Bash - assign array into variable as string

Question

I have this code, it prints the correct result, but I can't figure out how to get the echo from the last line into a variable.

# hostname is 'tech-news-blog-324344' . Setting it into array host_name_array IFS='-' read -r -a host_name_array <<< "$(hostname)" #removing the last part of string after last "-" unset 'host_name_array[${#host_name_array[@]}-1]' ( IFS=$'-'; echo "${host_name_array[*]}" ) #result is 'tech-news-blog' 

How could get the value of the last line into a variable? I've tried the following:

( IFS=$'-'; URL="${host_name_array[*]}" ) 

But I get the result "tech news blog" with spaces between pieces of array instead of '-'.

Answer 1

When IFS='-' read -r -a host_name_array <<< "$(hostname)" is ran, the array is (tech news blog 324344).

After the final element is removed with unset 'host_name_array[${#host_name_array[@]}-1]', the array is (tech news blog).

So, to get this to echo tech-news-blog, some substitution will have to be done, since echo "${host_name_array[*]}" will yield tech news blog:

With tr: echo "${host_name_array[*]}" | tr ' ' '-'

sed: echo "${host_name_array[*]}" | sed 's/ /-/g'

Answer 2

You could do this by outputting the elements separated by -, and then strip off the last - by parameter expansion:

$ var=$(printf '%s-' "${host_name_array[@]}") $ echo "$var" foo-bar-spam-egg- $ var="${var%-}" $ echo "$var" foo-bar-spam-egg 

Also you need ${host_name_array[@]} instead of ${host_name_array[*]} to prevent outputting the elements as a whole separated by first character of IFS.

---

What you are trying to do can be achieved by a simple parameter expansion:

${var%-*} 

Example:

$ var=$(hostname) $ echo "${var%-*}" 

Answer 3

(...) introduces a subshell. So $URL wouldn't be set after that (still have the value it had before the subshell). You want:

IFS=- read -r -a host_name_array <<< "$(hostname)" unset 'host_name_array[${#host_name_array[@]}-1]' URL="${host_name_array[*]}" 

"${host_name_array[*]}" joins the elements on the arrays on the first character of $IFS just like "$*" does in standard sh.

If the reason why you're using a subshell is because you don't want to modify $IFS globally, you could do that in a function where you give $IFS a local scope:

f() { local IFS=- ... } f 

Or use command substitution that also creates a subshell but allows passing data to the parent shell:

URL=$(IFS=-; printf '%s\n' "${host_name_array[*]}") 

Here though I'd use standard sh expansion to remove that trailing component:

URL=$(uname -n) # uname -n is the standard equivalent of hostname URL=${URL%-*} 

It has several advantages over the above:

• it works even if the text contains newline characters (very unlikely for host names here though);
• if the text doesn't contain -, it doesn't remove anything;
• it is more efficient. read <<< $(hostname) means running hostname, read its output through a pipe, storing that in a temp file and have read read the first line of that;
• it doesn't clobber the variable namespace with those temporary variables
• it is shorter;
• it is portable. No need to install bash to run that code. The system's sh will suffice.

In any case, remember to quote your variables when using them in list contexts:

printf '%s\n' "$URL" 

When doing:

echo $URL

You're invoking the split+glob operator. So, if $IFS still contains -, that $URL would be split on -, and echo would output those words separated by spaces.

Answer 4

bash specific answer: whipe forks!

As this already use bashisms, sed, tr or any other external tool are useless:

IFS='-' read -r -a host_name_array < <(hostname) #removing the last part of string after last "-" unset host_name_array[${#host_name_array[@]}-1] shorted=${host_name_array[*]} echo ${shorted// /-} 

Or simplier:

This is a lot shorter, quicker and simplier:

read myvar < <(hostname) echo ${myvar%-*} 

And if really you need host_name_array for other reasons:

read shorted < <(hostname) host_name_array=(${shorted//-/ }) shorted=${shorted%-*} 

So you have an array and a shorted hostname:

declare -p host_name_array shorted 

will return something like:

declare -a host_name_array='([0]="tech" [1]="news" [2]="blog" [3]="324344")' declare -- shorted="tech-news-blog"