[SGVLUG] shell script & nawk exposure

[Don Gibbs]

This is a marginally involved note, so skip it if you like.



Although a committed Perl hacker, I wanted to strengthen my skill set 
in Unix shell scripting, so a while ago I started tunneling through a 
couple of texts on the unix kernel and various shells, a few pages 
each day, focusing the Bourne shell.  A couple of days ago, I decided 
to commit some repetitive commands to a shell script (_not_ Perl). 
The rest is a summary of one part of that effort.

The first step of the repetitious process was to identify the latest 
*_data  file in a particular directory and place it in a shell 
variable -- a pretty typical thing to do.  This seemingly simple step 
was a bit harder than I'd have thought.  The following is actually 
the contents of a file I used with some trailing commentary.  After 
each success, and a few surprising dead-ends, I'd comment out the 
line and move on until I got to the last two and success!

..................................................
#!/usr/bin/sh

# dlf - display last (chronological) *_data file

#ls -lrt *_data
#echo ls -lrt *_data
#echo "ls -lrt *_data"
#echo `ls -lrt *_data`
#echo "`ls -lrt *_data`"
#echo "`ls -lrt *_data`" | nawk '{print}'

#echo `ls -lrt *_data  | nawk '{print $NF}'`

#echo ls -lrt *_data | nawk '{print $NF}'
#NAME=ls -lrt *_data | nawk '{print $NF}'
#NAME=`ls -lrt *_data  | nawk '{print $NF}'`

  NAME=`echo ls -lrt *_data | nawk '{print $NF}'`
  echo $NAME
.................................................


Commentary:

This ran on a solaris box but the results are probably the same for 
bash on linux.

I was a bit surprised that echo'ing "ls -lrt" caused the new-lines to 
be replaced with spaces.  Turned out to be a help.  Curious 
nonetheless.  (Back quotes inside double quotes reversed that effect.)

I went down a couple of other interesting dead ends, but I deleted them, alas.

I had to keep reminding myself that I had intentionally stepped out 
of a comfortable and productive environment (Perl) into one that kept 
feeding me frustration and humility.

I recall David Lawyer's comment regarding awk a while back that it 
was not worth the effort to learn.  In my case, I occasionally do 
have to understand shell scripts someone else wrote and I figure 
it'll pay off for me in an engineering office situation, but if I was 
just hacking for myself, David has point.

BTW, as I read through Bruce Blinn's _Portable Shell Programming_, I 
was struck by how much Larry Wall borrowed from 
sh/awk/sed/cut/grep/andonandonandon.  The slow read of how the shell 
works made Perl clearer, too.

I wonder how much other scripting languages are a derivation of shell 
scripting.

   --Don
-- 
========================================================================
Don Gibbs		| "Somehow it seems to fill my head with ideas
dgibbs at jpl.nasa.gov	| -- only I don't know exactly what they are!"
818 354-2990 - office	|
818 653-9531 - cell	|       Alice, after reading JABBERWOCKY

Sec 316, Flight S/W & Data Systems - Group B, GN&C and FSW Testing
Opinions expressed are not necessarily those of JPL/Caltech or NASA
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.sgvlug.net/pipermail/sgvlug/attachments/20060220/af2d283c/attachment.html