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## Acorn Validator

Edit: Corrected title and a bunch of grammatical mistakes. I typed this out on my phone with Monospace - and I seems that my keyboard (and Phone-Laptop Bluetooth connection!) leave something to be desired.....

Over the last week, I've been hard at work on an entry for #LOWREZJAM. While it's not finished yet (submission is on Thursday), I've found some time to write up a quick blog post about a particular aspect of it. Of course, I'll be blogging about the rest of it later once it's finished :D

The history of my entry is somewhat.... complicated. Originally, I started work on it a few months back as an independent project, but due to time constraints and other issues I was unable to get very far with it. Once I discovered #LOWREZJAM, I made the decision to throw away the code I had written and start again from (almost) scratch.

It is for this reason that I have a Javascript validator script lying around. I discovered when writing it originally that my editor Atom didn't have syntax validation support for Javascript. While there are extensions that do the job, it looked like a complicated business setting one up for just syntax checking (I don't want your code style guideline suggestions! I have my own style!). To this end, I wrote myself a quick bash script to automatically check the syntax of all my javascript files that I can then include as a build step - just before webpack.

Over the time I've been working on my #LOWREZJAM entry here, I've been tweaking and improving it - and thought I'd share it here. In the future, I'm considering turning it into a linting provider for the Atom editor I mentioned above (it depends on how complicated that is, and how good the documentation is to help me understand the process).

The script (which can be found in full at the bottom of this post), has 2 modes of operation. In the first mode, it acts as a co-ordinator process that starts all the sub-processes that validate the javascript. In the second mode, it validates a single file - outputting the result to the standard output and also logging any errors in a special place that the co-ordinator process can find them later.

It decides which mode to operate in based on whether it recieves an argument telling it which file to validate:

if [ "$1" != "" ]; then validate_file "$1" "$2"; exit$?;
fi

If it detects an argument, then it calls the validate_file function and exits with the returned exit code.

If not, then the script continues into co-ordinator mode. In this mode it chains a bunch of commands together like lego bricks to start subprocesses to validate all of the javascript files it can find a fast as possible - acorn, the validator itself, can only check one file as a time it would appear. It does this like so:

find . -not -path "./node_modules/" -not -path "./dist/" | grep -iP '\.mjs$' | xargs -n1 -I{} -P32$0 "{}" "${counter_dirname}"; This looks complicated, but it can be broken down into smaller, easy-to-understand chunks. explainshell.com is rather good at demonstrating this. Particularly of note here is the $0. This variable holds the path to the currently executing script - allowing the co-ordinator to call itself in validator mode.

The validator function itself is also quite simple. In short, it runs the validator, storing the result in a variable. It then also saves the exit code for later analysis. Once done, it outputs to the standard output, and then also outputs the validator's output - but only is there was an error to keep things neat and tidy. Finally, if there was an error, it outputs a file to a temporary directory (whose name is determined by the co-ordinator and passed to sub-processes via the 2nd argument) with a name of its PID (the content doesn't matter - I just output 1, but anything will do). This allows the co-ordinator to count the number of errors that the subprocesses encounter, without having to deal with complicated locks arising from updating the value stored in a single file. Here's that in bash:

