How to concatenate arrays in bash?

How can I read lines from the standard input into an array and then concatenate the array with itself?

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[@]}" 

3 Answers

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[@]}" ) 

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.

on ubuntu 14.04, the following would concatenate three elements (an element count would give :3), each element being an array countries:

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

while the below would concatenate all elements in one single array:

countries=( ${countries[*]} ${countries[*]} ${countries[*]} ) 

a count of this would be 30 (taken into account the countries specified in the original post).