BASH : How to make a script that make "tail -f" always logging the last file in a directory, live - linux

Question

I'm basically trying to make debugging easier for other scripts of mine.
(Centos 7.6)
What I need is a script doing :
tail -f the last file entry in a directory
if a new file appears in this directory, it logs this new file, smoothly
if I send a SIGINT (Ctrl+C), it doesn't leave orphans
with the less possible add-ons for the maximum portability
This is my non working solution :
CURRENT_FILE=`ls -1t | head -n1`
tail -n 100 -f "$CURRENT_FILE" &
PID=$!
while [ true ];
do
#is there a new file in the directory ?
NEW_FILE=`ls -1t | head -n1`
if [[ "$CURRENT_FILE" != "$NEW_FILE" ]]; then
#yes, so kill last tail
kill -9 $PID
clear
#tail on the new file
CURRENT_FILE=$NEW_FILE
tail -n 100 -f "$CURRENT_FILE"
PID=$!
fi
sleep 1s
done
The problem with this solution is that when I'm sending SIGINT (Ctrl+C), what I normally do when exiting a "tail -f", it leaves an orphan child in the background.
I've searched solution with "trap" but I don't get it well, and it doesn't seem to work with an eternal process like "tail -f".
I'll be glad to here your thoughts about that and get into advanced bash programming.

Answer 1

You can trap whenever the script exits and kill the process then. You don't need -9 to kill your tail though, that's overkill.
You can also use inotify to tell you when something happens in the directory instead of sleeping and rechecking. Here's a basic building block. inotify has a lot of events you can wait for. You can add detection if the file was moved/renamed so you don't have to restart the tail in those cases etc.
#!/bin/bash
killpid() {
if [[ -n $PID ]]; then
kill $PID
PID=""
fi
}
trap killpid EXIT
DIR="."
CURRENT_FILE="$(ls -1t "$DIR" | head -n1)"
tailit() {
echo "::: $CURRENT_FILE :::"
tail -n 100 -f "$CURRENT_FILE" &
PID=$!
}
tailit
# wait for any file to be created, modified or deleted
while EVENT=$(inotifywait -q -e create,modify,delete "$DIR"); do
# extract event
ev=$(sed -E "s/^${DIR}\/ (\S+) .+$/\1/" <<< "$EVENT")
# extract the affected file
NEW_FILE=${EVENT#${DIR}/ $ev }
case $ev in
MODIFY)
# start tailing the file if we aren't tailing it already
if [[ $NEW_FILE != $CURRENT_FILE ]]; then
killpid
CURRENT_FILE="$NEW_FILE"
tailit
fi
;;
CREATE)
# a new file, tail it
killpid
CURRENT_FILE="$NEW_FILE"
tailit
;;
DELETE)
# stop tailing if the file we are tailing was deleted
if [[ $NEW_FILE == $CURRENT_FILE ]]; then
echo "::: $CURRENT_FILE removed :::"
CURRENT_FILE=""
killpid
fi
;;
esac
done

Answer 2

You can use trap solution at the beginning of your shell.
#! /bin/bash
trap ctrl_c INT
function ctrl_c() {
if [[ -n "$PID" ]]; then
kill -9 $PID
fi
exit 0
}
CURRENT_FILE=`ls -1t | head -n1`
tail -n 100 -f "$CURRENT_FILE" &
PID=$!
while [ true ];
do
#is there a new file in the directory ?
NEW_FILE=`ls -1t | head -n1`
if [[ "$CURRENT_FILE" != "$NEW_FILE" ]]; then
#yes, so kill last tail
kill -9 $PID
clear
#tail on the new file
CURRENT_FILE=$NEW_FILE
tail -n 100 -f "$CURRENT_FILE" &
PID=$!
fi
sleep 1s
done