How to concatenate string variables in Bash

Question

In PHP, strings are concatenated together as follows:

$foo = \"Hello\";
$foo .= \" World\";

Here, $foo becomes \"Hello World\"

Answer 1

I prefer to use curly brackets ${} for expanding variable in string:

foo="Hello"
foo="${foo} World"
echo $foo
> Hello World

---

Curly brackets will fit to Continuous string usage:

foo="Hello"
foo="${foo}World"
echo $foo
> HelloWorld

Otherwise using foo = "$fooWorld" will not work.

Answer 2

I do it this way when convenient: Use an inline command!

echo "The current time is `date`"
echo "Current User: `echo $USER`"

Answer 3

Bash also supports a += operator as shown in this code:

A="X Y"
A+=" Z"
echo "$A"

output

X Y Z

Answer 4

If you want to append something like an underscore, use escape (\)

FILEPATH=/opt/myfile

This does not work:

echo $FILEPATH_$DATEX

This works fine:

echo $FILEPATH\\_$DATEX

Answer 5

There's one particular case where you should take care:

user=daniel
cat > output.file << EOF
"$user"san
EOF

Will output "daniel"san, and not danielsan, as you might have wanted. In this case you should do instead:

user=daniel
cat > output.file << EOF
${user}san
EOF

Answer 6

var1='hello'
var2='world'
var3=$var1" "$var2 
echo $var3