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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pwd

PWD(1)							    BSD General Commands Manual 						    PWD(1)

NAME

     pwd -- return working directory name

SYNOPSIS

     pwd [-L | -P]

DESCRIPTION

     The pwd utility writes the absolute pathname of the current working directory to the standard output.

     Some shells may provide a builtin pwd command which is similar or identical to this utility.  Consult the builtin(1) manual page.

     The options are as follows:

     -L      Display the logical current working directory.

     -P      Display the physical current working directory (all symbolic links resolved).

     If no options are specified, the -L option is assumed.

ENVIRONMENT

     Environment variables used by pwd:

     PWD  Logical current working directory.

EXIT STATUS

     The pwd utility exits 0 on success, and >0 if an error occurs.

SEE ALSO

     builtin(1), cd(1), csh(1), sh(1), getcwd(3)

STANDARDS

     The pwd utility conforms to IEEE Std 1003.1-2001 (``POSIX.1'').

BUGS

     In csh(1) the command dirs is always faster because it is built into that shell.  However, it can give a different answer in the rare case
     that the current directory or a containing directory was moved after the shell descended into it.

     The -L option does not work unless the PWD environment variable is exported by the shell.

BSD
								  April 12, 2003							       BSD