bash - add blank line to heredoc via variable

Question

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.

αғsнιη · Accepted Answer · 2017-10-01 17:35:05Z

1

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

answered Oct 1, 2017 at 17:35

αғsнιη

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Jeff Schaller · Accepted Answer · 2017-10-01 19:44:07Z

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

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