I initially started typing this as short -[ Contrafibularities ]- segment for my free, weekly newsletter. But it got a bit long, so I broke it out into a blog post instead.
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Going live!
I have a work-supplied M3 MacBook Pro. It’s a lovely device with ludicrous battery endurance, great screen, keyboard and decent connectivity. As an ex-Windows user at work, and predominantly Linux enthusiast at home, macOS throws curveballs at me on a weekly basis. This week, screenshots.
I scripted a ‘going live’ shell script for my personal live streams. For the title card, I wanted the script to take a screenshot of the running terminal, Alacritty. I went looking for ways to do this on the command line, and learned that macOS has shipped a screencapture command-line tool for some time now. Amusingly the man page for it says:
DESCRIPTION
The screencapture utility is not very well documented to date.
A list of options follows.
and..
BUGS
Better documentation is needed for this utility.
This is 100% correct.
How hard can it be?
Perhaps I’m too used to scrot on X11, that I have used for over 20 years. If I want a screenshot of the current running system, just run scrot and bang there’s a PNG in the current directory showing what’s on screen. Easy peasy.
On macOS, run screencapture image.png and you’ll get a screenshot alright, of the desktop, your wallpaper. Not the application windows carefully arranged on top. To me, this is somewhat obtuse. However, it is also possible to screenshot a window, if you know the <windowid>.
From the screencapture man page:
-l <windowid> Captures the window with windowid.
There appears to be no straightforward way to actually get the <windowid> on macOS, though. So, to discover the <windowid> you might want the GetWindowID utility from smokris (easily installed using Homebrew).
That’s fine and relatively straightforward if there’s only one Window of the application, but a tiny bit more complex if the app reports multiple windows - even when there’s only one. Alacritty announces multiple windows, for some reason.
$ GetWindowID Alacritty --list
"" size=500x500 id=73843
"(null)" size=0x0 id=73842
"[email protected] (192.168.1.170) - byobu" size=1728x1080 id=73841
FINE. We can deal with that:
$ GetWindowID Alacritty --list | grep byobu | awk -F '=' '{print $3}'
73841
You may then encounter the mysterious could not create image from window error. This threw me off a little, initially. Thankfully I’m far from the first to encounter this.
Big thanks to this rancher-sandbox, rancher-desktop pull request against their screenshot docs. Through that I discovered there’s a macOS security permission I had to enable, for the terminal application to be able to initiate screenshots of itself.
A big thank-you to both of the above projects for making their knowledge available. Now I have this monstrosity in my script, to take a screenshot of the running Alacritty window:
screencapture -l$(GetWindowID Alacritty --list | \
grep byobu | \
awk -F '=' '{print $3}') titlecard.png
If you watch any of my live streams, you may notice the title card. Now you know how it’s made, or at least how the screenshot is created, anyway.