Bash Cheatsheet

# no spaces around = in assignments
name="Alice"
age=30
PI=3.14
active=true

# use variables with $ prefix
echo "Hello, $name!"
echo "Age: $age"

# command substitution: capture command output
today=$(date +%Y-%m-%d)
files=$(ls | wc -l)
echo "Today is $today, $files files"

# readonly and environment variables
readonly MAX=100
export PATH="$PATH:/opt/bin"
env | grep PATH

s="Hello World"
echo ${#s}                # length: 11
echo ${s:0:5}             # substring: Hello
echo ${s:6}               # from index 6: World
echo ${s: -5}             # last 5 chars: World

# case conversion (Bash 4+)
echo ${s,,}               # lowercase: hello world
echo ${s^^}               # uppercase: HELLO WORLD
echo ${s^l}               # capitalize first if 'l': Hello World

# default values
echo ${name:-Guest}       # use Guest if name unset
echo ${name:=Guest}       # assign Guest if unset, then use
echo ${name:?required}    # error and exit if unset

s="Hello World Hello"

# replace first occurrence
echo ${s/Hello/Hi}        # Hi World Hello

# replace all occurrences
echo ${s//Hello/Hi}       # Hi World Hi

# replace at beginning / end
echo ${s/#Hello/Hi}       # Hi World Hello (prefix)
echo ${s/%Hello/Hi}       # Hello World Hello (no suffix match)

# delete patterns (replace with empty)
echo ${s//Hello/}         #  World 
path="/usr/local/bin/app"
echo ${path##*/}          # app (longest prefix)
echo ${path%/*}           # /usr/local/bin (shortest suffix)

# extract filename and extension
file="report.tar.gz"
echo ${file%.*}           # report.tar (remove ext)
echo ${file##*.}          # gz (keep ext)

# single quotes: literal, no expansion
echo 'Hello $USER'         # Hello $USER

# double quotes: expansion happens
echo "Hello $USER"         # Hello alice

# escape special chars with backslash
echo "Price: \\$5.00"      # Price: $5.00
echo 'It\'s a test'       # It's a test (escape inside single)

# concatenate strings
a="Hello"
b="World"
c="$a, $b!"
echo $c                    # Hello, World!

# multi-line string
msg="Line 1
Line 2
Line 3"
echo "$msg"

# read a single value
echo -n "Enter your name: "
read name
echo "Hi, $name!"

# read multiple values
read -p "First and last: " first last

# read with timeout (seconds)
read -t 5 -p "Quick! " answer || echo "too slow"

# read into an array
echo "Enter colors:"
read -a colors
echo "First: ${colors[0]}"

# read line by line from a file
while IFS= read -r line; do
    echo ">>> $line"
done < file.txt

# create and populate
fruits=("apple" "banana" "cherry")
fruits[3]="date"

# access elements
echo ${fruits[0]}         # apple
echo ${fruits[@]}         # all: apple banana cherry date
echo ${fruits[*]}         # same as @ in most contexts

# count and slice
echo ${#fruits[@]}        # count: 4
echo ${fruits[@]:1:2}     # elements 1-2: banana cherry

# append
fruits+=("elderberry")

# iterate
for f in "${fruits[@]}"; do
    echo "- $f"
done

# delete an element
unset fruits[1]            # removes "banana" (gap remains)

# must declare before use (Bash 4+)
declare -A ages

# key-value assignment
ages[alice]=30
ages[bob]=25
ages["carol smith"]=28

# access by key
echo ${ages[alice]}       # 30
echo ${ages[bob]}         # 25

# all keys and values
echo ${!ages[@]}          # keys: alice bob carol smith
echo ${ages[@]}           # values: 30 25 28
echo ${#ages[@]}          # count: 3

# iterate keys
for name in "${!ages[@]}"; do
    echo "$name is ${ages[$name]}"
done

# check if key exists
if [[ -v ages[alice] ]]; then
    echo "alice exists"
fi

arr=(one two three four five)

# iterate by value
for item in "${arr[@]}"; do
    echo "$item"
done

# iterate by index
for i in "${!arr[@]}"; do
    echo "[$i] = ${arr[$i]}"
done

# C-style indexed loop
for ((i=0; i<${#arr[@]}; i++)); do
    echo "${arr[$i]}"
done

# map/transform: build a new array
upper=()
for w in "${arr[@]}"; do
    upper+=("$(echo $w | tr a-z A-Z)")
done
echo "${upper[@]}"

# split a string into an array
csv="a,b,c,d"
IFS=',' read -ra parts <<< "$csv"
echo ${parts[2]}          # c

# read lines of a file into an array
mapfile -t lines < file.txt
echo "${lines[0]}"        # first line
echo "${#lines[@]}"       # line count

# alternative: readarray (same as mapfile)
readarray -t lines2 < file.txt

# from command output (words split on IFS)
files=($(ls *.txt))
echo "${files[@]}"

# safer: read null-delimited output
while IFS= read -r -d '' f; do
    files+=("$f")
done < <(find . -name "*.txt" -print0)

# positional parameters
echo $0       # script name
echo $1       # first argument
echo $#       # argument count
echo $@       # all arguments (separate words)
echo "$@"     # all arguments (preserves quoting)
echo $*       # all arguments (single string)

# process and shell info
echo $$       # current shell PID
echo $!       # PID of last background command
echo $?       # exit status of last command (0 = success)
echo $-       # current shell option flags
echo $_       # last argument of previous command

# shift arguments
echo "$1 $2"  # a b
shift
echo "$1 $2"  # b c

# iterate all args safely
for arg in "$@"; do
    echo "arg: $arg"
done

# basic if
if [ "$age" -ge 18 ]; then
    echo "adult"
elif [ "$age" -ge 13 ]; then
    echo "teen"
else
    echo "child"
fi

# modern [[ ]] test (Bash-specific, safer)
if [[ $name == "Alice" ]]; then
    echo "hi Alice"
fi

# pattern matching in [[ ]]
if [[ $file == *.txt ]]; then
    echo "text file"
fi

# regex matching
if [[ $email =~ ^[a-z]+@[a-z]+.[a-z]+$ ]]; then
    echo "valid email"
fi

# multiple conditions
if [[ -f file.txt && -r file.txt ]]; then
    echo "readable file"
fi

# file tests
[ -f file ]       # exists and is regular file
[ -d dir ]        # exists and is directory
[ -e path ]       # exists (any type)
[ -r file ]       # readable
[ -w file ]       # writable
[ -x file ]       # executable
[ -s file ]       # exists and is non-empty
[ file1 -nt file2 ]  # newer than
[ file1 -ot file2 ]  # older than

# string tests
[ -z "$s" ]       # empty string
[ -n "$s" ]       # non-empty string
[ "$a" = "$b" ]   # equal (use == in [[ ]])
[ "$a" != "$b" ]  # not equal

# integer tests
[ $n -eq 5 ]      # equal
[ $n -ne 5 ]      # not equal
[ $n -lt 5 ]      # less than
[ $n -gt 5 ]      # greater than
[ $n -le 5 ]      # less or equal
[ $n -ge 5 ]      # greater or equal

case $1 in
    start)
        echo "Starting service..."
        systemctl start app
        ;;
    stop)
        echo "Stopping service..."
        systemctl stop app
        ;;
    restart)
        $0 stop
        $0 start
        ;;
    status)
        systemctl status app
        ;;
    --help|-h)
        echo "Usage: $0 {start|stop|restart|status}"
        ;;
    *)
        echo "Unknown command: $1" >&2
        exit 1
        ;;
esac

# pattern matching in case
case $file in
    *.jpg|*.png)    echo "image" ;;
    *.mp3|*.wav)    echo "audio" ;;
    *.txt)          echo "text" ;;
    *)              echo "other" ;;
esac

# arithmetic expansion
x=5
y=$((x + 3))
echo $y              # 8
echo $((x * 2))      # 10
echo $((2 ** 10))    # 1024 (exponent)
echo $((17 % 5))     # 2 (modulo)
echo $((17 / 5))     # 3 (integer division)

# (( )) condition: no $ needed, returns exit status
x=10
if (( x > 5 && x < 20 )); then
    echo "in range"
fi

# increment / decrement
((x++))             # post-increment
((x--))             # post-decrement
((++x))             # pre-increment

# bitwise operators
echo $((5 & 3))     # 1 (AND)
echo $((5 | 2))     # 7 (OR)
echo $((5 << 1))    # 10 (left shift)

# && and || as short-circuit control flow
[[ -f file.txt ]] && echo "exists"
[[ -f file.txt ]] || echo "missing"

# combined: do this OR fail
mkdir -p build && cd build && make || exit 1

# ternary-like (Bash doesn't have a real ternary)
result=$([[ $x -gt 0 ]] && echo "positive" || echo "non-positive")

# default value via &&
echo ${name:-"anonymous"}

# run command, fallback on failure
ping -c1 host && echo "up" || echo "down"

# null command as no-op
if condition; then
    :    # do nothing
fi

# iterate a list
for fruit in apple banana cherry; do
    echo "$fruit"
done

# iterate an array
colors=("red" "green" "blue")
for c in "${colors[@]}"; do
    echo "Color: $c"
done

# range with brace expansion
for i in {1..5}; do
    echo "Number: $i"
done

# range with step
for i in {0..20..2}; do echo $i; done    # 0 2 4 ... 20

# C-style for loop
for ((i=0; i<5; i++)); do
    echo "Iteration $i"
done

# iterate command output
for file in *.txt; do
    echo "Processing $file"
done

# while: runs while condition is true
count=0
while [ $count -lt 5 ]; do
    echo "Count: $count"
    ((count++))
done

# read lines from a file
while IFS= read -r line; do
    echo "Line: $line"
done < input.txt

# infinite loop with break
while true; do
    read -p "Quit? (y/n) " ans
    [[ $ans == "y" ]] && break
done

# until: runs until condition is true (opposite of while)
n=0
until [ $n -ge 3 ]; do
    echo "n=$n"
    ((n++))
done

# while reading from a pipeline
seq 1 5 | while read n; do
    echo "got $n"
done

# break exits the loop
for i in {1..10}; do
    [[ $i -eq 5 ]] && break
    echo $i       # prints 1 2 3 4
done

# continue skips to next iteration
for i in {1..6}; do
    (( i % 2 == 0 )) && continue
    echo $i       # prints 1 3 5 (odd only)
done