counter_dirname=$2; # ....... # Use /dev/shm here since apparently while is in a subshell, so it can't modify variables in the main program O.o if ! [ "${validate_exit_code}" -eq 0 ]; then
echo 1 &gt;"${counter_dirname}/$$"; fi Once all the subprocesses have finished up, the co-ordinator counts up all the errors and outputs the total at the bottom: error_count=(ls {counter_dirname} | wc -l); echo echo Errors: error_count echo  Finally, the co-ordinator cleans up after the subprocesses, and exits with the appropriate error code. This last bit is important for automation, as a non-zero exit code tells the patent process that it failed. My build script (which uses my lantern build engine, which deserves a post of its own) picks up on this and halts the build if any errors were found. rm -rf "{counter_dirname}"; if [ {error_count} -ne 0 ]; then exit 1; fi exit 0; That's about all there is to it! The complete code can be found at the bottom of this post. To use it, you'll need to run npm install acorn in the directory that you save it to. I've done my best to optimize it - it can process a dozen or so files in ~1 second - but I think I can do much better if I rewrite it in Node.JS - as I can eliminate the subprocesses by calling the acorn API directly (it's a Node.JS library), rather than spawning many subprocesses via the CLI. Found this useful? Got a better solution? Comment below! #!/usr/bin/env sh validate_file() { filename=1; counter_dirname=2; validate_result=(node_modules/.bin/acorn --silent --allow-hash-bang --ecma9 --module filename 2&gt;&amp;1); validate_exit_code=?; validate_output=([ {validate_exit_code} -eq 0 ] &amp;&amp; echo ok || echo {validate_result}); echo "{filename}: {validate_output}"; # Use /dev/shm here since apparently while is in a subshell, so it can't modify variables in the main program O.o if ! [ "{validate_exit_code}" -eq 0 ]; then echo 1 &gt;"{counter_dirname}/$$"; fi } if [ "$1" != "" ]; then
validate_file "$1" "$2";
exit $?; fi counter_dirname=$(mktemp -d -p /dev/shm/ -t acorn-validator.XXXXXXXXX.tmp);
# Parallelisation trick from https://stackoverflow.com/a/33058618/1460422
# Updated to use xargs
find . -not -path "./node_modules/" -not -path "./dist/" | grep -iP '\.mjs$' | xargs -n1 -I{} -P32$0 "{}" "${counter_dirname}"; error_count=$(ls ${counter_dirname} | wc -l); echo echo Errors:$error_count
echo

rm -rf "${counter_dirname}"; if [${error_count} -ne 0 ]; then
exit 1;
fi

exit 0;

## Job Scheduling on Linux

Scheduling jobs to happen at a later time on a Linux based machine can be somewhat confusing. Confused by 5 4 8-10/4 6/4 * baffled by 5 */4 * * *? All will be revealed!

### cron

Scheduling jobs on a Linux machine can be done in several ways. Let's start with cron - the primary program that orchestrates the whole proceeding. Its name comes from the Greek word Chronos, which means time. By filling in a crontab (read cron-table), you can tell it what to do when. It's essentially a time-table of jobs you'd like it to run.

Your Linux machine should come with cron installed already. You can check if cron is installed and running by entering this command into your terminal:

