UNIX for Dummies Questions & Answers

Hi,
I need to truncate a file based on date.Suppose i have a log file which is getting updated every date,i need to keep 7 days worth of data(like sysdate-7) and rest i want to truncate it.Can some help me?

We need to see examples of the records in the logfile. If the record formats vary record by record, this could be reallly hard.

Thanks for the reply.Now we have agreed on the file size to be maintain,that i can do by the following

mv file file.tmp && tail -n 500 file.tmp > file && rm file.tmp

If whatever process that writes to the original log file keeps a file descriptor open on the original file, all that will do is cause all log entries to keep going to the original but-now-deleted-from-the-directory file.

Create a cron which runs daily at midnight and rotates the logs.
Note to "Parm0073" (as explained by "achenle") we never issue "mv logfile" (or "rm logfile") because this will create ghost files if the log is open by a process.

This is the longhand method. It should work whether or not the logfile exists each day and will correctly deal with a logfile which is open by a process.
If you currently have a large logfile, the issue will take 8 days to roll off that logfile. Be patient.

Code:

if [ -f logfile-7 ]
then
    rm logfile-7
fi
if [ -f logfile-6 ]
then
    mv logfile-6 logfile-7
fi
if [ -f logfile-5 ]
then
    mv logfile-5 logfile-6
fi
if [ -f logfile-4 ]
then
    mv logfile-4 logfile-5
fi
if [ -f logfile-3 ]
then
    mv logfile-3 logfile-4
fi
if [ -f logfile-2 ]
then
    mv logfile-2 logfile-3
fi
if [ -f logfile-1 ]
then
    mv logfile-1 logfile-2
fi
cp -p logfile logfile-1
>logfile

Just for interest. Personally I rotate system logs at midday. Though you get two half-days in each log it gives you chance to read what happened after you went home at some time before lunch the next day.

@thegeek
I've only seen "logrotate" in Linux not Unix (contrary to what the website says).