How can I tell if a file is older than 30 minutes from /bin/sh?
The following gives you the file age in seconds:
echo $(( `date +%s` - `stat -L --format %Y $filename` ))which means this should give a true/false value (1/0) for files older than 30 minutes:
echo $(( (`date +%s` - `stat -L --format %Y $filename`) > (30*60) ))30*60 -- 60 seconds in a minute, don't precalculate, let the CPU do the work for you!
If you're writing a sh script, the most useful way is to use test with the already mentioned stat trick:
if [ `stat --format=%Y $file` -le $(( `date +%s` - 1800 )) ]; then do stuff with your 30-minutes-old $filefiNote that [ is a symbolic link (or otherwise equivalent) to test; see man test, but keep in mind that test and [ are also bash builtins and thus can have slightly different behavior. (Also note the [[ bash compound command).