BASH：如何制作一个脚本，使“tail -f”始终记录目录中的最后一个文件，实时

Question

I'm basically trying to make debugging easier for other scripts of mine.

(Centos 7.6)

What I need is a script doing:

1. tail -f the last file entry in a directory
2. if a new file appears in this directory, it logs this new file, smoothly
3. if I send a SIGINT (Ctrl+C), it doesn't leave orphans
4. 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