Simple reactive statusbar

statusbar.adoc

Table of Contents

• The main event loop
• Using crond(8) or systemd.timer(5)
• Considerations
• Examples
  • Currently playing title from MPD
  • Real-time pulseaudio volume monitor

The main event loop

statusdir=$XDG_RUNTIME_DIR/statusbar
mkdir -pm0700 "$statusdir"

inotifywait -rqme close_write --format %f "$statusdir" | while read -r file; do
    if read -r -- "$file" < "$statusdir/$file"; then
        # ...
    fi
done

With inotifywait we can watch a directory of files. When one of those files triggers the close_write event we synthesise a new variable based on the file name using read if the file name is also a valid identifier. The new variable will contain the first line of data from the file itself.

These new variables can, for example, be used with printf to create a formatted string designed for use in a typical statusbar.

Note	This event loop can be arbitrarily fast. Any pre-formatting should be done before the values are written to files watched by inotifywait so only a minimum amount of work is needed to process them.

Using crond(8) or systemd.timer(5)

Some status updates which cannot be done using notification and have usually long intervals can be easily provided with per-user crond or systemd.timer services.

For example given a battery-status script, we could run it every five minutes using the following crontab:

*/5 *  *  *  * battery-status > "$XDG_RUNTIME_DIR"/statusbar/battery

Considerations

• Pre-initialised data

• Multi-user

• Persistence

Examples

Although all these examples are simple sh scripts it does not mean only shell can be used. Any language capable of writing files (even to stdout) is suitable for this.

Currently playing title from MPD

#!/bin/sh --

statusdir=$XDG_RUNTIME_DIR/statusbar
mkdir -pm0700 "$statusdir"

fmt='[[%artist% - ]%title%]|[%file%]'

mpc current -f "$fmt" > "%statusdir"/mpd

mpc idleloop | while read -r event; do
    case $event in
        player) mpc current -f "$fmt" > "%statusdir"/mpd
    esac
done

Real-time pulseaudio volume monitor

Use pactl subscribe to provide real-time volume notification for default sinks.

Note	One potential improvement would be to pair this with another pacmd running as a "daemon" using exec_with_pipe to accept commands over a control fifo thereby avoiding execution of multiple pulseaudio clients.

#!/bin/sh --

statusdir=$XDG_RUNTIME_DIR/statusbar
mkdir -pm0700 "$statusdir"

pastat() {
    local command=$1
    local fmt=$2

    pacmd list-"$command" | awk -v fmt="$fmt" '
        # Default indices are marked with an * so we use that as a sentinel.
        /\* index/              {m = 1; next}
        m == 1 && /muted/       {muted = $2}
        m == 1 && /^\s+volume:/ {volume = $5}

        END{
            if(muted == "no")
                printf fmt, volume
            else
                print ""
        }'
}

# If pulseaudio isn't running, print something out and exit.
if ! pulseaudio --check; then
    printf 'VOL:N/A\n' > "$statusdir"/volume
    exit 1
fi

# Initialise something before we begin monitoring events.
pastat sinks 'VOL:%s\n' > "$statusdir"/volume

pactl subscribe | while read -r _ event _ type _; do
    if [ "$event" = \'change\' ]; then
        case $type in
            sink)   pastat sinks 'VOL:%s\n' > "$statusdir"/volume
        esac
    fi
done