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Bash Scripting

Read user input​

Storing her user input into name variable

echo -n "Enter your name: "
read name
echo "You have entered: $name"

-n flag in echo removes the carriage return

read -p 'username: ' user_name
echo "username is: $user_name"

-p flag in read will remove the carriage return

Hiding input​

read -sp 'password: ' password
echo "Your password is: $password"

-sp flag input is hidden

array variable​

echo "Enter array names"
read -a names
echo "Names : ${names[0]}, ${names[1]}"

-a flag indicates the names variable an array

array length​

echo "The length : ${#names[@]}"

#name[@] returns length of the array

Passing args​

Accessing command line args passed to the script

echo "$0 $1 $2 $3"

Storing args in the array​

echo 'storing args in the array'
args=("$@")

# Doesn't throw an error if only 2 args are sent 2,3,4 will be empty
echo ${args[0]} ${args[1]} ${args[2]} ${args[3]} ${args[4]}
echo "To print all the args \"\$@\" $@"

Number of args​

echo $#

If then elif​

#!/bin/bash

echo "***This is if-then-else***"
echo
read -p 'Enter a value: ' var
echo you have entered $var
if [[ $var == 10 ]]
then
    echo **The values are equal to 10
elif [[ $var == 100 ]]
then
    echo ***The values are 100
elif [[ $var == "abcd" ]]
then
    echo Checking \$var == $var
elif [[ $var -eq "Hello" ]]
then
    echo Checking \$var -eq $var
else
    echo Nothing is equal

fi
note

if [[ $var == 10 ]] Space is needed after the open square bracket and before the closing square bracket

for loop in bash script​

Basic for loop​

Basic for loop in bash script

for (( expression; condition; expression))
    do
    command
done

The code below reads words at the command prompt into the names array then loops through the array and prints the data from the array.

#!/bin/bash

# -a for array in read
echo "Enter array names"
read -a names
echo "Names : ${names[0]}, ${names[1]}"
echo "The length : ${#names[@]}"
for (( i=0; i<${#names[@]}; i++ ))
do
    echo -n -e "${names[i]}\t"
done

for loop one-liner in bash​

for i in `seq 1 3`; do echo "Hello ${i}"; done
output
Hello 1
Hello 2
Hello 3

for loop - network scan one-liner​

Scan the network between the ip range ending with 192.168.84.100-200 with nc

for i in {100..200}; do nc -v -n -z -w 1 192.168.84.$i 443; done