How to concatenate string variables in Bash

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攒了一身酷
攒了一身酷 2020-11-22 03:40

In PHP, strings are concatenated together as follows:

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

Here, $foo becomes \"Hello World\"

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  • 2020-11-22 03:52

    I wanted to build a string from a list. Couldn't find an answer for that so I post it here. Here is what I did:

    list=(1 2 3 4 5)
    string=''
    
    for elm in "${list[@]}"; do
        string="${string} ${elm}"
    done
    
    echo ${string}
    

    and then I get the following output:

    1 2 3 4 5
    
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  • 2020-11-22 03:54
    bla=hello
    laber=kthx
    echo "${bla}ohai${laber}bye"
    

    Will output

    helloohaikthxbye
    

    This is useful when $blaohai leads to a variable not found error. Or if you have spaces or other special characters in your strings. "${foo}" properly escapes anything you put into it.

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  • 2020-11-22 03:54
    $ a=hip
    $ b=hop
    $ ab=$a$b
    $ echo $ab
    hiphop
    $ echo $a$b
    hiphop
    
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  • 2020-11-22 03:56

    Even if the += operator is now permitted, it has been introduced in Bash 3.1 in 2004.

    Any script using this operator on older Bash versions will fail with a "command not found" error if you are lucky, or a "syntax error near unexpected token".

    For those who cares about backward compatibility, stick with the older standard Bash concatenation methods, like those mentioned in the chosen answer:

    foo="Hello"
    foo="$foo World"
    echo $foo
    > Hello World
    
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  • 2020-11-22 03:56

    In my opinion, the simplest way to concatenate two strings is to write a function that does it for you, then use that function.

    function concat ()
    {
        prefix=$1
        suffix=$2
    
        echo "${prefix}${suffix}"
    }
    
    foo="Super"
    bar="man"
    
    concat $foo $bar   # Superman
    
    alien=$(concat $foo $bar)
    
    echo $alien        # Superman
    
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  • 2020-11-22 03:57

    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.

    See also:

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