MESSAGE="Hello, world!"
echo $MESSAGE
echo "$MESSAGE"
echo "${MESSAGE}"

echo "OS: $(uname)"
# 等价于
echo "OS: `uname`"

git commit && git push
git commit || echo "Commit failed"

print_info() {
  local message=$1
  echo "$(date) - INFO $message"
}

print_log "System launched"

if [[ -z "$string" ]]; then
  echo "String is empty"
elif [[ -n "$string" ]]; then
  echo "String is not empty"
fi

echo {A,B}.js

name="John"
echo ${name}
echo ${name/J/j}       #=> "john" (替换)
echo ${name:0:2}       #=> "Jo" (截取)
echo ${name::2}        #=> "Jo" (截取)
echo ${name::-1}       #=> "Joh" (截取)
echo ${name:(-1)}      #=> "n" (从右边截取)
echo ${name:(-2):1}    #=> "h" (从右边截取)
echo ${VERSION:-1.0.1} #=> "1.0.1" (若变量VERSION不存在或是空值， 则取1.0.1)

length=2
echo ${name:0:length}   #=> "Jo"
echo ${name:0:$length}  #=> "Jo" in zsh

STR="/path/to/foo.cpp"
echo ${STR%.cpp}    # /path/to/foo
echo ${STR%.cpp}.o  # /path/to/foo.o
echo ${STR%/*}      # /path/to

echo ${STR##*.}     # cpp (extension)
echo ${STR##*/}     # foo.cpp (basepath)

echo ${STR#*/}      # path/to/foo.cpp
echo ${STR##*/}     # foo.cpp

echo ${STR/foo/bar} # /path/to/bar.cpp

SRC="/path/to/foo.cpp"
BASE=${SRC##*/}   #=> "foo.cpp" (basepath)
DIR=${SRC%$BASE}  #=> "/path/to/" (dirpath)

MSG='/path/to/apple/apple/apple.txt'
SUFFIX='apple/apple/apple.txt'

echo ${MSG%apple.txt}     #=> /path/to/apple/apple/ (删除后缀)
echo ${MSG%*.txt}         #=> /path/to/apple/apple (通配符删除后缀)
echo ${MSG%$SUFFIX}       #=> /path/to/ (指定变量删除后缀)
echo ${MSG%apple*.txt}    #=> /path/to/apple/apple/ (一个 % 按最短字符串删除)
echo ${MSG%%apple*.txt}   #=> /path/to/ (两个 % 按最长字符串删除)

# 单行注释

: '
多
行
注
释
'

for i in /etc/rc.*; do
  echo $i
done

for ((i = 0 ; i < 100 ; i++)); do
  echo $i
done

for i in {1..5}; do
    echo "Welcome $i"
done

for i in {5..50..5}; do
    echo "Welcome $i"
done

cat file.txt | while read line; do
  echo $line
done

while true; do
  ···
done

myfunc() {
    echo "hello $1"
}

# Same as above (alternate syntax)
function myfunc() {
    echo "hello $1"
}

myfunc "John"

myfunc() {
    local myresult='some value'
    echo $myresult
}

result="$(myfunc)"

myfunc() {
  return 1
}

if myfunc; then
  echo "success"
else
  echo "failure"
fi

# String
if [[ -z "$string" ]]; then
  echo "String is empty"
elif [[ -n "$string" ]]; then
  echo "String is not empty"
else
  echo "This never happens"
fi

# Combinations
if [[ X && Y ]]; then
  ...
fi

# Equal
if [[ "$A" == "$B" ]]

# Regex
if [[ "A" =~ . ]]

if (( $a < $b )); then
   echo "$a is smaller than $b"
fi

if [[ -e "file.txt" ]]; then
  echo "file exists"
fi

Fruits=('Apple' 'Banana' 'Orange')

Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"

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[@]}" "Watermelon")    # 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
lines=(`cat "logfile"`)                 # Read from file

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

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

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

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

set -o noclobber  # Avoid overlay files (echo "hi" > foo)
set -o errexit    # Used to exit upon error, avoiding cascading errors
set -o pipefail   # Unveils hidden failures
set -o nounset    # Exposes unset variables

shopt -s nullglob    # Non-matching globs are removed  ('*.foo' => '')
shopt -s failglob    # Non-matching globs throw errors
shopt -s nocaseglob  # Case insensitive globs
shopt -s dotglob     # Wildcards match dotfiles ("*.sh" => ".foo.sh")
shopt -s globstar    # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')

$((a + 200))      # Add 200 to $a

$(($RANDOM%200))  # Random number 0..199

(cd somedir; echo "I'm now in $PWD")
pwd # still in first directory

python hello.py > output.txt   # stdout to (file)
python hello.py >> output.txt  # stdout to (file), append
python hello.py 2> error.log   # stderr to (file)
python hello.py 2>&1           # stderr to stdout
python hello.py 2>/dev/null    # stderr to (null)
python hello.py &>/dev/null    # stdout and stderr to (null)

python hello.py < foo.txt      # feed foo.txt to stdin for python

command -V cd
#=> "cd is a function/alias/whatever"

trap 'echo Error at about $LINENO' ERR

traperr() {
  echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
}

set -o errtrace
trap traperr ERR

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

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

source "${0%/*}/../share/foo.sh"

printf "Hello %s, I'm %s" Sven Olga
#=> "Hello Sven, I'm Olga

printf "1 + 1 = %d" 2
#=> "1 + 1 = 2"

printf "This is how you print a float: %f" 2
#=> "This is how you print a float: 2.000000"

DIR="${0%/*}"

while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in
  -V | --version )
    echo $version
    exit
    ;;
  -s | --string )
    shift; string=$1
    ;;
  -f | --flag )
    flag=1
    ;;
esac; shift; done
if [[ "$1" == '--' ]]; then shift; fi

cat <<END
hello world
END

echo -n "Proceed? [y/n]: "
read ans
echo $ans

read -n 1 ans    # Just one character

pwd # /home/user/foo
cd bar/
pwd # /home/user/foo/bar
cd -
pwd # /home/user/foo

if ping -c 1 google.com; then
  echo "It appears you have a working internet connection"
fi

if grep -q 'foo' ~/.bash_history; then
  echo "You appear to have typed 'foo' in the past"
fi