Shell Scripting — Bash Fundamentals

#!/usr/bin/env bash
# ↑ shebang: tells OS which interpreter to use (/usr/bin/env finds bash in PATH)

set -e          # exit immediately on any error (non-zero exit code)
set -u          # treat unset variables as errors (instead of empty string)
set -o pipefail # pipe fails if ANY command in the pipe fails (not just last)
# Shorthand: set -euo pipefail

# Debugging mode (print every command before executing)
set -x          # enable debug tracing
set +x          # disable debug tracing

# Script info
SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
SCRIPT_NAME="$(basename "$0")"

# Trap for cleanup on exit
cleanup() {
  echo "Cleaning up..."
  rm -f /tmp/my-lock-file
}
trap cleanup EXIT    # runs cleanup() on any exit (normal or error)
trap 'echo "Error at line $LINENO"' ERR  # print line number on error

echo "Script started from: $SCRIPT_DIR"

#!/usr/bin/env bash
set -euo pipefail

# Variables (no spaces around =)
NAME="Akshay"
AGE=30
GREETING="Hello, $NAME! You are $AGE."
echo "$GREETING"

# Command substitution
CURRENT_DATE=$(date +%Y-%m-%d)
FILE_COUNT=$(ls -1 /var/log/*.log | wc -l)

# Conditionals — prefer [[ ]] over [ ] (supports && || without escaping)
if [[ "$NAME" == "Akshay" ]]; then
  echo "Hello, Akshay"
elif [[ $AGE -gt 18 ]]; then   # -gt (>), -lt (<), -eq (==), -ne (!=) for numbers
  echo "Adult"
else
  echo "Other"
fi

# File tests
if [[ -f "/etc/nginx/nginx.conf" ]]; then   # -f: file exists
  echo "nginx.conf found"
fi
# -d: directory  -e: exists  -r: readable  -w: writable  -x: executable
# -z: string empty  -n: string non-empty

# For loops
for i in 1 2 3 4 5; do
  echo "Item: $i"
done

for file in /var/log/*.log; do
  echo "Processing: $file"
done

for i in $(seq 1 10); do    # seq generates a range
  echo "$i"
done

# While loop
COUNT=0
while [[ $COUNT -lt 5 ]]; do
  echo "Count: $COUNT"
  ((COUNT++))
done

#!/usr/bin/env bash
set -euo pipefail

# Functions
log_info()  { echo "[INFO]  $(date +%H:%M:%S) $*"; }
log_error() { echo "[ERROR] $(date +%H:%M:%S) $*" >&2; }  # >&2 = stderr

check_requirements() {
  local -a required=("docker" "git" "curl")
  for cmd in "${required[@]}"; do
    if ! command -v "$cmd" &>/dev/null; then
      log_error "Required command not found: $cmd"
      exit 1
    fi
  done
  log_info "All requirements met"
}

# Script arguments
# $0 = script name, $1 $2 ... = positional args
# $# = number of args, $@ = all args, $* = all args as one string
usage() {
  echo "Usage: $0 <environment> <image-tag>"
  echo "  environment: staging | production"
  echo "  image-tag:   docker image tag to deploy"
  exit 1
}

[[ $# -lt 2 ]] && usage    # exit with usage if fewer than 2 args

ENV="$1"
TAG="$2"

deploy() {
  local env="$1"
  local tag="$2"
  log_info "Deploying $tag to $env..."
  docker pull "myapp:$tag"
  docker stop "myapp-$env" 2>/dev/null || true   # ignore if not running
  docker run -d --name "myapp-$env" --restart=unless-stopped "myapp:$tag"
  log_info "Deploy complete"
}

check_requirements
deploy "$ENV" "$TAG"