Linux – Showing the line count of a specific file

You can do this using grep's -f option (that's lower-case -f, not -F):

% echo -e 'Dog\nFish\nCat\nShoes' > ./file1.txt 
% echo -e '1,shoes,red\n2,apple,black\n3,fog,blue' > ./file2.csv 

# Grab all lines from the CSV that match a pattern from file1:
% grep -if ./file1.txt ./file2.csv
1,shoes,red

# Grab all lines from the CSV that DON'T match a pattern from file1:
% grep -vif ./file1.txt ./file2.csv
2,apple,black
3,fog,blue

Detailed explanation:

• grep — self-explanatory
• -v — means 'return lines not matching the input pattern'
• -i — means 'use case-insensitive matching' (since your first file had capital letters and the CSV didn't)
• -f — means 'interpret each line in the specified file (file1.txt) as a pattern to use for matching'

Depending on the results you want and the contents of your files, you may also want to read into the -F and -w options.

If you need to edit the file in-place, i think you can do this with sed's -f option, but sed interprets each line of the file as a command rather than a simple pattern like grep does.

It might be easier if you use more than one line to do this. You could write a bash script:

#!/bin/bash

tail -f test.log | while read line; do
    if echo "$line" | grep -q "Error"; then
        echo "$line" >> error.log
    elif echo "$line" | grep -q "Warning"; then
        echo "$line" >> warning.log
    # The following is in case you want to print out lines that do not match 
    else
        echo "$line"
    fi
done