X clipboard#

The X Window System, the basis for the GUI on most desktop Linux systems, defines how the clipboard works for copying and pasting between applications in Linux. One notable quark of X clipboard is that there's actually two clipboards in common use: the one you expect explicitly accessed via Copy and Paste menu items or key shortcuts called the CLIPBOARD and another one where you copy by selecting text and paste by pressing the middle mouse button called the PRIMARY selection.

X clipboard utilities#

Occasionally it is useful to be able to read or write the clipboard at the command-line. For most uses, your terminal emulator's copy and paste options are probably enough. The primary use case I have for using a command-line program to interact with the clipboard is when I am uploading a file as a Gist:

<file xclip

The xclip utility will copy the contents of the file onto the clipboard (PRIMARY, not CLIPBOARD, by default) and then I can paste it on the Gist website.

My system also has xsel which is very similar to xclip. Wikipedia actually lists several such programs, including the unfortunately named xcopy, not to be confused with XCOPY.

GNU Screen copy mode#

GNU Screen provides its own clipboard for copying information between the different windows of a screen session. ctrl+a, [ enters copy mode. In copy mode you can move the cursor using the arrow keys and page up/page down keys. Screen keeps a history (of configurable size), so you can scroll back pretty far. In fact, I use Screen's copy mode far more often for viewing the history in a terminal than for actually copying anything. You can exit copy mode either by using esc to cancel or enter once to mark the start of the selection and again to mark the end of it. Once you have copied something, ctrl+a, ] pastes the contents of the clipboard.

Vim plugins#

The real power of Vim and the other text editor comes not from their complicated key combinations, but their programmability. Vim uses its own language Vim script while Emacs uses a dialect of Lisp called Emacs Lisp. Both have large communities who have authors many plugins.

A great source for Vim plugins is [Vim.org][vim-scipts], which also has a wiki about Vim with additional information on some of the plugins and lots of tips on how to better use Vim.

Plugin managers#

While, Vim does have basic support for plugins via .vba files, also known as vimball archives, it's easier to use a plugin manager. I recommend Vundle, which is easy to install and use.

The problem#

$ ls > file

doesn't do what you expect:

$ touch foo
$ touch bar
$ ls > filelist
$ cat filelist
bar
filelist
foo

You probably didn't expect, or want, filelist to be listed in filelist.

The solution#

$ filelist=$(ls); echo "$filelist" >filelist

Calculating π the hard way#

In honor of Pi Day, I was going to try to write a script that computed π in shell, but given the lack of floating point support, I decided it would be too messy. If you want to see hard to follow code to generate π, I highly recommend the IOCCC entry westley.c from 1998, the majority of which is an ASCII art circle which calculates its own area and radius in order to estimate π. The hint file suggests looking at the output of

$ cc -E westley.c

The 2012 entry, endoh2 is also a pretty amazing π calculator.

Getting π#

Instead, I will just generate π the shell way: using another program.

$ python -c 'import math; print(math.pi)'
3.14159265359

The problem#

After using Vim as my primary editor for a while, I find myself trying to use vi-style keyboard shortcuts in other contexts. Usually resulting in :w in middle of whatever I was writing as saving is a natural thing to do when pausing.

There's a few ways to fix this:

1. Get used to the fact that not everything supports vi-style keyboard shortcuts.
2. Change the keyboard shortcuts of the programs I do use.
3. Use Vim for everything.

The first option is suboptimal because vi-style keyboard shortcuts are very useful. Luckily, in many cases there's ways to get them.

The goal#

Given a series of images, present them in a video with a blend effect between the images. The frames between the input frames should gradually transition between the previous and next image. This, and other transition effects, could likely be implemented using a slideshow generator, but it can also be done quite easily using a shell script.

The script#

The final script I wrote is blend.sh. The following commands fetch and run it:

$ wget https://gist.githubusercontent.com/dperelman/2b4b86233aa13d13c0ab/raw/91c7988e27288af8aeb25ade11d0cab90355702f/blend.sh
$ chmod +x blend.sh
$ ./blend.sh slideshow.mkv first.png second.jpg third.gif
$ mplayer slideshow.mkv

The input images may be in any format. The extension of slideshow.mkv will be used by FFmpeg to guess the desired video format (H.264 in a Matroska Multimedia Container for .mkv).

The problem#

While trying to develop a modification to the Pelican source, I was unexpectedly having my installed version of Pelican get run instead of the local version:

$ which pelican
/usr/local/bin/pelican
$ command -v pelican
/usr/bin/pelican

For some reason, which was pointing to the executable I was expecting bash to run, but the Bash builtin command was telling me that bash was running the installed version instead.

The solution#

Use the hash builtin to clear bash's cache of the location of pelican:

$ hash -d pelican
$ command -v pelican
/usr/local/bin/pelican

CPU and memory#

On Linux, the basic way to monitor load is to use top. The only thing top really has going for it is that it is almost certainly available on any system you will ever use. Luckily, there's a better way: htop. htop supports colors and mouse clicks and lists the available key commands at the bottom of the terminal. It also can be customized to your liking. You can start by putting my htoprc in your ~/.config/htop/ directory:

$ mkdir -p ~/.config/htop/
$ cd ~/.config/htop/
$ wget https://gist.githubusercontent.com/dperelman/1e051f5705685cb41f31/raw/3ab9cf17b166120a805d5f76a71ce82452f553b4/htoprc

Or just explore the options yourself.

Hit F1 (or click Help in the bottom-left) to get an explanation of the colors used in the CPU and memory bars and a guide to keystrokes not listed at the bottom.

In my usage, I find insufficient memory is more often the problem than CPU, so I usually leave htop sorted by the MEM% column.

Other resources#

While CPU and memory are the easiest to monitor resources, they are not the only ones. Linux offers a wide variety of system monitors, depending on what resource you want to monitor and what format you want to view it in. This post focuses on real-time viewing with human-friendly displays but most of these have options or variants that support logging historical data in a more machine-friendly format as well.

The bug#

A couple days ago my Twitter screen scraper stopped working in Liferea. I hadn't changed anything, and the script's output at the command-line still looked okay to my inspection, but Liferea started giving the message

Podcasts are a simple extension to RSS: in addition to text and a link, posts can include a file to be downloaded. In the case of podcasts, this is an audio file. Due to this, while many specialized podcast applications exist, any news aggregator will work, although it might not have the best interface for that use case.

My normal workflow for podcasts is to keep track of them in a news aggregator and explicitly download the files to a local folder. While this isn't overly onerous for a weekly podcast, it is a repetitive task that could be automated. More importantly, downloading an entire backlog of dozens of episodes that way would take a while.