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

#!/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

• $(())

  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

  let x=1+10  
  echo $x  # output is 11

• expr

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

---

---

input

to take input from user we use

# 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

  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

  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

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

execution

to execute command on system $()

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

echo $(whoami)
#or
echo $WHOAMI

---

---

condition

• if condition

  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
• Test condition

  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

  seqq="hello world"
  for i in $seqq
  do
  echo -n $i "   "
  done
  
  #output -> hello    world  

• While loop

  • while [condition]

  • do

  • expiration

  • done

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

To Debug your code we use :

set -x
code here
set +x

# show all operation step by step

---

---

String

compare two strings

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

To count chars in word

x="hello world"

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

Split → IFS

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}

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

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

---

---

Function

function_name(){commands;}

or

function_name(){

commands

}

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

Notes

exit 0 → to exit from code