How to add the real hostname in the beginning of Linux cli command

You could run:

sar -p -d 1 1 | sed "s/^/$(hostname) /"

You can do:

sar -p -d 1 1 | sed "s,^,$(hostname) ,"

if you want to prepend hostname to only non-empty lines:

sar -p -d 1 1 | sed -E "s,^(.+),$(hostname) \1,"

An awk version:

sar -p -d 1 1 | awk -v HOSTNAME=$HOSTNAME '{print HOSTNAME " " $0}'

with the -v flag we set the bash $HOSTNAME variable to a variable that awk can reference in its print statement. With $0 we print the entire line

Or on ssh:

ssh user@remote "sar -p -d 1 1 | awk -v HOSTNAME=$HOSTNAME '{print HOSTNAME \" \" \$0}'"

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