Variable substitution with an exclamation mark in bash

Question

I have the following lines in my .cfg bash script file

DDF_SOURCE="siebel_DATA_DATE_FORMAT"
DATA_DATE_FORMAT=${!DDF_SOURCE}

how is ${!DDF_SOURCE} evaluated? It would be !siebel_DATA_DATE_FORMAT, which doesn't make sense to me.

Answer 1

That is an indirect expansion, documented in man bash section EXPANSION, subsection Parameter Expansion:

If the first character of parameter is an exclamation point (!), a level of variable indirection is introduced. Bash uses the value of the variable formed from the rest of parameter as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of parameter itself. This is known as indirect expansion.

bash-4.2$ DDF_SOURCE="siebel_DATA_DATE_FORMAT"

bash-4.2$ siebel_DATA_DATE_FORMAT='Hello Indirect Redirection'

bash-4.2$ DATA_DATE_FORMAT=${!DDF_SOURCE} # siebel_DATA_DATE_FORMAT must get value before this line

bash-4.2$ echo $DATA_DATE_FORMAT
Hello Indirect Redirection