Thursday, August 23, 2012

Find files modified on certain date or in a date range

When you want to find files matching certain conditions, this is most probably job for the find command. The problem, though, is that you can't specify a certain date.

Options like -atime, -ctime or -mtime receive an argument that specifies the number of 24-hour periods. This means that when you run the following command:

find /usr/log/mysql -type f -mtime -1 -exec ls -l {} \;

you will get not only the files modified today, but those modified within the last 24 hours. You can change this behaviour by adding the option -daystart, which means that the time periods are calculated from the beginning of the current day:

find /var/log/mysql -type f -daystart -mtime -1 -exec ls -l {} \;

This command will produce the list of files modified today. IMPORTANT! Note that the -daystart option must precede all date-comparing options to have an effect.

To find files modified between two dates you can join two conditions using -a option:

find /var/log/mysql -type f -daystart -mtime -3 -a -mtime +0 -exec ls -l {} \;

The result will include the files modified yesterday or the day before yesterday.

Sometimes, though, you may want to specify the dates as they are, not as relative number of days from today. Traditionally, it was done using an awkward technique that involved creating two empty files with modification dates corresponding to the lower and upper borders of the range (using touch -t filename) and then using these files in options -newer and -older:

touch temp -t 200604141130
touch ntemp -t 200604261630
find /data/ -cnewer temp -and ! -cnewer ntemp

(example taken from here)

New versions of find allow you to do just that using -newerXY. Letters X and Y here stand for some one-letter codes corresponding to various comparison types. The combinations are pretty much incomprehensible, but what we need is -newermt. With this option, life gets simple and sunny!

find /var/log/mysql -type f -newermt 2012-08-21 ! -newermt 2012-08-23 -exec ls -l {} \;

This command produces the list of files modified exactly between the beginning of August 21 and the beginning of August 23.

And a little bonus for those who made it to the end! To sum the sizes of the found files (so you can find out, for example, how many gigabytes were written to binlogs in the last two days) use du -c:

du -c `find /var/log/mysql -type f -newermt 2012-08-21 ! -newermt 2012-08-23`|tail -n1

Thursday, July 26, 2012

Chroot, but don't chroot

FTP, SFTP and SCP accounts are often restricted to their home directory, so the users don't mess around with the system. This is done using 'chroot'. You can set up SSH demon or FTP server so the user cannot leave his home directory. But what if you want to give him access to some other directory outside his home directory?

The first thing that comes to mind is links. Soft links don't work, though, because the user cannot see anything outside his directory. Hard links won't work, either, because you usually cannot hardlink directories. They'll do if all you want is access to a single file, though. Also, you can change the user's home directory to the directory you want him to modify. But you may want to grant access to more than one directory. Besides, the implementation of chroot requires that if the user is chrooted to /var/data/lib/img, all directories up the tree (i.e., /var, /var/data and /var/data/lib) must belong to root and nobody else should have write permissions there. This is not always possible.

But the right solution would be to mount a directory to your home directory using bind option. Create the mountpoint inside the home directory and mount:

mkdir /home/remote/img
mount --bind /var/data/lib/img /home/remote/img

Now, the user will be able to work with /var/data/lib/img, but not with any other data on the server.

To make the mount persistent across reboots, add the corresponding entry to /etc/fstab:

/var/data/lib/img /home/remote/img none bind 0 0

Friday, February 17, 2012

More than one 'exec' action in 'find' command

When you run find command, you can pass the names of the found files to an arbitrary command using -exec option:

find /tmp -mtime +3 -exec rm {} \;

The curly braces get replaced by the name of found files and the command is executed for every file. However, if you want to run more than one command on the file or use the filename more than once in one command, you cannot do that:

Only one instance of `{}' is allowed within the command.

To bypass the limitation, you can execute a shell, passing the filename as an argument. In the commands executed by the shell, the argument will be available as $0:

find /tmp -mtime +3 -exec sh -c 'ls -ld "$0"; rm "$0"' {} \;

Tuesday, January 3, 2012

Getting rid of GUI password prompt in EasyPG

EasyPG is a package used by GNU Emacs to simplify work with files encrypted by GnuPG (GPG). Extremely useful package, it has a bad habit of relying on external tools to prompt for the password. One of such tools is 'gpg-agent'. In Debian/Ubuntu this agent usually replaces 'ssh-agent'. It's a good tool, but it uses a GUI dialog to prompt for the password. This makes your work with EasyPG and Emacs rather uncomfortable.

There's a number of advices on how you can suppress that GUI dialog. I prefer another one, I simply removed 'gpg-agent'. It did not affect the functionality, since it was replaced by 'ssh-agent'. Now, EasyPG prompts for the password in Emacs mini-buffer