Your directory might have 50, 100, or more files. Which files haven't been used for a while? You might be able to save space by removing them. You read or edited a file yesterday but you can't remember its name? These commands will help you find it. (If you want a quick review of UNIX file times, see article 16.5.)
In this example, I'll show you mydirectory full of shell scripts and other programs-I want to see which programs I don't use very often. You can use the same technique for directories with text or other files.
The ls command has options to change the way it orders files. By default, ls lists files alphabetically - that probably won't help you find old files, but it's a good place to start this explanation. For finding old files, use the -t option. This sorts files by their modification time, or the last time the file was changed. The newest files are listed first. Here's what happens:
jerry@ora ~/.bin 60 %
ls -tweather unshar scandrafts rn2mh recomp crontab zloop tofrom rmmer mhprofile rhyes showpr incc mhadd append rhno rfl drmm fixsubj README pickthis maillog reheader distprompter rtfm cgrep c-w zrefile xmhprint saveart dirtop cw zscan replf echoerr which cx zfolders fols tcx showmult alifile incs
I just added a shell script named weather yesterday; you can see it as the first file in the first column. I also made a change to my script named crontab last week; it's shown next. The oldest program in here is echoerr; it's listed last. 
 On some systems, ls -t will list the files in one column, with the newest file first. Although that's usually a pain, I actually find that more convenient when I'm interested in the most recent files. If your system does that and you don't like the single-column display, you can use ls -Ct. On other systems, if a single column display would be handy, use ls -1t; the "1" option means "one column." Throughout this article, we'll assume you're using a multi-column display.
ls -t is also great for file time comparisons. [Personally, I find ls -t most useful when I've forgotten whether or not I've edited a file recently. If I've changed a file, it will be at or near the top of the ls -t listing. For example, I might ask, "Have I made the changes to that letter I was going to send?" If I haven't made the changes (but only think I have), my letter will most likely appear somewhere in the middle of the listing. -ML ]
The -u option shows the files' last-access time instead of the last-modification time. The -u option doesn't do anything with plain ls-you have to use it with another option like -t or -l. The next listing shows that I've recently used the rtfm and rmmer files. I haven't read README in a long time, though - oops:
jerry@ora ~/.bin 62 %
ls -turtfm cx drmm saveart fixsubj rmmer c-w zscan scandrafts echoerr rfl cw zrefile rhno dirtop mhprofile distprompter xmhprint rhyes cgrep showmult recomp zloop replf append tcx crontab zfolders reheader alifile tofrom mhadd which incs README rn2mh pickthis unshar maillog weather incc showpr fols
(Some UNIXeswhen you run them. Shell scripts are always read, so their last-access times will always be updated.)
The -c option shows when the file's was last changed. The inode time tells when the file was created, when you used chmod to change the permissions, and so on. That doesn't help you find "stale" files:
jerry@ora ~/.bin 64 %
ls -tcweather maillog reheader recomp incs crontab tcx rn2mh fols cx cgrep zscan tofrom rmmer cw zloop zrefile mhadd fixsubj c-w dirtop rfl drmm mhprofile echoerr pickthis showmult alifile append which rhno rtfm showpr saveart README unshar incc scandrafts distprompter rhyes zfolders xmhprint replf
If you're wondering just how long ago a file was modified (or accessed), add the -l option for a long listing. As before, adding -u shows the last-access time; -c shows inode change time. If I look at the access times of a few specific files, I find that I haven't read README since 1989...
jerry@ora ~/.bin 65 %
ls -ltu README alifile maillog-rwxr-xr-x 1 jerry ora 59 Feb 2 1991 maillog -rwxrwxr-x 1 jerry ora 213 Nov 29 1989 alifile -rw-r--r-- 1 jerry ora 3654 Nov 27 1989 README