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

Here is a concise summary of what most answers are talking about.

Let's say we have two variables and $1 is set to 'one':

set one two
a=hello
b=world

The table below explains the different contexts where we can combine the values of a and b to create a new variable, c.

Context                               | Expression            | Result (value of c)
--------------------------------------+-----------------------+---------------------
Two variables                         | c=$a$b                | helloworld
A variable and a literal              | c=${a}_world          | hello_world
A variable and a literal              | c=$1world             | oneworld
A variable and a literal              | c=$a/world            | hello/world
A variable, a literal, with a space   | c=${a}" world"        | hello world
A more complex expression             | c="${a}_one|${b}_2"   | hello_one|world_2
Using += operator (Bash 3.1 or later) | c=$a; c+=$b           | helloworld
Append literal with +=                | c=$a; c+=" world"     | hello world

A few notes:

• enclosing the RHS of an assignment in double quotes is generally a good practice, though it is quite optional in many cases
• += is better from a performance standpoint if a big string is being constructed in small increments, especially in a loop
• use {} around variable names to disambiguate their expansion (as in row 2 in the table above). As seen on rows 3 and 4, there is no need for {} unless a variable is being concatenated with a string that starts with a character that is a valid first character in shell variable name, that is alphabet or underscore.

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See also:

• BashFAQ/013 - How can I concatenate two variables?
• When do we need curly braces around shell variables?