No worries. I'm more than glad to be able to help.
Let me refer to the script itself to answer your questions:

Note that you don't need the .txt extension. You can leave it there if you want, though. Just a thing here, I assume you meant "Logcheck.txt" - without the uppercase "O". This kind of typo can cause your script to run with issues or not run at all.

You don't need to add the cd command - just the absolute path to the log. The loop

Code:

while read line
do
[...bunch of commands here :) ...]
done < Logcheck.txt

does this:
It reads the Logcheck.txt file (that's what the "<" does - it tells the while loop to use the file as input) line by line, one at a time, and during every run it assigns the value of each line to the line variable. Picture this scenario. Let's say the Logcheck.txt file looks exactly as I mentioned in my first post. During the first loop, the line variable (referenced in the rest of the loop as $line) will take the value /var/log/mail.log, and that value will be used until the loop ends and starts again, this time with the line variable taking the value of the 2nd line in your Logcheck.txt file.
That line saves the last 100 lines of each log (refer to the first answer - $line here will be replaced by the absolute path to each script during each loop) to the checklog file. (Note that I didn't use the extension here).
So during the first loop this will actually be:

Code:

tail -100 /var/log/mail.log > checklog

"checklog" is an auxiliary file where we're redirecting the output of each log and that's where we'll search for errors later.
That line will delete your auxiliary files mailbody (see the top of the script) and checklog once the loop has ended and you don't need them anymore. You can comment out that line if you want to keep those files.
Let me know if you need further help .


---------- Post updated at 09:38 PM ---------- Previous update was at 09:09 PM ----------

Updated version, tested . Works like a charm.
1) I created the file named mylogs.
2) Updated script:

Code:

#!/bin/bash
mailcontents=mailbody
while read line
do
    if [ ! -f $line ]; then
        echo "The file $line doesn't exist. Continuing with the next file..." >> $mailcontents
        echo "============" >> $mailcontents 
        continue
    else
        last_mod_time=$(stat -c '%Y' $line) # this line checks the log's last modification time and converts it to Unix's epoch
        last_24_hours=$(date +%s -d "24 hours ago") # this line returns the epoch for the current timestamp minus 24 hours

        if [ $last_mod_time -lt $last_24_hours ]; then
            echo "Log $line has NOT been updated in the last 24 hours" >> $mailcontents
        else
            echo "Log $line was updated during the last 24 hours" >> $mailcontents
        fi
    
        tail -100 $line > checklog

        error=$(grep ERROR checklog | wc -l) # We look for the lines containing the word "ERROR" in the checklog file.
                                             # Then we redirect the output to the wc -l command that will count the number
                                             # of lines where the word ERROR appears.
    
        if [ $error -gt 0 ]; then # If this condition is satisfied, that means the word ERROR appeared at least once in
                                  # the log that's being examined in the current loop.
            echo "ERROR found in log $line" >> $mailcontents
        else
            echo "No errors found in $line" >> $mailcontents
        fi
    fi
echo "============" >> $mailcontents
done < mylogs

if [ -s $mailcontents ]; then
    mail -s "Log errors and last modification times - $(date +'%A %B %d, %Y')" myemail@gmail.com < $mailcontents
fi

rm $mailcontents checklog # Delete auxiliary files when we're done.

The results:


---------- Post updated at 10:21 PM ---------- Previous update was at 09:38 PM ----------

Added file test.log with the word ERROR in 2 lines, and included it in the mylogs file (where the list of the logs is stored):

The results in the email: (note that the email includes the lines with the errors). You need to add the following line

Code:

echo -e "\t" $(grep ERROR checklog) >> $mailcontents

right beneath this one:

Code:

echo "ERROR found in log $line" >> $mailcontents