement

case EXPRESSION in

  PATTERN_1)
    STATEMENTS
    ;;

  PATTERN_2)
    STATEMENTS
    ;;

  PATTERN_N)
    STATEMENTS
    ;;

  *)
    STATEMENTS
    ;;
esac

#!/bin/bash

read -p "Are you 21 or over? Y/N " ANSWER

case "$ANSWER" in
        [yY] | [yY][eE][sS])
                echo "You can have beer :)"
                ;;
        [nN] | [nN][oO])
                echo "Sorry, no drinking"
                ;;
        *)
                echo "Please enter y/yes or n/no"
                ;;
esac

For Loop

#!/bin/bash

NAMES="Brad Kevin Alice Mark"

for NAME in $NAMES
        do 
                echo "Hello $NAME"
done

Rename Files

#!/bin/bash

FILES=$(ls *.txt)

NEW="new"

for FILE in $FILES
        do
                echo "Renaming $FILE to new-$FILE"
                mv $FILE $NEW-$FILE
done

While Loop

while [ condition ];
do
    # statements
    # commands
done

#!/bin/bash

LINE=1

while read -r CUR_LINE
        do
                echo "$LINE: $CUR_LINE"
                ((LINE++))
done < "./new-1.txt"

Functions

#!/bin/bash

function say_hello() {
        echo "Hello World!"
}

say_hello

Function with parameters

#!/bin/bash

function greet()
{
        echo "Hello, I am $1 and I am $2"
}

greet "Rohan" "36"