# shell-script

#!/bin/bash
# Author: Ankush Singal
# Date Created: 31/12/2020
# Last Modified: 31/13/2020

# Description:
# Create a backup in ~/bash_course folder of all files in the home directory

# Usage:
#backup_script
```echo "Hello, ${USER^}"```
``` echo "I will now backup your hime directory, $HOME" ```
```currentdir=$(pwd)```
```echo "You are running this script from $currentdir"```
```echo "Therefore, I will save the backup in $currentdir"```
- ```tar -cf $currentdir/my_backup_"$(date +%d-%m-%Y_%H-%M-%S)".tar $HOME/* 2>/dev/null```
```echo "Backup Completed Successfully"```
- exit 0

-- Permission Calculator

Adding scripts to PATH

-- echo "$PATH"

Add the path as below

--- vi ~/.profile

export PATH="$PATH:$HOME/bash_course/scripts"

-- The PATH variable tells the shell where to search for executable files -- We can make our scripts accessible system-wide by putting them into a folder covered by our PATH

Parameter Expansion

numbers=0123456789 ${parameters:offset:length}^C echo ${numbers:0:7}

--- time=$(date +%H:%m:%S)

--- echo "Hello $USER, the time right now is $time"

Arithmetic Expression

x=4 y=2 echo $(( 4 + 2 ))

echo $(( $x + $y ))

Tilde Expression

echo ~ echo $PWD

Brace Expression

echo month{1..12} month1 month2 month3 month4 month5 month6 month7 month

#BAsh processes command lines - Tokenisation, Command Identification, Shell expansions, Quote removal, redirections

if [[ 2 -gt 1]]; then 
  echo "hello world"
fi

A simple command is just a list of words separated by spaces and terminated by either a new line or one of the other control operators available in bash

Word splitting

Word splitting can have some very significant effects on how your command lines are interpreted

Globbing: It is also known as filename expansion, is used to generate an alphabetically-sorted list of file names that match a certain pattern exactly. Any word on the command line that contains an unquoted *,? or [ will be interpreted as a pattern

ls * ls file[ab].txt

ls file[abc][abc][abc].txt

ls file[a-g].txt

rm ~/Downloads/*.jpg

Types of Quotes

1. Backslashes: \
2. Single Quotes: ``
3. Double Quotes " "

EXAMPLES

echo $name > "~/$out" echo $name > "$HOME/$out"

Advanced

echo "$(ls *.txt)"

Requesting User input

#!/bin/bash
echo "My name is $1"
echo "My home directory is $2"
echo "My favourite color is $3"
echo "The 10th argument is ${10}

• Command line arguments are information you give to your script from your command line. Each argument is separated by a spac
• Positional parameters are parameters set by the shell to store the value of each of these command line arguments

#!/bin/bash

echo $(( {2:-0} $1 ${3:-0} $1 ${4:-0} $1 ${5:-0} $1 ${6:-0} $1 ${7:-0} $1 ${8:-0} $1 ${9:-0} $1 ${10:-0} ))

-- replace with

echo $(( $@ ))

chmod 744 calculator.sh

./calculator.sh + 5 5 5 5 ${parameter:-word}

Special Parameters

#!/bin/bash echo "My name is $1" echo "My home directory is $2" echo "My favourite color is $3" echo "The 10th argument is ${10} echo $#

$@: $1 $2 $3 ... $N..... Where N is the number of command line arguments provided

$*: $1 $2 $3... $N.. Where N is the number of command line arguments provided... All positional parameters as separate words.... Its exactly the same as $@

• That $@ and $* allow you to access all the positional parameters that are provided to your bash script
• The $@ expands to all the positional parameters provided to the script, but provides them as unquoted separate words , that are subject to word splitting
• "$@" stops subsequent word splitting and ensures the value of the positional parameters is exactly how they were provided by the user

READ COMMAND

echo $REPLY

read.... hello

Now when i type: echo $REPLY

it gives hellol