# Bash

From Ahmed Farag

To write bash code start with `#!/bin/bash` → because system show this file as ASCII text

`echo` or `printf` we use it for printing

**BASH** → use interpreter bash (print correct line then print the error is is found)

***

***

### implementation

to implement variable

```bash
#!/bin/bash
# no spaces before and after equal -> before space execute as a command 

name="irix"
echo $name  # to print the value for variable
printf $name
```

the difference between `echo`,`printf` :

* **printf** → print text in the same line
* **echo** → print text in new line (to print text in the same line we use option -n )

To save the value for execution command we use ``-> `var=`ls``

### mathematical operation

* **$(())**

  ```bash
  num1=5;num2=4
  echo $(($num1+$num2)) # sum num1,num2 the print the value for summation
  ```

  with all operation except postfix and prefix ((x++))
* **let**

  ```bash
  let x=1+10  
  echo $x  # output is 11
  ```
* **expr**

  ```bash
  # take care with space in expr
  x= expr 1 + 10   # same spaces
  echo $x
  ```

***

***

### input

to take input from user we use

```bash
# first way
echo "enter your name"
read name      # read use to read input from user and stored it in variable

# second way
num1=$1  # input in var1
num2=$2  # input in var2
# ./calc.sh 10 5  
```

**read** → we have more option for it like :

* -s —> make input non visible (like password in linux) `read -s $name`
* -p —> print text (like input in python) `read -p "what is your name: " $name`
* array

  we use `set` to implement array

  ```bash
  set 10 20 30 50 90
  echo $(($1+$5))  # the value is 100
  set what is your name
  echo $1  # the output is what
  ```

  implement array , we use `${}` to print array

  ```bash
  os=('linux' 'mac' 'windows' 'android')  # WE USE () TO implemant array
  echo ${os[0]}  # output linux

  # to print all element in the array we use *
  echo ${os[*]}

  #remove element from array we use
  unset os[1]

  #insert value 
  os[2]='hello'
  ```
* connection

  no `concatenation` in bash

  ```bash
  set 10 20 30 50 90
  echo $1 $2 $3   # output is 10 20 30
  ```

### execution

to execute command on system `$()`

```bash
var=$(whoami)
# or
var=`whoami`
echo $var 

echo $(whoami)
#or
echo $WHOAMI
```

***

***

### condition

* **if** condition

  ```bash
  if [ $num1 == $num2 ] # must space between []
  then
  	echo "yes"
  elif [ $num1>$num2 ]
  then
  	echo "keep"
  else
  	echo "no"
  fi  # to end condition
  ```

  The **-f** checked if the file existed.

  The **-w** checked if the file was writable, without write permissions we wouldn't be able to output our text into the file.

  ![image.png](https://prod-files-secure.s3.us-west-2.amazonaws.com/90e49eb5-9520-43b0-a51f-ec506194b09d/acdbb767-559d-4c3a-998e-535e9439a952/image.png)
* **Test** condition

  ```bash
  test 001==1 && echo "True" || echo "False"   # output is false (because it compare two string not two integer) To compare integer use -eq,-ge,etc..
  ```

  just check on two variable like `?` in cpp

***

***

### loop

`echo {1..6}` → regular expiration (out put from 1 to 6)

* **For** loop

  * for $variable\_name in 1 2 3 4
  * do
  * expiration
  * done

  ```bash
  seqq="hello world"
  for i in $seqq
  do
  echo -n $i "   "
  done

  #output -> hello    world  
  ```
* **While** loop

  * while \[condition]
  * do
  * expiration
  * done

  ```bash
  $i=0
  while [true]
  do
  echo $i
  i=$((i+1))
  done
  ```

To Debug your code we use :

```bash
set -x
code here
set +x

# show all operation step by step
```

***

***

### String

compare two strings

```bash
if [ $str1=$str2 ]
then
echo "print"
elif [ $str1\\<$str2 ]  # greeter than \\< , less than >\\
```

To count chars in word

```bash
x="hello world"

length=`echo $x | wc -c`  # -c is option refer to chars
echo $length
```

**Split → IFS**

```bash
x="what is the name of the index file"

IFS=" "  # setting space as a delimiter 
read -ra arr <<< "$x"  # reading str as an array as token sepeated by IFS

echo ${arr[0]}  # output -> what
```

Substring `${var:start:end}`

```bash
x="what is the name of the index file"

substr=${x:0:10}
echo $substr
```

***

***

### Function

`function_name(){commands;}`

**or**

function\_name(){

commands

}

```bash
name(){
	echo hello world
}
name # call function
```

### Notes

exit 0 → to exit from code