Re: Request for enhancement: links to 'true' and 'false' (original) (raw)

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From: Bob Proulx
Subject: Re: Request for enhancement: links to 'true' and 'false'
Date: Sat, 20 Apr 2013 11:28:04 -0600
User-agent: Mutt/1.5.21 (2010-09-15)

Jeffrey Streifling wrote:

This additional feature would be used with external scripting interpreters that use 0 and 1 as truth values, such as GNU bc. The useful idiom would be

if bc <<< "$VAL1 > $VAL2""

Of course the use of ... backticks makes even a traditionalist guy like me cringe. I much prefer to see $(...) used over .... (I do use bc all of the time as a quick desk calculator.)

If you are comparing values the bc program isn't the traditional way. It would be more traditional to use 'expr'.

if expr 1 '>' 2; then

A simple way to check and print the return:

$ expr 1 '>' 2 && echo yes || echo no 0 no

$ expr 1 '<' 2 && echo yes || echo no 1 yes

But even expr is now obsoleted by built in shell math. Use ((...)) to perform the math in the shell with no external program calls. (And no need to redirect the stdout output of expr to hide it either.) And best of all shell metacharacters do not need to be quoted since it is a shell builtin.

$ ((1 < 2)) && echo yes || echo no yes

$ ((1 > 2)) && echo yes || echo no no

if ((1 < 2)); then

Your example case would be:

if (($VAL1 > $VAL2)); then

Isn't that nicer than using the original?

if bc <<< "$VAL1 > $VAL2""

Inside ((...)) variables are automatically expanded. I like the consistency of names always having a '$' in front so I personally always include it. It makes searching for uses easier for me. But you can leave out the dollar signs in ((...)) and the variable will be expanded. I find the inconsistency grating though.

if ((VAL1 > VAL2)); then

Bob