sh.txt

# bash: the default shell in linux

# change the default shell
	$ chsh <username> -s /bin/sh 

# output to null
	$ <command> >/dev/null
# redirect stderr 2 to stdout
	$ <command> 2>&1
		# examples: 
		$ cat <filename> > output.txt 2>&1
		$ cat <filename> &> output.txt
	> # rediect and overwrite
	>> # redirect and append
# set time to seconds in UTC
	$ starttime=$(date +%s)
# output from last command
	$?
# all params
	$@ # $* all params separated by space
# the number of arguments, or the number of positional parameters passed to the script, shell or shell function
	$#

# Grammar
# During the start command like ./start.sh, the following sentence will give the value "./" to SDIR
SDIR=$(dirname "$0")
# assign the default value
: ${A:=value}

# Useful tools
# tr command is used to translate, for instance if I want to translate upper case to lower case, the txt file cantains string HELLOWORLD
	cat helloworld.txt | tr "[A-Z]" "[a-z]" # will output helloworld
# sed
	# -i option(-it for darwin): change the content directly w/o printing on console, otherwise it will just print on console and will not change the original file
	# general usage
		$ sed -i "s/<beReplacedString>/<replaceString>/g" <filaPath>
		# w/o g, it will only change if the beReplacedString shows up at head of each line 
# awk
# for more info about powerful "sed" and "awk", refer https://blog.csdn.net/gv7lzb0y87u7c/article/details/80490997

# shell if statements
# basic usages
	if [ condition ]; then
		commands
	elif  [ condition ]; then
		commands
	else
		commands
	fi
# check if the first arg is an empty string, where ! means 'not', -z meanns empty
    if [ ! -z $1 ]; then
# check the arguments number equals 3
    if [ $# -eq 3  ]; then
# check if the directory exists
	if [ -d "<filePath>" ]; then	
# comparison
	$ -lt # less than
	$ -ne # not equal
# AND/OR
	$ -o # or 
	# -a # and

# case usages
case ${VAR} in
	pattern1)
		command1
		;;
	pattern2)
		command2
		;;
esac

# while 
i=1
sum=0
while [ $i -le 100 ]
do 
	let sum=sum+$i
	let i++
done

# until
until [ ! $i -lt 100 ]
do 
	let sum=sum+$i
	let i++
done


# some usages
# get the OS ARCH(My laptop will have the value of darwin-amd64)
	$ OS_ARCH=$(echo "$(uname -s|tr '[:upper:]' '[:lower:]'|sed 's/mingw64_nt.*/windows/')-$(uname -m | sed 's/x86_64/amd64/g')" | awk '{print tolower($0)}')

# replace the first occurance of a pattern with a given string, use $(parameter/pattern/string}
# replace all occurances of a pattern, use $(parameter//pattern/string}
	$ echo $(STRING// /$';'}

# array definition
	arr=("a" "b" "c")
# array length
	${#arr[@]}

# set domain to ip address
	$ vim /etc/hosts # then type the mapping