There are many ways of doing this, depending on your needs.

• Simply print all lines containing the keyword test:

grep test file.txt

• The same but for many keywords. Create a file that has all the keywords, one per line and run:

grep -f keywords.txt file.txt

• Process a file line by line, exit if the line is found:

while read line; do echo $line | grep test && break; done < file.txt

• Exit with status 1 if the line matches. Unfortunately, if I remember correctly (not 100% sure), bash only allows you to set return values in functions. So, to get an exit status you need to use something else. Perl for example:

perl -ne '/test/ && exit(1)' file.txt

• Though you can't set an exit value, you can still do something similar with bash. The exit value of the last command run is always saved as $? in bash, that means you can test the value of $? and act accordingly:

while read line; do let c++; echo $line | grep test >/dev/null; 
  if [[ $? == 0 ]] ; then echo Match for line $c : $line;
  else echo No match for line $c; fi  
done < file.txt

07/17 21:04:01 sndc addr unit 1 : hu P1 (TempLinc)
07/17 21:04:02 sndc func StatusReq : hc P
07/17 21:04:04 rcvi addr unit 15 : hu P15 (TempAck_5)
07/17 21:04:04 rcvi func Preset : level 11
07/17 21:04:04 Temperature = 78 : hu P0 (office_temp)
07/17 21:19:01 sndc addr unit 1 : hu P1 (TempLinc)
07/17 21:19:02 sndc func StatusReq : hc P
07/17 21:19:05 rcvi addr unit 15 : hu P15 (TempAck_5)
07/17 21:19:05 rcvi func Preset : level 11
07/17 21:19:05 Temperature = 78 : hu P0 (office_temp)

#!/bin/bash

fil=/home/test.log

# Test for existence of the test file
if [ -f $fil ]
then

  # Read through the file looking for the word Temperature =

  while read line
  do
    echo $line | grep -q Temperature
    if [ $? == 0 ]; then
    mytemp=`echo $line  | cut -d = -f2 | cut -d : -f1`
    echo "Current temperature is: $mytemp"
    fi
  done < $fil

fi

Returns:

Current temperature is:  78 
Current temperature is:  78

#!/bin/bash
fil=/home/heyu.log.ttyS0

if [ -f $fil ]
then

mytemp=`grep Temperature $fil | tail -1 | cut -d = -f2 | cut -d : -f1`
echo "Current temperature is: $mytemp"

fi