bourne shell query

[Jeff Beadles]

fred at maccs.dcss.mcmaster.ca (Fred Whiteside) writes:
:>
:>	i have a simple little shell question.  I am attempting
:>to write a script that will perform various manipulations on news
:>articles.  my problem seems to be with understanding variable
:>substitution in sh.  a minimal example is the enclosed sh script.  what
:>it should do is set the variable files to be the names of those
:>articles in comp.std.c whose name (article number) is greater than or
:>equal to the numeric value of the scripts first argument.  this does
:>not behave as expected.  i have tried quite a unmber of variations on
:>this theme (mostly centering around multiple $'s and interesting
:>quoting of commands, but i tend to get all of the files or none of them
:>depending on (for some reason) the number of $'s preceding the zots
:>variable name (also on  the number in front of the 1, $1 or $$1, etc.
:>but i expected that ...) this is running under sunos4.1 if that is
:>interesting.  i have tried to read the manual, but with little
:>success.  anyone interested in pointing out my clear stupidity
:>publicly?  i *would* appreciate it ....
:>
:>	many thanks ...
:>
:>#!/bin/sh
:>cd /usr/spool/news/comp/std/c
:>zots=$1
:>echo "Should ignore files with name < $zots"
:>files=`ls -rt * |grep -v '.*[a-zA-Z].*' | awk $1 '>=' $zots {print}`
              I see a quoting problem here     ^^^^^^^^^^^^^^^^^^^^^
:>echo $files
:>
:>-- 
:>Fred Whiteside   Chedoke-McMaster Hospitals, Hamilton, Ontario, Canada
:>whitesid@{mcmaster.ca,darwin.cmh.mcmaster.ca,130.113.2.18}
:>fred at maccs.DCSS.McMaster.CA   ...!uunet!utai!utgpu!maccs!fred




>files=`ls -rt * |grep -v '.*[a-zA-Z].*' | awk $1 '>=' $zots {print}`

This line is the problem, replace it with:

files=`ls -rt * |grep -v '.*[a-zA-Z].*' | awk "\$1 >= $zots {print}"`

Or, to make it a little more simpler... (You don't need .*'s in the grep)

files=`ls -rt * |grep -v '[a-zA-Z]' | awk "\$1 >= $zots {print}"`

To explain:

The only real change was to change thw quoting around the awk script.
$1 was getting expanded to the shell's $1, not the 1st awk field.

By using " as a quoting character, the shell is able to expand $zot to whatever
number it represents.  The \$1 is to prevent the shell from trying to expand $1
to the shell's $1, rather than the 1st awk field.

After the shell is finished with variable substitution,

awk "\$1 >= $zots {print}"

is transformed, and awk see's:  (if zot = 10)

awk "$1 >= 10 {print}"


Hope this helps!  A decent book on shell programming and quoting is the Howard
Sam's book on shell programming.

	-Jeff
-- 
Jeff Beadles				jeff at quark.WV.TEK.COM 
Utek Engineering, Tektronix Inc.	+1 503 685 2568
			SPEEA - Just say no.