A simple Bash script to automatically git fetch (with delay), to be called in combination with __git_ps1 for your prompt.

autogitfetch

#!/usr/bin/env bash

set -eu

MIN_INTERVAL="${AUTOGITFETCH_MIN_INTERVAL:-300}" # seconds
TIMESTAMP_STORE_ROOT="${HOME}/.cache/autogitfetch/"

RESET='\e[0m'
GREY='\e[0;90m'
OUTPUT_TEXT_COLOR="$GREY"

# Note:
# Display every cached "last fetched" timestamp with:
# for f in ~/.cache/autogitfetch/*; do echo "$(date -d @$(cat "$f")) ($(basename "$f" | cut --delimiter=- --fields=4- | basenc --base64url --decode))"; done

# Will fail and therefore stop the while script if not in a git repository:
worktrees="$(git worktree list --porcelain 2>/dev/null)"
main_repo_root="$(echo "$worktrees" | head -n1 | cut --delimiter=' ' --fields=2)"

mkdir -p "$TIMESTAMP_STORE_ROOT"

enc_repo_ref="$(echo "$main_repo_root" | basenc --base64url)"
timestamp_file="${TIMESTAMP_STORE_ROOT}/${enc_repo_ref}"

timestamp="$(date +%s)"

if ! last_timestamp="$(cat "$timestamp_file" 2>/dev/null)"
then
    last_timestamp=0
fi

if (( $timestamp - $last_timestamp < $MIN_INTERVAL ))
then
    exit
fi

echo -n "$timestamp" > "$timestamp_file"

echo -en "${OUTPUT_TEXT_COLOR}Fetching git branches…${RESET}"

OUTPUT="$(git fetch --prune --progress 2>&1)"

if [[ $OUTPUT = '' ]]
then
    echo -e "${OUTPUT_TEXT_COLOR} done.${RESET}"
else
    echo -e "${OUTPUT_TEXT_COLOR} done:\n${OUTPUT}${RESET}"
fi

find "$TIMESTAMP_STORE_ROOT" -type f -name 'git-last-fetch-*' -mtime +30 -delete # clean old cache entries