Welcome to The Linux Page

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This site is a collection of my own work with Linux. Certain things are easy, others take time to decipher and if I ever need to repeat the work (which usually happens!), then I need to remember everything by memory or... have a site with all the steps taken and to take again.

The following are my most recent posts:

Useful Docker Commands

Retrieve a Docker Image

You found an image and you have a URL now you want to get a copy, you do a pull like so:

docker pull gcr.io/distroless/static

It will load that image in docker.

List Files in a Docker Image

I often would like to see the files inside a docker image. The fact is that Docker has a tarball of those files that one can export to see said files.

First you can try the save command like so:

docker save <image-uuid> > foo.tar

If that doesn't seem to work, you can try the following few steps to check out your image contents by first creating a ...

Parallelize a process in bash

Run processes in parallel in a bash script.

The other day, I worked on creating a video which means extract over 4,000 frames and then processing those frames. One of the processes is to overlay one image over another (using convert from ImageMagick). This process is very slow because each time the convert tool reloads the background image and the movie frame to overlay... so I thought I should run these commands in parallel. After all, I have 64 CPUs, let's use them!

In bash, there is a special option on the wait command: -n. This option means: wait for any one of the currently running jobs to finish. This is quite practical ...

Files piling up...

Once in a while, when I try to open a file, I get an error message saying that the file is already being edited in another instance of vim. I often have over 10 instances of vim opened so it makes it rather complicated to find out which one is the culprit.

In most likelihood, the error message includes all the information that you need. Here is an example:

E325: ATTENTION
Found a swap file by the name "/etc/apache2/sites-available/.linux.m2osw.com.conf.swp"
          owned by: root   dated: Fri Jan 22 16:19:51 ...

Flood waters, how a hacker can generate so many connections that your server runs out of ports.

Issue

I found an interesting concept which is to block a user¹ when they attempt to open too many TCP connections within one minute.

This comes down to a couple of rules:

iptables -A INPUT -p tcp \
    -m recent --update --name synflood --seconds 60 --hitcount 100 \
    -m tcp --syn -j add_to_denylist
iptables -A INPUT -p tcp \
    -m recent --set --name synflood -m tcp --syn

The second rule says to add an entry for the source IP in a table named synflood if the user is attempting to connect using the TCP protocol.

The first rule checks whether the same IP ...

Duplicated files can cause double the trouble when part of a dependent package.

Today I tried to install a new version of a Debian package I created.

That package, let's call it A, now depends on another new package, let's call it B.

A included a file that was installed under /etc/... I removed that file from A because it is also present in B. In other words, the exact same file was being installed by two packages:

A (old version)
B

Unbeknownst to me, when you try to do so, dpkg gets confused because A also included:

Depends: B

My idea was that since the new version of A does not include the file, it will get removed and then B gets ...

Istanbul, it's full.

Where Did My Disk Space Go?

I suppose with the title on this page, you already understood that Docker is the culprit... Yep! It's using a lot of disk space to store all of those images. One image can very easily be over 1Gb and if you have a VirtualBox system with many images, you end up eating a lot of your disk space very quicly!

How Did You Discover the Issue?

I ran the following command:

sudo ncdu -qx /

Did some other work for a little while, then checked the results. By default, the largest folder was shown at the top:

/var

You can simply select the folder and hit ...

Pigeon deliverying a message

It is possible to send a command to a running gvim instance.

Here is an example on how to send a Write + Quit:

vim --servername GVIM --remote-send '<C-\><C-N>:wq<CR>'

As we can see, the syntax for the command is very similar to what we usually have in our macros.

The name "GVIM" is what you need to use (i.e. it's not the name of the server on which the gvim editor is running (a.k.a. NOT localhost).

I have not yet tested if I have multiple windows, would the instructions be sent to all or just one. I suppose I should look at the documentation ...

Once in a while, I click on something and the mouse literally dies.

From what I've seen the last time it happened, Opera had a lock issue. It could not create a lock and went on and on about it. Killing Opera did not help, even though the issue happened when I clicked on a button in Opera.

That last click happened in Opera and that's when the mouse froze. This is why I thought killing Opera would help... The good thing, though, is that the keyboard is running.

On that day, I decided to reboot the whole computer anyway because it had been running for over 42 days and so I needed to ...

I have a CMakeLists.txt file and I run a command while building and if one of my libraries has an issue, then it will crash and it becomes close to impossible to debug unless you can run the debugger.

The fact is that I could tweak my CMakeLists.txt to run the command with gdb and get a stacktrace which helped much.

Here is the command I ran:

     add_custom_command(
        OUTPUT
            ${CMAKE_CURRENT_BINARY_DIR}/${ATOMIC_NAMES_BASENAME}.cpp
            ${CMAKE_CURRENT_BINARY_DIR}/${ATOMIC_NAMES_BASENAME}.h

        COMMAND
           "gdb"
                ...

Broken Dockerfile generates this strange error which means roughly nothing.

Docker is still a very strange beast to me and some of the errors it generates are even stranger.

Today I got this one:

failed to solve with frontend dockerfile.v0: failed to solve with frontend gateway.v0: rpc error: code = Unknown desc = failed to create LLB definition: no build stage in current context

which means roughly nothing to a normal human being.

The fact is that it happens when there is an error in the setup.

In my case, I added this line:

ENV BUILDKIT_PROGRESS=plain

before this one:

FROM ...

and as a result got that message as shown above.

I've found a post ...

Unfreeze your Mouse when it Froze

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Main Menu

Retrieve a Docker Image

List Files in a Docker Image

Issue

Where Did My Disk Space Go?

How Did You Discover the Issue?