Extract a single directory from a tar.gz file

tar -zvxf myfile.tar.gz path/in/tarfile/to/extract

Purge mail from exim queue

mailq | awk '{print $3}' | xargs exim  -Mrm

Strip crtl-M characters from files on the command line (recurses)

find . -name '*.html' | xargs perl -pi -e 's/\r//g;'

File images in a horizontal or vertical subdirectory - uses ImageMagick

#!/usr/bin/perl

# check the image size and move to a horizontal
# or vertical subdirectory
use Image::Size;
use File::Copy;
use strict;

my $dir = "/home/rory/Jobs/images/cig_images/";
my $horiz = $dir . "horizontal";
my $vert = $dir . "vertical";

# read the files
opendir(DIR, $dir) || die "can't opendir $dir: $!";
my @images = readdir(DIR);
closedir(DIR);

# loop thru images
foreach my $image (@images) {
    my $fullpath = $dir . $image;
    if (not(-d $fullpath)) {    # exclude directories
       
        # check what dimension of the image is longer
        my ($x, $y) = imgsize("$fullpath");
        if ($x>$y) {
            move("$fullpath", "$horiz/$image") || die "move failed: $!";
        } else {
            move("$fullpath", "$vert/$image") || die "move failed: $!";   
        }
        print "$fullpath $x $y\n";
       
    } # end if for not a directory
   
} # end foreach loop thru images

Using crypt to encrypt/decrypt files

A file can be encrypted in the shell mode using crypt, or in the edit mode using the -x or X option. When you are ready to decrypt the file, you can use the crypt command in the shell mode. The following is the command format to encrypt a file:

crypt < oldfile > newfile

The system prompts you for a password.



Before removing the unencrypted oldfile, make sure the encrypted newfile can be decrypted using the appropriate password.



To decrypt a file, redirect the encrypted file to a new file you can read. The command to decrypt a file is as follows:

crypt < encrypted_file > new_filename 

Use wget to retrieve a directory of mp3s

wget -r -l1 --no-parent -A.mp3 -R.html,.gif http://www.mysite.com/mp3/

Block a particular IP address with iptables

iptables -I INPUT -s 25.25.25.25 -j DROP

In java, take a string and strip alphanumeric characters

Call it like this:
String newString = reformat(categoryName);

// for stripping strings of non alphanumeric stuff and replacing with +
// should really be a class

public String reformat(String categoryName)
{
StringBuffer reformattedString = new StringBuffer();
if (categoryName != null)
{
char[] chars = categoryName.toCharArray();
for (int ii = 0; ii < chars.length; ii++)
{
if (Character.isLetterOrDigit(chars[ii]))
{
reformattedString.append(chars[ii]);
}
else if (Character.isSpaceChar(chars[ii]))
{
if (reformattedString.length() > 0 && reformattedString.charAt(reformattedString.length() - 1) != '_')
{
reformattedString.append('_');
}
}
}
}
return reformattedString.toString();
}

Display last n lines of a file

tail -n /path/to/file

Parse IP from ifconfig with sed (excluding your internal IP)

/sbin/ifconfig | sed -n -e "s/^.*addr://g" -n -e "s/ Bcast.*//p" | grep -v '192.168.0.20'