# break out of nested loops
for i in 1 2 3; do
    for j in a b c; do
        [[ $j == "b" ]] && break 2   # break both loops
        echo "$i$j"
    done
done

# select menu
select opt in "Start" "Stop" "Quit"; do
    case $opt in
        Start) echo "starting" ;;
        Stop)  echo "stopping" ;;
        Quit)  break ;;
        *)     echo "invalid" ;;
    esac
done

# BAD: breaks on filenames with spaces
for f in $(ls *.txt); do
    cat "$f"
done

# GOOD: glob expands safely
for f in *.txt; do
    [[ -f "$f" ]] || continue
    cat "$f"
done

# BEST: handle spaces, newlines, and no matches
shopt -s nullglob dotglob
for f in *.txt; do
    echo "Processing: $f"
done

# find with -print0 and while read -d ''
while IFS= read -r -d '' f; do
    echo "Found: $f"
done < <(find . -type f -name "*.txt" -print0)

# process files modified today
for f in $(find . -mtime -1 -print); do
    echo "Recent: $f"
done

# brace expansion (not a loop, but generates lists)
echo {1..10}                 # 1 2 3 4 5 6 7 8 9 10
echo {a..e}                  # a b c d e
echo {01..10}                # zero-padded: 01 02 ... 10
echo file{1..3}.txt          # file1.txt file2.txt file3.txt

# seq command
seq 1 5                      # 1 2 3 4 5
seq 1 2 10                   # 1 3 5 7 9 (step 2)
seq -f "%03d" 1 5            # 001 002 003 004 005

# use seq in a for loop
for i in $(seq 1 5); do
    echo $i
done

# generate a list of dates
for d in $(seq -f "2024-01-%02g" 1 31); do
    echo "$d"
done

# two equivalent syntaxes
greet() {
    echo "Hello, $1!"
}
function greet2() {
    echo "Hi, $1!"
}

# call with arguments (no parens)
greet "Alice"               # Hello, Alice!
greet2 "Bob"                # Hi, Bob!

# local variables (function-scoped)
counter() {
    local count=0
    ((count++))
    echo $count
}

# return a value via echo + command substitution
add() {
    echo $(($1 + $2))
}
result=$(add 3 4)           # 7

# all positional params work inside functions
show_args() {
    echo "Function name: $FUNCNAME"
    echo "First arg:  $1"
    echo "Second arg: $2"
    echo "All args:   $@"
    echo "Arg count:  $#"
}

show_args a b c
# Function name: show_args
# First arg:  a
# Second arg: b
# All args:   a b c
# Arg count:  3

# pass all script args to a function
process "$@"

# shift within a function (local to it)
parse() {
    while (($#)); do
        echo "arg: $1"
        shift
    done
}

# return sets exit status (0-255), not a value
is_even() {
    if (($1 % 2 == 0)); then
        return 0    # true/success
    else
        return 1    # false/failure
    fi
}

# use as a condition
if is_even 4; then
    echo "4 is even"
fi

# chain with && and ||
is_even 4 && echo "yes" || echo "no"

# return a string via echo (capture with $())
to_upper() {
    echo "${1^^}"
}
UP=$(to_upper hello)        # HELLO

# return multiple values via global or nameref
set_pair() {
    local -n _r=$1
    _r=(10 20)
}
set_pair result
echo "${result[@]}"       # 10 20

# global by default
x=1
modify() {
    x=2          # changes the GLOBAL x!
    local y=3    # local to function
    echo "inside: x=$x y=$y"
}

modify
echo "outside: x=$x"       # x=2 (modified!)
echo "outside: y=$y"       # empty (y is local)

# declare -g for global inside a function
set_global() {
    declare -g NEW_VAR=42
}
set_global
echo $NEW_VAR              # 42

# recursion with local
factorial() {
    local n=$1
    if (( n <= 1 )); then
        echo 1
    else
        local prev=$(factorial $((n - 1)))
        echo $((n * prev))
    fi
}
echo $(factorial 5)        # 120

# utils.sh — a reusable library
#!/bin/bash
log() {
    echo "[$(date +%H:%M:%S)] $*" >&2
}

die() {
    log "ERROR: $*"
    exit 1
}

confirm() {
    read -p "$1 [y/N] " ans
    [[ $ans =~ ^[Yy]$ ]]
}

# main.sh — use the library
source ./utils.sh        # or . ./utils.sh

log "Starting script"
confirm "Continue?" || die "aborted"
log "Done"

# source vs execute
# source: runs in current shell (shares variables)
# ./script.sh: runs in a subshell (isolated)

# basic search
grep "error" app.log
grep -i "error" app.log          # case-insensitive
grep -v "debug" app.log          # invert (non-matching lines)
grep -c "error" app.log          # count matches
grep -n "error" app.log          # line numbers
grep -w "error" app.log          # whole word only

# recursive search
grep -rn "TODO" src/             # recursive + line numbers
grep -rl "TODO" src/             # files with matches only

# extended regex (or use egrep)
grep -E "error|warn|fatal" log
grep -E "^[0-9]{4}-" log         # lines starting with date

# context lines
grep -A 2 "error" log            # 2 lines after
grep -B 2 "error" log            # 2 lines before
grep -C 2 "error" log            # 2 lines before and after

# search multiple files
grep "function" *.js --include="*.js" -r .

# substitute (replace)
sed 's/old/new/' file.txt           # first occurrence per line
sed 's/old/new/g' file.txt          # all occurrences (global)
sed 's/old/new/3' file.txt          # 3rd occurrence only
sed 's/old/new/gi' file.txt         # global + case-insensitive

# in-place editing
sed -i 's/foo/bar/g' file.txt       # edit file in place
sed -i.bak 's/foo/bar/g' file.txt   # keep a backup

# delete lines
sed '/^#/d' file.txt                # delete comment lines
sed '/^$/d' file.txt                # delete blank lines
sed '5d' file.txt                   # delete line 5
sed '5,10d' file.txt                # delete lines 5-10

# print specific lines
sed -n '10,20p' file.txt            # print lines 10-20
sed -n '/pattern/p' file.txt        # print matching lines

# use a different delimiter for paths
sed 's|/usr/local|/opt|g' paths.txt

# multiple commands
sed -e 's/a/A/g' -e 's/b/B/g' file.txt

# print columns (default separator: whitespace)
awk '{print $1}' file.txt           # first column
awk '{print $1, $3}' file.txt       # columns 1 and 3
awk '{print $NF}' file.txt          # last column
awk -F: '{print $1}' /etc/passwd    # split on :

# filter and print
awk '$3 > 100' file.txt             # lines where col 3 > 100
awk '$1 == "ERROR" {print $2}' log  # col 2 of ERROR lines
awk 'NR == 5' file.txt              # 5th line only
awk 'NR >= 10 && NR <= 20' file.txt # lines 10-20

# BEGIN and END blocks
awk 'BEGIN {print "Start"} {sum += $1} END {print sum}' nums.txt

# field separator and output
awk -F, 'BEGIN {OFS="|"} {print $1, $2}' csv.txt

# count lines matching a pattern
awk '/error/' file.txt              # like grep
awk '/error/ {count++} END {print count}' log

# built-in variables: NR (row), NF (fields), FS, OFS
awk '{print NR, NF, $0}' file.txt

# cut: extract fields or characters
cut -d: -f1 /etc/passwd            # field 1, delimiter :
cut -c1-5 file.txt                 # characters 1-5
cut -d, -f2,3 data.csv             # fields 2 and 3

# tr: translate or delete characters
echo "Hello" | tr 'a-z' 'A-Z'      # HELLO
echo "a,b,c" | tr ',' ' '          # a b c
echo "hello" | tr -d 'l'           # heo (delete)
tr -s ' ' < file.txt               # squeeze repeated spaces

# sort lines
sort file.txt                      # alphabetical
sort -n nums.txt                   # numeric
sort -rn nums.txt                  # reverse numeric
sort -t: -k3 -n /etc/passwd        # by 3rd field, numeric
sort -u file.txt                   # sort + unique

# uniq: filter or count adjacent duplicates
sort file.txt | uniq               # unique lines
sort file.txt | uniq -c            # count occurrences
sort file.txt | uniq -d            # only duplicates
sort file.txt | uniq -u            # only unique lines

# pipe: chain commands (stdout of one -> stdin of next)
cat file | grep "x" | sort | uniq -c | sort -rn | head

# redirect stdout
echo "hello" > file.txt            # overwrite
echo "world" >> file.txt           # append

# redirect stderr
command 2> error.log               # stderr to file
command 2>&1                       # stderr to stdout
command > all.log 2>&1             # both to same file
command &> all.log                 # both (Bash shortcut)

