Shell

Variables

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

Shell Variables

• $#: 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[@]:3:2}       # Range (from position 3, length 2)
echo ${!Fruits[@]}          # Keys of all elements, space-separated



Fruits=("${Fruits[@]}" "Grape")         # Push
Fruits+=('Watermelon')                  # Also Push
Fruits=( ${Fruits[@]/Ap*/} )            # Remove by regex match
unset Fruits[2]                         # Remove one item
Fruits=("${Fruits[@]}")                 # Duplicate
Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate

TempFiles=($(ls /tmp))

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

if [[ " ${Fruits[*]} " =~ " Banana " ]]; then
  echo "Apple 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

if

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

case

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

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

exitcode

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

Loop

# index
for i in `seq 1 10`; do
  echo $i 
done

for i in `seq 1 2 10`; do
  echo $i
done

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

# array
arr=( a b c )
for v in ${arr[@]}; do
  echo $v
done

# lines
cat file.txt | while read l; do
  echo $l
done

# forever
while true; do
  sleep 1
done

Function

Returns value

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

Returns boolean

_is_win() {
    if [[ "$(uname)" =~ "_NT" ]]; then
        return 0
    else
        return 1
    fi
}

if _is_win; then
  echo 'windows'
fi

Value

Condition

Math

a=$(($RANDOM%200))  # Random number 0..199
$((a + 200))        # Add 200 to $a

Redirect

• >: equal to 1>
• 1>&2： redirect stdout to stderr
• 2>&1： redirect stderr to stdout
• >& file: redirect stdout/stderr to file

Heredoc

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

Yes/No

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

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