How is the name of a variable expanded in Bash?

How is the name of a variable expanded in Bash?

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.

Is the exclamation point subject to expansion in Bash?

The exceptions to this are the expansions of $ {! prefix *} and $ {! name [@]} described below. The exclamation point must immediately follow the left brace in order to introduce indirection. In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.

What happens if the expansion of a parameter is unset?

Otherwise, the value of parameter is substituted. $ {parameter:=word}. If parameter is unset or null, the expansion of word is assigned to parameter . The value of parameter is then substituted. Positional parameters and special parameters may not be assigned to in this way.

Which is an example of substring expansion in Bash?

Here are some examples illustrating substring expansion on parameters and subscripted arrays: If parameter is ‘ @ ’, the result is length positional parameters beginning at offset . A negative offset is taken relative to one greater than the greatest positional parameter, so an offset of -1 evaluates to the last positional parameter.

Is the assignment to script exploitable in shell?

The following invocation is better — it’s not exploitable: Note the use of single ticks in the assignment to script, which means that it’s taken literally, without variable expansion or any other form of interpretation. The repo command can’t care what kind of quotes it gets.

When do you use indirect expansion in Bash?

If the first character of “PARAMETER” is an exclamation point, 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.

What are the Order of the expansions in Bash?

The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion); word splitting; and filename expansion. On systems that can support it, there is an additional expansion available: process substitution .

When to use parameter expansion in a shell?

Shell variables are allowed as operands; parameter expansion is performed before the expression is evaluated. Within an expression, shell variables may also be referenced by name without using the parameter expansion syntax. The value of a variable is evaluated as an arithmetic expression when it is referenced.