Shell Scripting Fundamentals

What are the essential components of a shell script? Explain variables, conditionals, and loops.

Linuxbeginner

// interview question

What are the essential components of a shell script? Explain variables, conditionals, and loops.

Sample answer

Shell scripts automate command sequences. Essential components: 1) Shebang (#!/bin/bash) - specifies interpreter. 2) Variables - store values, accessed with $. 3) Conditionals (if/elif/else) - control flow based on conditions. 4) Loops (for, while) - iterate over items or until condition. 5) Functions - reusable code blocks. 6) Exit codes - indicate success (0) or failure (non-zero). Always quote variables and use 'set -e' for error handling.

Why this matters

Shell scripting is the glue of DevOps - automating repetitive tasks, deployment scripts, cron jobs, and system administration. Bash is most common on Linux, but principles apply to other shells. Well-written scripts include error handling, logging, and are idempotent when possible.

Code examples

Basic script structure

bash

#!/bin/bash
set -e  # Exit on error

# Variables
NAME="webapp"
ENV=${1:-"dev"}  # Default to 'dev' if not provided

# Conditional
if [[ "$ENV" == "prod" ]]; then
    echo "Deploying to production..."
    REPLICAS=3
else
    echo "Deploying to $ENV..."
    REPLICAS=1
fi

# Loop
for i in {1..5}; do
    echo "Attempt $i"
done

echo "Deployed $NAME with $REPLICAS replicas"

Functions and error handling

bash

#!/bin/bash
set -euo pipefail  # Strict mode

# Function definition
log() {
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] $1"
}

# Check command exists
check_dependency() {
    if ! command -v "$1" &> /dev/null; then
        log "ERROR: $1 is required but not installed"
        exit 1
    fi
}

# Usage
check_dependency docker
check_dependency kubectl
log "All dependencies satisfied"

Common mistakes to avoid

Not quoting variables, causing word splitting issues
Forgetting to handle error cases (missing 'set -e')
Using 'rm -rf $VAR' without checking if VAR is set

Likely follow-ups

What does 'set -euo pipefail' do and why is it important?
How do you pass arguments to a shell script?
What is the difference between single and double quotes in bash?

Answer out loud first, then check yourself against the model answer.

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