Many modern systems provide a way to watch a directory for events (new files, reading the directory, modification of a file in the directory, etc.). This facility can be done in various ways, from providing hooks in the filesystem code itself to something that watches for inode changes. Linux and BSD have several possibilities in that regard, but Mac doesn't give us much at the shell level.
There is FileMonitor (shareware), and it's certainly possible that some of the BSD tools might compile on OS X. But what do you do when there is no support in the OS?
Typically you are interested in new files in a particular directory. You can do something like this:
touch /tmp/testdirb.$$ while true do ls /testdir > /tmp/testdira.$$ diff /tmp/testdira.$$ /tmp/testdirb.$$ || echo "changed" cp /tmp/testdira.$$ /tmp/testdirb.$$ sleep 300 done
If you were looking for a particular file to change, you'd use "ls -l", and if your interest was in if the file was being used or executed, "ls -lut" would give you that. This example just echoes when something changes, but you would more likely call some other script that did more testing. One obvious issue that comes up if a file has been created is waiting for the creating program to have finished up: lsof or fuser can help you with that.
But this is all pretty crude. Sometimes crude is fine, but if you need to know more often, there's a fair amount of overhead in this that you really don't need.
If it is just one file, and your interest is additions to it, the mailchecking utility in your shell can give you alerts. For example, in bash:
MAIL=/tmp/watchthis MAILCHECK=10
You can watch multiple files, each with its own message, by using MAILPATH instead of MAIL. You need to "unset MAIL", and set and export MAILPATH:
unset MAIL MAILPATH='/tmp/foo/h?"H is changed":/tmp/foo/a?"A has changed"' export MAILPATH
That's a little clumsy and interfers with real mail if you use it.
Mac OS X does have a command line "stat" that can make checking changes a little less intensive. "stat testdir" looks something like this:
234881029 7966131 drwxr-xr-x 5 apl staff 0 170 "Sep 4 15:40:50 2005" "Sep 4 15:47:31 2005" "Sep 4 15:47:31 2005" 4096 0 0 testdir
which is everything you want to know about the file or directory for this purpose. Read the man page for details; there are quite a few options and control of output format. You'd need to build a checking loop something like this:
OLD=`stat testdir` while true do NEW=`stat testdir` [ "$NEW" == "$OLD" ] || echo "changed!" sleep 3 OLD=$NEW done
From Perl or C (and of course many other languages), you can get access to the stat information a bit more easily. Here's a simple Perl example:
#!/usr/bin/perl @info=stat("testdir") or die "Can't stat testdir $!"; while (1) { @newinfo=stat("testdir") or die "Can't stat testdir $!"; @what=qw(Device Inum Mode Links Owner Group Rdev Size Atime Mtime Ctime PBlock Blocks); $x=0; foreach(@info) { print "Change $what[$x] $info[$x] $newinfo[$x]\n" if ($info[$x] ne $newinfo[$x]); $x++; } @info=@newinfo; sleep 1; }
Of course you'd probably do more than just announce the change.
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More Articles by Tony Lawrence © 2009-11-07 Tony Lawrence
Simplicity is prerequisite for reliability. ((Edsger W. Dijkstra)
Sun Dec 30 18:49:51 2007: 3359 TonyLawrence
See "man opensnoop" if you have Leopard. Also see article here : (link)
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Watching File or Directory Changes Copyright © December 2005 Tony Lawrence
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