How can I tell if a file is older than 30 minutes from /bin/sh?

Here's one way using find.

if test "`find file -mmin +30`"

The find command must be quoted in case the file in question contains spaces or special characters.

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 $file
fi

Note 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).

Ok, no stat and a crippled find. Here's your alternatives:

Compile the GNU coreutils to get a decent find (and a lot of other handy commands). You might already have it as gfind.

Maybe you can use date to get the file modification time if -r works?

(`date +%s` - `date -r $file +%s`) > (30*60)

Alternatively, use the -nt comparision to choose which file is newer, trouble is making a file with a mod time 30 minutes in the past. touch can usually do that, but all bets are off as to what's available.

touch -d '30 minutes ago' 30_minutes_ago
if [ your_file -ot 30_minutes_ago ]; then
    ...do stuff...
fi

And finally, see if Perl is available rather than struggling with who knows what versions of shell utilities.

use File::stat;
print "Yes" if (time - stat("yourfile")->mtime) > 60*30;

You can do this by comparing to a reference file that you've created with a timestamp of thirty minutes ago.

First create your comparison file by entering

touch -t YYYYMMDDhhmm.ss /tmp/thirty_minutes_ago

replacing the timestamp with the value thirty minutes ago. You could automate this step with a trivial one liner in Perl.

Then use find's newer operator to match files that are older by negating the search operator

find . \! -newer /tmp/thirty_minutes_ago -print

Difference in seconds between current time and last modification time of myfile.txt:

echo $(($(date +%s)-$(stat -c "%Y" myfile.txt)))

you can also use %X or %Z with the command stat -c to get the difference between last access or last status change, check for 0 return!

%X time of last access, seconds since Epoch
%Y time of last data modification, seconds since Epoch
%Z time of last status change, seconds since Epoch

The test:

if [ $(($(date +%s)-$(stat -c "%Y" myfile.txt))) -lt 600 ] ; then echo younger than 600 sec ; else echo older than 600 sec ; fi

What do you mean by older than 30 minutes: modified more than 30 minutes ago, or created more than 30 minutes ago? Hopefully it's the former, as the answers so far are correct for that interpretation. In the latter case, you have problems since unix file systems do not track the creation time of a file. (The ctime file attribute records when the inode contents last changed, ie, something like chmod or chown happened).

If you really need to know if file was created more than 30 minutes ago, you'll either have to scan the relevant part of the file system repeatedly with something like find or use something platform-dependent like linux's inotify.

You can use the find command.
For example, to search for files in current dir that are older than 30 min:

find . -type f -mmin +30 

You can read up about the find command HERE