EN1
 EN2
 EN3
 EN4
 EN5

EN1,EN2,EN3,EN4,EN5

awk 'BEGIN { OFS = ","} { printf $1}' input.txt

awk 'BEGIN{ORS=","}1' input.txt

EN1,EN2,EN3,EN4,EN5,

tr 'n' ',' <input.txt

<input.txt head -c -1 | tr 'n' ,

columns=$(<input.txt tr 'n' ',')
echo "${columns%,}"

$ xargs <input.txt | sed -e 's/ /,/g'
EN1,EN2,EN3,EN4,EN5

EN1 foo bar
EN2 bar foo
EN3 baz quux
EN4 abc def
EN5 hij klm

$ awk '{print $1}' input.txt | xargs | sed -e 's/ /,/g'
EN1,EN2,EN3,EN4,EN5

$ sed -e 's/ /--space--/g' input.txt | xargs | sed -e 's/ /,/g' -e 's/--space--/ /g'
EN1 foo bar,EN2 bar foo,EN3 baz quux,EN4 abc def,EN5 hij klm

$ paste -sd, input.txt
EN1,EN2,EN3,EN4,EN5
$

perl -pe '(eof)?s/s+$//:s/s+$/,/' input.txt

EN1,EN2,EN3,EN4,EN5

sed -z 's/n/,/g' input.txt

sed -z 's/n/,/g' input.txt | sed 's/,$/n/'