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Comment by mortar | Hacker News Reader

Just learnt about whiteout files from this, thanks! Trying to understand if you purposely included a filename into a layer with the same whiteout prefix “.wh.”, if it would mess with the process that is meant to obfuscate that prefix from subsequent layers.

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I learned about $_

  echo abc && echo $_
  abc
  abc

except it's used with wget...

  wget URL && tar -xvf $_

does this work? Shouldn't tar take a filename?

hmm... also, it says there is an alpine layer with "FROM scratch"??

$_ is the last argument. Here's a better example to illustrate

  > echo 'Hello' 'world' 'my' 'name' 'is' 'godelski'
  Hello world my name is godelski
  > echo $_
  godelski
  > !:0 !:1 !:2 "I'm" "$_"
  Hello world I'm godelski

The reference manual is here[0] and here's a more helpful list[1]

One of my favorites is

  > git diff some/file/ugh/hierarchy.cpp
  > git add $_
  ## Alternatively, but this is more cumbersome (but more flexible)
  !!:s^diff^add

So what is happening with wget is

  > wget https://dl-cdn.alpinelinux.org/alpine/v3.18/releases/x86_64/alpine-minirootfs-3.18.4-x86_64.tar.gz && tar -xvf $_
  ## Becomes
  > wget https://dl-cdn.alpinelinux.org/alpine/v3.18/releases/x86_64/alpine-minirootfs-3.18.4-x86_64.tar.gz
  > tar -xvf https://dl-cdn.alpinelinux.org/alpine/v3.18/releases/x86_64/alpine-minirootfs-3.18.4-x86_64.tar.gz

Which you are correct, doesn't work.

It should actually be something like this

  > wget https://dl-cdn.alpinelinux.org/alpine/v3.18/releases/x86_64/alpine-minirootfs-3.18.4-x86_64.tar.gz -O alpine.tar.gz && tar xzf $_

This would work as the last parameter is correct. I also added `z` to the tar and removed `-` because it isn't needed. Note that `v` often makes untaring files MUCH slower

[0] https://www.gnu.org/software/bash/manual/html_node/Bash-Vari...

[1] https://www.gnu.org/software/bash/manual/html_node/Variable-...

If you want to add in another bash trick called Parameter Expansion[0] you can parse out the filename automatically with the special variable $_. Something like:

  > wget https://dl-cdn.alpinelinux.org/alpine/v3.18/releases/x86_64/alpine-minirootfs-3.18.4-x86_64.tar.gz && tar xzf ${_##*/}

[0] https://www.gnu.org/software/bash/manual/html_node/Shell-Par...

I want to make a small note, in that `$_` is a special Bashism (though it is supported widely), but Parameter Expansion is POSIX-standard and will work on all POSIX-compliant shells, not just Bash.

https://pubs.opengroup.org/onlinepubs/009604499/utilities/xc...

You know I just realized after all these years I still don't quite know what a shell is.

You have iTerm, Terminal, etc. But what do those do? Those are not the shells themselves right?

iTerm and Terminal are pieces of software emulate a physical terminal environment. They take the output of programs/shells output characters and control codes to render text, clear the screen, etc.

The terminal emulator receives keyboard input via your operating system, and passes it to the shell program via stdin.

The shell is responsible for prompting you and handling whatever you type. For example the “$ “ waits for next character from the terminal emulator until you hit newline.

The shell is responsible for parsing your input, executing any child programs “ls” for example, outputting their content to stdout, and prompting you again.

I wanted to offer that (if you are "C handy") writing your own shell is a super informative exercise. We had to write our own shell in my operating system class at GT and I actually got it working well enough that I could "exec ./myshell" and use it for some day to day stuff. I felt empowered

I tried to dig up the course but naturally things are wwaaaaaaay different now than back in my day. But OCW has something similar https://ocw.mit.edu/courses/6-828-operating-system-engineeri... and does ship the source files https://ocw.mit.edu/courses/6-828-operating-system-engineeri... although I have no idea why that's only present in a graduate level class

##*/ is probably one of my most used parameter expansions. This was definitely a better solution than the one I proposed lol

I am surprised that this is working, as I always thought that variables get initialized after the full command is parsed. So, I would assume that $_ would be related to the previous command (defined by a new line) and not this one, because there's no newline character here, but only an ampersand.

&& means there's a sequence. So the second statement will only execute conditioned on the first sequence. So...

  > thisFunctionFails && echo "Hello world" && echo "I SAID $_"
  
  > thisFunctionSucceeds && echo "Hello world" && echo "I SAID $_"
  Hello World
  I SAID Hello World

The left function has to get evaluated before the next function. So it is still related to the previous command.

  !!:s^diff^add

This is enough:

  ^diff^add

TIL! I use alt-. for that when running interactively, good to know there's a way to do that in a script

It's not an alpine layer, it's a Dockerfile construct representing basically an empty tar file layer: <https://docs.docker.com/build/building/base-images/#create-a...> and <https://github.com/moby/moby/pull/8827>

He says:

  FROM scratch

  COPY ./hello /root/

  ENTRYPOINT ["./hello"]

> Here, our image contains 2 layers. The first layer comes from the base image, the alpine official docker image i.e. the root filesystem with all the standard shell tools that come along with an alpine distribution.

But I thought "FROM scratch" was an empty container, while "FROM alpine" is a container with alpine libs/executables.

otherwise using "FROM scratch" to populate for example an ubuntu image would pollute the container.

You're right, that doesn't work the way it is shown. Thankfully, a reader(I'm not sure if it's you) pointed this out with a solution[1] that I plan to add to the post shortly. Again, thanks for pointing this out!

[1] - https://github.com/danishprakash/danishpraka.sh/issues/30

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