Concatenating multiple line variables using newline

Question

I have two variables var1 and var2 that could either be multiple lines or could be completely empty. For example, either one could look like:

line/1
line/2

or

I want to concatenate them so that when put together var1 outputs as is, and var 2 outputs as is right below the last line of var1. There are 4 cases, either var1 is empty and var2 is not, var 2 is empty and var 1 is not, both are empty, or neither are empty. I dont want any whitespace or an empty line if one of the variables is empty. So if var 1 is empty I do not want,

line/1

or vice versa.

Other than using a if, elif, else block, is there a way I could do this or do I HAVE to use an if else block.

In addition, for the last case where neither are empty how can I concatenate these two? I have tried

var3="${var1}\n${var2}" 

but that doesn't seem to work. Any tips would be much appreciated.

Answer 1

Using bash, you can use this function for this job:

concat() {
   printf '%s' "${1:+$1$'\n'}" "${2:+$2$'\n'}";
}

Use of "${1:+$1$'\n'}" appends \n to $1 only if $1 is null/unset.

As per man bash:

${parameter:+word} Use Alternate Value. If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted.

Examples:

concat 'line1' 'line2'

line1
line2

concat 'line1' ''

line1

concat '' 'line2'

line2

concat '' ''

concat 'line1
line2
line3' ''

line1
line2
line3