# stdin from file
command < input.txt

# here-string
grep "x" <<< "some text with x"

# here-document (multi-line input)
cat << EOF
Line 1
Line 2 with $VAR expansion
EOF

# quoted delimiter disables expansion
cat << 'EOF'
Literal $VAR no expansion
EOF

# process substitution
diff <(ls dir1) <(ls dir2)         # compare outputs
while read line; do echo "$line"; done < <(grep x file)

# search by name
find . -name "*.js"
find . -iname "*.JS"               # case-insensitive
find / -name "*.conf" 2>/dev/null  # suppress permission errors

# search by type
find . -type f -name "*.txt"       # files only
find . -type d -name src           # directories only
find . -type l                     # symlinks

# search by time and size
find . -mtime -7                   # modified < 7 days ago
find . -mtime +30                  # modified > 30 days ago
find . -mmin -60                   # modified < 60 min ago
find . -size +10M                  # larger than 10MB
find . -size -1k                   # smaller than 1KB
find . -empty                      # empty files/dirs

# act on results (-exec)
find . -name "*.log" -exec rm {} \;
find . -name "*.js" -exec wc -l {} +
find . -type f -exec chmod 644 {} \;

# safe deletion with -delete
find /tmp -type f -mtime +7 -delete

# view permissions
ls -l file.txt
# -rw-r--r-- 1 user group 1024 Jan 1 10:00 file.txt
#  ^^^ ^^^ ^^^
#  owner group others

# chmod: change permissions
chmod 755 script.sh        # rwxr-xr-x
chmod 644 file.txt         # rw-r--r--
chmod +x script.sh         # add execute for all
chmod u+x,g-w file.txt     # symbolic: user +x, group -w
chmod -R 755 directory/    # recursive

# chown: change owner
chown alice file.txt
chown alice:developers file.txt
chown -R alice:developers project/

# umask: default permissions for new files
umask                      # show current (e.g. 022)
umask 077                  # new files: 600, dirs: 700

# numeric permission reference:
# 7=rwx 6=rw 5=rx 4=r 3=wx 2=w 1=x 0=none

# copy
cp file.txt backup.txt
cp -r src/ dest/            # recursive (directories)
cp -i file.txt dest/        # interactive (prompt before overwrite)
cp -u file.txt dest/        # update (only if newer)
cp -p file.txt dest/        # preserve attributes

# move / rename
mv old.txt new.txt
mv file.txt /other/dir/
mv -i file.txt dest/        # prompt before overwrite

# remove
rm file.txt
rm -r directory/            # recursive
rm -f file.txt              # force (no error if missing)
rm -rf node_modules/        # force + recursive (DANGER)

# links
ln target hardlink          # hard link (same inode)
ln -s target symlink        # symbolic link (path reference)
ln -sf target symlink       # force recreate symlink

# view link target
readlink symlink
readlink -f symlink         # canonical absolute path

# tar + gzip (most common)
tar -czf archive.tar.gz dir/      # create
tar -xzf archive.tar.gz           # extract
tar -xzf archive.tar.gz -C /opt/  # extract to /opt
tar -tzf archive.tar.gz           # list contents

# tar + bzip2 (better compression)
tar -cjf archive.tar.bz2 dir/
tar -xjf archive.tar.bz2

# zip / unzip
zip -r archive.zip dir/
unzip archive.zip
unzip archive.zip -d /target

# gzip / gunzip (single files)
gzip file.txt           # -> file.txt.gz
gunzip file.txt.gz      # -> file.txt
gzip -k file.txt        # keep original

# xz (best compression)
tar -cJf archive.tar.xz dir/
tar -xJf archive.tar.xz

# mnemonic: eXtract, Create, List, File, gZip, bZip2, xz

# view file contents
cat file.txt               # entire file
head -n 20 file.txt        # first 20 lines
tail -n 20 file.txt        # last 20 lines
tail -f app.log            # follow (live updates)
less file.txt              # pager (q to quit, / to search)

# line/word/char count
wc file.txt                # lines words bytes
wc -l file.txt             # lines only
wc -w file.txt             # words only

# compare files
diff file1.txt file2.txt
diff -u file1.txt file2.txt    # unified format (for patches)
diff -r dir1/ dir2/            # recursive

# patch
diff -u old new > change.patch
patch < change.patch
patch -R < change.patch        # reverse

# find duplicate lines across files
comm -12 <(sort a) <(sort b)   # lines in both
comm -23 <(sort a) <(sort b)   # only in a
comm -13 <(sort a) <(sort b)   # only in b

# run in background (append &)
sleep 100 &
# [1] 12345   (job 1, PID 12345)

# list jobs
jobs
jobs -l            # with PIDs

# bring to foreground / send to background
fg                 # foreground the current job
fg %1              # foreground job 1
bg %2              # background job 2 (continue running)

# suspend a running foreground job: Ctrl+Z
# then resume in background:
bg

# wait for background jobs
sleep 5 &
wait                # wait for all background jobs
wait $!             # wait for the last background job
wait 12345          # wait for specific PID

# disown: detach from shell (survives logout)
long_task &
disown %1

# send signals to processes
kill 12345              # SIGTERM (graceful, default)
kill -15 12345          # SIGTERM explicitly
kill -9 12345           # SIGKILL (force, cannot be caught)
kill -HUP 12345         # SIGHUP (reload config)

# kill by name
killall nginx           # kill all nginx processes
pkill -f "python app.py"  # match full command line

# common signals
# SIGTERM (15) - graceful termination (default)
# SIGKILL (9)  - force kill (cannot be caught/ignored)
# SIGINT (2)   - interrupt (Ctrl+C)
# SIGHUP (1)   - hangup (often reload config)
# SIGSTOP (19) - pause (cannot be caught)
# SIGCONT (18) - resume
# SIGTSTP (20) - terminal stop (Ctrl+Z)

# list all signals
kill -l

# priority: try SIGTERM first, SIGKILL as last resort
kill 12345; sleep 2; kill -9 12345 2>/dev/null

# clean up on exit (EXIT signal)
trap 'rm -f $TMPFILE' EXIT
TMPFILE=$(mktemp)
echo "working with $TMPFILE"
# when script exits (normally or via error), TMPFILE is removed

# handle Ctrl+C (SIGINT)
trap 'echo "Caught Ctrl+C, exiting"; exit 130' INT

# handle multiple signals
cleanup() {
    echo "Cleaning up..."
    rm -rf "$WORKDIR"
    exit
}
trap cleanup INT TERM EXIT

# ignore a signal
trap '' INT              # ignore Ctrl+C
trap - INT               # restore default behavior

# reload on SIGHUP
trap 'echo "reloading config"; source config.sh' HUP

# list current traps
trap -p

# list processes
ps aux                   # all processes (BSD style)
ps -ef                   # all processes (System V style)
ps -ef | grep nginx      # find nginx processes

# interactive viewer (like Task Manager)
top                      # real-time, all processes
htop                     # nicer interactive viewer (install separately)

# process tree
pstree                   # tree view
pstree -p                # with PIDs

# find process by name/port
pgrep -f "node app"      # print PIDs matching
pgrep -fl "node"         # full command line

# who's listening on a port
lsof -i :8080            # process using port 8080
ss -tlnp                 # all listening TCP ports
netstat -tlnp            # legacy alternative

# process resource usage
top -b -n 1 | head       # snapshot
ps aux --sort=-%mem | head  # top memory users

# nohup: run immune to hangups (survives logout)
nohup ./long_script.sh &
# output goes to nohup.out by default
nohup ./server.sh > server.log 2>&1 &

# disown: detach an already-running job
./long_task &
disown %1

# setsid: start in a new session
setsid ./daemon.sh &

# tmux: terminal multiplexer (persistent sessions)
tmux                    # start new session
tmux new -s work        # named session
tmux ls                 # list sessions
tmux attach -t work     # reattach
# inside tmux: Ctrl+B then D to detach
# processes keep running after detach

# screen: alternative to tmux
screen -S work
screen -ls
screen -r work

# check if a process is running
pgrep -x nginx && echo "running" || echo "stopped"

#!/usr/bin/env bash
# ^ portable shebang (finds bash via PATH)
# #!/bin/bash          # absolute path (common)
# #!/usr/bin/bash       # on some systems
# #!/bin/sh             # POSIX sh (most portable)

set -euo pipefail        # strict mode (see below)

# script metadata
readonly PROGNAME=$(basename "$0")
readonly VERSION="1.0.0"

usage() {
    cat << EOF
Usage: $PROGNAME [OPTIONS] FILE
Options:
  -h, --help     Show this help
  -v, --version  Show version
  -o OUTPUT      Output file
EOF
}

# parse options with getopts
while getopts "hvo:" opt; do
    case $opt in
        h) usage; exit 0 ;;
        v) echo $VERSION; exit 0 ;;
        o) output=$OPTARG ;;
        *) usage; exit 1 ;;
    esac
done
shift $((OPTIND - 1))

# the 'strict mode' trio
set -e             # exit on error (any non-zero status)
set -u             # error on unset variable
set -o pipefail    # pipeline fails if any command fails
# combined: set -euo pipefail

# example: without -e, errors are silently ignored
mkdir /root/nope   # fails (permission denied)
echo "still running"  # this runs anyway!

