# BASH -Conditions and Loops ### Working with Arrays The Array is a type of variable which is the collection of multiple items of the same type. The counter in the array starts with zero –0. Declared as `depts= (“cse” “cnd” “ece”)` as `depts` being the name of the array. To fetch the output - `echo $depts`. Below are the variations on working with arrays. * `echo $depts` – will return 0th value or first value in the array i.e. *cse* * `echo ${depts[]}`*–*returns error as bad substitution * `echo ${depts[2]}` – returns third value i.e. * `echo ${depts[3]}` – returns nothing as there is no value for `dept[3]` * `depts[3] = "research"` – to insert the fourth element in the `dept` array * `echo ${#depts[*]}` – returns the count of elements in the array i.e. returns * `echo ${depts[*]:2:1}` – skips the first two elements and only prints the next one value i.e. returns * `echo ${depts[]/research/phd}` – replaces the “research” element in the array with `“phd”` on runtime. However, `echo ${depts[]}`still shows research as the change is only for runtime. * `depts2 = echo ${depts[*]/research/phd}`- to store the changed `depts`array into `depts2` array. `echo ${depts2[*]}` returns values with phd in the fourth position. * `echo sort <<<"${depts[*]}"` - triple redirection to sort a text in array argument but not in a file argument. ### If and Else Conditions If and Else helps us handle and judge the conditions on data, we manage. Below are a few examples of Syntax. Highlighted are the required syntax constructs of if & else. ```bash #!/bin/bash if test $USER := "root" #to open if. Used $USER env variable to check user then { clear echo "Your are not root" } else # to open else { clear echo "Your are root" } fi#close the if ``` Running If and Else on command like as below. As soon as you close if using fi keywords, the if and else logic will get executed directly. ```bash root@ubuntu1:/scripts# numdays=400 root@ubuntu1:/scripts# echo $numdays 400 root@ubuntu1:/scripts# iftest "$numdays" -gt 100 >then >{ >echo "Over Budget!" >} >fi Over budget! root@ubuntu1:/scripts# ``` ### Looping Structure - for For loop is used to repeat a task in a specific number of times we would like to process something. Below `for.sh` is an example to demo a FOR loop with conduction using IN keyword to list of .sh files in each sub directory based on the total size of the file. The below script will print the size of all the .sh files in a list and then gives us the sum of the total size. Highlighted are the required syntax constructs for the For Loop. ```bash #!/bin/bash clear #declaring variables so that can be used totalsize=0 currentfilesize=0 #currentfile is a temporary variable used for counter for currentfile in/scripts/*.sh do currentsize=`ls -l $currentfile | tr -s " " | cut -f5 -d " "` #Gets file size and cuts it with space delimiter #tr command to squeeze the space on the file name spaces let totalsize=$totalsize+$currentsize #calculate the totalsizeecho $currentsize done echo echo "Total space used by Script files is:" $totalsize ``` Below is an example for `FOR` loop using an array.The Array is a variable contains multiple items of the same type. For example, `cities=("Chennai", "Hyderabad")` is an array with two items. `echo $cities` will print the first value in the cities array i.e. Chennai. Highlighted are the required syntax constructs for theFor Loop.@ value in the array to call each element based on the increment value of i. ```bash echo ${cities[0]} #return Chennai echo $(cities[1]} #will return "Hyderabad" root@Ubuntu:/scripts# for((i=0;i<${#cities[@]};i++));do >echo ${cities[$i]} >done ChennaiHyderabad ``` ### More examples - For loop ```bash #!/bin/bash echo "Starting FOR loop" for val in 0 1 2 3 4 5 do echo "Printing Value:" $val done echo " Finished FOR loop" ``` ```bash #!/bin/bash echo "Starting FOR loop" for files in $PWD/*.sh do echo "Printing File names:" $files done echo "Finished FOR loop" ``` ```bash #!/bin/bash echo "Printing FOR loop" for x in {1..5} do echo "Printing $x times" done echo "Finished FOR loop" ``` ```bash #!/bin/bash echo "Starting FOR loop" for j in {0..10..3} do echo "Printing $j value" done echo "Finished FOR loop" ``` ```bash #!/bin/bash echo "Starting FOR loop" for a in $(seq 1 4 20) do echo "Printing FOR value $a times" done echo "Finished FOR loop" ``` ```bash #!/bin/bash echo "Starting FOR loop" for ((i=1;i<10;i++)) do echo "Printing $i entry in the loop" done echo "Finished FOR loop" ``` ### Looping Structure - While Unlike a `FOR` loop, when we use `WHILE` loop –we don’t necessarily know how many times we are going through the loop. `While.sh` is an example script that prompts the user to select an option, so while the user doesn’t choose to exit and live in the loop. Highlighted are the required syntax constructs of `WHILE` Loop. ```bash #!/bin/bash choice="0" while (( $choice != "1")) do clear echo echo "Please select and Option" echo echo "1 –Exit" echo read choice done ``` The script waits for the input choice from the user. Until the input is given as1 the loop is not broken. The loop will execute initially as the choice value is zero by default and waits for the input, which will be in the loop unless the user gives the value as 1. ### Break and Continue inside Loop Depending on the code, we might wish to break out of the loop or continue the loop so that we go back to loop statements and run them again. Below is an example for a break and continue. Unless we type exit, we will not be able to break the loop, else it still continues to run the same while again and again. ```bash #!/bin/bash while true do clear echo "To leave, Type exit" echo read -p "What did you say?" choice if test $choice = "exit" then { break } else { continue } done ``` ```bash #!/bin/bash while true do echo "Welcome NEO - The Matrix has you" echo "Choose wisely - Red Pill or Blue Pill?" echo read -p "What is your choice? - Type Red or Blue: " pill if test $pill = "Blue" then { break } else { echo "Welcome to Matrix. Thank you for choosing us" continue } fi ``` ### Case Statements Case statements help us to test the value of something for multiple possible items, instead of using a bunch of If-else, we can use Case statements. Below is an example `case.sh` for your reference. Highlighted are the constructs used in the case statements. ```bash #!/bin/bash clear echo "City Type" echo read -p "Enter your City:" city case $city in "Hyderabad") city_type ="Tier 2";; # Pipe Symbol is a OR command "Bengaluru" |"Mumbai") city_type="Tier 1";; "New Delhi") city_type= "Capital";; # here star is to consider anything else *)clear; echo "Invalid City $city";exit;; esac #case in reverse to close the case statement clear echo "City Typefor $city has been set to $city_type" echo ``` --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://sai11101989.gitbook.io/devops/terminal-programming/bash-conditions-and-loops.md?ask= ``` The question should be specific, self-contained, and written in natural language. 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