Calling echo with a single expandable string unexpectedly results in output with a newline in the middle

Martin Brandl's helpful answer bypasses your problem with a solution that is preferable anyway (no need for Write-Output a.k.a echo).
This answer explains the problem with your approach.

tl;dr

• Use double quotes:

  • echo "$($var):123:zzz" > test.txt

• Simplified (no need for echo):

  • "$($var):123:zzz" > test.txt

• Simplest (note the {}, which are needed to disambiguate the variable name):

  • "${var}:123:zzz" > test.txt
  • "$var`:123:zzz" > test.txt would work too (escaping the :)

Your specific problem is that $($var):123:zzz is parsed as 2 arguments, due to not being enclosed in ".

In other words: The following 2 statements are equivalent:

echo $($var):123:zzz

echo $($var) :123:zzz    

Both yield the following (wether in the console or when redirected to a file), because Write-Output (that echo is an alias for) writes its (stringified) arguments separated by a line break:

abcd
:123:zzz

Since you're passing $($var):123:zzz as an argument to the echo (Write-Output) command, it is parsed in argument mode.

In argument mode, an unquoted subexpression-operator expression such as $($var) at the start of a token is always treated as a separate argument, even if followed directly by more characters.

^{This answer of mine has more information about how PowerShell parses unquoted tokens in argument mode.}

By contrast, double-quoting (enclosing the token in "...") ensures parsing as a single argument (though note that the subexpression will then be stringified first).

Thus, this command would work:

echo "$($var):123:zzz" > test.txt

Or, more simply, given that PowerShell outputs an expression's result by default (Write-Output is usually not needed):

"$($var):123:zzz" > test.txt

Note that while double-quoted strings are used in both commands, the latter command is technically parsed in expression mode (which is PowerShell's other, programming language-like parsing mode), because the command line starts with a ".

If you are trying to add ":123:zzz" to the end of $var you can do this as well;

$var = 'abcd'
echo ($var+':123:zzz') > test.txt

or if you want to add a new line (append) add a second ">";

$var = 'abcd'
echo ($var+':123:zzz') >> test.txt