set output into variable from grep from input variable

Question

whats wrong with this bash script:

acme2=$(dig txt @$1 _acme-challenge.$1.de)                                            
acme3=$(echo $acme2 | grep "^_acme")                                                    

acme2 has the whole output, but acme3 is always empty

I searched several solutions, and tried other possibilities, but nothing works...

acme3=echo $acme2|grep acme                                                          
acme3=$($acme2|grep "^_acme")                                                        
acme3=$(grep "acme" $acme2)                                                          
acme3=$(echo "$acme2" | grep "^_acme")                                              

Answer 1

We really can't help since we have no idea what $acme2 contains, but I can tell you that your grep command is looking for lines that start with _acme. Since the output of dig is multiple lines and you are echoing the variable unquoted, that will put everything on a single line, so unless the very first string in the output is _acme, your grep will match nothing. To illustrate:

$ seq 3
1
2
3

$ acme=$(seq 3)

$ echo $acme
1 2 3

$ echo "$acme"
1
2
3

$ echo $acme | grep '^2'
$

$ echo "$acme" | grep '^2'
2

So, assuming you have at least one line in your dig output that begins with _acme, correctly quoting your variable (which you should ALWAYS do, by the way) should work:

acme3=$(echo "$acme2" | grep "^_acme")

Or, better, especially for arbitrary data like this:

acme3=$(printf '%s\n' "$acme2" | grep "^_acme")                 

Answer 2

Both lines could be coalesced by telling dig you're only interested in its direct output:

#!/bin/sh
acme3=$(dig +short txt @"$1" "_acme-challenge.$1.de")

This also sidesteps the issue with echo $acme2, which should almost certainly have been echo "$acme2" or even printf '%s\n' "$acme2". (Omitting the double quotes causes the contents of the variable $acme2 to be split on characters of $IFS (space, tab and newline by default) and subject to glob expansion which when passed to echo likely results in the full output ending up as a single line, possibly modified beyond recognition.)