Hack 8 Create a Trash Directory


figs/moderate.gif figs/hack8.gif

Save "deleted" files until you're really ready to send them to the bit bucket.

One of the first things Unix users learn is that deleted files are really, really gone. This is especially true at the command line where there isn't any Windows-style recycling bin to rummage through should you have a change of heart regarding the fate of a removed file. It's off to the backups! (You do have backups, don't you?)

Fortunately, it is very simple to hack a small script that will send removed files to a custom trash directory. If you've never written a script before, this is an excellent exercise in how easy and useful scripting can be.

1.9.1 Shell Scripting for the Impatient

Since a script is an executable file, you should place your scripts in a directory that is in your path. Remember, your path is just a list of directories where the shell will look for commands if you don't give them full pathnames. To see your path:

% echo $PATH PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/games:/usr/local/sbin:/usr/ local/bin:/usr/X11R6/bin:/home/dru/bin

In this output, the shell will look for executables in the bin subdirectory of dru's home directory. However, it won't look for executables placed directly in my home directory, or /home/dru. Since bin isn't created by default, I should do that first:

% cd % mkdir bin

As I create scripts, I'll store them in /home/dru/bin, since I don't have permission to store them anywhere else. Fortunately, no one else has permission to store them in my bin directory, so it's a good match.

The scripts themselves contain at least three lines:

#!/bin/sh # a comment explaining what the script does the command to be executed

The first line indicates the type of script by specifying the program to use to execute the script. I've chosen to use a Bourne script because that shell is available on all Unix systems.

Your script should also have comments, which start with the # character. It's surprising how forgetful you can be six months down the road, especially if you create a lot of scripts. For this reason, you should also give the script a name that reminds you of what it does.

The third and subsequent lines contain the meat of the script: the actual command(s) to execute. This can range from a simple one-liner to a more complex set of commands, variables, and conditions. Fortunately, we can make a trash script in a simple one-liner.

1.9.2 The Code

Let's start with this variant, which I found as the result of a Google search:

% more ~/bin/trash #!/bin/sh # script to send removed files to trash directory mv $1 ~/.trash/

You should recognize the path to the Bourne shell, the comment, and the mv command. Let's take a look at that $1. This is known as a positional parameter and specifically refers to the first parameter of the trash command. Since the mv commands takes filenames as parameters, the command:

mv $1 ~/.trash/

is really saying, mv the first filename, whatever it happens to be, to a directory called .trash in the user's home directory (represented by the shell shortcut of ~). This move operation is our custom "recycle."

Before this script can do anything, it must be set as executable:

% chmod +x ~/bin/trash

And I must create that trash directory for it to use:

% mkdir ~/.trash

Note that I've chosen to create a hidden trash directory; any file or directory that begins with the . character is hidden from normal listings. This really only reduces clutter, though, as you can see these files by passing the -a switch to ls. If you also include the F switch, directory names will end with a /:

% ls -aF ~ .cshrc    .history    .trash/ bin/      images/     myfile

1.9.3 Replacing rm with ~/bin/trash

Now comes the neat part of the hack. I want this script to kick in every time I use rm. Since it is the shell that executes commands, I simply need to make my shell use the trash command instead. I do that by adding this line to ~/.cshrc:

alias rm        trash

That line basically says: when I type rm, execute trash instead. It doesn't matter which directory I am in. As long as I stay in my shell, it will mv any files I try to rm to my hidden trash directory.

1.9.4 Running the Code Safely

Whenever you create a script, always test it first. I'll start by telling my shell to reread its configuration file:

% source ~/.cshrc

Then, I'll make some test files to remove:

% cd % mkdir test % cd test % touch test1 % rm test1 % ls ~/.trash test1

Looks like the script is working. However, it has a flaw. Have you spotted it yet? If not, try this:

% touch a aa aaa aaaa % rm a* % ls ~/.trash test1         a % ls test aa         aaa         aaaa

What happened here? I passed the shell more than one parameter. The a* was expanded to a, aa, aaa, and aaaa before trash could execute. Those four parameters were then passed on to the mv command in my script. However, trash passes only the first parameter to the mv command, ignoring the remaining parameters. Fortunately, they weren't removed, but the script still didn't achieve what I wanted.

You can actually have up to nine parameters, named $1 to $9. However, our goal is to catch all parameters, regardless of the amount. To do that, we use $@:

mv $@ ~/.trash/

Make that change to your script, then test it by removing multiple files. You should now have a script that works every time.

1.9.5 Taking Out the Trash

You should occasionally go through your trash directory and really remove the files you no longer want. If you're really on your toes you may be thinking, "But how do I empty the trash directory?" If you do this:

% rm ~/.trash/*

your trash directory won't lose any files! This time you really do want to use rm, not trash. To tell your shell to use the real rm command, simply put a \ in front of it like so:

% \rm /trash/*

Voila, empty recycling bin.

1.9.6 Hacking the Hack

One obvious extension is to keep versioned backups. Use the date command to find the time of deletion and append that to the name of the file in the trash command. You could get infinitely more complicated by storing a limited number of versions or deleting all versions older than a week or a month. Of course, you could also keep your important files under version control and leave the complexity to someone else!



BSD Hacks
BSD Hacks
ISBN: 0596006799
EAN: 2147483647
Year: 2006
Pages: 160
Authors: Lavigne

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