if [[ "$(pgrep -c cron)" -gt 0 ]]; then echo "Cron is installed :D"; else echo "Cron is not installed :-("; fi If it isn't installed or running, then you'll have to investigate why this isn't the case. The most common is that it isn't installed. It's normally in the official repositories for most distributions - on Debian-based system sudo apt install cron should suffice. Arch-based users may need to check to make sure that the system service is enabled and do so manually. With cron setup and ready to go, we can start adding jobs to it. This is done by way of a crontab, as explained above. Each user has their own crontab such that they can each configure their own individual sets jobs. To edit it, type this: crontab -e This will open your favourite editor with your crontab ready for editing (if you'd like to change your editor, do sudo update-alternatives --config editor or change the EDITOR environment variable). You should see a bunch of lines like this: # Edit this file to introduce tasks to be run by cron. # # Each task to run has to be defined through a single line # indicating with different fields when the task will be run # and what command to run for the task # # To define the time you can provide concrete values for # minute (m), hour (h), day of month (dom), month (mon), # and day of week (dow) or use '*' in these fields (for 'any').# # Notice that tasks will be started based on the cron's system # daemon's notion of time and timezones. # # Output of the crontab jobs (including errors) is sent through # email to the user the crontab file belongs to (unless redirected). # # For example, you can run a backup of all your user accounts # at 5 a.m every week with: # 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/ # # For more information see the manual pages of crontab(5) and cron(8) # # m h dom mon dow command I'd advise you keep this for future reference - just in case you find yourself in a pinch later - so scroll down to the bottom and start adding your jobs there. Let's look at the syntax for telling cron about a job next. This is best done by example: 0 1 * * 7 cd /root && /root/run-backup This job, as you might have guessed, runs a custom backup script. It's one I wrote myself, but that's a story for another time (comment below if you'd like me to post about that). What we're interested in is the bit at the beginning: 0 1 * * 7. Scheduling a cron job is done by specifying 5 space-separated values. In the case of the above, the job will run at 1am every Sunday morning. The order is as follows: • Minute • Hour • Day of the Month • Month • Day of the week For of these values, a number of different specifiers can be used. For example, specifying an asterisk (*) will cause the job to run at every interval of that column - e.g. every minute or every hour. If you want to run something on every minute of the day (such as a logging or monitoring script), use * * * * *. Be aware of the system resources you can use up by doing that though! Specifying number will restrict it to a specific time in an interval. For example, 10 * * * * will run the job at 10 minutes past every hour, and 22 3 * * * will run a job at 03:22 in the morning every day (I find such times great for maintenance jobs). Sometimes, every hour or every minute is too often. Cron can handle this too! For example 3 */2 * * * will run a job at 3 minutes past every second hour. You can alter this at your leisure: The value after the forward slash (/) decides the interval (i.e. */3 would be every third, */15 would be every 15th, etc.). The last column, the day of the week, is an alternative to the day of the month column. It lets you specify, as you may assume, the day oft he week a job should run on. This can be specified in 2 way: With the numbers 0-6, or with 3-letter short codes such as MON or SAT. For example, 6 20 * * WED runs at 6 minutes past 8 in the evening on Wednesday, and 0 */4 * * 0 runs every 4th hour on a Sunday. The combinations are endless! Since it can be a bit confusing combining all the options to get what you want, crontab.guru is great for piecing cron-job specifications together. It describes your cron-job spec in plain English for you as you type! (Above: crontab.guru displaying a random cronjob spec) ### What if I turn my computer off? Ok, so cron is all very well, but what if you turn your machine off? Well, if cron isn't running at the time a job should be run, then it won't get executed. For those of us who don't leave their laptops on all the time, all is not lost! It's time to introduce the second piece of software at our disposal. Enter stage left: anacron. Built to be a complement to cron, anacron sets up 3 folders: • /etc/cron.daily • /etc/cron.weekly • /etc/cron.monthly Any executable scripts in this folder will be run at daily, weekly, and monthly intervals respectively by anacron, and it respects the hash-bang (that #! line at the beginning of the script) too! Most server systems do not come with anacron pre-installed, though it should be present if your distributions official repositories. Once you've installed it, edit root's crontab (with sudo crontab -e if you can't remember how) and add a job that executes anacron every hour like so: # Run anacron every hour 5 * * * * /usr/sbin/anacron This is important, as anacron does not in itself run all the time like cron does (this behaviour is called a daemon in the Linux world) - it needs a helping hand to get it to run. If you've got more specific requirements, then anacron also has it's own configuration file you can edit. It's found at /etc/anacrontab, and has a different syntax. In the anacron table, jobs follow the following pattern: • period - The interval, in days, that the job should run • delay - The offset, in minutes, that the job should run at • job identifier - A textual identifier (without spaces, of course) that identifies the job • command - The command that should be executed You'll notice that there are 3 jobs specified already - one for each of the 3 folders mentioned above. You can specify your own jobs too. Here's an example: # Do the weekly backup 7 20 run-backup cd /root/data-shape-backup && ./do-backup; The above job runs every 7 days, with an offset of 20 minutes. Note that I've included a command (the line starting with a hash #) to remind myself as to what the job does - I'd recommend you always include such a comment for your own reference - whether you're using cron, anacron, or otherwise. I'd also recommend that you test your anacron configuration file after editing it to ensure it's valid. This is done like so: anacron -T #### I'm not an administrator, can I still use this? Sure you can! If you've got anacron installed (you could even compile it from source locally if you haven't) and want to specify some jobs for your local account, then that's easily done too. Just create an anacrontab file anywhere you please, and then in your regular crontab (crontab -e), tell anacron where you put it like this: # Run anacron every hour 5 * * * * /usr/sbin/anacron -t "path/to/anacrontab" ### What about one-off jobs? Good point. cron and anacron are great for repeating jobs, but what if you want to set up a one-off job to auto-disable your firewall before enabling it just in case you accidentally lock yourself out? Thankfully, there's even an answer for this use-case too: atd. atd is similar to cron in that it runs a daemon in the background, but instead of executing jobs specified in a crontab, you tell it when you want it to execute a series of commands, and then enter the commands themselves. For example: $ at now + 10 minutes
warning: commands will be executed using /bin/sh
at> echo -e "Testing"
at> uptime
at> <EOT>
job 4 at Thu Jul 12 14:36:00 2018

In the above, I tell it to run the job 10 minutes from now, and enter a pair of commands. To end the command list, I hit CTRL + D on an empty line. The output of the job will be emailed to me automatically if I've got that set up (cron and anacron also do this).

