# How Long Is A Millisecond?

“Time is relative; its only worth depends upon what we do as it is passing.”
― Albert Einstein

Different trades and professions have their favorite units of measurement. For astronomers it’s light years, for farmers it’s acres, and inches for carpenters. For us systems programmers it’s clearly milliseconds.

Images from a video camera running at 60 FPS arrive every 16.7 milliseconds. An operating system’s time-sliced task scheduler might switch tasks every 50 milliseconds. During full-breaking, an anti-lock breaking system typically applies/releases breaking pressure every 75 milliseconds. Milliseconds seem to be perfectly suited for close-to-the-metal programming and for smoothly controlling systems.

But how long is a millisecond, actually? Sure, everybody knows that it’s one thousandth of a second, but I’m rather curious as to how much work can be accomplished in a millisecond. Is it, for instance, a waste of CPU cycles if a thread wakes up every 5 ms to poll a sensor?

People often use “dog years” to better understand the behavior of dogs. We can apply the same idea and use “CPU years” to better understand CPUs.

For us humans, a second is a very important unit of measurement as the human heart beats (roughly) once every second. Let’s use a fictitious modern-day RISC CPU that runs at a clock rate of one GHz and requires one clock cycle to execute an instruction as an example. If we view the clock frequency as the heartbeat of a CPU, then a CPU’s heart beats every nanosecond[1]. Thus, one millisecond corresponds to one million CPU hearbeats.

One million human heartbeats equal roughly 12 days (i. e. 1,000,000 / 60 / 60 / 24), a time-span in which a human being can get a lot of work done[2]. Likewise, for our CPU, a millisecond corresponds to 12 CPU days. A full second corresponds to — lo and behold! — 32 CPU years (i. e. 12,000 CPU days).

Now, you might be coding for an embedded system that runs at “only” 100 MHz, so in this case a millisecond is “just” 1.2 CPU days. Or, you might have a Raspberry Pi 4 running at 1.5 GHz whose Cortex-A72 CPU sports a 3-way out-of-order superscalar pipeline, in which case you can probably multiply the 12 days by three.

While not perfect, this “a millisecond is a dozen CPU days” rule-of-thumb is a nice reminder that on a modern CPU, a millisecond really stands for “a lot of time”.

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[1] A nanosecond is such a mind-boggingly short period of time that even the fastest thing in the universe (i. e. “light”) can only move one foot (30 cm) in a nanosecond.

[2] At least theoretically. Definitely not if you work for a large corporation and your schedule is packed with meetings.

# Working the Bash Shell Like a Pro, Service Pack 1

“Man is a tool-using animal. Without tools he is nothing, with tools he is all.”
— Thomas Carlyle

“A fool with a tool is still a fool.”
— anonymous

“A fool with a tool is a dangerous fool.”
— anonymous

Five years ago, I wrote a popular post called “Working the Bash Shell Like a Pro“, a short collection of essential tips and tricks for working efficiently with the Bash shell.

Like everyone, during my daily interaction with Bash I use many “tricks”, however, only few of them qualify for being added to my original list. Now, the time has come for a small update.

1. The Bash Curse

As you already know from the previous post, ‘!#’ selects the “entire command-line typed so far”, ‘:1’ takes the first argument in the “command-line typed so far” and ‘:r’ strips the extension from it. So changing the file extension of a file can be done with a fairly short command-line:

which is — depending on the path length — a lot shorter than

Instead of picking the first argument by employing the ‘:1’ word designator you could equally well take the last argument by specifying ‘:\$’. Why? There is only one argument typed so far (“some/long/path/to/file.c”), so the first argument is the last argument. Consequently, this achieves the same effect as the previous command:

You probably think that this hasn’t gained us much since the number of characters to type hasn’t changed. That’s true but Bash has a special shortcut for ‘!#:\$’ which is ‘!#\$’. This saves us the colon and hence one character:

Since ‘!#\$’ looks like a grawlix, I call this character sequence the “Bash curse”. I often use it many times a day, whenever I need to reuse (parts) of the argument preceding the argument I’m currently typing.

