Real-time scenarios where VARIABLE SUBSTITUTION/EXPANSION is useful Post: 302946288

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Top Forums Shell Programming and Scripting Real-time scenarios where VARIABLE SUBSTITUTION/EXPANSION is useful Post 302946288 by Scrutinizer on Sunday 7th of June 2015 08:04:33 AM

06-07-2015

Moderator

One practical application for ${parameter:=word} I can think of is is with the use of $PWD which is not always defined in every shell, whereas `pwd` is alway available (as long as the search path is set correctly in the script).

However, the latter is expensive since it involves a subshell and an external utility.
By using this construct we can use it multiple times (e.g. in a loop):

Code:

printf "%s\n" "${PWD:=`pwd`}"

And still be sure that if PWD is not set, `pwd` will only be called once in the script.

Likewise for the COLUMNS variable, etcetera..

---
Another possible application is when using numerical comparisons to make sure that a variable contains a numerical value.

Code:

if [ ${var:=0} -ge 1 ]; then

Last edited by Scrutinizer; 06-07-2015 at 09:12 AM..

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LEARN ABOUT CENTOS

pwd

PWD(1)								   User Commands							    PWD(1)

NAME

       pwd - print name of current/working directory

SYNOPSIS

       pwd [OPTION]...

DESCRIPTION

       Print the full filename of the current working directory.

       -L, --logical
	      use PWD from environment, even if it contains symlinks

       -P, --physical
	      avoid all symlinks

       --help display this help and exit

       --version
	      output version information and exit

       NOTE:  your shell may have its own version of pwd, which usually supersedes the version described here.	Please refer to your shell's docu-
       mentation for details about the options it supports.

       GNU coreutils online help: <http://www.gnu.org/software/coreutils/> Report pwd translation bugs to <http://translationproject.org/team/>

AUTHOR

       Written by Jim Meyering.

COPYRIGHT

       Copyright (C) 2013 Free Software Foundation, Inc.  License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
       This is free software: you are free to change and redistribute it.  There is NO WARRANTY, to the extent permitted by law.

SEE ALSO

       getcwd(3)

       The full documentation for pwd is maintained as a Texinfo manual.  If the info and pwd programs are properly installed at  your	site,  the
       command

	      info coreutils 'pwd invocation'

       should give you access to the complete manual.

GNU coreutils 8.22						     June 2014								    PWD(1)