[Bash] Shell Scripting in Linux Hands-On

What is shell scripting?

Shell Scripting is a program to write a series of commands for the shell to execute or simply we can say that shell scripting is writing a bunch of command and executing them from shell itself.

Shell: A shell is a computer program that presents a command line interface which allows you to control your computer using commands entered with a keyboard instead of controlling graphical user interfaces (GUIs) with a mouse/keyboard/touchscreen combination.

Script: A script is a program or sequence of instructions that is interpreted or carried out by another program.

Type of Shells:

Depending on different OS there are different type of shells. To check type of shells offered by your OS just type:

cat /ect/shells

To check path of your Shell

which bash

Bash: Bourne-Again Shell

Dash: Debian Almquist shell

rbash: If Bash is started with the name rbash , or the --restricted or -r option is supplied at invocation, the shell becomes restricted. A restricted shell is used to set up an environment more controlled than the standard shell.

Let's make our first shell script

Just open your Terminal and follow the steps:

Simply type in terminal:

touch hello.sh

Now, open this file with any editor here I am using VsCode but you can use any IDE according to your preference.

Now, from here we will be writing the commands in our hello.sh file.

before starting with your shell script file always specify the path of your shell (in my case I am using bash so I am specifying bash path followed by #!)

Printing Hello-World using hello.sh

```
#! /bin/bash
```
declaring comments using #
```
# it is a comment
```
printing hello-world using
```
echo "Hello-World"
```
Now save the changes in hello.sh script
```
#! /bin/bash 
# it is a comment
echo "Hello-World"
```
and run the script file in terminal using
```
./hello.sh
```

System defined variables

Build-in or system defined variables will always be in Capital letters i.e BASH, HOME and variables will always be called after $

echo Our shell is: $BASH #give bash shell name
echo Our shell version is: $BASH_VERSION
echo Our home directory is: $HOME
echo Our current working directory is: $PWD

Declaring global variables

NOTE: there variables can be called globally

name=Mark
echo The name is: $name
value=10
echo The value is: $value

Taking input from user for variables

echo "Enter names: "
read n1 n2 n3 n4
echo "Names: $n1,$n2,$n3,$n4"

Taking silent input and in same line

-p for taking input in same line -s for taking silent input i.e data will not be visible while inserting

read -p "username: " u_name
read -sp "password: " pass 
echo 
echo "username: $u_name"       
echo "password: $pass"

Declaring arrays

-a for array @ for printing all values of array

echo "Enter names: "
read -a name
echo Names are : ${name[@]}

Printing specific values of array

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

Printing index of array

echo "Enter names: "
read -a name
echo Index of array : ${!name[@]}

Printing length of array

echo "Enter names: "
read -a name
echo Length of array is : ${#name[@]}

Adding values to array

os=('ubuntu' 'windows' 'mac')
os[3]='kali'         #--> setting value
echo Array is : ${os[@]}

Removing values from array

os=('ubuntu' 'windows' 'mac')
unset os[2] #--> remove value from 2nd index
os[3]='kali' #--> setting value
echo Array is : ${os[@]}

Reading input by default variable

By default data value will be stored in a default variable REPLY
```
echo "Enter name: "
read 
echo "Name is : $REPLY"
```

Taking input as argument

echo Names are : $1 $2 $3

Alternate method

args=("$@") #arguments as an array
echo Names are : $@

if statement

-ne for not equal comparison fi for telling system that if statement ends here.

count=10
if (( $count == 1 ))
# or if [ $count -eq 10 ]
then
  echo "equal"
elif [ $count -ne 1 ]
then 
  echo "not equal"
else
  echo "Invalid"
fi         #--> to specify end of if statement

Here is the list of specifying operators in shell scripting

File/Folder test operators

Before executing the below code make sure that you have created some files/folders. -e for taking input in same line -f for checking there is file or not -w for checking write permissions cat >> for inserting and appending data.

echo -e "Enter the name of the file : \c"
read file_name
if [ -e $file_name ] # -e for exist or not
# -f for it is file or not
# -d for it is directory or not
# -s for file is empty or not
then 
  echo "$file_name found"
else
  echo "$file_name not found"
fi

In my case I have a file as hi

Inserting/Appending data to file

```
echo -e "Enter the name of the file : \c"
read file_name
if [ -f $file_name ]
then 
  if [ -w $file_name ]
  then
      echo "type some text data. Press ctrl+d to exit."
      cat >> $file_name
  else
      echo "File don't have write permissions."
  fi
else
  echo "$file_name not found"
fi
```
Before moving ahead you have to make sure that you have a file to enter some data and the file mush have write permissions. To check write permissions just type ls -la If your file don't have any write permissions to that file just type chmod +w <file_name> Now executing the bash file Even if you try to add text again the text will not be overwritten rather it will be appended to the file

switch case

echo Hey choose an option
echo
echo a=To see current date
echo b=list all the files in current dir
echo -e "Enter choice : \c"
read choice
case $choice in
  a) date;;
  b) ls;;
  *) echo "Invalid output" 
esac   #--> specifying end of switch case

pattern based case

echo -e "Enter some character : \c"
read value
case $value in
  [a-z] )
      echo "a-z";;
  [A-Z] )
      echo "A-Z";;
  [0-9] )
      echo "0-9";;
  ? )
      echo "specical char";;
  * )
      echo "invalid"
esac

for loop

Method 1

for i in 1 2 3 4 5
do 
  echo the number is $i
done

Method 2

for j in eat your lunch 
do echo $j
done

Method 3

for p in {1..10..2} # {start..end..increment}--> not work in version less than 4
do echo $p
done