2. Skipping History

All of the mentioned tricks depend heavily on Bash’s history feature. After all, we want to save time by reusing something we typed earlier. Sometimes, however, we would rather not mess up our history. Take this command-line, for example:

Let’s further assume that you need to execute this regularly, many times a day. If you haven’t created an alias for it, you’ll probably pull the CTRL-R stunt*, something which I also explained in my previous post. So you hit CTRL-R and start typing “conan install”. Immediately, the desired command-line shows up and all you have to do is hit “Enter” to execute it.

So far so good. Sometimes, however, you want to fine-tune the command-line, for instance, to do a release build:

After you executed this command, your next attempt to retrieve the original command-line via CTRL-R will first find the one containing ‘-s build_type=Release’ which is not what you want as you only rarely want to do release builds. So it would have been better if the release build command-line had never been recorded in Bash’s history.

Another example of when you don’t want something to be entered in you Bash history is when you provide passwords/credentials on the command-line, as in

So how do you avoid that something is remembered in Bash? It’s easy. Just put a single space before the command you are about to execute:

This concludes service pack 1. Don’t expect service pack 2 anytime soon…

*) If I had to give up all the Bash tricks and only keep one, I’d would keep — hands down — the CTRL-R trick [back]

# To Pluralize, Or Not To Pluralize, That Is The Question

“Consciousness is a singular for which there is no plural”
— Erwin Schrödinger

When it comes to naming directories and files, some folks seem to insist on adding plural ‘s’ letters, as in

docs/, tests/, srcs/, recipes.txt

People like this have a collection-oriented view of the world. To them, “docs” is a label for a folder holding documents, just like a label on a box saying “screws” denotes that there’s a collection of screws in it.

When “pluralists” come across a directory named “doc”, it causes them grief. Why do people do that, they whine, there’s clearly an ‘s’ missing — is it just a typo or was it done deliberately, to save typing?

Let me tell you this: it’s usually done deliberately, but not to save a measly character. It’s done by individuals who have an identity-oriented view of the world and don’t care about containment and multiplicity. To such people, a folder named “doc” is the documentation of a project. It may hold a single text file, multiple PDFs and even some markdown documents. Likewise, “src” is the “source code” and “test” is the corresponding test in its entirety.

So there you have it: the reason why there is no ‘s’ in a name is just a coincidence in cases where the abbreviated name of what something is looks like the singular form of an item of a collection. “doc”, mind you, is not the abbreviation of “document” — it’s meant to be the abbreviation of “documentation”. (Incidentally, the Linux project avoids this confusion by keeping all the documentation in a folder named “Documentation“.)

My general advice is to strive hard to name something after what it is for the sake of better abstraction. Clearly, “essay” is more meaningful than “characters” and by the same token, I prefer “cookbook.txt” over “recipes.txt”. Only when a container has no higher-level purpose other than containment it should be given the plural form of items it contains, but this should rarely be the case.

# Flashme

“It is the flash which appears,
the thunderbolt will follow.”
– Voltaire

Flashme is a flashcard program for command-line lovers, just like me. To avoid repeating myself, here’s the introduction from the GitHub project’s README file:

“I felt compelled to write flashme because I couldn’t find anything like it. While there are many freeware flashcard programs out there, none met my chief requirement: it must be fully operable from the command-line. My next most important requirement was that cards must be kept in plain text files (I call them “deckfiles”): I want to be able to diff, grep, and edit deckfiles with standard Unix command-line tools and editors. Binary file formats, including zipped XML files, are a nuisance and make it difficult to store flashcards in version control systems.”

I’ve used flashme extensively (and successfully) for more than a year to learn English, Linux command-line options, vi hacks, and much more. I think the time has come to release it to the public.

Share and enjoy!

# Grokking Git Merge Commits And Combined Diffs

“A poor workman blames his tools.”
— anon

The other day, I overheard two developers discussing pros and cons of various version control systems. I only caught this fragment: “… What sucks about Git is that when you look at a merge commit, you can’t really see what changed!”. It wasn’t the first time I heard such complaints. Time to debunk Git merge commits.

