shell dsl

sunday, 31 january 2016

inevitably, shell scripting a suite of herbstluftwm window manager hidd functions led to considering recoding the scripts, originally written in zsh to dash. The decision to move to dash for all user shell scripts was predicated on its superior speed.

While zsh is feature laden and still used as my interactive shell, dash is lightweight and more fleet of foot. Of course, some shell scripts challenged their conversion to POSIX with features like arrays and globbing not part of the POSIX shell language specification, but the exercise was worth it for the portability – and efficiency.

Post conversion to dash, a $HOME/bin directory hierarchy was created to organize the shell scripts by function. That led to rethinking the naming of the herbstluftwm scripts residing in $HOME/.config/herbstluftwm whose functions were identified by common prefixes such as draw, toggle, query, etc.

A pseudo high level shell language DSL was created with the syntax..

Field Value for herbstluftwm
command draw, toggle, query …
object monitor, panel, tag …
optional parameters fullscreen, kill …

for a command syntax..

command object command object parameters..


by refactoring the window manager scripts by their function, and sourcing the scripts from a common shell script, a simple DSL is created.

Normally, shell scripts are written to source one or more scripts containing a library of common functions and settings. Instead, the command script containing the common functions and settings sources the object script providing the command syntax described above..

function() {...} export ... ... msg=$1 shift if [ -e ${0%/command}/functions/command/$msg ] ;then . ${0%/command}/functions/command/$msg $@ else usage $0 fi

The usage script lists the valid objects for the command command when incorrectly called i.e. the object does not exist..

name=$(basename $@) dir=$(dirname $@) funcs=$(ls -1 $dir/functions/$name) echo ".. $name $(echo $funcs | sed 's/ / \| /g')"

By having a common command script source the relevant object script, a certain language like elegance is achieved. Of course, an argument could be easily be made to simply retain unique descriptive script names, sourcing the relevant script library and foregoing the directory hiearchy.

In this implementation for herbstluftwm, not all command scripts contained refactored code common to their objects. Regardless, this approach allows a natural directory hierarchy with a negligible execution time penalty.


refactoring the scripts for herbstluftwm created the following script organization under $HOME/.config/herbstluftwm/ (commands) and $HOME/.config/herbstluftwm/functions/[command]/ (objects)..

Command Object
draw border, monitor, panel, refresh, root
focus frame, hlwm, window
hide window
query conky, count, desktop, display, fullscreen, hidden, monocle, multihead, primary, rez, tag, ttag, window, winids
switch frames, layout, monitor, out, tag, window
restore frames, hidden, window, windows
toggle cinema, compton, conky, focus, fullscreen, max, monocle, panel, scratchpad, unclutter

Refer to the dotfiles for examples of the actual scripts. Note the toggle script which includes logic to spawn system wide applications not specific to herbstluftwm.

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