Replacing Variables in Bash/shell

Last update on August 13, 2009 10:58 AM by jak58
Published by jak58

Replacing Variables in Bash/shell






Intro:


First of all make sure that you the variable is defined before replacing it, or in other cases determine that it is not empty, and to initialize with a default value if necessary.

Expression:Defininition


$var: replace by the value of the variable $var if defined. Otherwise nothing.


${var}: replace by the value of the variable $var if defined. Otherwise nothing. Better use this, in order to protect from certain inconsistencies when using Strings.


${var:-default}: replace by the value of the variable $ var "if defined. Otherwise the value default.


${var:+default}: Set the variable $var with the value default if defined. Otherwise leave empty.


${var:?"Message"} :replace by the value of the variable $var if defined. Otherwise, the shell displays specific error message instead of "Message".


${var:=default} : replace by the value of the variable $var if defined. Otherwise create and initialize the variable var with the Value default.
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