Specifying a time can be somewhat fiddly, but its also quite flexible:

• at tomorrow
• at now + 5 hours
• at 16:06
• at next month
• at 2018 09 25

....and so on. Listing the current scheduled jobs is also just as easy:

atq

This will output a list of scheduled jobs that haven't been run yet. You can't see any jobs that aren't created by you unless you're root (use sudo), though. You can use the job ids listed here to cancel a job too:

# Remove job id 4:
atrm 4

### Conclusion

That just about concludes this whirlwind tour of job scheduling on Linux systems. We've looked at how to schedule jobs with cron, and how to ensure our jobs get run - even when the target machine isn't turned on all the time with anacron. We've also looked at one-time jobs with atd, and how to manage the job queue.

As usual, this is a starting point - not an ending point! Job scheduling is just the beginning. From here, you can look at setting up automated backups. You could investigate setting up an email server, and how that integrates with cron. You can utilise cron to perform maintenance for your next great web (or other!) application. The possibilities are endless!

Found this useful? Still confused? Comment below!

## Read / Write Disk Performance Testing in Bash

Recently I needed to quickly (and non-destructively) test the read / write performance of a flash drive of mine. Naturally, I turned my attention to my terminal. This post is me documenting what I did so that I can remember for next time :P

Firstly, to test the speed of a disk, we need some data to test with. Since lots of small files will inevitably cause slowdowns due to the overhead of writing the file metadata and inode information to the superblock, it makes the most sense to use one gigantic file rather than tons of small ones. Here's what I did to generate a 1 Gigabyte file filled with zeroes:

dd if=/dev/zero of=/tmp/testfile.bin bs=1M count=1024

Cool. Next, we need to copy it to the target disk and measure the time it took. Then, since we know the size of the file (1073741824 bytes, to be exact), we can calculate the speed at which the copy took place. Here's my first attempt:

time dd if=/tmp/testfile.bin >testfile.bin

If you run this, you might find that it doesn't take it very long at all, and you get a speed of something like ~250MiB / sec! While impressive, I seriously doubt that my flash drive has that kind of speed behind it. Typically, flash memory takes longer to write to and read from - and I'm pretty sure that it can't read from it that fast either. So what's going on?

Well, it turns out that Linux is caching the disk write operations in a buffer, and then doing them in the background for us. Whilst fine for ordinary operation, this doesn't give us an accurate representation of how fast it's actually writing to the disk. Thankfully, there's something we can do about this: Use the sync command. sync will flush all cached write operations to disk for us, giving us the actual time it took to write the 1 GiB file to disk. Here's the altered command:

sync;
time sh -c 'dd if=/tmp/testfile.bin >testfile.bin; sync'

Very cool! Now, we can just take the time it took and do some simple maths to calculate the write speed of our disk. What about the read speed though? Well, to test that, we'll first need to clear out the page cache - another one of Linux's (many) caches that holds portions of files that have recently been accessed for faster retrieval - because as before, we're not interested in the speed of the cache! Here's how to do that:

echo 1 | sudo tee /proc/sys/vm/drop_caches

With the correct cache cleared, we can test the read speed accurately. Here's how I did it:

time dd if=testfile.bin of=/dev/null

Fairly simple, right? At a later date I might figure out a way of automating this, but for the occasional use now and again this works just fine :)

Found this useful? Got a better way of doing it? Want to say hi? Post in the comments below!

## Securing a Linux Server Part 2: SSH

Wow, it's been a while since I posted something in this series! Last time, I took a look at the Uncomplicated Firewall, and how you can use it to control the traffic coming in (and going out) of your server. This time, I'm going to take a look at steps you can take to secure another vitally important part of most servers: SSH. Used by servers and their administrators across the world to talk to one another, if someone manages to get in who isn't supposed to, they could do all kinds of damage!

