Shell Scripting — Special Variables & Environment

#!/usr/bin/env bash
# positional.sh — Demonstrate positional parameters

echo "Script name  : $0"
echo "Script name  : $(basename "$0")"   # Just the filename
echo "First arg    : $1"
echo "Second arg   : $2"
echo "Tenth arg    : ${10}"              # Braces required for 2+ digits
echo "Arg count    : $#"               # Number of arguments
echo "All args (@) : $@"               # All args, individually quoted
echo "All args (*) : $*"               # All args, as single string

# Loop over all arguments safely — use "$@" not "$*"
echo "Processing all arguments:"
for arg in "$@"; do
  echo "  → ${arg}"
done

# Check exit status after every important command
ls /etc/passwd
echo "Exit code: $?"     # 0 — success

ls /nonexistent/path
echo "Exit code: $?"     # 2 — failure (file not found)

# Use in conditionals
ping -c1 -W1 8.8.8.8 > /dev/null 2>&1
if [[ $? -eq 0 ]]; then
  echo "Network is reachable"
else
  echo "Network is unreachable"
fi

# More idiomatic — check directly in if
if ping -c1 -W1 8.8.8.8 > /dev/null 2>&1; then
  echo "Network OK"
fi

# Exit with a specific code
validate_input() {
  [[ -z "${1}" ]] && return 1    # Return 1 for failure
  return 0                         # Return 0 for success
}
validate_input ""
echo "Validation result: $?"     # 1 (failed)

# $$ — PID of the current script (current shell)
echo "My PID: $$"

# Used for unique temp files — prevents collisions in parallel runs
TMPFILE="/tmp/myapp_$$.tmp"
echo "data" > "${TMPFILE}"
# ... use file ...
rm -f "${TMPFILE}"

# $! — PID of the last background process
sleep 60 &
BG_PID=$!
echo "Background PID: ${BG_PID}"
wait "${BG_PID}"              # Wait for it to finish

# $_ — last argument of the previous command
mkdir -p /tmp/myproject/logs
cd $_                       # cd to /tmp/myproject/logs

# $PPID — Parent process ID
echo "Parent PID: $PPID"

# $BASHPID — PID in subshells (different from $$)
( echo "Subshell PID: $BASHPID, Parent $$: $$" )

# Default IFS — splits on space, tab, newline
sentence="one two three"
for word in $sentence; do    # Splits on spaces
  echo "  → ${word}"
done

# Change IFS to split on colon — useful for PATH parsing
OLD_IFS="${IFS}"              # Save original
IFS=':'
for dir in $PATH; do
  echo "  PATH entry: ${dir}"
done
IFS="${OLD_IFS}"              # Always restore!

# Use IFS with read to parse CSV lines
while IFS=',' read -r name age city; do
  echo "Name=${name}, Age=${age}, City=${city}"
done < users.csv

# Best practice — use IFS=$'\n\t' at script top
# to prevent word splitting on spaces in filenames
IFS=$'\n\t'

# ── Navigation ─────────────────────────────────────────────
echo "Home dir    : $HOME"
echo "Current dir : $PWD"
echo "Previous dir: $OLDPWD"
echo "PATH        : $PATH"

# ── User info ──────────────────────────────────────────────
echo "Username    : $USER"
echo "Effective UID: $EUID"    # 0 = root
echo "Hostname    : $HOSTNAME"

# ── Shell info ─────────────────────────────────────────────
echo "Shell       : $SHELL"
echo "Bash version: $BASH_VERSION"
echo "Bash source : ${BASH_SOURCE[0]}"
echo "Line number : $LINENO"    # Current line (useful in debug)

# ── Terminal ───────────────────────────────────────────────
echo "Term type   : $TERM"
echo "Term width  : $COLUMNS"
echo "Term height : $LINES"

# ── Runtime ────────────────────────────────────────────────
echo "Seconds up  : $SECONDS"   # Seconds since shell started
echo "Random num  : $RANDOM"    # Random int 0-32767

# Check if running as root
if [[ "${EUID}" -ne 0 ]]; then
  echo "ERROR: This script must be run as root" >&2
  exit 1
fi

#!/usr/bin/env bash
# Using BASH_SOURCE and LINENO in error messages

die() {
  # Prints: [filename:linenum] ERROR: message
  echo "[${BASH_SOURCE[1]}:${BASH_LINENO[0]}] ERROR: $*" >&2
  exit 1
}

check_file() {
  local file="${1}"
  [[ -f "${file}" ]] || die "File not found: ${file}"
  echo "File OK: ${file}"
}

check_file "/etc/passwd"    # OK
check_file "/no/such/file"  # Fails with location

# BASH_SOURCE array — useful in sourced libraries
# BASH_SOURCE[0] = current file
# BASH_SOURCE[1] = file that sourced this one
SCRIPT_DIR=$( cd "$(dirname "${BASH_SOURCE[0]}")" && pwd )
echo "This script lives in: ${SCRIPT_DIR}"

#!/usr/bin/env bash
# show_special_vars.sh — Print all special variables

echo "═══════════ POSITIONAL ═══════════"
echo "\$0  Script name   : $0"
echo "\$1  First arg     : ${1:-'(none)'}"
echo "\$#  Arg count     : $#"
echo "\$@  All args      : $@"

echo "═══════════ PROCESS ══════════════"
echo "\$$  Current PID   : $$"
echo "\$PPID Parent PID  : $PPID"

echo "═══════════ STATUS ═══════════════"
true
echo "\$?  After true    : $?"
false
echo "\$?  After false   : $?"

echo "═══════════ ENVIRONMENT ══════════"
echo "\$USER    : $USER"
echo "\$HOME    : $HOME"
echo "\$PWD     : $PWD"
echo "\$SHELL   : $SHELL"
echo "\$RANDOM  : $RANDOM"
echo "\$SECONDS : $SECONDS"
echo "\$LINENO  : $LINENO"