TL;dr

# quietly add a user without password
adduser --quiet --disabled-password --shell /bin/bash --home /home/newuser --gecos "User" newuser
# set password
echo "newuser:newpassword" | chpasswd
nc -w 1 qq.com 80 < /dev/null && echo "tcp port ok"
pstree -c -p -s -S -h -a 17648
[ -f /etc/passwd ] && ( echo "1" ) || :
sudo pipinstall ngxtop
sudo yum install -y libpcap libpcap-devel
sudo yum install -y epel-release
sudo yum install -y nethogs nload

/usr/bin/ncat --listen -4 -vvv --keep-open --wait 50 --sh-exec "cat -n | tee -a ./out.txt" 8080
/usr/bin/ncat --listen -4 --verbose --keep-open --wait 50 --sh-exec "cat -n | tee -a ./out.txt" 9090

/usr/bin/nping -4 --verbose -c 1 -p 9090 localhost

python3 -c "import socket;print([(s.connect(('8.8.8.8', 53)), s.getsockname()[0], s.close()) for s in [socket.socket(socket.AF_INET, socket.SOCK_DGRAM)]][0][1])"

ip a |grep <ip> |awk '{print $NF}'
ip r |grep <ip> |cut -d " " -f 3

systemd-cgls --no-pager


sysctl -a --ignore |& grep ipv4.ip_forward
sysctl -a --ignore 2>&1 | grep ipv4.ip_forward
sysctl -a --ignore 2>/dev/null | grep rp_filter
sysctl -a --ignore 2>&1 | grep ipv4.ip_

sysctl --all --ignore 2>&1 |wc -l
## shell下循环输出 1-10
# 方法1

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

# 方法2

for i in $(seq 1 10);do echo $i;done

# 方法3

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

## shell下计算 5+6
# 方法1

let a=5;b=6;let c=a+b;echo $c

# 方法2

a=5;b=6;c=$((a+b)); echo $c


## shell下判断 /tmp/f 目录是否存在,如果不存在则创建目录
# 方法1

[ -d /tmp/f ] || { mkdir -p /tmp/f; }

# 方法2

if [ ! -d "/tmp/f" ]; then mkdir /tmp/f; fi

ref