Lazy score in Bash

Question

Is there a more elegant way to lazy evaluate than the following:

 pattern = '$ x and $ y'
 x = 1
 y = 2
 eval "echo $ pattern"

results:

 1 and 2

This works, but eval "echo ..." just feels messy and might be somehow insecure. Is there a better way to do this in bash?

Answer 1

You can use the envsubst command from gettext , for example:

 $ pattern='$x and $y' $ x=1 y=2 envsubst <<< $pattern 1 and 2 

Answer 2

One safe option is to use the function:

 expand_pattern() { pattern="$x and $y" } 

It's all. Then use the following:

 x=1 y=1 expand_pattern echo "$pattern" 

You can even use x and y as environment variables (so that they are not set in the main area):

 x=1 y=1 expand_pattern echo "$pattern" 

Answer 3

You are right, eval is a security risk in this case. Here is one possible way:

 pattern='The $a is $b when the $z is $x $c $g.' # simulated input from user (use "read") unset results for word in $pattern do case $word in \$a) results+=($(some_command)) # add output of some_command to array (output is "werewolf" ;; \$b) results+=($(echo "active")) ;; \$c) results+=($(echo "and")) ;; \$g) results+=($(echo "the sky is clear")) ;; \$x) results+=($(echo "full")) ;; \$z) results+=($(echo "moon")) ;; *) do_something # count the non-vars, do a no-op, twiddle thumbs # perhaps even sanitize %placeholders, terminal control characters, other unwanted stuff that the user might try to slip in ;; esac done pattern=${pattern//\$[abcgxz]/%s} # replace the vars with printf string placeholders printf "$pattern\n" "${results[@]}" # output the values of the vars using the pattern printf -v sentence "$pattern\n" "${results[@]}" # put it into a variable called "sentence" instead of actually printing it 

The result will be “A werewolf is active when the moon is full and the sky is clear.” The program itself, if the pattern is equal to '$ x $ z outside $ c $ g, therefore $ a must be $ b.' then the output will be "The full moon will disappear and the sky will become clear, so the werewolf must be active."