Basic of Shell Scripting

#! /bin/bash This statement marks the start of shell script.

Echo command:

Echo used for returning/printing the information on stdout(screen).
echo Hello World!

Variables:

Variables used for holding a value in memory. Should be Uppercase by convention. Any letters, numbers and underscores can be used. For accessing the variables, always use '$' or '${VAR-NAME}' in front of variable name.

NAME="Adam"
echo "My name is $NAME."
echo "My name is ${NAME}."

User Input:

read command is used for taking the input from stdin.
-p is used for prompting the text to the stdout.
NAME is used as variable for capturing the user entered data.

read -p "Enter your name:" NAME
echo "Your name is ${NAME}."

Incase you do not want to prompt any text at stdout then use following: Here read will read the user provided input and store it in variable NAME.

read NAME
echo "Your name is ${NAME}."

IF statement:

if condition then result and fi

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

IF-ELSE statement:

if [ "$NAME" == "Thomas" ]
then
echo "Your name is Thomas"
else
echo "Your name is something else"
fi

IF-ELSE-IF-ELSE statement:

if [ "$NAME" == "Thomas" ]
then
echo "your name is Thomas"
elif [ "$NAME" == "Fred" ]
then
echo "your name is Fred"
else
echo "your name is not Thomas nor Fred"
fi

COMPARISON

NUM1=40
NUM2=35

if [ "$NUM1" -gt "$NUM2" ]
then
echo "$NUM1 is greater than $NUM2."
else
echo "$NUM2 is greater than $NUM1."
fi

Different comparison operators:

• val1 -eq val2 => Returns true if the values are equal

• val1 -ne val2 => Returns true if the values are not equal

• val1 -gt val2 => Returns true if val1 is greater than val2

• val1 -ge val2 => Returns true if 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

CASE STATEMENT:

read -p "Is your age above 18? Y/N" ANSWER
case "$ANSWER" in
[yY]|[yY][eE][sS])
echo "You are allowed to have drinks"
;;
[nN]|[nN][oO])
echo "You can enjoy juices"
;;
*)
echo "Enter your age"
;;
esac

FILE CONDITIONS:

FILE="file.txt"
if [ -e "$FILE" ]
then
echo "$FILE exists"
else
echo "$FILE does not exists"
fi

Different file operators:

• -d file => True if file is an directory
• -e file => True if file exists
• -f file => True if provided string is file
• -g file => True if the group id is set on file
• -r file => True if file is readable
• -s file => True if file has non zero size
• -u file => True if the user id is set on a file
• -w file => True if file is writtable
• -x file => True is file is an executable

FOR LOOP:

NAMES="Brad Kevin Alice Mark"
for NAME in $NAMES
do
echo "Hello $NAME"
done

WHILE LOOP:

LINE=1
WHILE read -r CURRENT_LINE
do
echo "$LINE: $CURRENT_LINE"
((LINE++))
done < "./shell-script.sh"

FUNCTION:

function helloWorld(){
echo "Hello World"
}
helloWorld
helloWorld will call the function.

FUNCTION WITH PARAMS:

function intro(){
echo "Hello I am $1 and I am $2 years old"
}
intro "Brad" "36"
intro Brad will call the function with param Brad. You can pass in multiple params and access them using numbers $1, $2 and so on.

CREATE FOLDER AND WRITE TO A FILE

mkdir hello
touch hello/world.txt
echo "Hello World" >> hello/world.txt
echo "created hello/world.txt"