How to exit a FOR loop on detecton of "error" stream using TAIL

Question

I have a "for" loop which passes the results to a file called "results" Occasionally an error stream is detected and the loop continues trying within the loop for a number of times before it exits and moves onto the next iteration. I'd like to detect an error result when it first occures and move onto the next iteration. I've had three attempts to do this - none of which have worked.

edit:- To put it in context this code preceding;

wl_pass=input.txt
for fn in `cat $wl_pass`; do
wpscan --url $fn -e  u vp vt >> "results"

Attempt 1

if [ "tail -1 -f results" == "Error" ]; 
then
        echo "error detected"
        continue
fi

Attempt 2

while [ $("tail -1 -f results" | grep -o0 -c "Error") -eq 0 ]; do continue; done

Attempt 3

tail -f "results" | grep --line-buffered "Error" | while read line ; do
      continue
done

Any advice would be welcome

Thanks for the input -->> this is what I have so far. I'm still having problems with it:-

wl_pass=input.txt

wl_pass="${wl_pass}"

count_pass=$(wc -l $wl_pass | cut -d " " -f1)

while true
do
        timestamp=`date +%s`
        echo start $timestamp >> results
        for fn in `cat $wl_pass`
        do
        wpscan --url $fn -e  u vp vt >> results
        echo end $timestamp >> results

        if sed -n '/start $timestamp/,/end $timestamp/p' results | grep -E 'Error:'; then
                break
#               continue
        fi
        done

done

Why use tail here? Is your log file millions of lines? Wouldn't it simpler to just grep the whole thing? — terdon
– terdon ♦, Commented Jan 17, 2023 at 15:53
Please edit your question and add more context. Show the code that writes to the file results. What should happen when Error is found or otherwise? Do you want to change the behavior of the for loop or the processing of the output? — Bodo
– Bodo, Commented Jan 17, 2023 at 15:53
You have a loop that writes to the file, ok. That loop then sometimes faces an error which you want to deal with? But here, what you're showing is commands reading that output, apparently outside that mentioned loop. Is that right? Trying to affect a loop writing to the file from the outside doesn't seem at all simple, so you might want reconsider what you're doing, or at least consider showing more context on what you're actually doing there. — ilkkachu
– ilkkachu, Commented Jan 17, 2023 at 16:10
Please be careful to edit your question in response to clarifying comments. Do not reply in the Answer box -- that is only for Answers to the question. Thank you! — Jeff Schaller
– Jeff Schaller ♦, Commented Jan 19, 2023 at 16:40

White Owl · Accepted Answer · 2023-01-17 16:11:06Z

Not clear why you are using tail.

Assuming you have an app which produces text file result and you need to check for word "Error" in it:

while true
do
    ./app > result
    if grep Error result ; then
       break
    fi
done

If your main app is producing a continuous log and the word "Error" can appear in it as a result of some event, and you want to check that particular section of the log. And ignore word "Error" before the event if it exist there, you can do something like:

./app > result
while true
do
    timestamp=`date +%s`
    echo start $timestamp >> result
    ./trigger_event_in_app
    echo end $timestamp >> result
    
    if sed -n '/start $timestamp/,/end $timestamp/p' result | grep Error ; then
       break
    fi
done

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