Allow lastdir/selection to be written to stdout

[joelim-work]

• Fixes lfcd improvement proposal #1393

In order for lf to successfully write information to /dev/stdout when using the -last-dir-path/-selection-path options, the following changes needs to be made:

• Writing the information needs to happen after the tcell screen is cleaned up, as pointed out in question: Is there a way i can output the last-dir-path to STDOUT #472 (comment).
• The output of shell commands needs to be connected to stderr instead of stdout so as not to conflict with the actual information being written. It might be possible in future to change the output of shell commands with a setting, but I think hardcoding it to stderr should suffice for now.

This change makes scripting with lf much easier, as there is no longer any need to create a temporary file to store the information and read it afterwards. For example the lfcd command can be changed from this:

lfcd () {
    tmp="$(mktemp)"
    # `command` is needed in case `lfcd` is aliased to `lf`
    command lf -last-dir-path="$tmp" "$@"
    if [ -f "$tmp" ]; then
        dir="$(cat "$tmp")"
        rm -f "$tmp"
        if [ -d "$dir" ]; then
            if [ "$dir" != "$(pwd)" ]; then
                cd "$dir"
            fi
        fi
    fi
}

To this:

lfcd() {
    # `command` is needed in case `lfcd` is aliased to `lf`
    cd "$(command lf -last-dir-path /dev/stdout "$@")"
}

[gokcehan]

@joelim-work I wasn't aware of the issue with the order of cleanup. For me cd $(lf -last-dir-path /dev/stdout) already works when I don't use any commands, though I see now it does not print anything to the terminal when used outside of a subshell with lf -last-dir-path /dev/stdout. This patch makes it possible to see the output outside of subshells as well. Everything seems good to me.

[nicolasdumitru]

Hello! Does this mean that starting with the next release lfcd will be usable without temporary files? Also, I'd just like to point out that /dev/stdout is not POSIX (they specifically mention it). I use Linux and /dev/stdout is available to me, but what about the other Unix-like operating systems?

[joelim-work]

I responded in #1393 (comment) (ticket reopened)

[ilyagr]

This is a very interesting solution. I'm still worried that it might not work on some weird system (Windows?), but it works perfectly for me for now.

I'm wondering if this would allow some generalization beyond printing the last visited dir. I don't have any immediate specific applications in mind, and I don't propose implementing anything before we have a compelling usecase, but it would be straightforward to have a "print to actual stdout" lf command that would allow lf to be put into some elaborate pipelines.