Merge pull request Tikam02#171 from rahulgore7/master

Bash Scripting Cheatsheet Added

Terminal/bash scripting.md

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+## Introduction
+
+No matter if you are a DevOps/SysOps engineer, developer, or just a Linux enthusiast, you can use Bash scripts to combine different Linux commands and automate boring and repetitive daily tasks, so that you can focus on more productive and fun things.
+
+Here you can find the completed Bash Scripting cheat sheet 👇
+
+## Bash Script Header (Shebang)
+
+Option 1:
+
+```
+#!/bin/bash
+
+```
+
+Option 2:
+
+```
+#!/usr/bin/env bash
+
+```
+
+## Variables
+
+```
+#!/bin/bash
+name="DevDojo"
+echo "Hi there $name"
+
+```
+
+## User Input
+
+```
+#!/bin/bash
+
+echo "What is your name?"
+read name
+
+echo "Hi there $name"
+echo "Welcome to DevDojo!"
+
+```
+
+## Comments
+
+To do that in bash you need to add the # symbol at the beginning of the line. Comments will never be rendered on the screen.
+
+```
+# This is a comment and will not be rendered on the screen
+
+```
+
+## Arguments
+
+```
+#!/bin/bash
+
+echo "Argument one is $1"
+echo "Argument two is $2"
+echo "Argument three is $3"
+
+```
+
+Then run the file and pass 3 arguments:
+
+```
+bash ./arguments.sh dog cat bird
+
+```
+
+## Arrays
+
+```
+my_array=("value 1" "value 2" "value 3" "value 4")
+
+# Access a single element, this would output: value 2
+echo ${my_array[1]}
+
+# This would return the last element: value 4
+echo ${my_array[-1]}
+
+# This would output the total number of elements in the array, in our case, it is 4:
+echo "${#my_array[@]}"
+
+# This would output all of the elements of the array:
+printf '%s\n' "${my_array[@]}"
+
+```
+
+## Conditional Expressions
+
+### File expressions
+
+```
+## True if file exists.
+[[ -a ${file} ]]
+
+## True if file exists and is a block special file.
+[[ -b ${file} ]]
+
+## True if file exists and is a character special file.
+[[ -c ${file} ]]
+
+## True if file exists and is a directory.
+[[ -d ${file} ]]
+
+## True if file exists.
+[[ -e ${file} ]]
+
+## True if file exists and is a regular file.
+[[ -f ${file} ]]
+
+## True if file exists and is a symbolic link.
+[[ -h ${file} ]]
+
+## True if file exists and is readable.
+[[ -r ${file} ]]
+
+## True if file exists and has a size greater than zero.
+[[ -s ${file} ]]
+
+## True if file exists and is writable.
+[[ -w ${file} ]]
+
+## True if file exists and is executable.
+[[ -x ${file} ]]
+
+## True if file exists and is a symbolic link.
+[[ -L ${file} ]]
+
+```
+
+### String expressions
+
+```
+# True if the shell variable varname is set (has been assigned a value).
+[[ -v ${varname} ]]
+
+# True if the length of the string is zero.
+[[ -z ${string} ]]
+
+# True if the length of the string is non-zero.
+[[ -n ${string} ]]
+
+# True if the strings are equal. = should be used with the test command for POSIX conformance. When used with the [[ command, this performs pattern matching as described above (Compound Commands)
+[[ ${string1} == ${string2} ]]
+
+# True if the strings are not equal.
+[[ ${string1} != ${string2} ]]
+
+# True if string1 sorts before string2 lexicographically.
+[[ ${string1} < ${string2} ]]
+
+# True if string1 sorts after string2 lexicographically.
+[[ ${string1} > ${string2} ]]
+
+```
+
+### Arithmetic operators
+
+```
+# Returns true if the numbers are equal
+[[ ${arg1} -eq ${arg2} ]]
+
+# Returns true if the numbers are not equal
+[[ ${arg1} -ne ${arg2} ]]
+
+# Returns true if arg1 is less than arg2
+[[ ${arg1} -lt ${arg2} ]]
+
+# Returns true if arg1 is less than or equal arg2
+[[ ${arg1} -le ${arg2} ]]
+
+# Returns true if arg1 is greater than arg2
+[[ ${arg1} -gt ${arg2} ]]
+
+# Returns true if arg1 is greater than or equal arg2
+[[ ${arg1} -ge ${arg2} ]]
+
+# As with other programming languages you can use AND & OR conditions:
+[[ test_case_1 ]] && [[ test_case_2 ]] # And
+[[ test_case_1 ]] || [[ test_case_2 ]] # Or
+
+```
+
+## Conditionals
+
+```
+#!/bin/bash
+
+# Bash if statement example
+
+read -p "What is your name? " name
+
+if [[ -z ${name} ]]
+then
+    echo "Please enter your name!"
+else
+    echo "Hi there ${name}"
+fi
+
+```
+
+## Loops
+
+### For loops
+
+```
+#!/bin/bash
+
+users="devdojo, bobby, tony"
+
+for user in ${users}
+do
+    echo "${user}"
+done
+
+```
+
+### While loops
+
+```
+#!/bin/bash
+
+counter=1
+while [[ $counter -le 10 ]]
+do
+    echo $counter
+    ((counter++))
+done
+
+```
+
+### Until Loops
+
+```
+#!/bin/bash
+
+count=1
+until [ $count -gt 10 ]
+do
+    echo $count
+    ((count++))
+done
+
+```
+
+## Functions
+
+```
+function function_name() {
+    your_commands
+}
+
+```
+
+Example
+
+```
+#!/bin/bash
+
+function hello(){
+    echo "Hello World Function!"
+}
+
+hello
+```