Robust Bash Options

# set -e (errexit)
# Exit immediately if a command exits with non-zero status
set -e
false          # Script exits here
echo "never"  # Never executed

# set -u (nounset)
# Treat unset variables as errors
set -u
echo "$UNDEFINED_VAR"  # Error: variable not set

# set -o pipefail
# Return exit code of last failed command in pipeline
set -o pipefail
false | true   # Exit code is 1, not 0

# Combined (use in every script)
set -euo pipefail

# Disable for specific commands
command_that_may_fail || true
set +e
risky_command
set -e

# Default IFS: space, tab, newline
echo "hello   world" | read -r first second
echo "$first"  # hello
echo "$second" # world

# IFS splitting behavior
data="one:two:three"
IFS=: read -r a b c <<< "$data"
echo "$a"  # one
echo "$b"  # two
echo "$c"  # three

# Save and restore IFS
OLD_IFS=$IFS
IFS=','
for item in "apple,banana,cherry"; do
    echo "$item"
done
IFS=$OLD_IFS

# Use IFS= to preserve whitespace
while IFS= read -r line; do
    echo "Line: $line"
done < file.txt

# Split string into array
IFS=' ' read -ra items <<< "one two three"
echo "${items[0]}"  # one

# Always quote variables
name="hello world"
echo $name          # WRONG: splits into two arguments
echo "$name"        # CORRECT: one argument

# Quote command substitution
files=$(ls)
echo "$files"       # Preserves whitespace

# Quote array expansions
arr=("one two" "three four")
echo "${arr[0]}"   # "one two"
echo ${arr[0]}     # one two (wrong)

# Quote in conditionals
if [[ "$var" == "value" ]]; then
    echo "matched"
fi

# Avoid quoting in [[ ]] for pattern matching
if [[ $var == hello* ]]; then
    echo "starts with hello"
fi

# Quote in for loops (preserves whitespace)
for file in "/path with spaces"/*; do
    echo "$file"
done

# Install shellcheck
sudo apt install shellcheck
sudo dnf install ShellCheck

# Check a script
shellcheck myscript.sh

# Check with severity levels
shellcheck -S warning myscript.sh    # warnings and errors
shellcheck -S info myscript.sh       # info, warnings, and errors

# Fix automatically
shellcheck -f diff myscript.sh | patch -p0

# Check inline (for editors)
# Add shellcheck directives
# shellcheck disable=SC2086
echo $unquoted_var

# Common issues shellcheck catches:
# SC2086: Double quote to prevent globbing and word splitting
# SC2046: Quote to prevent word splitting
# SC2006: Use $(..) instead of legacy backticks
# SC2034: Variable appears unused
# SC2155: Declare and assign separately

# Trap-based cleanup
cleanup() {
    local exit_code=$?
    rm -f "$TEMP_FILE"
    exit $exit_code
}
trap cleanup EXIT

# Error handler with line numbers
error_handler() {
    echo "Error on line $1, exit code $2" >&2
    exit $2
}
trap 'error_handler $LINENO $?' ERR

# Retry with backoff
retry() {
    local max_attempts=$1; shift
    local attempt=1
    while (( attempt <= max_attempts )); do
        if "$@"; then
            return 0
        fi
        echo "Attempt $attempt failed, retrying in $((attempt * 2))s..."
        sleep $((attempt * 2))
        ((attempt++))
    done
    return 1
}

retry 3 curl -s http://example.com

# Assert function
assert() {
    if ! "$@"; then
        echo "Assertion failed: $*" >&2
        exit 1
    fi
}
assert test -f "/etc/passwd"
assert grep -q "root" /etc/passwd

# Trace execution (show each command before running)
set -x
# + echo hello
# hello

# Disable tracing
set +x

# Debug specific function
debug_func() {
    set -x
    # function body
    set +x
}

# Check variable values
echo "DEBUG: var=$var" >&2

# Print function arguments
debug_args() {
    echo "DEBUG: $@=($*)" >&2
    echo "DEBUG: $#=($#)" >&2
}

# Verbose flag in scripts
VERBOSE=false
log() { $VERBOSE && echo "[DEBUG] $*" >&2; }

# Debug with bash -x
bash -x myscript.sh

# Debug only a portion
bash -x myscript.sh 2>&1 | grep "+"