Regular expressions in bash

You can perform regular expression matching on a variable within an extended test command (see the Conditional Constructs part of the bash manual).

e.g.

prompt> name=foobar.blarg;if [[ $name =~ foo ]]; echo yep; else echo nope; fi
yep
prompt> name=foobar.blarg;if [[ $name =~ foo[a-c] ]]; echo yep; else echo nope; fi
yep
prompt> name=foobar.blarg;if [[ $name =~ foo[d-z] ]]; echo yep; else echo nope; fi
nope

Convert tab data into HTML tables with a Perl one-liner

Quick one-liner for generating a HTML table from tab delimited input. Either pipe in your data or include the file as a command line argument.

perl -F'\t' -lane 'BEGIN{print "<table border=1 cellpadding=3 cellspacing=0>"}print "<tr>", (map {"<td>$_</td>"} @F), "</tr>";END{print "</table>}'

The map is in parentheses so that the closing '<tr>' tag is not slurped in as part of it's input array.

Setting AVR Clock Speed

I'm using gcc/WinAVR to program my AVRs and have just been discovering how to program the clock speed. I'm playing with an ATtiny13 and an ATmega8. Both of these ship with their clocks set to 1MHz by default but both can be clocked to a higher speed.

There are a few things to note:

1. F_CPU is used by the compiler for calculating timings (most obvious example is in the delay.h routines). Setting it has no effect on the actual clock speed. This needs to be set to the correct value.

2. Easiest way (for me) to set the clock speed is to program the relevant fuse bits. This was different for the two chips I've been using - ATtiny13 and ATmega8). Note: for fuse bits 1 = unprogrammed and 0 = programmed (this is due to the nature of EEPROM).

This step is made easy when using the Eclipse avr plugin. There's a GUI/wizard for setting them in Project->Properties->AVR->AVRDude->Fuses. Accessible by selecting the "direct hex values" radio option and then clicking the "start editor" button.

3. For some MCUs you can dynamically adjust the clock speed in software (I know this is true for the ATtiny13 at least). However, this has to be done within 4 clock cycles of setting the CLKPCE bit (again, this is for the Atiny13). See this forum post on avrfreaks.net and pg.28 of the datasheet.

Here's a good overview of setting the clock for the ATmega8: Electrons - AVR Fuses HOWTO Guide.

Bash: reading lines from a file

I guess I don't do this enough to remember it:

When reading a file in a for loop in bash, the following idiom will read each work (whitespace delimited):

for word in `cat file.txt`; do echo $word; done

If you want to grab the whole line then you can do this:

while read line; do echo $line; done < file.txt

or

cat file.txt | while read line; do echo $line; done

Essential Eclipse Keyboard Shortcuts (navigation)

There are a few shortcuts I use all the time for navigating in Eclipse and I've just learned a few more useful ones (I was looking for quick ways to jump between editor windows).

Here's the essential list (IMO):

Ctrl+E (go to other open editors - opens selection box)
Ctrl+Q (jump to last edit location)
Crtl+O (jump to any member/method/inner-class in the current editor)
Ctrl+shift+T (open any type)
Ctrl+shift+R (open any file)
Ctrl+L (jump to a particular line number)
Ctrl+T (go to a supertype/subtype - multiple presses toggle between super/sub)
Alt+left/right arrow (jump through visited files)
Ctrl+. Ctrl+, (navigate up and down through error/warning locations)

C rand() and random() functions

There are two primary random functions to be aware of in stdlib.h: random() and rand() the main difference is in the range of values returned by the two functions.

random() returns a pseudo random number in the range 0 -> 0x7FFFFFFF = 0 -> 1,879,098,192 (RANDOM_MAX).
rand() returns values in the range 0 -> 0x7FFF = 0 -> 28,672 (RAND_MAX).

I had mistakenly been using one in place of the other in some micro-controller code and had spent some time wondering why it wasn't behaving as expected...