The first, and easiest thing we can do it improve security is to prevent the root user logging in. If you haven't done so already, you should create a new user on your server, set a good password, and give it superuser privileges. Login with the new user account, and then edit /etc/ssh/sshd_config, finding the line that says something like

PermitRootLogin yes

....and change it to

PermitRootLogin no

Once done, restart the ssh server. Your config might be slightly different (e.g. it might be PermitRootLogin without-password) - but the principle is the same. This adds an extra barrier to getting into your server, as now attackers must not only guess your password, but your username as well (some won't even bother, and keep trying to login to the root account :P).

Next, we can move SSH to a non-standard port. Some might argue that this isn't a good security measure to take and that it doesn't actually make your server more secure, but I find that it's still a good measure to take for 2 reasons: defence in depth, and preventing excessive CPU load from all the dumb bots that try to get in on the default port. With that, it's make another modification to /etc/ssh/sshd_config. Make sure you test at every step you take, as if you lock yourself out, you'll have a hard time getting back in again....

Port 22

Change 22 in the above to any other number between about 1 and 65535. Next, make sure you've allowed the new port through your firewall! If you're using ufw, my previous post (link above) gives a helpful guide on how to do this. Once done, restart your SSH server again - and try logging in before you close your current session. That way if you make a mistake, you can fix through your existing session.

Once you're confident that you've got it right, you can close port 22 on your firewall.

So we've created a new user account with a secure password (tip: use a password manager if you have trouble remembering it :-)), disabled root login, and moved the ssh port to another port number that's out of the way. Is there anything else we can do? Turns out there is.

Passwords are not the only we can authenticate against an SSH server. Public private keypairs can be used too - and are much more secure - and convenient - than passwords if used correctly. You can generate your own public-private keypair like so:

ssh-keygen -t ed25519

It will ask you a few questions, such as a password to encrypt the private key on disk, and where to save it. Once done, we need to tell ssh to use the new public-private keypair. This is fairly easy to do, actually (though it took me a while to figure out how!). Simply edit ~/.ssh/config (or create it if it doesn't exist), and create (or edit) an entry for your ssh server, making it look something like this:

Host bobsrockets.com
Port            {port_name}
IdentityFile    {path/to/private/keyfile}

It's the IdentityFile line that's important. The port line simply makes it such that you can type ssh bobsrockets.com (or whatever your server is called) and it will figure out the port number for you.

With a public-private keypair now in use, there's just one step left: disable password-based logins. I'd recommend trailing it for a while to make sure you haven't messed anything up - because once you disable it, if you lose your private key, you won't be getting back in again any time soon!

Again, open /etc/ssh/sshd_config for editing. Find the line that starts with PasswordAuthentication, and comment it out with a hash symbol (#), if it isn't already. Directly below that line, add PasswordAuthentication no.

Once done, restart ssh for a final time, and check it works. If it does, congratulations! You've successfully secured your SSH server (to the best of my knowledge, of course). Got a tip I haven't covered here? Found a mistake? Let me know in a comment below!

## Rendering LaTeX documents to PDF: Attempt #2

It was all going rather well, actually - until I discovered that pandoc doesn't support regular bibliographies / references. Upon discovering this, I ended up with a bit of problem. Thankfully, the answer lay in pdflatex - but getting to the point where I could use it without having it crash on me (which, by the way it can't accomplish properly - it gives an exit code of 0 when crashing! O.o) was not a trivial journey.

This blog post is a follow up to my first post on rendering LaTeX documents with pandoc, and is my attempt to document what I did to get it to work. To start with, I installed texlive properly. Here's how to do that on apt-based systems:

sudo apt install texlive-latex-extra --no-install-recommends

The no-install-recommends is useful here to avoid ~450MiB of useless documentation (in PDF form, apparently) being dumped to your hard drive. I've also got an arch-based system (it's actually Artix Linux, that I've blogged about) which I've done this on, so here's the install command for those kind of systems:

sudo pacman -S texlive-latexextra

Once that's installed, we can use it to render our LaTeX document to PDF. Upon discussing my issues with my Lecturer at University, I discovered that you actually have to run 3 commands in succession in order to render a single PDF. Here they are:

bibtex filename
pdflatex --output-directory=. filename.tex
pdflatex --output-directory=. filename.tex

The first one compiles the bibliography using BiBTeX. If it isn't installed already, you might need to search your distribution's repositories and install it. Next, we run the LaTeX file through pdflatex from TeXLive not once but twice - as it apparently needs to resolve the references on the first pass (why it can't do them all in once pass I have no idea :P).