I’ll use this repository which contains two merge commits (labelled ‘M’):

In my initial commit I added a file called ‘song_of_the_bell.txt’ which contains the first stanza of an English translation of a famous poem by Friedrich Schiller:

The next commit (ddfebcd) just added blank lines after every other line and was done on a branch called ‘better_formatting’. You don’t see the branch ‘better_formatting’ (graphically as a branch) because the merge to ‘master’ was a so-called fast-forward merge.

Then, I made a change on ‘master’ that replaced the comma on the last line with a period:

and a concurrent change on branch ‘add_title’:

Afterwards, ‘add_title’ was merged into ‘master’. Since there were modifications on both branches, a fast-forward merge was not possible, so there’s an explicit merge commit (fd63230). What do you think you’ll see when you show this merge commit?

Nothing, or rather, not much!

You don’t see the typical diff-like line changes and that’s what the developers lamented about. Other version control systems would give them what they want, namely the delta between the merge commit and the previous commit on ‘master’. This would allow them to easily figure out what was changed on ‘master’. Git can do it as well, but for merge commits you have to be explicit, ‘git show’ doesn’t cut it:

fd63230^- is a shortcut and translates to “show the difference between the predecessor of commit fd63230 and the commit fd63230 itself”. In general, ^- is short for hash^-n where n defaults to 1 (the first parent, aka. merge-to parent). You can show the delta between the second (merge-from) parent and the merge commit like this:

There’s a reason why a regular ‘git show’ doesn’t show much. In order to understand, we need to talk about combined diffs first. Combined diffs show the delta between the merge commit and the merge commit’s both parents in a single diff. Let’s produce a combined diff for our merge commit by using the -c option:

First of all, notice the line containing “Merge: 69a7968 6be3af1”: The first hash is the hash of the first parent of the merge commit (aka. the merge-to parent, fd63230^1), while the second hash belongs to the second parent of the merge commit (aka. the merge-from parent, fd63230^2). Next comes the diff output, in which the first column is used to show the delta between the merge commit and the first parent, whereas the second column is used to display the delta between the merge commit and the second parent:

The +/- markers are in the first column (i. e. markers are not indented) which means that “SONG OF THE BELL” and a single blank line were added to the first parent (on master). This change must have come from the merge-from commit (the branch ‘add_title’). Conversely,

shows the difference between the second parent (on ‘add_title’) and the merge commit, since the +/- markers are in the second column (i. e. markers are indented by one space). These are the changes that were done on master.

There’s also a –cc option (think “compact combined”) that gives an even tighter output than -c in that it only shows modifications that occur on both parents in the same lines. In other words, it’s a combined diff showing only merge conflicts. Since the changes in the merge commit fd63230 are non-conflicting (they’re on different lines), –cc produces no diff at all:

Wait a minute! Isn’t this the same output that we got above when we executed a plain ‘git show fd63230’ without the –cc option? Precisely! When showing merge conflicts, ‘git show’ defaults to “compact combined format”, which displays only conflicts. That’s why most merge commits are empty and that’s why there’s so much whining. On the other hand, this little feature makes the life of an integrator much easier, as (s)he can focus on the parts of a merge commit that are criticial: conflicts.

Now let’s take a look at the other merge commit, the one at the top of the history:

Here you do have some output, which means there was a conflict. Again, the first column shows what changed between the first parent (merge-to parent) and the merged version, which is just the addition of the author name “Friedrich Schiller”. Obviously, this change originated from the ‘add_author’ branch. The second column shows what has changed between the second parent (merge-from parent) and the merged version. Clearly, the title “SONG OF THE BELL” was indented on ‘master’. But why is the author name “Friedrich Schiller” marked as a change in the second column as well? It shouldn’t appear, because this is the change that was done in the merge-from parent, right? As always, Git is right, it should. During the merge, as part of the conflict resolution, the author name “Friedrich Schiller” was indented (in the spirit of the change in the merge-to parent which indented the title). It’s this indentation that has changed in the merge commit compared to the merge-from parent.

