Execution flow in shell scripting relies on three fundamental mechanisms: sequential processing, conditional selection, and iterative loops. Selection logic allows scripts to make decisions based on runtime states, primarily utilizing logical operatros, conditional blocks, and match statements.

Conditional Logic in Bash

Conditional blocks evaluate expressions to determine which code path executes. There are three primary structures for handling conditions.

Single-Branch Condition Executes a block only when the expression evaluates to true.

if [[ condition ]]; then
    # actions when true
fi

Double-Branch Condition Provides an alternative path when the expression is false.

if [[ condition ]]; then
    # actions when true
else
    # actions when false
fi

Multi-Branch Condition Checks multiple expressions sequentially. Execution stops at the first true condition.

if [[ condition1 ]]; then
    # actions for condition 1
elif [[ condition2 ]]; then
    # actions for condition 2
else
    # default actions
fi

Example: File Type Identification The following script validates input arguments and identifies the nature of the specified path.

#!/bin/bash

target_path="${1}"

if [[ -z "${target_path}" ]]; then
    echo "Error: No path provided."
    exit 1
fi

if [[ ! -e "${target_path}" ]]; then
    echo "Error: Path does not exist."
    exit 2
fi

if [[ -f "${target_path}" ]]; then
    echo "Type: Regular File"
elif [[ -d "${target_path}" ]]; then
    echo "Type: Directory"
elif [[ -L "${target_path}" ]]; then
    echo "Type: Symbolic Link"
elif [[ -b "${target_path}" ]]; then
    echo "Type: Block Device"
elif [[ -c "${target_path}" ]]; then
    echo "Type: Character Device"
elif [[ -S "${target_path}" ]]; then
    echo "Type: Socket"
else
    echo "Type: Unknown"
fi

Example: User Role Classification This script accepts a username and categorizes the account based on its User ID (UID).

#!/bin/bash

query_user="${1}"

if [[ -z "${query_user}" ]]; then
    echo "Usage: Provide a username."
    exit 1
fi

if ! id "${query_user}" &>/dev/null; then
    echo "Error: User not found."
    exit 2
fi

uid_value=$(id -u "${query_user}")

if [[ ${uid_value} -eq 0 ]]; then
    echo "Role: Superuser"
elif [[ ${uid_value} -ge 1000 ]]; then
    echo "Role: Standard Login User"
else
    echo "Role: System Service Account"
fi

Example: Interactive System Monitor Users can select specific hardware information to display via a text menu.

#!/bin/bash

cat << MENU
1) disk) Display partition table
2) mem) Display memory usage
3) cpu) Display processor info
4) quit) Exit script
MENU

read -p "Select an option: " user_choice

if [[ "${user_choice}" =~ ^disk$ ]]; then
    lsblk
elif [[ "${user_choice}" =~ ^mem$ ]]; then
    free -h
elif [[ "${user_choice}" =~ ^cpu$ ]]; then
    nproc
else
    echo "Exiting."
    exit 0
fi

Iterative Processes in Bash

Loops allow repeated execution of code blocks. Bash supports for, while, and until constructs. The for loop is commonly used for iterating over lists or sequences.

List Iteration The loop continues as long as there are items in the provided list.

for variable in list; do
    # commands
done

Lists can be generated via explicit strings, brace expansion, command output, or glob patterns.

Example: Batch User Provisioning Checks a list of usernames and creates them if they do not already exist.

#!/bin/bash

user_list=("dev01" "dev02" "dev03")

for uname in "${user_list[@]}"; do
    if id "${uname}" &>/dev/null; then
        echo "Skip: ${uname} exists."
    else
        useradd "${uname}" && echo "Created: ${uname}"
    fi
done

Example: Arithmetic Accumulation Calculates the sum of integers within a specific range.

#!/bin/bash

total_sum=0

for num in $(seq 1 100); do
    (( total_sum += num ))
done

echo "Total: ${total_sum}"

Example: Directory Content Analysis Iterates through files in a specific directory to identify their types.

#!/bin/bash

search_dir="/etc"

for entry in "${search_dir}"/*; do
    if [[ -f "${entry}" ]]; then
        echo "File: ${entry}"
    elif [[ -d "${entry}" ]]; then
        echo "Dir: ${entry}"
    elif [[ -L "${entry}" ]]; then
        echo "Link: ${entry}"
    else
        echo "Other: ${entry}"
    fi
done