Shell Scripting

#!/bin/bash
echo "Hello, World!"

chmod +x script.sh
./script.sh

echo "Hello, World!"
printf "Name: %s\n" "Alice"

name="Alice"
echo "Hello, ${name}"

name="World"
echo 'Hello $name'   # prints: Hello $name  (no expansion)
echo "Hello $name"   # prints: Hello World  (variable expanded)

read -p "Enter your name: " name
echo "Hello, $name"

# This is a single-line comment
echo "Running script..."  # inline comment

a=5
b=3
sum=$((a + b))
echo "Sum: $sum"        # prints: Sum: 8

# Alternative with expr
result=$(expr $a \* $b)
echo "Product: $result" # prints: Product: 15

if [ $a -gt $b ]; then
  echo "$a is greater than $b"
fi

age=18
if [ $age -ge 18 ]; then
  echo "Adult"
elif [ $age -ge 13 ]; then
  echo "Teenager"
else
  echo "Child"
fi

# for loop
for i in 1 2 3 4 5; do
  echo "Number: $i"
done

# while loop
count=1
while [ $count -le 5 ]; do
  echo "Count: $count"
  ((count++))
done

# until loop (runs until condition becomes true)
until [ $count -gt 5 ]; do
  echo "Count: $count"
  ((count++))
done

#!/bin/bash
echo "Script name : $0"
echo "First arg   : $1"
echo "Second arg  : $2"
echo "All args    : $@"
echo "Arg count   : $#"

ls /nonexistent
echo "Exit code: $?"  # prints a non-zero value (e.g. 2)

# [[ ]] supports regex matching and logical operators without quoting issues
name="Alice"
if [[ $name == A* ]]; then
  echo "Name starts with A"
fi

greet() {
  local name="$1"
  echo "Hello, $name!"
}

greet "Alice"
greet "Bob"

# Command substitution
today=$(date +%Y-%m-%d)
echo "Today is: $today"

# Backtick style (older, avoid)
files=`ls`

cat <<EOF
Line 1
Line 2
Today is $(date)
EOF

# Declare an array
fruits=("apple" "banana" "cherry")

echo "${fruits[0]}"     # apple
echo "${fruits[@]}"     # all elements
echo "${#fruits[@]}"    # number of elements

# Append an element
fruits+=("date")

# Loop over array
for fruit in "${fruits[@]}"; do
  echo "$fruit"
done

#!/bin/bash
set -e          # exit immediately on error
set -u          # treat unset variables as errors
set -o pipefail # catch errors in pipelines

cp source.txt dest.txt || { echo "Copy failed"; exit 1; }

# Compare output of two commands
diff <(ls dir1) <(ls dir2)

# Read from a process as if it were a file
while IFS= read -r line; do
  echo "Line: $line"
done < <(grep "ERROR" logfile.txt)

source script.sh  # or: . script.sh
./script.sh

# Test conditions
[ -f file.txt ]   && echo "Is a regular file"
[ -d /tmp ]       && echo "Is a directory"
[ -r file.txt ]   && echo "Is readable"
[ -w file.txt ]   && echo "Is writable"
[ -x script.sh ]  && echo "Is executable"
[ -s file.txt ]   && echo "Is non-empty"
[ -e path ]       && echo "Exists"

str="Hello, World!"

echo "${#str}"           # Length: 13
echo "${str:7:5}"        # Substring: World
echo "${str,,}"          # Lowercase: hello, world!
echo "${str^^}"          # Uppercase: HELLO, WORLD!
echo "${str/World/Bash}" # Replace first: Hello, Bash!
echo "${str//l/L}"       # Replace all:   HeLLo, WorLd!
echo "${str#Hello, }"    # Remove prefix: World!
echo "${str%!}"          # Remove suffix: Hello, World

read -p "Enter a fruit: " fruit
case "$fruit" in
  apple)
    echo "You chose apple."
    ;;
  banana | mango)
    echo "You chose a tropical fruit."
    ;;
  *)
    echo "Unknown fruit."
    ;;
esac

IFS=',' read -ra parts <<< "one,two,three"
for part in "${parts[@]}"; do
  echo "$part"
done

ln -s target.txt symlink.txt  # soft (symbolic) link
ln   target.txt hardlink.txt  # hard link

#!/bin/bash
tmpfile=$(mktemp)

cleanup() {
  echo "Cleaning up..."
  rm -f "$tmpfile"
}

trap cleanup EXIT        # runs on any exit
trap 'echo "Interrupted"' INT  # runs on Ctrl+C (SIGINT)

echo "Working..." > "$tmpfile"
# ... rest of script ...

mkfifo mypipe
echo "Hello from producer" > mypipe &   # writer (runs in background)
cat mypipe                               # reader (blocks until writer writes)
rm mypipe

LOCKFILE="/tmp/myscript.lock"

acquire_lock() {
  if ! mkdir "$LOCKFILE" 2>/dev/null; then
    echo "Script is already running. Exiting."
    exit 1
  fi
}

release_lock() {
  rmdir "$LOCKFILE"
}

trap release_lock EXIT
acquire_lock

echo "Critical section running..."

#!/bin/bash
pids=()

for i in 1 2 3 4; do
  sleep "$i" &
  pids+=($!)
done

for pid in "${pids[@]}"; do
  wait "$pid" && echo "PID $pid finished OK" || echo "PID $pid failed"
done

x=10
(
  x=20
  echo "Inside subshell: x=$x"   # 20
)
echo "Outside subshell: x=$x"    # 10 — unchanged

#!/bin/bash
usage() { echo "Usage: $0 [-n name] [-v]" >&2; exit 1; }

verbose=0
name=""

while getopts ":n:v" opt; do
  case $opt in
    n) name="$OPTARG" ;;
    v) verbose=1 ;;
    :) echo "Option -$OPTARG requires an argument." >&2; usage ;;
   \?) echo "Invalid option: -$OPTARG" >&2; usage ;;
  esac
done

shift $((OPTIND - 1))
echo "Name: $name, Verbose: $verbose, Remaining args: $*"

coproc my_proc { cat; }

echo "Hello" >&"${my_proc[1]}"   # write to coprocess stdin
read line <&"${my_proc[0]}"      # read from coprocess stdout
echo "Got: $line"

declare -A capitals

capitals["France"]="Paris"
capitals["Germany"]="Berlin"
capitals["Japan"]="Tokyo"

echo "${capitals["France"]}"       # Paris

# Iterate over key-value pairs
for country in "${!capitals[@]}"; do
  echo "$country -> ${capitals[$country]}"
done

#!/bin/bash
set -x    # print each command and its expanded form before execution
set -v    # print shell input lines as they are read

# Measure execution time of a section
time {
  for i in $(seq 1 1000); do
    :
  done
}

exec /bin/bash    # replaces current shell — the original process is gone

#!/bin/bash
SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
echo "Script is located in: $SCRIPT_DIR"

# Unsafe
tmpfile="/tmp/myapp-$$"

# Safe
tmpfile=$(mktemp /tmp/myapp-XXXXXX)