Bash script - Check if a file contains a specific line

From the grep's man:

-q, --quiet, --silent
    Quiet; do not write anything to standard output.  Exit immediately with zero status if any match is found, even if an error was detected.  Also see the -s or --no-messages option.
-F, --fixed-strings
    Interpret PATTERNS as fixed strings, not regular expressions.
-x, --line-regexp
    Select only those matches that exactly match the whole line.  For a regular expression pattern, this is like parenthesizing the pattern and then surrounding it with ^ and $.

# Inline
grep -q -F "$STRING" "$FILE" && echo 'Found' || echo 'Not Found'
grep -q -F "$STRING" "$FILE" || echo 'Not Found'

# Multiline
if grep -q -F "$STRING" "$FILE"; then
  echo 'Found'
else
  echo 'Not Found'
fi

if ! grep -q -F "$STRING" "$FILE"; then
  echo 'Not Found'
fi

If you need to check the whole line use -x flag:

grep -q -x -F "$STRING" "$FILE" && echo 'Found' || echo 'Not Found'

if $FILE contains the file name and $STRING contains the string to be searched, then you can display if the file matches using the following command:

if [ ! -z $(grep "$STRING" "$FILE") ]; then echo "FOUND"; fi

Poll the file's modification time and grep for the string when it changes:

while :; do
    a=$(stat -c%Y "$FILE") # GNU stat
    [ "$b" != "$a" ] && b="$a" && \
        grep -q "$STRING" "$FILE" && echo FOUND
    sleep 1
done

Note: BSD users should use stat -f%m