Category: Sysadmin

System administration is a special are of IT. It also has a special place in my heart. It is an interesting mixture of all the other disciplines, both common across the whole industry, and at the same time unique for each person, company, and geographical location. When I have something to say or share about system administration, I use this category.

Interpretation of NTFS Timestamps

“Interpretation of NTFS Timestamps” is a fascinating technical dive into the NTFS filesystem and the way it stores file and directory timestamps. Let me just leave you with this quote:

NTFS file timestamps, according to the documentation of the ‘FILETIME’ data structure in the Windows Software Development Toolkit, is a  “64-bit value representing the number of 100-nanosecond intervals since January 1, 1601 (UTC)”.

WTF?

Lazydocker – a simple terminal UI for both docker and docker-compose

Lazydocker is a simple terminal UI for easier management of Docker. This is particularly useful for new Docker users, but can as well save plenty of keystrokes to the seasoned administrators.

How HTTPS Works in 10 Minutes

“How HTTPS Works in 10 Minutes” is a simple, high-level overview of how HTTPS works. It doesn’t dive into too much detail or heavy math. But it does cover the main stages of how the connection is established, verified, and encrypted. These are the stages that are covered:

  1. You go to an HTTPS website via your browser
  2. The Client says “Hello”
  3. The Server says “Hello”
  4. The Client makes sure the SSL certificate is legitimate
  5. The Client gets the public key from the SSL certificate
  6. The Client uses the public key to make more random bytes
  7. The Client and Server make session keys
  8. The Client and Server compare session keys
  9. If the session keys match, game on

Bash trick: Repeat last command until success

More and more often I come across a scenario where I need to repeat the shell command until it succeeds. Here are a couple of examples:

  • Reboot a server. Try to remotely login to it via ssh. This fails until the server actually boots up. Keep trying until connected.
  • Start an application that writes to the log file. Run “tail -f some.log” to watch the log messages. This fails if the log file does not exist yet. Keep trying until the application creates the log file and writes something into it.

Sure, I can always press the up arrow key and Enter, to repeat the last command from the history. But it is a tiny bit annoying.

Today I came across this little trick, that solves the problem. Add the following function to your .bashrc:

rpt() {
  CMD=$(fc -ln | tail -n 2 | head -n 1)
  echo "repeating until success: $CMD"
  until $CMD
  do
    sleep 1
  done
}

Now you can run “rpt” to repeat the latest command until it succeeds.

Handy!