Category Archives: Sys Admin

Material related to systems administration.

Linux File Timestamps Explained: atime, mtime, and ctime

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Today I discovered an article explaining Linux file system timestamps: Linux File Timestamps Explained: atime, mtime, and ctime; and a good thing too, because I always thought ‘ctime’ was ‘create time’, but not so! Turns out that ‘ctime’ is ‘change time’, similar to ‘mtime’ but apparently cannot be set from userspace. My investigations were prompted by my investigations info how `borg create` identified changed files.

watch catting together HTML head/foot and MySQL information_schema.processlist

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This came up back on August 9th 2020 in #lobsters on freenode. They were doing a system upgrade and providing a report by using `watch` to `cat` together a HTML header and footer with `mysql -e ‘select * from information_schema.processlist’` to provide a status report. Thought that was a neat hack.

Bridge sniffer

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I’ve been having trouble getting my Pioneer FreedomBox to work and I found myself wanting a network protocol analyser so I could see what was going on.

I got myself two of these Simplecom USB 3.0 to Gigabit RJ45 Ethernet LAN Adapters and attached them to my MacBook Pro, like this:

network sniffing setup

Then I spun up a Kubuntu system in a VMWare Fusion virtual machine and created a bridge like this (thanks to my mate Raz for the instructions):

   ifconfig eth0 up
   ifconfig eth1 up
   brctl addbr br0
   brctl addif br0 eth0
   brctl addif br0 eth1
   ifconfig br0 up

Then I was able to run WireShark to monitor traffic across the br0 bridge which helped me to diagnose the issues I was facing.

…and in the end I got my FreedomBox operational, so got to be happy about that! :)

The problem I was having was that my FreedomBox would work okay for a few minutes and then lose network connectivity. I confirmed this with my WireShark logs. You could see the DHCP and MDNS working at first and then see things stop working after some time.

The problem turned out to be related to the fact that I was running an old version of the FreedomBox software. The reason my software was out of date was that I had downloaded a “nightly build” which was well out of date. I think maybe the nightly builds are no longer maintained. After I upgraded to a stable build my networking problems went away.

NetBeans red line

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There is a feature of NetBeans where it puts a red line as the right margin in the code editor. By default this margin is at 80 characters but I prefer it at 99 characters. To change the setting in NetBeans see Tools -> Options -> Editor -> Formatting -> All Languages -> Tabs And Indents -> Right Margin. For example:

NetBeans formatting options