if condition;then
  pass 
fi

if condition
then
  pass 
fi

if condition1
then
  pass1 
elif condition2
  pass2
else
  pass3
fi

case $var in
"var1")
  pass1
;;
"var")
  pass2
;;
*)
  pass
;;
esac

for((初始值;判断;变量变化)) # 因为(()),因此内部可以直接使用数学上的运算式
do
  pass
done

# for var in {1..100} # 遍历从1到100这个序列
# for var in `ls` # 
for var in var1 var2 var3
do
  pass
done

while condition
do
  pass
done

# reading from a File
while read line; do
    echo "$line"
done < filename.txt

function add()
{
  s=$1 # $1为传入的参数
  echo $s 
  # return 0 # 如果是返回状态码，建议使用return
}
sum=$(add $a)
echo $sum

# extract a string surround by "=> " and" (" from a long string

# Perl regex, available in GNU grep
# (?<==> ): positive lookbehind for =>
# .*?: minimal match
# (?= \(): positive lookahead for " ("
echo 'some text => desired_string (more text)' | grep -oP '(?<==> ).*?(?= \()'

# sed(more portable)
echo 'some text => desired_string (more text)' | sed -n 's/.*=> \(.*\) (.*/\1/p'

# awk
# -F'=> | \\(': splits by either => or (
# {print $2}: prints the part between them
echo 'some text => desired_string (more text)' | awk -F'=> | \\(' '{print $2}'

#!/bin/bash

files=$(find "." -type f)

# 遍历文件
for file in $files; do
    # 检查文件名是否符合特定首尾条件
    if [[ "$file" == *_posts* && "$file" == *.md ]]; then
        # 获取文件的最近修改时间
        last_modified=$(stat -c "%Y" "$file")

        # 将时间戳转换为日月年时分秒的格式
        last_modified_formatted=$(date -d @"$last_modified" +"%Y-%m-%d %H:%M:%S %z")

        # 替换文件的第四行为最近修改时间
        sed -i "4s/.*/date:   $last_modified_formatted/" "$file"

        echo "Updated date in $file to $last_modified_formatted"
    fi
done

ls | while read line
do
  file=$line
  if echo $file | grep -q -E '\.bag$'
  then
    if [ 18 = `echo ${#file}` ] 
      then
      # time=`rosbag info $file | grep start`
      # str=`date -d @${time: 0-14: 13} "+%Y-%m-%d %H:%M:%S"`
      time=${file: 0: 14}
      echo $time >> date_t.txt
    fi
  fi
done

mkdir -p /home/bs/.config/systemd/user/ 
cd /home/bs/.config/systemd/user/

[Unit]
Description=describe your service here
# This service starts your custom script after the network is available. If this service id fully independent and does not rely on the state of any other services, you can delete this line
After=network.target

[Service]
Type=simple
# Use `ExecStart` to call a wrapper script to ensure that the script handles the termination of all its child processes upon receiving a signal to stop.
# shell script, it must be absolute path
ExecStart=/home/bs/.config/systemd/user/shell.sh
# ensures that all processes started by the service (including any child processes started by the script) will be terminated when the service itself is stopped.
KillMode=control-group
# on-failure: restart this service if this service dies; no: Don't restart the service automatically
Restart=on-failure

[Install]
WantedBy=default.target
# This service will be started at the default runlevel

systemctl --user daemon-reload

systemctl --user enable myservice.service

#!/bin/bash

# Function to terminate all roslaunch processes
cleanup() {
    echo "Stopping all ROS nodes..."
    kill -SIGINT "$roslaunch1_pid" "$roslaunch2_pid" "$roslaunch3_pid"
}

# The trap command in the script sets up a handler for SIGTERM, which is the signal sent by systemd when the service is stopped. This handler function, cleanup, will explicitly kill all the background processes.
trap cleanup SIGTERM

# Start roslaunch commands in background
sleep 1 && roslaunch your_package launch_file1.launch & roslaunch1_pid=$!
sleep 2 && roslaunch your_package launch_file2.launch & roslaunch2_pid=$!
sleep 3 && roslaunch your_package launch_file3.launch & roslaunch3_pid=$!

# The wait command makes the main script wait for all the background processes, which means the main service process remains active as long as the background processes are running.
wait $roslaunch1_pid $roslaunch2_pid $roslaunch3_pid

systemctl --user start myservice.service # start service
systemctl --user status myservice.service # check service
systemctl --user restart myservice.service # restart service
systemctl --user stop myservice.service # stop service

sudo apt install ntp # install ntp
sudo systemctl start ntp # start ntp

sudo nano /etc/systemd/timesyncd.conf # you can also use vim or gedit

[Time]
NTP=bs 192.168.1.100
FallbackNTP=bs 192.168.1.100

sudo systemctl restart systemd-timesyncd

timedatectl status

sudo chmod +x /usr/local/func/myscript

crontab -e

@reboot /usr/local/func/myscript

crontab -l

@reboot /usr/local/func/myscript >> /var/log/myscript.log 2>&1

#!/bin/bash

# colorful output
RED='\033[0;31m'
GREEN='\033[0;32m'
YELLOW='\033[0;33m'
BLUE='\033[0;34m'
NC='\033[0m'

if [ "$#" -eq 0 ]; then
  echo -e "${RED}Should contain project name.${NC}"
  exit
fi

# (absolute path (relative path))
SHELL_DIR=$(realpath $(dirname "${BASH_SOURCE[0]}"))

name=$1

# whether file exists
# if [ -e "$name" ]; then
#   echo -e "${BLUE}File ${name} exists, skip generation.${NC}"
#   exit
# fi

# if [ -d "$name" ]; then
#     echo -e "${YELLOW}Directory ${name} exists, skip creation.${NC}"
#     exit
# fi

# Internal Field Separator
IFS='_' 
# Convert string into an array
read -ra PARTS <<< "$name"
# Reset IFS if needed
unset IFS

# STRING="part1_part2_part3"
# # Get specific parts
# FIRST_PART=$(echo "$STRING" | cut -d '_' -f 1)
# SECOND_PART=$(echo "$STRING" | cut -d '_' -f 2)
# THIRD_PART=$(echo "$STRING" | cut -d '_' -f 3)

# traverse an array 
for PART in "${PARTS[@]}"; do
  # echo "$PART"
  # Convert first character to uppercase
  Name=${Name}$(echo "$PART" | sed 's/./\U&/') 
  # convert all character to uppercase
  NAME=${NAME}${PART^^} 
  NA_ME=${NA_ME}${PART^^}"_"
done

mkdir -p $name
cd $name

echo $Name
echo $NAME
echo $NA_ME

# "eval" allows you to construct and execute commands dynamically.
eval "${NAME}VERSION=1.0"
echo "${COVFERVERSION}"

date "+%Y-%m-%d %H:%M" # use date as separator

var_name="${NAME}VERSION"
# Create the variable dynamically
declare "$var_name=2.0"
# Access the newly created variable using indirect reference
echo "${!var_name}"  # Correct way to reference it