Method 4

for (( i-0; i<5; i++ ))
do 
  echo $i
done

for loop to execute command

for command in ls pwd date
do  
  echo -------------$command------------  #--->printing command
  $command    # --->running command
done

printing folders from for loop

for i in *
do
  if [ -d $i ]  #---> -d for folders and -f for files
  then
      echo $i
  fi
done

iterating values from file ; hosts is the path of file including file_name at last i.e where I have written hi

hosts="/home/shankar/Desktop/shell_script/hi"
for i in $(cat $hosts)
do echo $i
done

while loop

count=0
num=10
while [ $count -le $num ]  # ---> -le for less than equal to
do 
  echo Numbers are $count
  let count++
done

select loop

select name in mark john ben julie
do 
  case $name in 
  mark ) echo "mark selected" ;;
  john ) echo "john selected" ;;
  ben ) echo "ben selected" ;;
  julie ) echo "julie selected" ;;
  * ) echo "invalid" ;;
  esac
done

until loop

n=0
until [ $n -ge 10 ]
do 
  echo $n
  n=$(( n+1 )) ## or let n++
done

Logical Operators

AND operator -gt for greater than -lt for less than && or -a for AND logical operator

a=6
if [ $a -gt 5 ] && [ $a -lt 7 ]
then
  echo "true"
else
  echo "false"
fi

OR operator || or -o for OR logical operator

a=1
if [ $a -gt 5 ] || [ $a -lt 7 ]
then
  echo "true"
else
  echo "false"
fi

Arithmetic operators

a=20
b=10
echo $(( a+b ))
echo $(expr $a - $b )     #-->alternate way
echo $(( a/b ))
echo $(( a*b ))
echo $(( a%b ))

floating point operations bc for basic calculation in float scale is the number of values after decimal point -l for calling math library

n=2.5
m=4.4
echo "$n+$m" | bc
echo "$n-$m" | bc
echo "$n*$m" | bc
echo "scale=2;$m/$n" | bc
echo "$m%$n" | bc
num=4
echo "scale=2;sqrt($num)" | bc -l   #--> -l for calling math library
echo "3^3" | bc  -l

Reading a file's content

Put file name in place of hi Method 1

while read p
do echo $p
done < hi

Method 2

cat hi | while read p
do echo $p
done

Method 3 -r for mentioning end of line

while IFS=' ' read -r line 
do
  echo $line
done < hi

Functions

Method 1

function greet(){
  echo "Hello-World"
}
greet  # --> calling function

Method 2

name(){
  echo $1
}
name Ram  # --> calling function

Declaring local variables

while declaring function the variable we use are global i.e can be used outside the function but sometime we don't want such behavior so we define local variables followed by local keyword.
```
id(){
      local name=$1
      echo $name
}
name=Ram
echo names is $name before
id Ben
echo names is $name after
```

Declaring readonly variables

after making variables readonly you would not be able to change its value
```
var=31
readonly var
var=50 
echo var is : $var
```

Declaring readonly functions

after making function readonly we will not be able to overwrite this function

hello(){
      echo Hello guys
}
readonly -f hello  #--> -f for function
hello

Signal and Trap

trap "echo file deleted" 0 2 15
echo "pid is $$"   
while (( COUNT < 10 ))
do
  sleep 1
  (( COUNT ++ ))
  echo $COUNT
done
exit 0

Debugging

Method 1

bash -x ./hello.sh

Method 2

set -x    #--> from
echo hi  #--> script
set +x   #--> to

That's it You are now done with shell scripting. Hope the above information was valuable to you.

Shankar's blog

Shankar's blog

[Bash] Shell Scripting in Linux Hands-On

What is shell scripting?

Type of Shells:

To check path of your Shell

Let's make our first shell script

Printing Hello-World using hello.sh

System defined variables

Declaring global variables

Taking input from user for variables

Taking silent input and in same line

Declaring arrays

Reading input by default variable

Taking input as argument

if statement

File/Folder test operators

Inserting/Appending data to file

switch case

for loop

while loop

select loop

until loop

Logical Operators

Arithmetic operators

Reading a file's content

Functions

Declaring local variables

Declaring readonly variables

Declaring readonly functions

Signal and Trap

Debugging