Evaluating shell variables

The following set of rules govern the evaluation of all shell variables.


Definition            Description

$var signifies the value of var or nothing, if var is undefined. ${var} same as above except the braces enclose the name of the variable to be substituted. ${var-thing} value of var if var is defined; otherwise thing. $var is not set to thing. ${var=thing} value of var if var is defined; otherwise thing. If undefined $var is set to thing. ${var?message} If defined, $var; otherwise print message and exit the shell. If the message is empty, print a standard message. ${var+thing} thing if $var is defined, otherwise nothing.

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