Linux – Prefix to each output of a command on runtime

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

Yes, you can do this with for. Even if there is "some simpler way to do this with sort" (but I don't think so), this is also quite simple:

# cd to the directory you want to process

mkdir sorted    
for file in *; 
do
   printf 'Processing %s\n' "$file"
   [ -f "$file" ] && sort -u "$file" > "./sorted/$file"
done

Notes:

• for file in * doesn't process files in subdirectories.
• printf is only to report progress. In fact it should be placed after [ ... ] (see below) but I don't want to overcomplicate the code. You can just remove the printf line, if you want the whole thing to be silent.
• [ -f "$file" ] checks if $file is a regular file. With the most general pattern (i.e. *) we need this condition at least to avoid running sort with the sorted directory as an argument (this would throw an error, harmless but non-elegant). Most likely this test won't be needed if you use a more specific glob like *.txt or *.db instead of * (e.g. to skip a stray desktop.ini file that shouldn't be processed). In this case you can omit [ ... ] && and start the line with sort (leaving the line intact shouldn't hurt though).

• sort supports various options and you may want to use some of them, depending on how you need to sort.

• sort -u de-duplicates entries right after sorting them, and when already using sort is a less redundant alternative to using the uniq command.

If you needed to pick files according to conditions more complex than a simple glob, find might be better to start with. For your current task for should be fine.