start dockerfile and script for running a terminal and screenshotting it

test/Dockerfile

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+FROM alpine:3.16
+
+# Base packages. A good set of this is just to get X (xvfb) and OpenGL
+# software rendering working (all the mesa drivers).
+RUN apk add --no-cache \
+    bash \
+    grep \
+    procps \
+    imagemagick \
+    libxrandr \
+    mesa-dev \
+    mesa-dri-gallium \
+    mesa-gl \
+    xvfb \
+    xvfb-run \
+    xdotool
+
+# Our terminals
+RUN apk add --no-cache \
+    alacritty
+
+COPY ./run.sh /run.sh
+COPY ./colors.sh /colors.sh
+
+ENTRYPOINT ["/bin/bash"]

test/colors.sh

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+#!/usr/bin/env bash
+
+# Tom Hale, 2016. MIT Licence.
+# Print out 256 colours, with each number printed in its corresponding colour
+# See http://askubuntu.com/questions/821157/print-a-256-color-test-pattern-in-the-terminal/821163#821163
+
+set -eu # Fail on errors or undeclared variables
+
+printable_colours=256
+
+# Return a colour that contrasts with the given colour
+# Bash only does integer division, so keep it integral
+function contrast_colour {
+    local r g b luminance
+    colour="$1"
+
+    if (( colour < 16 )); then # Initial 16 ANSI colours
+        (( colour == 0 )) && printf "15" || printf "0"
+        return
+    fi
+
+    # Greyscale # rgb_R = rgb_G = rgb_B = (number - 232) * 10 + 8
+    if (( colour > 231 )); then # Greyscale ramp
+        (( colour < 244 )) && printf "15" || printf "0"
+        return
+    fi
+
+    # All other colours:
+    # 6x6x6 colour cube = 16 + 36*R + 6*G + B  # Where RGB are [0..5]
+    # See http://stackoverflow.com/a/27165165/5353461
+
+    # r=$(( (colour-16) / 36 ))
+    g=$(( ((colour-16) % 36) / 6 ))
+    # b=$(( (colour-16) % 6 ))
+
+    # If luminance is bright, print number in black, white otherwise.
+    # Green contributes 587/1000 to human perceived luminance - ITU R-REC-BT.601
+    (( g > 2)) && printf "0" || printf "15"
+    return
+
+    # Uncomment the below for more precise luminance calculations
+
+    # # Calculate percieved brightness
+    # # See https://www.w3.org/TR/AERT#color-contrast
+    # # and http://www.itu.int/rec/R-REC-BT.601
+    # # Luminance is in range 0..5000 as each value is 0..5
+    # luminance=$(( (r * 299) + (g * 587) + (b * 114) ))
+    # (( $luminance > 2500 )) && printf "0" || printf "15"
+}
+
+# Print a coloured block with the number of that colour
+function print_colour {
+    local colour="$1" contrast
+    contrast=$(contrast_colour "$1")
+    printf "\e[48;5;%sm" "$colour"                # Start block of colour
+    printf "\e[38;5;%sm%3d" "$contrast" "$colour" # In contrast, print number
+    printf "\e[0m "                               # Reset colour
+}
+
+# Starting at $1, print a run of $2 colours
+function print_run {
+    local i
+    for (( i = "$1"; i < "$1" + "$2" && i < printable_colours; i++ )) do
+        print_colour "$i"
+    done
+    printf "  "
+}
+
+# Print blocks of colours
+function print_blocks {
+    local start="$1" i
+    local end="$2" # inclusive
+    local block_cols="$3"
+    local block_rows="$4"
+    local blocks_per_line="$5"
+    local block_length=$((block_cols * block_rows))
+
+    # Print sets of blocks
+    for (( i = start; i <= end; i += (blocks_per_line-1) * block_length )) do
+        printf "\n" # Space before each set of blocks
+        # For each block row
+        for (( row = 0; row < block_rows; row++ )) do
+            # Print block columns for all blocks on the line
+            for (( block = 0; block < blocks_per_line; block++ )) do
+                print_run $(( i + (block * block_length) )) "$block_cols"
+            done
+            (( i += block_cols )) # Prepare to print the next row
+            printf "\n"
+        done
+    done
+}
+
+print_run 0 16 # The first 16 colours are spread over the whole spectrum
+printf "\n"
+print_blocks 16 231 6 6 3 # 6x6x6 colour cube between 16 and 231 inclusive
+print_blocks 232 255 12 2 1 # Not 50, but 24 Shades of Grey

test/run.sh

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+#!/usr/bin/env bash
+
+# The child program to execute
+CHILD="alacritty"
+
+#--------------------------------------------------------------------
+# Some terminals require XDG be properly setup. We create a new
+# set of XDG directories for this.
+export XDG_BASE_DIR="/work/xdg"
+export XDG_RUNTIME_DIR="${XDG_BASE_DIR}/runtime"
+mkdir -p ${XDG_BASE_DIR} ${XDG_RUNTIME_DIR}
+chmod 0700 $XDG_RUNTIME_DIR
+
+#--------------------------------------------------------------------
+
+# Start up the program under test
+CHILD_LOG="${XDG_BASE_DIR}/child.log"
+${CHILD} -o "window.start_maximized=true" >${CHILD_LOG} 2>&1 &
+CHILD_PID=$!
+echo "Child pid: ${CHILD_PID}"
+echo "Child log: ${CHILD_LOG}"
+
+sleep 2
+
+xdotool type "/colors.sh"
+xdotool key Return
+
+sleep 1
+
+import -window root /src/screen.jpeg