waitfor

#!/usr/bin/env bash
#
# Wait until a process exits by PID.
#
# Accepts either --pid PID or a positional PID argument.
# If no PID is given, opens an interactive fzf picker from `ps ax`
# and infers the PID from the selected row.
#
# Useful for simple shell chaining, for example:
#
#   waitfor 1234; echo 'process 1234 finished - do something after'
#   waitfor; echo 'process finished!'  # fzf-powered interactive process selection
#
# author: andreasl

show_usage() {
    echo "Usage: ${0##*/} [--pid PID] [PID]"
    echo
    echo 'Wait until the target process exits.'
    echo 'If no PID is provided, an interactive fzf picker opens.'
    echo
    echo 'Examples:'
    echo "  ${0##*/} --pid 1234"
    echo "  ${0##*/} 1234"
    echo "  ${0##*/}"
    echo "  ${0##*/} 1234; echo 'process finished - do something after'"
    echo "  ${0##*/}; echo 'interactively chosen process finished!'"
}

pid=''

while [ "$#" -gt 0 ]; do
    case "$1" in
    --pid)
        shift
        if [ -z "${1:-}" ]; then
            echo 'Error: --pid requires a value.' >&2
            exit 2
        fi
        pid="$1"
        ;;
    --pid=*)
        pid="${1#--pid=}"
        if [ -z "$pid" ]; then
            echo 'Error: --pid requires a value.' >&2
            exit 2
        fi
        ;;
    -h | --help)
        show_usage
        exit 0
        ;;
    -*)
        echo "Error: unknown option '$1'." >&2
        show_usage >&2
        exit 2
        ;;
    *)
        if [ -n "$pid" ]; then
            echo 'Error: PID specified more than once.' >&2
            exit 2
        fi
        pid="$1"
        ;;
    esac
    shift
done

if [ -z "$pid" ]; then
    if ! command -v fzf >/dev/null 2>&1; then
        echo 'Error: fzf is required for interactive selection.' >&2
        exit 127
    fi

    selection="$(ps ax -o pid= -o command= | fzf --prompt='waitfor> ' --header='Select process to wait for')"
    [ -z "$selection" ] && {
        echo 'No process selected.' >&2
        exit 1
    }

    read -r pid _ <<EOF
$selection
EOF
fi

[[ ! "$pid" =~ ^[0-9]+$ ]] && {
    echo "Error: PID must be numeric, got '$pid'." >&2
    exit 2
}

tail -f /dev/null --pid="$pid"

printf '%s: Process %s finished.\n' "$(date)" "$pid"