It's also worth noting that the bibtex command doesn't like you to append the filename extension - it does it automatically, apparently.

That's about everything I've got on the process so far. If you've got anything else to add, please let me know in the comments below (I'm rather new to this whole LaTeX thing....)

## Rendering LaTeX documents to PDF on Linux (and maybe Windows too)

I'm starting to write another report for University, and unlike other reports, this one apparently has to be a rather specific format. To that end, I've got two choices, apparently: Use the provided Word / LibreOffice template, or use a LaTeX template instead. After the trouble and frustration I had with LibreOffice for my previous report, I've naturally decided that using the LaTeX template might be a good idea.

After downloading it, I ended up doing some research and troubleshooting to get it to render properly to a PDF. Now that I've figured it out, I thought I'd share it here for anyone else who ends up experiencing difficulties or is unsure on how it's done.

The way I'm going to be using it is with a tool called Pandoc. First, install it like so:

sudo apt install pandoc texlive-fonts-recommended

Adjust as necessary for your distribution - Windows users will need to read the download instructions. The texlive-font-recommended package is ~66MiB(!), but it contains a bunch of fonts that are needed when you're rendering LaTeX documents, apparently.

With the dependencies installed, here's the command to convert a LaTeX document to a PDF:

pandoc -s input.tex -o output.pdf

Replace index.tex with the path to your input file, and output.pdf with the desired path to the output file. I haven't figured out how to set the font to sans-serif yet, but I'll probably make another post about it when I do.

Found this helpful? Still having issues? Let me know below! I don't have analytics on here, so that's the only way I'll know if anyone reads this :-)

## Jump around a filesystem with a bit of bash

(Banner remixed from images found on openclipart)

I've seen things like jump, which allow you to bookmark places on your system so that you can return to them faster. The trouble is, I keep forgetting to use it. I open the terminal and realise that I need to be in a specific directory, and forget to bookmark it once I cd to it - or I forget that I bookmarked it and cd my way there anyway :P

To solve the problem, I thought I'd try implementing my own simplified system, under the name teleport, telepeek, and telepick. Obviously, we'll have to put these scripts in something like .bash_aliases as functions - otherwise it won't cd in the terminal itself. Let's start with teleport:

function teleport() {
cd "$(find . -type d | grep -iP "$@" | head -n1)";
}

Not bad for a first attempt! Basically, it does a find to list all the subdirectories in the current directory, filters the results with the specified regex, and changes directory to the first result returned. Here's an example of how it's used:

~ $teleport 'pep.*mint' ~/Documents/code/some/path/pepperminty-wiki/$ 

We can certainly improve it though. Let's start by removing that head call:

function teleport() {
cd "$(find . -type d | grep -m1 -iP "$@")";
}

What about all those Permission denied messages that pop up when you're jumping around places that you might not have permission to go everywhere? Let's suppress those too:

function teleport() {
cd "$(find . -type d 2>/dev/null | grep -m1 -iP "$@")";
}

Much better. With a teleport command in hand, it might be nice to inspect the list of directories the find + grep combo finds. For that, let's invent a telepeek variant:

function telepeek() {
find . -type d 2>/dev/null | grep -iP "$@" | less } Very cool. It doesn't have line numbers though, and they're useful. Let's fix that: function telepeek() { find . -type d 2>/dev/null | grep -iP "$@" | less -N
}

Better, but I'd prefer them to be highlighted so that I can tell them apart from the directory paths. For that, we've got to change our approach to the problem:

function telepeek() {
find . -type d 2>/dev/null | grep -iP "$@" | cat -n | sed 's/^[ 0-9]*[0-9]/\o033[34m&\o033[0m/' | less -R } By using a clever combination of cat -n to add the line numbers and a strange sed recipe (which I found in a comment on this Stack Overflow answer) to highlight the numbers themselves, we can get the result we want. This telepeek command has given me an idea. Why not ask for an index to jump to after going to the trouble of displaying line numbers and jump to that directory? Let's cook up a telepick command! function telepick() { telepeek$1;
cd "$(find . -type d 2>/dev/null | grep -iP "$@" | sed "${line_number}q;d")"; } That wasn't too hard. By using a few different commands rather like lego bricks, we can very easily create something that does what we want with minimal effort. The read -p "jump to index: " line_number bit fetches the index that the user wants to jump to, and sed comes to the rescue again to pick out the line number we're interested in with sed "${line_number}q;d".

