How to insert variables inside a string containing ""?

Question

I want to construct an xml string by inserinting variables:

str1="Hello"
str2="world"

xml='<?xml version="1.0" encoding="iso-8859-1"?><tag1>$str1</tag1><tag2>$str2</tag2>'

echo $xml

The result should be

<?xml version="1.0" encoding="iso-8859-1"?><tag1>Hello</tag1><tag2>world</tag2>

But what I get is:

<?xml version="1.0" encoding="iso-8859-1"?><tag1>$str1</tag1><tag2>$str2</tag2>

I also tried

xml="<?xml version="1.0" encoding="iso-8859-1"?><tag1>$str1</tag1><tag2>$str2</tag2>"

But that removes the inner double quotes and gives:

<?xml version=1.0 encoding=iso-8859-1?><tag1>hello</tag1><tag2>world</tag2>

Solution 1

You can embed variables only in double-quoted strings.

An easy and safe way to make this work is to break out of the single-quoted string like this:

xml='<?xml version="1.0" encoding="iso-8859-1"?><tag1>'"$str1"'</tag1><tag2>'"$str2"'</tag2>'

Notice that after breaking out of the single-quoted string, I enclosed the variables within double-quotes. This is to make it safe to have special characters inside the variables.

Since you asked for another way, here's an inferior alternative using printf:

xml=$(printf '<?xml version="1.0" encoding="iso-8859-1"?><tag1>%s</tag1><tag2>%s</tag2>' "$str1" "$str2")

This is inferior because it uses a sub-shell to achieve the same effect, which is an unnecessary extra process.

As @steeldriver wrote in a comment, in modern versions of bash, you can write like this to avoid the sub-shell:

printf -v xml ' ... ' "$str1" "$str2"

Since printf is a shell builtin, this alternative is probably on part with my first suggestion at the top.

Solution 2

Variable expansion doesn't happen in single quote strings.

You can use double quotes for your string, and escape the double quotes inside with \. Like this :

xml="<?xml version=\"1.0\" encoding=\"iso-8859-1\"?><tag1>$str1</tag1><tag2>$str2</tag2>"

The result output :

<?xml version="1.0" encoding="iso-8859-1"?><tag1>hello</tag1><tag2>world</tag2>