Shell

Variables

# Default value
echo ${value:-xa} # xa
value=xyz/z-a/abc
# Head
echo ${value%/*} # xyz/z-a
echo ${value%%/*} # xyz
# Tail
echo ${value#*/} # z-a/abc
echo ${value##*/} # abc
# Truncate
echo ${value:7} # /abc
echo ${value:7:4} # /abc
echo ${value:(-4):4} # /abc
# Replace
echo ${value/a/A} # xyz/z-A/abc
echo ${value//a/A} # xyz/z-A/Abc

str="Hello World"
echo ${str,,}   # hello world (Lowercase)
echo ${str^^}   # HELLO WORLD (Uppercase)
echo ${str^}    # Hello World (Only first char)

Shell Variables

• $@: args list (recommended)
• $*: spconcatenate into a string (easy to mess up)
• $#: count args
• $n: nth arg
• $?: last exit code
• $!: last pid

Array

fruits=(Apple Banana Orange Pear)

echo "${fruits[0]}"           # Element #0
echo "${fruits[-1]}"          # Last element
echo "${fruits[@]}"           # All elements, space-separated
echo "${#fruits[@]}"          # Number of elements
echo "${#fruits}"             # String length of the 1st element
echo "${#fruits[3]}"          # String length of the Nth element
echo "${fruits[@]:1:2}"       # Range (from index 1 (second element), length 2)
echo "${!fruits[@]}"          # All keys: 0 1 ...

fruits=("${fruits[@]}" Grape)          # Push
fruits+=(Watermelon)                   # Also Push
unset fruits[2]                         # Remove one item
fruits=("${fruits[@]}")                 # Duplicate
fruits=("${fruits[@]}" "${Veggies[@]}") # Concatenate

for i in "${fruits[@]}"; do
  echo "$i"
done

if [[ " ${fruits[*]} " =~ " Banana " ]]; then
  echo "Banana in fruits"
fi

Dictionary

declare -A sounds

sounds[dog]="bark"
sounds[cow]="moo"
sounds[bird]="tweet"
sounds[wolf]="howl"

echo "${sounds[dog]}" # Dog's sound
echo "${sounds[@]}"   # All values
echo "${!sounds[@]}"  # All keys
echo "${#sounds[@]}"  # Number of elements
unset sounds[dog]     # Delete dog

# Value loop
for val in "${sounds[@]}"; do
  echo "$val"
done

# Key loop
for key in "${!sounds[@]}"; do
  echo "$key"
done

Condition

Test

Test equal with glob

• *: Any length of any character, e.g. [[ "$v" == *abc* ]]
• ?: any single character, e.g. [[ "$v" == rc? ]]
• [] any character in set, e.g. [[ "$v" == [a-z] ]]
• [!]: any character not in set, e.g. [[ "$v" == [!0-9] ]]

Test equal with extglob

shopt -s extglob

• ?(pattern)：0 or 1
• *(pattern)：0 or many
• +(pattern)：1 or many, e.g. [[ "$v" == +([0-9]) ]]
• @(pattern)：exact, e.g. [[ "$v" == @(apple|banana|orange) ]]
• !(pattern): exclude, e.g. [[ "$v" == !(tmp|cache) ]]

Use if expr

if [[ "foo" == "$1" ]]; then
  echo foo
elif [[ "bar" == "$1" ]]; then
  echo bar
else
  echo "not found"
  exit 1
fi

if grep -q 'alice' /etc/passwd; then
  echo "User alice exists"
fi

read -r -p "Are you sure (y/n)? " choice
if [ "$choice" = "y" ]; then
  echo choice yes
fi

Use case expr

case "$1" in
  start | up)
    echo start
    ;;
  stop | down)
    echo stop
    ;;

  *)
    echo "Usage: $0 {start|stop}"
    ;;
esac

Loop

# index
for i in {1..10}; do
  echo "$i"
done

for ((i=1; i<=10; i+=2)); do
  echo "$i"
done

# fs
for f in ./*; do
  echo "$f"
done

# array
IFS=, read -r -a arr <<< "a,b,c"
for v in "${arr[@]}"; do
  echo "$v"
done

# lines
while IFS= read -r l; do
  echo "$l"
done < file.txt

# forever
while true; do
  sleep 1
done

Function

Shell function has no real return value

• return can only return an exit code (0-255)
• echo is the actual way to return data

foo() {
    local msg='world'
    echo "$1 $msg"
}
result="$(foo hello)"
echo "$result"

_is_win() {
    [[ "$(uname)" =~ "_NT" ]]
}

if _is_win; then
    echo 'windows'
fi

Math

$((1 + 2))
$((10 - 3))
$((4 * 5))
$((20 / 3))   # Integer division => 6
$((RANDOM % 100))  # Random number 0..99

$((a = 5 + 3))
$((a++))
$((a--))
$((a += 10))
$((a *= 2))

((a == b))
((a != b))
((a < b))
((a <= b))
((a > b))
((a >= b))

Redirect

• <: stdin
• >: stdout
• >>: stdout in append mode
• 2>: stderr
• 1>&2： redirect stdout to stderr
• 2>&1： redirect stderr to stdout
• >file 2>&1: redirect stdout/stderr to file

Here String

grep foo <<<"$HOME"

Here Document

cat << EOF | sudo tee -a /etc/sudoers.d/$USER
$USER ALL=(ALL) NOPASSWD:ALL
EOF 

• <<EOF: will replace variable
• <<'EOF': not replace variable
• <<-EOF: will replace variable trim \t only
• <<-'EOF': not replace variable，trim \t only

Process Substitution

• <(cmd)：Use command stdout as a file, e.g. diff <(sort a.txt) <(sort b.txt)
• >(cmd): Write to the command stdin, e.g. tar cf >(gzip > out.tar.gz) dir/

Subshell

Modification of internal variables does not affect external ones

(cd /tmp && touch test.txt)

Command group

Executes in the current shell, used only for logical grouping

{ echo "header"; cat data.txt; echo "footer"; } > output.txt

There must be a space after {, and a semicolon or newline before }

Set Options

Misc

• source: Used for Running script files in the current Shell environment

source file.sh
. file.sh

• trap: Used to capture signals (such as EXIT, INT, TERM) and automatically execute specified cleanup commands when the script exits or is interrupted.

cleanup() {
    rm -f "$tmpfile"
    echo "Cleaned up on exit"
}
trap cleanup EXIT       # Execute when the script exits (including normal and error exits)
trap 'echo Ctrl+C' INT  # Capture SIGINT

• Export all environments/variables/functions/options of the current shell

( set -o posix; set )

• Load dotenv file

[ -f .env ] && { set -a; source .env; set +a; }

Common Tools

• Character related: find/sed/awk
• Network related: curl/wget
• Argument parser related: getopt/argc
• JSON/YAML/TOML related: jq/yq
• Interative related: gum