Using tac on most recent log file out of several log files in a directory

Question

I have several log files in a directory that we’ll call path/to/directory that are in the following format after long listing in Red Hat Enterprise 6:

-rw-r——-. 1 root root 17096 Sep 30 11:00 logfile_YYYYDDMM_HHMMSS.log

There are several of these log files that are generated everyday. I need to automatically tac the most recently-modified file without typing the exact name of the log file. For example, I’d like to do:

tac /path/to/directory/logile*.log | grep -m 1 keyword

And have it automatically tac the most recently modified file and grep the keyword in the reverse direction from the end of the log file so it runs quicker. Is this possible?

The problem I’m running into is that there is always more than one log file in the /path/to/directory and I can’t get Linux to automatically tac the most recently modified file as of yet. Any help would be greatly appreciated.

I’ve tried:

tac /path/to/directory/logfile_$(date +%Y%m%d)*.log 

which will tac a file created on the present date but the part that I’m having trouble with is using tac on the newest file (by YYYYMMDD AND HHMMSS) because multiple files can be generated on the same date but only one of them can be the most current and the most current log file is the only one I care about. I can’t use a symbolic link either.. Limitations, sigh.

Answer 1

The problem you seem to be expressing in your question isn't so much about tac, but rather .. how to select the most recent of a set of predictably named files in a directory.

If your filenames really are in the format logfile_YYYYDDMM_HHMMSS.log, then they will sort lexically without the need for an innate understanding of dates. Thus, if your shell is bash, you might:

shopt -s nullglob
for x in /path/to/logfile_*.log; do
  [[ "$x" > "$file" ]] && file="$x"
done

The nullglob option tells bash to expand a glob matching no files as a null rather than as a literal string. Following the code above, you might want to test for the existence of $hit before feeding it to tac.