Bash expand variable in a variable

Question

I'm trying to set up my PS1 prompt variable to dynamically choose a color. To do this, I've defined a bunch of local variables with color names:

$ echo $Green \033[0;32m 

but I was hoping to use those in dynamically assigning variables, but I can't figure out how to expand them properly:

> colorstr="\${$color}" > echo $colorstr ${Green} 

I've tried a dozen combinations of eval, echo, and double-quotes, and none seem to work. The logical way (I thought) to expand the variable results in an error:

> colorstr="${$color}" -bash: ${$color}: bad substitution 

(for clarity I've used > instead of $ for the prompt character, but I am using bash)

How can I expand that variable? i.e., somehow get the word "Green" to the value \033[0;32m? And prefereably, have bash or the terminal parse that \033[0;32m as the color green too.

EDIT: I was mis-using ${!x} and eval echo $x previously, so I've accepted those as solutions. For the (perhaps morbidly) curious, the functions and PS1 variable are on this gist: https://gist.github.com/4383597

Answer 1

Using eval is the classic solution, but bash has a better (more easily controlled, less blunderbuss-like) solution:

• ${!colour}

The Bash (4.1) reference manual says:

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.

For example:

$ Green=$'\033[32;m' $ echo "$Green" | odx 0x0000: 1B 5B 33 32 3B 6D 0A                              .[32;m. 0x0007: $ colour=Green $ echo $colour Green $ echo ${!colour} | odx 0x0000: 1B 5B 33 32 3B 6D 0A                              .[32;m. 0x0007: $ 

(The odx command is very non-standard but simply dumps its data in a hex format with printable characters shown on the right. Since the plain echo didn't show anything and I needed to see what was being echoed, I used an old friend I wrote about 24 years ago.)

Answer 2

Using eval should do it:

green="\033[0;32m" colorstr="green" eval echo -e "\$$colorstr" test           # -e = enable backslash escapes test 

The last test is in color green.