A chronological documentation test project, nothing serious, really!

15 Aug 2007 Verifying DNS mappings

An improperly configured DNS setup can be really annoying. You want to make sure that your mappings work both ways:

  1. Each hostname should resolve to an address, and that address ought to resolve back to the proper hostname.
  2. If an address on your subnet(s) has been assigned a reverse pointer to a hostname, that hostname ought to point back to the original address.

There are exceptions to those two rules, of course. A CNAME will resolve to another hostname first, and only then to an address. Sometimes multiple hostnames will point to the same address, but that address will have only one reverse pointer.

Still, it’s good to know that your basic mappings work as expected.

You can script such a test if you build a file containing your known hostnames. The example script below is pretty simple; it will break if fed a CNAME, and it’ll report a failure somewhere if multiple hostnames point to the same address. Let’s assume the file containing your hostnames is named named-hosts.

# test DNS forward- and reverse-mapping

# edit this variable to reflect local class C subnet(s)
NETS="192.168.1 192.168.2"

# Test name to address to name validity
echo -e "tname -> address -> name"
echo '----------------------------------'
while read H; do
  ADDR=$(dig $H +short)
  if test -n "$ADDR"; then
    HOST=$(dig -x $ADDR +short)
    if test "$H" = "$HOST"; then
      echo -e "okt$H -> $ADDR -> $HOST"
    elif test -n "$HOST"; then
      echo -e "failt$H -> $ADDR -> $HOST"
      echo -e "failt$H -> $ADDR -> [unassigned]"
    echo -e "failt$H -> [unassigned]"
done < named-hosts

# Test address to name to address validity
echo -e "taddress -> name -> address"
echo '-------------------------------------'
for NET in $NETS; do
  for n in $(seq 1 254); do
    HOST=$(dig -x $A +short)
    if test -n "$HOST"; then
      ADDR=$(dig $HOST +short)
      if test "$A" = "$ADDR"; then
        echo -e "okt$A -> $HOST -> $ADDR"
      elif test -n "$ADDR"; then
        echo -e "failt$A -> $HOST -> $ADDR"
        echo -e "failt$A -> $HOST -> [unassigned]"


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15 Aug 2007 Current directory name

pwd | sed s///n/g | tail -n 1 | sed s/\ /_/g


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14 Aug 2007 Watermark pictures using exiv2

Quick and dirty batch script to watermark and add IPTC data to a set of files in the supplied dir to -Copyright.

# Copyright Andy Wright - 2006.
# You have permission to do what you want with it as long as you don't
# blame me if it insults your mother etc. :).
# Std disclaimers apply.
# Quick and dirty batch script to watermark and add IPTC data to a set of
# files in the supplied dir to -Copyright.

# Needs:
# ImageMagick -
# Exiv2 -

CopyrightStr="Copyright String"
CreditStr="Credit String"


[ -z "$inDir" ] && cat < < EOF && exit 1
Add watermark and IPTC data to all the files in  -> -Copyright
Warning probably only works on relative directories.
Usage: $0 

test -d $outDir || mkdir $outDir

renice 15 -p $$

for file in $(ls -1 $inDir/*.{jpg,JPG} | sed -e "s/^$inDir\///")
    echo "Adding IPTC data to $file"
    $Exiv2 -M"set Iptc.Application2.Copyright String $CopyrightStr" $infile
    $Exiv2 -M"set Iptc.Application2.Credit String $CreditStr" $infile
    $Exiv2 -M"set Iptc.Application2.TransmissionReference String $file" $infile

    echo "Copyrighting $file"
    $Composite -quality $Quality -gravity southeast $CopyrightImg $infile $outfile


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13 Aug 2007 Crontab explained

The time and date fields are:

field          allowed values
-----          --------------
minute         0-59
hour           0-23
day of month   1-31
month          0-12 (or names, see below)
day of week    0-7 (0 or 7 is Sun, or use names)

A field may be an asterisk (*), which always stands for “first-last”.

Ranges of numbers are allowed. Ranges are two numbers separated with a hyphen. The specified range is inclusive. For example, 8-11 for an “hours” entry specifies execution at hours 8, 9, 10 and 11.

Lists are allowed. A list is a set of numbers (or ranges) separated by commas. Examples: “1,2,5,9′‘, “0-4,8-12”.

Step values can be used in conjunction with ranges. Following a range with “/” specifies skips of the number’s value through the range. For example, “0-23/2” can be used in the hours field to specify command execution every other hour (the alternative in the V7 standard is “0,2,4,6,8,10,12,14,16,18,20,22”). Steps are also permitted after an asterisk, so if you want to say “every two hours”, just use “*/2”.

Names can also be used for the “month” and “day of week” fields. Use the first three letters of the particular day or month (case doesn’t matter). Ranges or lists of names are not allowed.

The “sixth” field (the rest of the line) specifies the command to be run. The entire command portion of the line, up to a newline or % character, will be executed by /bin/sh or by the shell specified in the SHELL variable of the cronfile. Percent-signs (%) in the command, unless escaped with backslash (\), will be changed into newline characters, and all data after the first % will be sent to the command as standard input.

Note: The day of a command’s execution can be specified by two fields — day of month, and day of week. If both fields are restricted (ie, aren’t *), the command will be run when either field matches the current time. For example, “30 4 1,15 * 5” would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus every Friday.

Instead of the first five fields, one of eight special strings may appear:

string            meaning
------            -------
@reboot           Run once, at startup.
@yearly           Run once a year, "0 0 1 1 *".
@annually         (same as @yearly)
@monthly          Run once a month, "0 0 1 * *".
@weekly           Run once a week, "0 0 * * 0".
@daily            Run once a day, "0 0 * * *".
@midnight         (same as @daily)
@hourly           Run once an hour, "0 * * * *".


# use /bin/sh to run commands, no matter what /etc/passwd says
# mail any output to `paul', no matter whose crontab this is
# run five minutes after midnight, every day
5 0 * * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
# run at 2:15pm on the first of every month -- output mailed to paul
15 14 1 * *     $HOME/bin/monthly
# run at 10 pm on weekdays, annoy Joe
0 22 * * 1-5   mail -s "It's 10pm" joe%Joe,%%Where are your kids?%
23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday"
5 4 * * sun     echo "run at 5 after 4 every sunday"


/etc/crontab   System crontab file

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10 Aug 2007 CSS Shorthand Guide

Ok. Let’s set the record straight. There is no official guide for each and every CSS shorthand property value. So let’s work together and put one together shall we? Ok. Straight to the business. Anytime I’ve ran into a specification (besides the confusing mess at the W3C), it turns into showing off a couple of examples and you’re supposed to be set on your way. Well well. Over the years, I’ve found quite some interesting unknown quirky facts about these shorthands… hence this Guide was born.


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