bgcmd

#!/bin/bash

usage () {
    cat << 'eof'
bgcmd manages a long running, interactive command in background

usage:

    bgcmd START command [args...]    # starts a command in background

    bgcmd your text here             # sends this line

environment:

    $BGCMDPROMPT    used as delimiter to determine where the output ends
                    (cannot be empty)

    $BGCMDSUFFIX    used to specify a private directory
                    (can be empty)

important:

since this does not spawn a pty, you need to make sure that your program
actually prints the prompt when stdin/stdout/stderr are not pty's

example usage:

    export BGCMDPROMPT='bash-5.2$ '

    # start an interactive shell in background:
    bgcmd START bash --norc --noediting -i

    # set a variable in that shell:
    bgcmd var=7

    # run a simple command:
    bgcmd ls /

    # show that our variable is still there:
    bgcmd 'echo $var'

    # run python in a second session at the same time
    BGCMDSUFFIX=py BGCMDPROMPT='>>> ' bgcmd START python -i

    # show an example calculation with python
    BGCMDSUFFIX=py BGCMDPROMPT='>>> ' bgcmd '2**100'

    # show that our shell variable is still there:
    bgcmd 'echo $var'
eof
}

dir=.bgcmd$BGCMDSUFFIX

if [[ $1 = -h || $1 = --help ]]; then
    usage
    exit
elif [[ ! $BGCMDPROMPT ]]; then
    echo '$BGCMDPROMPT is empty' >&2
    usage >&2
    exit 1
elif [[ $1 = START ]]; then
    mkdir -p "$dir" || { echo could not create directory "$dir" >&2; exit 1; }

    # lhs can't know when rhs has exited, it must be killed manually
    # with this pidfile the next invocation can clean up an old one
    [[ -e .bgcmd/pid ]] && kill "$(<.bgcmd/pid)" 2>/dev/null

    rm -f "$dir"/*
    mkfifo "$dir"/{in,out} || { echo could not create fifos >&2; exit 1; }

    {
        echo "$BASHPID" > "$dir"/pid

        for fd in /proc/self/fd/*; do
            fd=${fd##*/}
            (( fd != 1 )) && exec {fd}>&-
        done

        while read -r < "$dir"/in; do
            printf '>%s\n' "$REPLY" >> "$dir"/log
            printf '%s\n' "$REPLY"
        done
    } | exec "${@:2}" > "$dir"/out 2>&1 &
else
    printf %s\\n "$*" > "$dir"/in
fi

text=

while read -r -n1 -d ''; do
    text+=$REPLY
    [[ $text = "$BGCMDPROMPT" ]] && break
    (( ${#text} < ${#BGCMDPROMPT} )) && continue
    printf %s "${text::1}"
    text=${text:1}
done < "$dir"/out