Bash. stdout from background piped commands

Question

I don't know how to recover the stdout of commands "piped" launched in background. Let's explain. I have this command:

iface=$(airmon-ng start wlan0 2> /dev/null | grep monitor)

And i want to send all to the background to recover the pid using $!, but when i put & anywhere, the grep stop working and the iface var is empty. Any idea? Thank you.

Answer 1

If you want to just get status that the script is still running while waiting for the output from grep you can use an existing tool like pv that will print a progress meter to stderr so it won't interfere with your capturing the output.

Failing that, you could write a function that would be slower than the pv solution I imagine, but will let you update the spinner after, say, each line like

get_monitor() {
    printf ' ' >&2 # to put a first char there to backspace the spinner over
    while read -r line; do
        if [[ $line =~ monitor ]]; then
            printf '%s\n' "$line"
        fi
        update_spinner
    done < <(airmon-ng start wlan0 2>/dev/null)
}

sp_ind=0
sp_chars='/-\|'
sp_num=${#sp_chars}
update_spinner() {
    printf '\b%s' "${sp_chars:sp_ind++%sp_num:1}" >&2
}

iface=$(get_monitor)

or you could have your backgrounded command write to a temporary file and get the answer after like

airmon-ng start wlan2 2>/dev/null >/tmp/airmon_out &
# your spinner stuff
iface=$(cat /tmp/airmon_out)

or perhaps you wouldn't even need it in a variable any more as many things know how to operate on files