Understanding combined diffs definitely takes a little getting used to. That’s the reason why most people only care about what changed between the merge-to commit and the merge commit. You already know how to obtain these changes painlessly:

# Working the Bash Shell Like a Pro

“People drive cars with steering wheels and gas pedals. Does that mean you don’t need wrenches?”
— Rob Pike

I’ve always preferred command-line interfaces (CLI) over GUIs. If I use GUIs at all then it’s mostly for browsing the web. Luckily, there is a plugin for my web browser that allows me to do most of my surfing using vi keystrokes. Yes, I try to avoid the mouse as much as I can.

I believe that most people who prefer GUIs either are bad at typing or haven’t taken the time to learn to use a CLI in an idiomatic way. With this post, I want to share some Bash CLI idioms that can significantly improve your efficiency. I don’t aim for a complete list — I rather present a compilation of the most frequently “not-known” techniques that I’ve encountered when I observed people working with Bash.

THE BASICS

First of all, make sure that you have enabled the command-line editing mode that suits you best. You are much more productive when your favorite shortcuts are available:

Often, we need to do something that we’ve already done before. You can easily re-execute the previous command with this shortcut:

Courtesy of this operator, forgetting to put ‘sudo’ in front of a command is not a big deal:

If you want to re-execute “one of the previous commands” instead of the last one, you could use the following abbreviations:

However, I don’t find these history expansions particularly useful. Often, going through the history by pressing ‘Arrow Up’ is equally fast.

The real game changer, however, is CTRL-R. Press CTRL-R and start typing any characters from one of your previous commands. This will start a backwards search through your command-line history and present you with the most recent command containing the characters you typed (or continue to type). Pressing CTRL-R again will find the next most recent command and so on. If you only partly need what you typed in the past, no problem — you can always edit the command-line that CTRL-R found before executing it.

MORE FUN WITH WORD DESIGNATORS

If you want to rename a file, please don’t do it like this:

Even with TAB completion, this requires too much typing. Why not reuse the path of the first argument?

‘!#’ is a shortcut for “the command-line typed so far” ‘:1’ selects the first argument and ‘:h’ strips off the last component (ie. “oldfile”).

In some cases, you don’t want to rename the file entirely but only change the extension. This can be achieved in a similar fashion:

You guessed it: ‘:r’ removes the file extension.

What if you did a mistake and wanted to undo this change? Again, that’s quite easy if you know the trick:

Which translates to “do another move but swap the arguments from the previous command”.

Sometimes, my fingers get ahead of me and I type ‘vm’ instead of ‘mv’:

Of course, you can always edit the last command be pressing ‘Arrow Up’ and change ‘vm’ to ‘mv’, but the following is much easier to type:

‘!*’ is a placeholder for “all arguments of the previous command”.

The word designator that I use the most — by far — is ‘!\$’; it expands to the last argument of the last command:

Many times, people gratuitously reach for the mouse to copy the output of a previous command in order to use it as an argument for another command. Why not use ‘\$()’ to capture command output?

SOME EXTERNAL HELP

If I was asked to name my favorite standard command-line tool, no doubt I would pick ‘xargs‘. Even though it is largely useless by itself, it’s the superglue that allowes you to build powerful command-lines. It takes the output of a command and uses it as arguments for another one.

Here’s an example that uses ‘xargs’ to build a tar archive containing all the C files that contain C++ comments:

In rare cases, when I have to do work that involves complicated selection/filtering, I reach out for TUI (usually ncurses-based) applications like ‘tig‘, ‘vifm‘, or ‘mc‘ that run in the shell and can be fully operated by the keyboard. Nevertheless, I first try to get by with the simpler ‘menucmd‘ tool. Here’s an example that builds a menu from all shell script files found in a directory. If the user selects an item, the specified action (‘cp -t /tmp’) is executed on it.

There you go. Even if this bag of tricks is not complete I hope it will serve you well. As always, in order to use a tool efficiently, you have to invest in learning how to use it idiomatically*. But this investment pays nice dividends in the medium-to-long term.

[Note — There is an update to this post here]

*) What’s the idiomatic way for vi users to underline a headline? 1. Yank the headline (‘yy’). 2. Paste the yanked headline under the exiting headline (‘p’). 3. Select the second headline (‘V’). 4. Replace every character in selected line with an underscore (‘r-‘) — that’s only six keystrokes! Awesome![back]