# with set -e, the script exits at the failure
set -e
mkdir /root/nope   # script exits here
echo "never reached"

# handle expected failures
set -e
if ! grep -q "x" file; then
    echo "not found"   # grep returning 1 won't kill script
fi

# temporarily disable -e
set +e
risky_command
set -e

# || true to ignore a command's failure
mkdir -p dir || true

# trace execution: print each command before running
set -x              # turn on
set +x              # turn off
bash -x script.sh   # run with tracing from the start

# verbose: print input lines as read
set -v

# combine: -xv for full detail
bash -xv script.sh

# print commands with line numbers
export PS4='+ ${BASH_SOURCE}:${LINENO}: '
set -x

# check syntax without executing
bash -n script.sh   # syntax check only

# step through interactively
bash -x script.sh 2>&1 | less

# debug a specific function
debug_func() {
    set -x
    actual_func "$@"
    set +x
}

# use trap ERR to catch errors
trap 'echo "Error on line $LINENO" >&2' ERR

# integer arithmetic
echo $((5 + 3))         # 8
echo $((10 / 3))        # 3 (integer division)
echo $((10 % 3))        # 1 (modulo)
echo $((2 ** 8))        # 256 (exponent)

# floating point with bc
echo "scale=2; 10/3" | bc       # 3.33
echo "3.14 * 2" | bc            # 6.28
result=$(echo "scale=4; $a / $b" | bc)

# floating point with awk
awk 'BEGIN {printf "%.2f\n", 10/3}'   # 3.33
awk "BEGIN {print $a * $b}"

# math functions with bc -l
echo "scale=4; s(0)" | bc -l    # 0 (sine, radians)
echo "scale=4; sqrt(16)" | bc -l # 4.0000

# random numbers
echo $RANDOM              # 0-32767
echo $((RANDOM % 100))    # 0-99
head -c 4 /dev/urandom | od -An -tu4   # 32-bit random

# increment/decrement
n=5
((n++)); echo $n          # 6
((n+=10)); echo $n        # 16

# glob patterns (filename matching)
*.txt          # any .txt file
file?.txt      # single char: file1.txt, fileA.txt
file[0-9].txt  # digit: file0.txt ... file9.txt
file[!0-9].txt # NOT a digit

# extended globs (shopt -s extglob)
shopt -s extglob
echo !(*.bak)       # everything except .bak files
echo @(*.jpg|*.png) # jpg or png
echo *(foo)         # zero or more "foo"

# regex with =~ in [[ ]]
if [[ $email =~ ^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$ ]]; then
    echo "valid email"
fi

# capture groups with BASH_REMATCH
if [[ $date =~ ^([0-9]{4})-([0-9]{2})-([0-9]{2})$ ]]; then
    echo "Year: ${BASH_REMATCH[1]}"
    echo "Month: ${BASH_REMATCH[2]}"
    echo "Day: ${BASH_REMATCH[3]}"
fi

# case-insensitive regex
shopt -s nocasematch
if [[ $s =~ ^hello ]]; then echo "starts with hello"; fi

# Basic search (literal string)
grep "error" logfile.txt

# Case-insensitive
grep -i "error" logfile.txt

# Whole word match
grep -w "error" logfile.txt

# Invert match (lines NOT matching)
grep -v "debug" logfile.txt

# Count matches
grep -c "error" logfile.txt

# Show line numbers
grep -n "error" logfile.txt

# Multiple files with filename prefix
grep "error" *.log

# Suppress filename prefix (single file behavior)
grep -h "error" *.log

# Only show filenames with matches
grep -l "error" *.log

# Extended regex (-E or egrep)
grep -E "error|warning|fatal" logfile.txt

# Anchors: ^ start, $ end
grep -E "^ERROR:" logfile.txt      # lines starting with ERROR:
grep -E "completed$" logfile.txt   # lines ending with completed

# Character classes
grep -E "[0-9]{4}-[0-9]{2}-[0-9]{2}" dates.txt  # date format
grep -E "[A-Z][a-z]+" names.txt                  # Capitalized words

# Quantifiers
grep -E "ab+c" file.txt    # one or more b's
grep -E "ab*c" file.txt    # zero or more b's
grep -E "ab?c" file.txt    # zero or one b

# Alternation and grouping
grep -E "(cat|dog) food" file.txt

# PCRE (-P, GNU grep only)
grep -P "\d{3}-\d{4}" phones.txt  # \d for digits

# Show context: lines before/after match
grep -B 2 "error" log.txt    # 2 lines Before
grep -A 3 "error" log.txt    # 3 lines After
grep -C 2 "error" log.txt    # 2 lines Context (both)

# Only show matched part (not full line)
grep -o "https?://[^ ]+" urls.txt

# Color highlight matches
grep --color=auto "error" log.txt

# Recursive search
grep -r "TODO" ./src/
grep -R "TODO" ./src/        # follows symlinks

# Include/exclude file patterns
grep -r --include="*.py" "import" .
grep -r --exclude="*.test.js" "TODO" .
grep -r --exclude-dir=node_modules "TODO" .

# Binary files
grep -a "pattern" binary_file  # treat as text

# Search command output
ps aux | grep nginx
ps aux | grep "[n]ginx"  # trick: prevents matching grep itself

# Search compressed logs
zcat log.gz | grep "error"
zgrep "error" log.gz     # direct

# Chain multiple greps
cat log.txt | grep "error" | grep -v "timeout" | grep -c

# Extract and filter
grep -oE "[0-9.]+" response.txt | sort -n | uniq

# Find processes excluding grep
pgrep -f "node server.js"  # better than ps | grep

# Search command history
history | grep "git rebase"

# Find files containing pattern, then search more
grep -rl "config" . | xargs grep "database"

# grep returns exit codes for scripting
# 0 = match found, 1 = no match, 2 = error

if grep -q "error" /var/log/syslog; then
    echo "Errors found!"
    # send alert, etc.
fi

# Silent check (-q), no output
grep -q "^$" file.txt && echo "has blank lines"

# Use in while loop
grep "pattern" file.txt | while read -r line; do
    echo "Found: $line"
done

# Count and branch
errors=$(grep -c "ERROR" log.txt)
if [ "$errors" -gt 10 ]; then
    echo "Too many errors: $errors"
fi

# grep with set -e (exits on non-zero)
set -e
grep "missing" file.txt || true  # prevent exit

# Basic substitution (first occurrence per line)
sed 's/old/new/' file.txt

# Replace all occurrences
sed 's/old/new/g' file.txt

# Case-insensitive (GNU sed)
sed 's/old/new/gi' file.txt

# Replace Nth occurrence
sed 's/old/new/2' file.txt  # 2nd occurrence only

# Only on lines matching pattern
sed '/error/s/old/new/' file.txt

# Only on specific line numbers
sed '5s/old/new/' file.txt      # line 5 only
sed '5,10s/old/new/g' file.txt  # lines 5-10
sed '$s/old/new/' file.txt      # last line

# In-place edit (modifies file)
sed -i 's/old/new/g' file.txt
sed -i.bak 's/old/new/g' file.txt  # keeps backup

# Extended regex (-E or -r)
sed -E 's/[0-9]+/NUMBER/g' file.txt

# Capture groups with \1, \2 (BRE)
sed 's/\(foo\)\(bar\)/\2\1/' file.txt  # swap: foobar -> barfoo

# Capture groups with () (ERE, cleaner)
sed -E 's/(foo)(bar)/\2\1/' file.txt

# Reformat date: 2025-01-15 -> 15/01/2025
sed -E 's/([0-9]{4})-([0-9]{2})-([0-9]{2})/\3\/\2\/\1/' file.txt

# Add prefix to lines
sed 's/^/>> /' file.txt

# Add suffix
sed 's/$/ <<</' file.txt

# Remove leading/trailing whitespace
sed 's/^[ \t]*//;s/[ \t]*$//' file.txt

# Use & for full match
sed 's/[0-9]\{3\}/[&]/g' file.txt  # wrap 3 digits in brackets

# Delete lines
sed '5d' file.txt              # delete line 5
sed '5,10d' file.txt           # delete lines 5-10
sed '/pattern/d' file.txt      # delete matching lines
sed '/^$/d' file.txt           # delete blank lines
sed '/^[ \t]*$/d' file.txt    # delete whitespace-only lines
sed '$d' file.txt              # delete last line

# Print only specific lines (like head/tail)
sed -n '5p' file.txt           # print line 5 only
sed -n '5,10p' file.txt        # print lines 5-10
sed -n '/pattern/p' file.txt   # print matching lines (like grep)
sed -n '5,${/pattern/p}' file.txt  # from line 5 to end

# Print with line numbers
sed '=' file.txt | sed 'N;s/\n/\t/'

# Print every Nth line
sed -n '0~3p' file.txt  # every 3rd line (GNU)

# Join lines (replace newline with space)
sed ':a;N;$!ba;s/\n/ /g' file.txt

# Join every 2 lines
sed 'N;s/\n/ /' file.txt

# Print paragraph (blocks separated by blank lines)
sed '/./{H;d;};x;s/\n/ /g' file.txt

# Reverse line order (tac alternative)
sed '1!G;h;$!d' file.txt

# Double-space a file
sed 'G' file.txt

# Remove last line of each paragraph
sed -n '/^$/{p;h;};/./{H;};/^$/{x;s/\n.*//;p;}' file.txt

# Hold space (h,H,g,G,x) stores lines between cycles
# h: copy pattern space to hold
# H: append pattern space to hold
# g: copy hold to pattern space
# G: append hold to pattern space
# x: exchange pattern and hold

