Bash script — determine if file modified?

Question

I have a Bash script that repeatedly copies files every 5 seconds. But this is a touch overkill as usually there is no change.

I know about the Linux command watch but as this script will be used on OS X computers (which don’t have watch, and I don’t want to make everyone install macports) I need to be able to check if a file is modified or not with straight Bash code.

Should I be checking the file modified time? How can I do that?

Edit: I was hoping to expand my script to do more than just copy the file, if it detected a change. So is there a pure-bash way to do this?

Answer 1

I tend to agree with the rsync answer if you have big trees of files to manage, but you can use the -u (--update) flag to cp to copy the file(s) over only if the source is newer than the destination.

cp -u

Edit

Since you've updated the question to indicate that you'd like to take some additional actions, you'll want to use the -nt check in the [ (test) builtin command:

#!/bin/bash

if [ $1 -nt $2 ]; then
  echo "File 1 is newer than file 2"
else
  echo "File 1 is older than file 2"
fi

From the man page:

file1 -nt file2
         True if file1 is newer (according  to  modification  date)  than
         file2, or if file1 exists and file2 does not.

Hope that helps.

Answer 2

OS X has the stat command. Something like this should give you the modification time of a file:

stat -f '%m' filename

The GNU equivalent would be:

stat --printf '%Y\n' filename