Loops

`for` Loop

to iterate over something..
can iterate over arrays or iterator sequences..
2 ways to use..

fruits=(apple bananna guva)

for i in ${fruits[@]}
do
        echo $i
done

printf "\n";

for ((i=0; i<${#fruits[@]}; i++))
do
    echo ${fruits[$i]}
done

Real world example..

# Accepts any no.of Args and prints sum, avg, total args..
SUM=0

for i in "$@"; do
(( SUM += i))
done

AVG=$((SUM / $#))

echo "SUM = ${SUM}"
echo "AVG = ${AVG}"
echo "ARGS = $#"

`while` loop

The condition is evaluated before executing the commands. If it is true, the commands execute; if false, the loop terminates immediately.

Basic Syntax:

while <condition> do <commands> done

Use Cases…

Basic Counter Loop: This example iterates as long as the variable i is less than or equal to two.

i=0 while [ $i -le 2 ] do echo Number: $i ((i++)) done

#!/bin/bash

# Infinite loop to simulate a console [cite: 973, 981]
while true
do
    # Use -p for a prompt and read the whole line [cite: 503]
    read -p "console> " input

    # Use 'set' to break the input into positional parameters
    set -- $input
    cmd=$1      # The first word is the command
    shift       # Shift left so $@ now contains only the arguments

    case "$cmd" in
        ls)
            # If no args are provided, it runs 'ls'. If args exist, they are passed.
            ls "$@"
            ;;
        pwd)
            pwd
            ;;
        hi)
            # Use the USER environment variable
            echo "Hello $USER"
            ;;
        exit)
            echo "Exiting..."
            exit 0 # Ends the script [cite: 42]
            ;;
        *)
            # Provide feedback for invalid options
            echo "Enter Valid Options: ls, pwd, hi, exit"
            ;;
    esac
done

Infinite Loop: An infinite loop repeats indefinitely. This can be achieved using the built-in : command or true, both of which always return true.

while : do echo "Press <CTRL+C> to exit." sleep 1 done
Reading a File Line-by-Line: One of the most common usages is reading a file using input redirection (<) and the read command.

file=/etc/passwd while IFS= read -r line; do echo $line done < "$file"
Automated Webcam Archiver: This practical script runs in the background to organize webcam images into date and hour-specific directories.
- Logic: It uses an outer while true loop for continuous execution and an inner while loop that runs until the hour reset (00) to manage hourly subdirectories.

`Until` Loop

reverse of while loop
i.e until loop executes until the condition is false..
means.. when condition becomes TRUE loop terminates..

Example Task…

Goals:

learn to use until loop

Tasks:

create a script that does the following:
reads a filename from user input
combines specifyed file with itself until it reaches a size greater than 1024 KB
create a file using head -c 4KB /dev/urandom > file.txt command
execute your script passing the file you’ve just created

Self-check:

script returns the following output:

Filesize: 8 Filesize: 16 Filesize: 32 Filesize: 64 Filesize: 128 Filesize: 256 Filesize: 504 Filesize: 1004 Filesize: 2004

#!/bin/bash

FILE=$1

# 1. Verification: Check if file exists [cite: 34]
if [[ ! -f "$FILE" ]]; then
    echo "Error: File $FILE not found."
    exit 1
fi

# 2. Initialize current size (using du -k for Kilobytes)
# du -k provides the size in KB [cite: 395]
CURRENT_SIZE=$(du -k "$FILE" | cut -f1)

# 3. Until loop: Executes as long as size is NOT greater than 1024 [cite: 346]
until [ "$CURRENT_SIZE" -gt 1024 ]
do
    # Append content of file to itself using a buffer to avoid input/output errors
    # we can't say -> "cat FILE >> FILE" -> this will raise some error, so use some temp file as buffer
    cat "$FILE" >> "$FILE.tmp"
    cat "$FILE.tmp" >> "$FILE"
    rm "$FILE.tmp"

    # Update size and print output [cite: 359]
    CURRENT_SIZE=$(du -k "$FILE" | cut -f1)
    echo "Filesize: $CURRENT_SIZE"
done

Real-world scenario..

#!/bin/bash

# this means, until loop first executes that command and it fails..
# so FALSE means it executes the body..
# until that command succeeds it will execute the same loop body

until git clone https://github.com/pavan-epam/task.git &> /dev/null;
do
    echo "Waiting for Git.. Retrying Again after 1s..";
    sleep 1
done

echo "🎉 Repo Cloned!!"

Exercise..

Goals:

learn to use positional arguments

Tasks:

create a script that does the following:
accepts any number of arguments
prints all arguments in the following format “Arg1: <arg1 value>”, “Arg2: <arg2 value>”
adds the value of the next argument to the previous one and prints it out (for the last argument add the value of the first one)
run the script with 7 1 5 7 4 3 6 arguments

Self-check:

script run returns the following results:

Arg1: 7

Arg2: 1

Arg3: 5

Arg4: 7

Arg5: 4

Arg6: 3

Arg7: 6

8 6 12 11 7 9 13

#!/bin/bash

for((i = 1; i <= $#; i++));
do
    echo Arg$i: ${!i}
done

for ((i = 1; i <= $#; i++ ));
do
    if (( i == $# ));
    then
        echo $(( ${!i} + $1 ))
        break;
    fi

    IND=$((i+1)) #make next index
    echo -n "$(( ${!i} + ${!IND} )) "
done

Loops

for Loop

while loop

Use Cases…

Until Loop

`for` Loop

`while` loop

`Until` Loop