# Clean up output in pipeline
cat messy.txt | sed 's/[ \t]*$//' | sed '/^$/d' | sort

# Extract and transform
echo "name=John;age=30" | sed 's/;/\n/g' | sed 's/=/: /'

# Modify config files in scripts
sed -i "s/PORT=8080/PORT=$NEW_PORT/" config.env

# Escape special chars for safe substitution
ESCAPED=$(printf '%s\n' "$URL" | sed 's/[&\]/\\&/g')
sed "s|URL|$ESCAPED|g" template.txt

# Multiple substitutions
sed -e 's/foo/bar/g' -e 's/baz/qux/g' file.txt

# Read commands from file
sed -f script.sed input.txt

# Convert CSV to TSV
sed 's/,/\t/g' data.csv

# Print specific fields (default delimiter: whitespace)
awk '{print $1}' file.txt        # first field
awk '{print $1, $3}' file.txt    # first and third
awk '{print $NF}' file.txt       # last field
awk '{print $(NF-1)}' file.txt   # second-to-last

# Custom field separator
awk -F: '{print $1, $7}' /etc/passwd   # colon-separated
awk -F',' '{print $2}' data.csv        # CSV
awk -F'\t' '{print $1}' data.tsv      # TSV

# Print entire line ($0)
awk '{print}' file.txt
awk '{print NR, $0}' file.txt  # with line number

# NR = record (line) number
# NF = number of fields in current record
# NR and NF are built-in variables

# Change output separator
awk 'BEGIN{OFS="|"} {print $1, $2, $3}' file.txt

# Pattern: only process matching lines
awk '/error/ {print}' log.txt          # like grep
awk '$3 > 100' data.txt                # 3rd field > 100
awk 'NR == 5' file.txt                 # line 5 only
awk 'NR >= 10 && NR <= 20' file.txt    # lines 10-20
awk 'NF == 0' file.txt                 # empty lines
awk '$1 == "ERROR"' log.txt            # first field equals ERROR

# Range pattern: from start to end
awk '/start/,/end/' file.txt

# Combine conditions
awk '$3 > 50 && $3 < 100 {print $1, $3}' data.txt
awk '$1 == "GET" || $1 == "POST" {print}' access.log

# Negation
awk '!/debug/ {print}' log.txt  # lines NOT containing debug

# Field comparison with regex
awk '$2 ~ /^[0-9]+$/' file.txt   # 2nd field is numeric
awk '$2 !~ /pattern/' file.txt   # 2nd field doesn't match

# BEGIN: runs before processing (setup)
# END: runs after processing (summary)
awk 'BEGIN {print "Total:"} {sum += $1} END {print sum}' nums.txt

# Count lines
awk 'END {print NR " lines"}' file.txt

# Sum a column
awk '{sum += $2} END {print "Total:", sum}' sales.csv

# Average
awk '{sum += $2; count++} END {print sum/count}' data.txt

# Find max value
awk 'NR == 1 {max = $1} $1 > max {max = $1} END {print max}' nums.txt

# User-defined variables (no declaration needed)
awk '{total += $1; if ($1 > max) max = $1}
     END {print "Sum:", total, "Max:", max, "Avg:", total/NR}'

# Pass variables with -v
awk -v threshold=50 '$1 > threshold {print}' data.txt
awk -v date="$(date +%Y-%m-%d)" '{print date, $0}' file.txt

# if-else
awk '{
  if ($3 >= 90) grade = "A"
  else if ($3 >= 80) grade = "B"
  else if ($3 >= 70) grade = "C"
  else grade = "F"
  print $1, grade
}' students.txt

# for loop
awk 'BEGIN {
  for (i = 1; i <= 5; i++) print i, i*i
}'

# for-in (iterate array)
awk '{count[$1]++} END {
  for (key in count) print key, count[key]
}' access.log | sort -rnk2

# while loop
awk '{
  i = 1
  while (i <= NF) {
    print i, $i
    i++
  }
}' file.txt

# Arrays (associative)
awk '{words[$1]++} END {
  for (w in words) print words[w], w
}' file.txt | sort -rn | head

# Top 10 IP addresses from access log
awk '{print $1}' access.log | sort | uniq -c | sort -rn | head
# Or entirely in awk:
awk '{ip[$1]++} END {for (i in ip) print ip[i], i}' access.log | sort -rn | head

# Sum file sizes by extension
ls -l | awk '{split($NF, a, "."); ext=a[2]; size[ext]+=$5}
  END {for (e in size) print e, size[e]}' | sort -rnk2

# Filter CSV by column value
awk -F',' '$3 == "active" {print $1, $2}' users.csv

# Calculate response time percentiles from log
awk '{times[NR] = $2} END {
  n = asorti(times, sorted)
  print "p50:", sorted[int(n*0.5)]
  print "p95:", sorted[int(n*0.95)]
  print "p99:", sorted[int(n*0.99)]
}' response_times.txt

# Reformat columns
awk -F',' 'BEGIN{OFS="|"} {print $1, $2, $4}' data.csv

# Find by name (exact)
find . -name "config.txt"

# Case-insensitive name
find . -iname "config.txt"

# Wildcard pattern
find . -name "*.py"
find . -name "test_*"

# Find by type
find . -type f -name "*.js"   # files only
find . -type d -name "node_modules"  # directories
find . -type l                # symlinks

# Find by path (full path match)
find . -path "*/src/*.test.js"

# Find by regex
find . -regex ".*\.\(py\|js\)$"

# Multiple name patterns
find . \( -name "*.py" -o -name "*.js" \)

# Max depth (don't recurse too deep)
find . -maxdepth 2 -name "*.txt"

# Find by modification time
find . -mtime -1    # modified in last 24 hours
find . -mtime +7    # modified more than 7 days ago
find . -mtime 7     # modified exactly 7 days ago

# Access time (-atime) and change time (-ctime)
find . -atime -1    # accessed in last 24h
find . -ctime -1    # status changed in last 24h

# Minutes instead of days
find . -mmin -30    # modified in last 30 minutes

# Find by size
find . -size +100M   # larger than 100MB
find . -size -1k     # smaller than 1KB
find . -size 10M     # exactly 10MB

# Find empty files/directories
find . -empty -type f
find . -empty -type d

# Find by permissions
find . -perm 644     # exactly 644
find . -perm -u+x    # has execute for user
find / -perm /4000   # SUID files (security audit)

# Execute command on each result
find . -name "*.log" -exec ls -lh {} \;

# Execute with + (batch, more efficient)
find . -name "*.log" -exec ls -lh {} +

# Delete files (DANGEROUS - test first!)
find /tmp -name "*.tmp" -type f -delete
# Safer: use -exec rm
find /tmp -name "*.tmp" -type f -exec rm -i {} \;

# Execute a complex command
find . -name "*.py" -exec wc -l {} \; | sort -rn | head

# Change permissions
find . -type d -exec chmod 755 {} \;  # dirs to 755
find . -type f -exec chmod 644 {} \;  # files to 644

# Find and grep
find . -name "*.py" -exec grep "TODO" {} +

# Print before deleting (confirmation)
find . -name "*.bak" -print -delete

# Basic: take stdin and pass as arguments
echo "file1 file2 file3" | xargs rm

# One argument per line
find . -name "*.bak" | xargs rm

# Control arguments per command (-n)
echo "1 2 3 4 5" | xargs -n 2 echo  # echo 1 2; echo 3 4; echo 5

# Show commands before running (-t, verbose)
find . -name "*.log" | xargs -t rm

# Prompt before each command (-p)
find . -name "*.tmp" | xargs -p rm

# Handle filenames with spaces (-0 with find -print0)
find . -name "*.txt" -print0 | xargs -0 grep "pattern"

# Replace string (-I)
find . -name "*.py" | xargs -I {} cp {} /backup/

# Parallel execution (-P)
find . -name "*.png" | xargs -P 4 -I {} convert {} {}.thumb.png

# Limit with -L (lines per command)
cat urls.txt | xargs -L 1 curl -O

# Find and bulk grep (efficient)
find . -name "*.py" -print0 | xargs -0 grep -l "import django"

# Find large files and show sizes
find . -type f -size +100M -print0 | xargs -0 ls -lhS

# Find and archive
find . -name "*.log" -mtime +30 -print0 |
  xargs -0 tar -czf old_logs.tar.gz

# Find and batch rename
find . -name "*.JPG" | xargs -I {} mv {} {}.bak
# Or with rename command:
find . -name "*.JPG" -exec rename 's/\.JPG$/.jpg/' {} +

# Find empty directories and remove
find . -type d -empty -print0 | xargs -0 rmdir

# Find recently modified files and copy
find . -name "*.py" -mtime -1 -print0 |
  xargs -0 -I {} cp {} /tmp/recent/

# Parallel processing
find . -name "*.wav" -print0 |
  xargs -0 -P 8 -I {} ffmpeg -i {} {}.mp3

# Here document: multi-line input to a command
cat << EOF
Line 1
Line 2 with $VARIABLE expansion
Line 3
EOF

# Quoted delimiter: no variable expansion
cat << 'EOF'
This $won't expand
\n stays literal
EOF

# Write to file
cat > config.txt << EOF
host=localhost
port=8080
EOF

# Here string: single string as stdin
grep "pattern" <<< "search this text"
read -r line <<< "hello world"

# With variables
name="Alice"
greeting=$(cat <<< "Hello, $name!")

# Pipe multi-line to command
mysql << EOF
USE mydb;
SELECT * FROM users;
EOF

# Default values
echo ${name:-"default"}    # use "default" if name unset/empty
echo ${name:="default"}    # set name to "default" if unset/empty

# String length
echo ${#var}

# Substring extraction
var="Hello World"
echo ${var:0:5}    # Hello (start:length)
echo ${var:6}      # World (start to end)

# Remove from beginning (# shortest, ## longest)
file="archive.tar.gz"
echo ${file#*.}     # tar.gz (remove up to first .)
echo ${file##*.}    # gz (remove up to last .)