Update April 20th 2018: I've updated the approach here to support spaces everywhere by adding additional quotes, and utilising $@ instead of $1.

## Getting an updated version of MonoDevelop on Ubuntu

Thanks to the AUR (The Arch User Repository), an updated version of MonoDevelop is never very far away on Manjaro, Artix, Arch Linux and friends. Unfortunately, such delights can't be found so easily in the Ubuntu and Debian ecosystem. On a check inspection of the official repositories, the latest version available is 5.10, while the AUR is offering 7.1.2!

In addition, the newer versions offered via this 'flatpak' thing are sandboxed, so you can't combined multiple technologies like Node.JS and the amazing npm in your C♯ applications with it.

Thankfully, all is not lost. A clever GitHub user by the name of cra0zy has put together a script that will install the latest version of MonoDevelop for us - without all the hassle of sandboxes or manually compiling from source. Here's a link to this installer:

cra0zy/monodevelop-run-installer

This post is mainly for my own reference, but I'd love to know if it helped you out too. If it did, leave a comment below!

## The Great Migration of Manjaro

It was just before lunch in the library, and I was checking my university emails on my travelling laptop that runs Manjaro OpenRC. While that was going on, I was inducing a few updates that it notified me about - and I started to install them with yaourt -Syua. First mistake.

During the installation, it decided to upgrade OpenRC to the version in the AUR (Arch User Repository), but I didn't think anything of it particularly - I knew that Manjaro OpenRC was dying deprecated. Second mistake.

Once the updates were complete, I shut it down and sent on my way - or at least I tried to - it wouldn't shut down, instead proceeding to log out and leave it at that. I resolved to investigate the problem when I got home. Third mistake.

By the time I came to use it again, I was greeted with an ominous message:

[Firmware Bug]: TSC_DEADLINE disabled due to Errata; please update microcode to version: 0x52 (or later)
Failed to execute /init (error -2)
Kernel panic - not syncing: No working init found. Try passing init= option to kernel. See Linux Documentation/admin-guide/init.rst
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.13.2-1-MANJARO #1
Hardware name: Entroware Apollo/Apollo, BIOS 1.05.05 04/27/2017

Hrm. That's odd. Maybe something went wrong in the update? Linux has what's called kernel parameters that tell it how to boot. They specify things like "here's the root partition of the system", and "please let me edit files on the system after booting". To understand how this fits into the next part of the story, it's first necessary to take a quick retour and look at how, precisely the linux kernel goes about booting a system. This is best explained with a diagram:

(Rendered with Ascidia. Textual diagram source available here)

1. BIOS / UEFI POST - The starting point of the boot process. The BIOS / UEFI turns on all the devices, runs some basic hardware checks, and (usually) gives the user a choice of what they want to boot from.
2. rEFInd - grub may be used instead of rEFInd, but the basic principle is the same: it asks the user how they want to boot from the hard drive. Kernel parameters are decided on here.
3. Initialisation: The Linux kernel is executed by the bootloader, and it proceeds to initialise itself and the connected devices.
4. Mount initial RAM disk: The Linux encounters a chicken-and-egg problem rather early on: How can it start talking to the connected devices if it doesn't know how to talk to them? The initial RAM disk solves that problem: It contains a bunch of drives and other such components that the kernel needs to initialise all the connected devices. It's like a cut-down root file system, in a sense.
5. Load drivers: The Linux kernel loads the drivers from the initial RAM disk (aka initrd) and starts initialising all the connected devices.
6. Mount root (read-only): The main root file system is mounted next, but only in read-only mode while the boot process finishes.
7. Execute init: It is at this point that the very first process is executed. It usually presides at /sbin/init, but this can be changed through the init kernel parameter.
8. Mount root (read-write): The init process (under SysVinit at least) then remounts the root filesystem such that it is writeable.
9. Mount other partitions: The next job is the mounting of the other partitions in /etc/fstab. This is also done by SysVinit if I recall correctly.
10. Reach runlevels: The main runlevels managed by the service manager (e.g. OpenRC) are now executed in order by the service manager.

