Real-time scenarios where VARIABLE SUBSTITUTION/EXPANSION is useful Post: 302946075

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Top Forums Shell Programming and Scripting Real-time scenarios where VARIABLE SUBSTITUTION/EXPANSION is useful Post 302946075 by bakunin on Saturday 6th of June 2015 05:28:45 PM

06-06-2015

Registered User

Quote:

Originally Posted by ab_2010

at academic level I am familiar with how variable substitution/expansion feature works. From your live experience, can you please tell what are real-time scenarios where ${variable:=} ${variable%pattern} ${variable:=word} sort of features can be used?

First off, I think it is one of the defining qualities of a software engineer to come up with useful applications of a language feature once he gets to know it. By analogy, Beethoven had the same 12 tones to work with as any other but what made him a composer was that he started having ideas on how to combine them in an original and inventive way.

Here are, off the top of my head, some threads where i mentioned applications for variable expansion:

walking back up a directory path

Storing string space

Not to forget the obvious application:

Code:

var="${othervar:=defaultvalue}"

which assigns a variable a default value in case "$othervar" is not set so far. This way you can avoid lengthy checks for completeness of sets of input parameters.

Finally, you can use variable expansion for everything where you would normally use string functions in other languages or string-manipulating commands (grep, awk, sed, tr, ...) in shell. Variable expansion is very fast and even if the code is ten times as long it will perhaps be executed faster than any external program. I learned this lesson myself right here.

I hope this helps.

bakunin

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dwww

DWWW(1) 							      Debian								   DWWW(1)

NAME

       dwww - access documentation via WWW

SYNOPSIS

       http://localhost/dwww/index.html
       dwww
       dwww program-name | package-name

DESCRIPTION

       A  typical Linux system has documentation in many formats (manual pages, info files, READMEs, and so on).  dwww makes it possible to access
       all of these via the same interface, a WWW browser.  This makes it easier to use the documentation.

       To use dwww, load the URL given in the SYNOPSIS.  If you have a web browser installed, you can also just run the dwww command  which  loads
       the  URL.   If  BROWSER	environment  variable  is  set, dwww uses sensible-browser(1) to load the URL.	Otherwise, dwww first tries to use
       browser specified by DWWW_BROWSER or DWWW_X11_BROWSER configuration variable (cf.  dwww(7)), than will use sensible-browser(1) command.

       If optional argument program-name or package-name is specified, dwww will search all documentation related to given program or package.

ENVIRONMENT

       BROWSER
	      Program used to load the above mentioned URL.

FILES

       /etc/dwww/dwww.conf
	      Configuration file for dwww.

SEE ALSO

       dwww-find(8), dwww(7), sensible-browser(1).

AUTHOR

       Lars Wirzenius <liw@iki.fi>. Modified by Robert Luberda <robert@debian.org>.
       Bugs should be reported via the	Debian Bug Tracking System at <URL:http://bugs.debian.org/>.

       dwww is licensed via the GNU General Public License.  While it has been written for Debian, porting it to other systems is strongly encour-
       aged.

dwww 1.11.1							February 15th, 2009							   DWWW(1)