# Remove from end (% shortest, %% longest)
echo ${file%.*}     # archive.tar (remove from last .)
echo ${file%%.*}    # archive (remove from first .)

# Replace
echo ${var/World/Bash}    # Hello Bash (first match)
echo ${var//l/L}          # HeLLo WorLd (all matches)

# Case conversion (Bash 4+)
echo ${var^^}   # HELLO WORLD (uppercase)
echo ${var,,}   # hello world (lowercase)

# Clean up on exit (any reason)
cleanup() {
  echo "Cleaning up..."
  rm -f /tmp/myapp.lock
  exit 0
}
trap cleanup EXIT

# Handle specific signals
trap 'echo "Interrupted!"; cleanup' INT TERM
trap 'echo "Ctrl-C pressed"' INT

# Ignore a signal
trap '' INT  # ignore Ctrl-C

# Reset trap
trap - INT  # restore default behavior

# Common pattern: temp file cleanup
TMPFILE=$(mktemp)
trap 'rm -f "$TMPFILE"' EXIT
echo "data" > "$TMPFILE"
# ... use TMPFILE ...
# File is removed when script exits (even on error)

# Debug trap (trace execution)
trap 'echo "DEBUG: $BASH_COMMAND"' DEBUG

# Trace execution: print each command before running
bash -x script.sh
# Or within script:
set -x   # enable tracing
set +x   # disable tracing

# Verbose: print input lines as read
bash -v script.sh

# Strict mode (recommended for all scripts)
set -euo pipefail
# -e: exit on error
# -u: error on undefined variable
# -o pipefail: pipe fails if any command fails
# -o pipefail: pipe fails if any command fails

# Print specific debug info
echo "DEBUG: var=$var" >&2  # to stderr

# Check syntax without executing
bash -n script.sh

# PS4: customize trace prompt
export PS4='+${BASH_SOURCE}:${LINENO}: '
bash -x script.sh

# Trap ERR for error handling
trap 'echo "Error on line $LINENO"' ERR

# Basic HTTP request
curl https://api.example.com/data
curl -O https://example.com/file.zip  # save to file

# HTTP methods
curl -X POST https://api.example.com/users
curl -X PUT -d '{"name":"Alice"}' https://api.example.com/users/1
curl -X DELETE https://api.example.com/users/1

# Headers and JSON
curl -H "Content-Type: application/json" \
     -H "Authorization: Bearer TOKEN" \
     -d '{"key":"value"}' \
     https://api.example.com/data

# Download with wget
wget https://example.com/file.zip
wget -q https://example.com/file.zip  # quiet
wget -c https://example.com/file.zip  # continue/resume

# Follow redirects
curl -L https://short.url/abc

# Save response, show headers
curl -s -o response.json -w "%{http_code}" https://api.example.com

# Download with progress bar
curl --progress-bar -O https://example.com/largefile.iso

# POST form data
curl -d "name=Alice&[email protected]" https://api.example.com/form

cleanup() {
  echo "Cleaning up..."
  rm -f /tmp/myapp.lock
  exit 0
}

# catch INT (Ctrl-C), TERM, EXIT
trap cleanup INT TERM EXIT

# normal exit also triggers EXIT trap
echo "Working..."
sleep 30
# cleanup runs automatically

# remove trap
trap - EXIT                      # disable
trap - INT TERM EXIT             # disable all

TMPDIR=$(mktemp -d)
TMPFILE=$(mktemp)

cleanup() {
  rm -rf "$TMPDIR" "$TMPFILE"
}
trap cleanup EXIT

# use temp files safely
echo "data" > "$TMPFILE"
process_data < "$TMPFILE"

# mktemp is safer than $RANDOM — avoids symlink attacks
# -d creates a directory
# -p /var/tmp specifies parent
# mktemp -t myapp.XXXXXX   # use TMPDIR env

#!/bin/bash
# graceful shutdown for a long-running script

RUNNING=1
stop() {
  echo "Stopping..."
  RUNNING=0
}
trap stop INT TERM

while [ $RUNNING -eq 1 ]; do
  echo "Tick $(date)"
  sleep 1
done

echo "Done."

# send signals: kill -TERM <pid> or Ctrl-C

# timeout kills a command after N seconds
timeout 30 slow_command
timeout 5s curl https://slow.example.com
timeout --signal=KILL 60 build.sh  # escalate to SIGKILL
timeout -k 5 30 command            # kill -9 after 5s grace

# exit status: 124 if timed out
timeout 2 sleep 10
echo $?                            # 124

# watch reruns a command
watch -n 1 date
watch -n 5 'kubectl get pods'

# tlimit-style loops
while true; do
  timeout 5 curl -s https://api.example.com/health || echo "down"
  sleep 60
done

# ignore a signal entirely
trap '' INT                       # ignore Ctrl-C
trap - INT                        # restore default

# re-raise / propagate to children
trap 'kill -TERM $childpid; wait $childpid' TERM

# forward signals to a subprocess
childpid=
run_child() {
  some_server &
  childpid=$!
  wait $childpid
}
trap 'kill -TERM $childpid' TERM INT
run_child

# exit on any error
set -e
trap 'echo "Failed at line $LINENO"' ERR

# two equivalent syntaxes
greet() {
  echo "Hello, $1!"
}

function greet {                  # also valid
  echo "Hello, $1!"
}

greet Alice                       # Hello, Alice!
greet "Bob Smith"                 # quoted args

# arguments: $1, $2, ... $@, $#, $*
echo_args() {
  echo "Count: $#"
  echo "All: $@"
  echo "First: $1"
  for arg in "$@"; do echo "- $arg"; done
}

# return is exit status (0-255), not a value
is_even() {
  if (( $1 % 2 == 0 )); then return 0; else return 1; fi
}

if is_even 4; then echo "even"; fi

# capture stdout as a value
get_date() { date +%Y-%m-%d; }
today=$(get_date)

# local variables
counter() {
  local count=0                  # scoped to function
  ((count++))
  echo $count
}

# library file: lib/utils.sh
log() {
  echo "[$(date +%H:%M:%S)] $*" >&2
}

CONFIG_PATH="/etc/myapp"

# main script
source lib/utils.sh              # or: . lib/utils.sh
log "Starting..."
echo "Config: $CONFIG_PATH"

# check if sourced vs executed
if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
  echo "Running directly"
else
  echo "Sourced"
fi

# factorial
factorial() {
  local n=$1
  if (( n <= 1 )); then
    echo 1
  else
    local prev=$(factorial $((n - 1)))
    echo $((n * prev))
  fi
}
factorial 5                      # 120

# fibonacci
fib() {
  if (( $1 < 2 )); then echo $1; return; fi
  echo $(( $(fib $(($1 - 1))) + $(fib $(($1 - 2))) ))
}

# caution: bash recursion is slow and limited depth
# for heavy work, use awk, python, or external tools

# default values
greet() {
  local name="${1:-World}"        # default if unset or empty
  local greeting="${2:-Hello}"
  echo "$greeting, $name!"
}
greet                            # Hello, World!
greet Alice                      # Hello, Alice!
greet Alice Hi                   # Hi, Alice!

# required argument
require_arg() {
  : "${1:?Usage: require_arg <name>}"   # exits with message if missing
  echo "Got: $1"
}

# shift through args
process() {
  while (( $# )); do
    case "$1" in
      -v) verbose=1; shift ;;
      *)  files+=("$1"); shift ;;
    esac
  done
}

# declare and assign
fruits=("apple" "banana" "cherry")
fruits+=("date")                 # append
fruits[10]="kiwi"                # sparse

# access
echo "${fruits[0]}"             # first
echo "${fruits[@]}"             # all values
echo "${#fruits[@]}"            # count
echo "${!fruits[@]}"            # indices

# iterate
for f in "${fruits[@]}"; do
  echo "$f"
done

# slice
echo "${fruits[@]:1:2}"         # 2 items starting at index 1

# delete
unset 'fruits[1]'

# requires bash 4+
declare -A ages
ages["alice"]=30
ages["bob"]=25
ages["carol"]=42

# access
echo "${ages["alice"]}"
echo "${ages[@]}"               # all values
echo "${!ages[@]}"              # all keys

# iterate keys
for name in "${!ages[@]}"; do
  echo "$name => ${ages[$name]}"
done

# check key exists
if [[ -v ages["alice"] ]]; then echo "yes"; fi

# count
echo "${#ages[@]}"

# delete
unset 'ages[bob]'

# read lines into array
mapfile -t lines < file.txt      # bash 4+
echo "${lines[0]}"

# alternative (portable)
lines=()
while IFS= read -r line; do
  lines+=("$line")
done < file.txt

# split string into array
IFS=',' read -ra csv <<< "a,b,c,d"
echo "${csv[2]}"                # c

# from command
mapfile -t files < <(find . -name "*.js")

# process each
for f in "${files[@]}"; do
  echo "Processing $f"
done

# append
arr=(a b c)
arr+=(d e)                       # (a b c d e)

# length
echo "${#arr[@]}"

# slice
echo "${arr[@]:1:2}"            # (b c)

# find index (no built-in; loop)
idx=-1
for i in "${!arr[@]}"; do
  if [[ "${arr[$i]}" == "b" ]]; then idx=$i; break; fi
done

