Uly.me

cloud engineer

Check Open Ports Remotely

Here are a few tools to check if ports are open from a remote host.

nc -zvw10 192.168.0.1 22
nmap 192.168.0.1 -p 22
telnet 192.168.0.1 22

You may have to install netcat and nmap if missing in your distro.

yum install nc
yum install nmap

Bash Variable Empty

Here’s how to check in Bash if a variable is empty or unset.

if [ -z "${VAR}" ]; 
then
  echo 'do something'
else
  echo 'do another'
fi

One liner

if [ -z "${VAR}" ]; then echo 'do something'; else echo 'do another'; fi

The inverse

if [ ! -z "${VAR}" ]; 
then
  echo 'do something'
else
  echo 'do another'
fi
if [ ! -z "${VAR}" ]; then echo 'do something'; else echo 'do another'; fi

Check Reboot After Updates

Here’s the command to check if the system needs a reboot after running yum update.

Must be root or run it sudo.

$ needs-restarting -r ; echo $?
Core libraries or services have been updated:
  openssl-libs -> 1:1.0.2k-21.el7_9
  kernel -> 3.10.0-1160.31.1.el7
  dbus -> 1:1.10.24-15.el7
  linux-firmware -> 20200421-80.git78c0348.el7_9
  glibc -> 2.17-324.el7_9
  systemd -> 219-78.el7_9.3
 
Reboot is required to ensure that your system benefits from these updates.
 
More information:
https://access.redhat.com/solutions/27943
1

In this instance, it requires a reboot due to a new kernel.