Phew, that took more explaining than I thought! And to think it all happens in the span of about 10 seconds....! With that out of the way, let's continue with the story.

Let's try specifying the init kernel parameter - maybe the update cleared it for some random reason....? I had no idea what I was getting myself into :P

Unexpectedly, specifying init=/sbin/init didn't work. Neither did specifying init=/bin/sh. At this point, I suspected that there was something seriously wrong. I (correctly) guessed that it was the update I performed that morning that was to blame. After a bunch of backing and forthing, I managed to get hold of a previous copy of the openrc package that was replaced by the 0.27 version from the AUR. After doing a full backup, I tried installing it and removing the new openrc-sysvinit package that was also installed.

Before we continue further, I should probably explain how I managed to install the previous package version. Didn't I just explain that my system wasn't bootable? Well, yes. But I also had the original manjaro-architect installation media that I used to build the system in the first place. With that in hand, I could use rEFInd to boot from that (my UEFI firmware makes it a bit of a pain otherwise!), and then mount the root partition of the broken system and chroot into it. This process allows me to pretend that the system is actually booted, while piggybacking off the live installation media of the boot process. It works a bit like this:

lsblk # Find the root partition
mkdir /mnt/os;
mount /dev/sdZY /mnt/os # Mount the root partition
mount /dev/sdAB /mnt/os/boot/efi # Mount the EFI partition
manjaro-chroot /mnt/os bash # Enter the chroot and execute bash

Back to the story. Sadly, valiant though my effort was to replace the openrc and openrc-sysvinit packages was, it did not solve the problem. Eventually, I ended up having to perform a blind migration to Artix Linux, the spiritual successor to both Manjaro OpenRC and Arch OpenRC (apparently the developers of both came together to create Artix Linux).

Eventually, I ended up with a successful migration that I performed inside the chroot, and the system was bootable again! Next time, I'll always run pacman -Syu before yaourt -Syua. I'll also set up a temporary backup solution for my system files (I've already got one in place for my personal files) while I figure out a more permanent one that backs up across the network.

## Manjaro OpenRC Cheat Sheet

Amidst preparations for my third year at university, I've put together a sort of reference sheet to help me remember all the common commands needed when using Manjaro with OpenRC. It's not complete, but I'll continue to update it with various useful commands I stumble upon. You can find it below.

If you have any that you find useful, post a comment below! I'd love to see what you come up with - I might even add it to this list (crediting you of course)!

### Cast List

• sudo pacman: Main package manager
• yaourt: pacman wrapper, also has AUR support. Swap out pacman for yaourt to include the AUR.
• sudo rankmirrors: Finds and selects the fastest repository mirrors.
• sudo rc-update - Enable and disable services
• sudo service - Start, stop, and query the status of services

### Commands

#### Package Management

Command Description
rankmirrors -i -m rank -d Interactively re-find the fastest mirrors
pacman -Sy Synchronise local repository metadata
pacman -Syy Redownload all repository metadata
pacman -Syua Sync with mirrors and update all packages
pacman -Fs filename Search repositories for packages that contain filename
pacman -Ss search_string Search repositories for package name or description that contain a search string
pacman -S package_name Install package_name and any dependencies required
pacman -Rs package_name Remove package_name and all dependencies not needed by anything else
pacman -Dk Check that all required dependencies are installed
pacman -Q List all installed packages and their versions
pacman -Qe List all packages that were installed manually
pacman -Qd List all packages that were isntalled automatically
pacman -Sii package_name See which packages require package_name to be installed

#### Services

Command Description
rc-update List all services and their runlevels
rc-update add service_name default Add service_name to the default runlevel
rc_update delete service_name default Remove service_name from the default runlevel
service service_name start Start service_name
service service_name stop Stop service_name
service service_name status Query the status of service_name