automated, shell

Quiet grep

Usually during an cron job, one needs to check for an expression in a log file, or in the output of a command, without generating any cumbersone new output (such as a standard grep would do): this is exactly the use of the -q option of grep. For example:

for i in `grep -v '^#' /etc/passwd | cut -f1 -d:`
    grep $i /var/log/samba/log.smbd | grep -q 'failed to authenticate'
    if [ $? == 0 ] ; then
        mail -s 'Error in samba' $i <<EOM
        You failed to authenticate at least once. Please be more careful...
EOM
    fi
done

The above script will send a short notice to any user whose samba session produced a “failed to authenticate” message. Obviously, the log.smbd file should be rotated frequently (at least as much as the script’s frequence).

Strictly speaking, man grep specifies that neither the -q nor the -s options should be used if compatibility with old versions of grep is desired… One should redirect all output to /dev/null, but we have already spoken about this, and it is likely that you do not need that compatibility in your new home-made scripts.

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