# reverse
n=${#arr[@]}
for ((i=n-1; i>=0; i--)); do
  echo "${arr[$i]}"
done

# unique (preserve order)
declare -A seen
unique=()
for x in "${arr[@]}"; do
  [[ -v seen[$x] ]] && continue
  seen[$x]=1
  unique+=("$x")
done

# array of "records" (delimiter-based)
users=("alice:30:admin" "bob:25:user")
for u in "${users[@]}"; do
  IFS=':' read -r name age role <<< "$u"
  echo "$name ($age) - $role"
done

# simulate 2D with name-mangled vars
declare -A grid
grid["0,0"]=1; grid["0,1"]=2
grid["1,0"]=3; grid["1,1"]=4
for key in "${!grid[@]}"; do
  IFS=',' read -r r c <<< "$key"
  echo "[$r,$c] = ${grid[$key]}"
done

# arrays of arrays (not directly supported)
# workaround: use indirection
rows=("row0" "row1")
row0=(a b c)
row1=(d e f)
ref="${rows[0]}[@]"
echo "${!ref}"                   # a b c

path="/var/log/app.log"

# length
echo "${#path}"                 # 16

# substring (offset:length)
echo "${path:5}"                # log/app.log
echo "${path:5:3}"              # log

# remove from front (shortest/longest)
echo "${path#*/}"               # var/log/app.log
echo "${path##*/}"              # app.log (basename)

# remove from end
echo "${path%/*}"               # /var/log (dirname)
echo "${path%%.*}"              # /var/log/app

# replace
echo "${path/log/LOG}"          # /var/LOG/app.log (first)
echo "${path//log/LOG}"         # all occurrences

s="Hello World"

# bash 4+
echo "${s,,}"                   # hello world (lower)
echo "${s^^}"                   # HELLO WORLD (upper)
echo "${s^}"                    # Hello World (capitalize first)
echo "${s,}"                    # hello World (lowercase first)

# pattern-based
echo "${s^^[aeiou]}"            # hEllO wOrld (vowels upper)

# portable (pre-bash 4)
echo "$s" | tr 'a-z' 'A-Z'
echo "$s" | awk '{print toupper($0)}'

# declare typed variables
declare -u UPPER="hello"         # HELLO
declare -l LOWER="WORLD"         # world

# split on delimiter
csv="a,b,c,d"
IFS=',' read -ra parts <<< "$csv"
echo "${parts[2]}"              # c
for p in "${parts[@]}"; do echo "$p"; done

# join with delimiter (bash 4+)
arr=(a b c d)
joined=$(IFS=,; echo "${arr[*]}")
echo "$joined"                   # a,b,c,d

# custom joiner
join_by() {
  local d=$1; shift
  local first=$1; shift
  printf '%s' "$first"
  printf '%s' "$d%s" "$@"
}
join_by " | " a b c              # a | b | c

# split into lines
tr ',' '\n' <<< "a,b,c"

s="Hello World"

# substring search
if [[ "$s" == *"World"* ]]; then echo "contains"; fi
if [[ "$s" == Hello* ]]; then echo "starts with"; fi
if [[ "$s" == *World ]]; then echo "ends with"; fi

# regex match
if [[ "$s" =~ ^H[a-z]+\sW[a-z]+$ ]]; then echo "match"; fi
echo "${BASH_REMATCH[0]}"       # whole match
echo "${BASH_REMATCH[1]}"       # first group

# case-insensitive
shopt -s nocasematch
[[ "$s" == *world* ]] && echo "yes"
shopt -u nocasematch

# index (no built-in; use expr)
expr "$s" : ".*World"            # length up to match

# basic
printf '%s\n' "hello" "world"   # one per line
printf '%s = %d\n' "count" 42

# width and padding
printf '%10s\n' "right"         # right-aligned, width 10
printf '%-10s|\n' "left"        # left-aligned
printf '%05d\n' 42              # 00042 (zero-padded)
printf '%.2f\n' 3.14159         # 3.14

# date formatting
printf 'Today: %(%Y-%m-%d)T\n' -1

# to variable
printf -v today '%(%Y%m%d)T' -1
echo "$today"                    # 20260621

# repeat
printf '=%.0s' {1..40}; echo     # 40 equals signs

s="user_42"

if [[ "$s" =~ ^([a-z]+)_([0-9]+)$ ]]; then
  echo "Match!"
  echo "Name: ${BASH_REMATCH[1]}"   # user
  echo "ID: ${BASH_REMATCH[2]}"     # 42
fi

# email-ish check
email="[email protected]"
if [[ "$email" =~ ^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$ ]]; then
  echo "valid"
fi

# case-insensitive
shopt -s nocasematch
[[ "$s" =~ USER ]] && echo "yes"
shopt -u nocasematch

# basic (BRE) — default
grep 'foo' file
grep 'a\.' file                 # literal dot (escape in BRE)
grep 'a*b' file                  # zero or more a

# extended (ERE) — -E
grep -E 'a+b' file               # one or more
grep -E 'a|b' file               # alternation
grep -E '(ab)+' file             # grouping
grep -E '[0-9]{3}' file          # quantifier

# perl (PCRE) — -P
grep -P '\d{3}' file            # digits
grep -P '\bword\b' file        # word boundary
grep -P '(?<=foo)bar' file       # lookbehind

# anchors
grep -E '^Error' file            # line start
grep -E 'done$' file             # line end
grep -E '^Error.*timeout$' file

# BRE (default)
sed 's/foo/bar/' file
sed 's/a\.b/x/' file            # literal dot
sed 's/a\(b\)c/\1/' file      # capture with backrefs

# ERE with -E
sed -E 's/a+b/x/' file
sed -E 's/(foo|bar)/X/g' file
sed -E 's/([0-9]{4})/Year: \1/' file

# case-insensitive
sed 's/foo/bar/I' file
sed 's/[fF][oO][oO]/bar/g' file

# common recipes
sed -E 's/^[[:space:]]+//' file  # trim leading whitespace
sed -E 's/[[:space:]]+$//' file  # trim trailing
sed -E 's/ +/ /g' file           # collapse spaces
sed -E 's#https?://[^ ]+##g' file # strip URLs

# match operator ~
awk '$1 ~ /^[0-9]+/' file        # field 1 is numeric
awk '$0 !~ /debug/' file         # line doesn't contain
awk '/^Error/,/^$/' file         # from Error to blank line

# case-insensitive
awk 'BEGIN { IGNORECASE=1 } /error/' file
awk 'tolower($0) ~ /error/' file

# gsub (global substitute)
awk '{ gsub(/foo/, "bar"); print }' file
awk '{ gsub(/[0-9]+/, "N"); print }' file

# match with capture (gawk)
awk 'match($0, /id=([0-9]+)/, m) { print m[1] }' file

# split on regex
awk '{ n = split($0, parts, /[,;\t]/); print n }' file

# IPv4
grep -E '([0-9]{1,3}\.){3}[0-9]{1,3}' access.log

# URL
grep -Eo 'https?://[^[:space:]]+' file

# email
grep -Eo '[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}' file

# date YYYY-MM-DD
grep -E '[0-9]{4}-[0-9]{2}-[0-9]{2}' file

# hex color
grep -Eo '#[0-9A-Fa-f]{6}' file

# strong password (8+, upper, lower, digit)
grep -E '^(?=.*[a-z])(?=.*[A-Z])(?=.*[0-9]).{8,}$' file

# trim whitespace
sed -E 's/^[[:space:]]+//; s/[[:space:]]+$//' file

# extract domain from URL
sed -E 's#https?://([^/]+).*#\1#' file

#!/bin/bash
set -e                           # exit on error
set -u                           # error on unset variable
set -o pipefail                  # pipe fails if any command fails
set -x                           # print commands (trace)
# or combined:
set -euo pipefail

# shorthand
set -Eeuxo pipefail              # -E: ERR trap inherits

# strict mode for robust scripts
set -euo pipefail
IFS=$'\n\t'

# trace specific section
set -x
complex_logic
set +x

# debug a script
bash -x script.sh
bash -xv script.sh               # verbose + trace

# trace entire script
bash -x script.sh

# trace inside script
set -x
echo "tracing on"
set +x

# customize PS4 (the trace prompt)
export PS4='+ ${BASH_SOURCE:-shell}:${LINENO}: ${FUNCNAME[0]:-main}() '
set -x
my_function
set +x

# trace to file
exec 5> /tmp/trace.log
BASH_XTRACEFD=5
set -x
# ... commands ...
set +x
exec 5>&-

set -e
set -E                           # ERR trap inherits functions

on_error() {
  local exit_code=$?
  local line=$1
  echo "ERROR: command failed at line $line with exit code $exit_code" >&2
  echo "Stack:" >&2
  local i=0
  while caller $i >&2; do ((i++)); done
  exit $exit_code
}
trap 'on_error $LINENO' ERR

# now any failing command triggers the handler
false                            # triggers on_error

# caller builtin gives file:line:func
# stack trace via: while caller $i; do ((i++)); done

# install: apt install shellcheck (or brew, or use shellcheck.net)
# run:
shellcheck script.sh
shellcheck -x script.sh          # trust includes (no SC1090 warnings)
shellcheck -S warning script.sh  # only warnings+
shellcheck --exclude=SC2086 *.sh # exclude specific rules

# common warnings:
# SC2086: double-quote to prevent globbing (use "$var")
# SC2046: quote $(cmd) to prevent word splitting
# SC2034: unused variable
# SC2155: declare and assign separately (masks return code)
# SC2004: $/${} unnecessary in arithmetic

