mjhjmtu

If I am using this scenario in a script:

#!/bin/bash

addvline=$(if [ "$1" ]; then echo "$1"; echo; fi)

cat << EOF
this is the first line
$addvline
this is the last line
EOF

if $1 is emty I get a blank line.

But how can I add a blank line after $1 for the case it is not emty?

So in the case running the script like:
bash script.sh hello

I would get:

this is the first line
hello

this is the last line

I tried to achieve this with using a second echo in the if statement, but the newline does not get passed.

Let if decide to set your variable content not to use command substitution.

if [ "$1" ]; then addvline=$1$'n'; fi

Then:

#!/bin/bash
if [ "$1" ]; then addvline=$1$'n'; fi
cat << EOF
this is the first line
$addvline
this is the last line
EOF

There are several solutions to this. First, let's create a variable that contains a newline to be used later (in bash):

nl=$'n'

then it could either be used to construct the variable to be printed:

#!/bin/bash
nl=$'n'
if [ "$1" ]; then
 addvline="$1$nl"
else
 addvline=""
fi

cat << EOF
this is the first line
$addvline
this is the last line
EOF

Or you could avoid the if entirely if you use the correct parameter expansion:

#!/bin/bash
nl=$'n'
addvline="$1:+$1$nl"

cat << EOF
this is the first line
$addvline
this is the last line
EOF

Or, in one simpler code:

#!/bin/bash
nl=$'n'

cat << EOF
this is the first line
$1:+$1$nl
this is the last line
EOF