tr

replace newline with comma

tr '\n' ','

strip newline from input

If you do this and paste it afterwards, it’ll add a newline or will even be used as return in some forms:

head -c 32 /dev/urandom|base64|pbcopy

strip the newline

do it like this, tr -d '\n' is the key:

head -c 32 /dev/urandom|base64|tr -d '\n'|pbcopy