# in script: disable inline
# shellcheck disable=SC2086
echo $unquoted_var

# disable for next line
# shellcheck disable=SC2086
echo $args

# logging function with levels
log() {
  local level=$1; shift
  local msg="$*"
  local ts=$(date '+%Y-%m-%d %H:%M:%S')
  local color
  case $level in
    DEBUG) color='\033[90m' ;;
    INFO)  color='\033[32m' ;;
    WARN)  color='\033[33m' ;;
    ERROR) color='\033[31m' ;;
  esac
  printf "%b[%s] [%s] %s\033[0m\n" "$color" "$ts" "$level" "$msg" >&2
}

log INFO "Starting process"
log WARN "Disk space low"
log ERROR "Connection failed"

# verbose flag
[[ $VERBOSE ]] && set -x

# dry-run mode
if [[ $DRY_RUN ]]; then
  echo "Would: $cmd"
else
  eval "$cmd"
fi

# ALWAYS quote variables
file="my file.txt"
rm $file                         # BAD: rm my file.txt (3 args!)
rm "$file"                       # GOOD: rm "my file.txt"

# use arrays for command + args
files=("a.txt" "b file.txt" "c.txt")
rm "${files[@]}"                # each file as separate arg, safely

# avoid eval
eval "echo $user_input"          # DANGEROUS
echo "$user_input"               # safe

# avoid command substitution in unsafe places
# BAD: filename from user, executed
cmd="ls $userdir"
$cmd                             # word-splitting + globbing

# GOOD: use array
cmd=(ls "$userdir")
"${cmd[@]}"

# safe temp files
tmp=$(mktemp)                    # atomic, unpredictable name
trap 'rm -f "$tmp"' EXIT

# safe temp dir
tmpdir=$(mktemp -d)
trap 'rm -rf "$tmpdir"' EXIT

# secrets: read from env, never hardcode
api_key="${API_KEY:?Set API_KEY env var}"

# don't leak secrets to process list (ps)
# BAD: curl -H "Authorization: Bearer $TOKEN" ...
# GOOD: pass via stdin or config file
curl -H @- <<EOF https://api.example.com
Authorization: Bearer $TOKEN
EOF

# restrict file perms for secrets
umask 077
echo "$api_key" > ~/.secrets/api
chmod 600 ~/.secrets/api

# validate numeric input
read -p "Enter a number: " n
if ! [[ "$n" =~ ^[0-9]+$ ]]; then
  echo "Not a number" >&2; exit 1
fi

# validate range
if (( n < 1 || n > 100 )); then
  echo "Out of range" >&2; exit 1
fi

# validate file path (no traversal)
path="$1"
case "$path" in
  *..*|/*) echo "Invalid path" >&2; exit 1 ;;
esac

# sanitize filename
safe=$(echo "$name" | tr -cd 'A-Za-z0-9._-')

# whitelist allowed values
case "$action" in
  start|stop|restart) ;;
  *) echo "Invalid action" >&2; exit 1 ;;
esac

# drop privileges in a script running as root
if [[ $EUID -eq 0 ]]; then
  exec sudo -u appuser "$0" "$@"
fi

# check for root
if [[ $EUID -ne 0 ]]; then
  echo "Requires root" >&2; exit 1
fi

# setuid scripts are DANGEROUS — avoid them
# instead, use sudoers entries
# /etc/sudoers.d/myapp:
#   appuser ALL=(root) NOPASSWD: /usr/local/bin/myapp.sh

# restrict script permissions
chmod 700 script.sh              # only owner can run
chown appuser:appuser script.sh

# use capsh instead of full root for fine-grained caps
# setcap 'cap_net_bind_service=+ep' /usr/bin/myapp

# verify checksum
sha256sum -c file.sha256
# file.sha256 contains: <hash>  <filename>

# generate checksums
sha256sum release.tar.gz > release.tar.gz.sha256
sha256sum *.iso > isos.sha256

# GPG sign and verify
gpg --sign file.txt              # creates file.txt.gpg
gpg --verify file.txt.gpg
gpg --detach-sig --armor file.tar.gz
gpg --verify file.tar.gz.asc file.tar.gz

# download + verify in one go
curl -fsSL https://example.com/install.sh | sha256sum
curl -fsSL https://example.com/install.sh -o install.sh
sha256sum -c <(echo "expected_hash  install.sh")

# POST with data
curl -X POST -d "name=Alice" https://api.example.com
# JSON POST
curl -H "Content-Type: application/json" \
     -d '{"name":"Alice"}' https://api.example.com
# Download with progress
curl -O https://example.com/file.zip
# Headers only
curl -I https://example.com

# Download file
wget https://example.com/file.zip
# Mirror website
wget -m https://example.com
# Resume download
wget -c https://example.com/largefile.zip
# Recursive download
wget -r -l 2 https://example.com
# Background
wget -b https://example.com/file.zip

# SSH connect
ssh user@host
# SSH with key
ssh -i ~/.ssh/id_rsa user@host
# Port forwarding
ssh -L 8080:localhost:80 user@host
# Copy file
scp file.txt user@host:/path/
# Copy directory
scp -r dir/ user@host:/path/

# List listening ports
netstat -tlnp
ss -tlnp
# Show all connections
netstat -an
ss -tunap
# Show routing table
netstat -r
# Show interface stats
netstat -i

# Check connectivity
ping -c 4 google.com
# Trace route
traceroute google.com
# DNS lookup
dig example.com
dig +short example.com
# Get DNS records
dig MX example.com
dig NS example.com

greet() {
    local name="$1"
    echo "Hello, $name"
    return 0
}
greet "Alice"
# Return value via $?
result=$(greet "Bob")

if [ -f "/path/file" ]; then
    echo "File exists"
elif [ -d "/path" ]; then
    echo "Directory exists"
else
    echo "Not found"
fi
# String comparison
if [ "$str" = "hello" ]; then ... fi
# Numeric
if [ "$num" -gt 10 ]; then ... fi

# For loop
for i in 1 2 3; do
    echo $i
done
# C-style
for ((i=0; i<5; i++)); do echo $i; done
# While loop
while read line; do
    echo "$line"
done < file.txt
# Iterate files
for f in *.txt; do process "$f"; done

# Declare array
fruits=("apple" "banana" "cherry")
# Access
echo ${fruits[0]}      # apple
echo ${fruits[@]}      # all
echo ${#fruits[@]}     # length
# Iterate
for fruit in "${fruits[@]}"; do
    echo "$fruit"
done
# Associative array
declare -A config
config[key]="value"

set -e  # Exit on error
set -u  # Error on undefined variable
set -o pipefail  # Pipeline fails if any command fails
set -euo pipefail  # All three (recommended)
trap 'echo "Error on line $LINENO"' ERR
trap 'cleanup' EXIT
# Check command result
if ! command -v docker &> /dev/null; then
    echo "docker not installed"
fi

# By name
find . -name "*.txt"
# By type and size
find . -type f -size +1M
# By modification time
find . -mtime -7  # Modified in last 7 days
# Execute command
find . -name "*.log" -exec gzip {} \;
# Delete
find . -name "*.tmp" -delete

# Create tar.gz
tar -czvf archive.tar.gz dir/
# Extract
tar -xzvf archive.tar.gz
# List contents
tar -tzvf archive.tar.gz
# zip/unzip
zip -r archive.zip dir/
unzip archive.zip

# Sync local
rsync -av src/ dest/
# Sync to remote
rsync -avz src/ user@host:/path/
# Delete files not in source
rsync -av --delete src/ dest/
# Dry run
rsync -av --dry-run src/ dest/
# Exclude
rsync -av --exclude='*.log' src/ dest/

# Symbolic
chmod u+x file      # User execute
chmod g-w file      # Group write off
chmod a=r file      # All read only
# Numeric
chmod 755 file      # rwxr-xr-x
chmod 644 file      # rw-r--r--
# Change owner
chown user:group file
# Recursive
chmod -R 755 dir/

# Hard link
ln file.txt hardlink.txt
# Symbolic link
ln -s /path/to/target symlink
# Find broken symlinks
find . -type l ! -exec test -e {} \; -print
# Copy symlink target
cp -L symlink dest/

# BUG: breaks with spaces
for f in $(ls); do echo "$f"; done
# GOOD: handle spaces
for f in *.txt; do echo "$f"; done
# Or with find
find . -name "*.txt" -print0 | while IFS= read -r -d '' f; do
    echo "$f"
done

# cp: copy (original remains)
cp file.txt backup.txt
# mv: move/rename (original removed)
mv file.txt newname.txt
# Common mistake: overwriting
cp important.txt /tmp/  # If /tmp/important.txt exists, overwritten!
# Use -i for safety
cp -i file.txt /tmp/

# DANGEROUS: recursive force
rm -rf /  # Never do this!
rm -rf $VAR/  # If $VAR is empty, becomes rm -rf /
# Safer: check first
if [ -n "$VAR" ]; then
    rm -rf "$VAR/"
fi
# Use trash-cli instead
trash-put file.txt

#!/bin/bash          # bash script
#!/usr/bin/env bash  # Portable bash
#!/usr/bin/env python3  # Python
#!/bin/sh            # POSIX sh (more portable)
# Make executable
chmod +x script.sh
./script.sh

# Check last command
if [ $? -eq 0 ]; then
    echo "Success"
fi
# Better: direct check
if command; then
    echo "Success"
else
    echo "Failed"
fi
# Exit with status
exit 0  # Success
exit 1  # Failure