How to concatenate arrays in bash?

Question

I am a new to Bash. I have an array taking input from standard input. I have to concatenate itself twice. Say, I have the following elements in the array:

Namibia Nauru Nepal Netherlands NewZealand Nicaragua Niger Nigeria NorthKorea Norway 

Now, The output should be:

Namibia Nauru Nepal Netherlands NewZealand Nicaragua Niger Nigeria NorthKorea Norway Namibia Nauru Nepal Netherlands NewZealand Nicaragua Niger Nigeria NorthKorea Norway 

My code is:

countries=() while read -r country; do     countries+=( "$country" ) done countries=countries+countries+countries # this is the wrong way, i want to know the right way to do it echo "${countries[@]}" 

Note that, I can print it thrice like the code below, but it is not my motto. I have to concatenate them in the array.

countries=() while read -r country; do     countries+=( "$country" ) done echo "${countries[@]} ${countries[@]} ${countries[@]}" 

Answer 1

First, to read your list into an array, one entry per line:

readarray -t countries 

...or, with older versions of bash:

# same, but compatible with bash 3.x; || is to avoid non-zero exit status. IFS=$'\n' read -r -d '' countries || (( ${#countries[@]} )) 

---

Second, to duplicate the entries, either expand the array to itself three times:

countries=( "${countries[@]}" "${countries[@]}" "${countries[@]}" ) 

...or use the modern syntax for performing an append:

countries+=( "${countries[@]}" "${countries[@]}" ) 

Answer 2

Simply write this:

countries=$(cat) countries+=( "${countries[@]}" "${countries[@]}" ) echo ${countries[@]} 

The first line is to take input array, second to concatenate and last to print the array.