= Backtick-quoted string

.Syntax
`{backtick}string{backtick}`

The backtick operator is used to perform a run-time expansion
similar to what is done with the Unix shell. The contents of the string
are split into one or more sub-strings, based on intermediate
whitespace. Each substring is then expanded as described above for
double quoted strings. The resulting set of strings is used to execute a
program with the associated arguments.

The output of the program is recorded, and the resulting data is
used in place of the input string value. Where the output is composed of
multiple lines, any carriage returns and line feeds are replaced by
spaces.

For safety reasons, the full path to the executed program should be
given. In addition, the string is split into arguments _before_ the
substrings are dynamically expanded.  This step is done both to allow
the substrings to contain spaces, and to prevent spaces in the
expanded substrings from affecting the number of command-line
arguments.

For performance reasons, we recommend that the use of back-quoted
strings be kept to a minimum. Executing external programs is
relatively expensive, and executing a large number of programs for
every request can quickly use all of the CPU time in a server. If many
programs need to be executed, it is suggested that alternative ways to
achieve the same result be found. In some cases, using a real
programming language such as `lua`, `perl` or `python` may be better.

.Examples

`{backtick}/bin/echo hello{backtick}`

// Copyright (C) 2020 Network RADIUS SAS.  Licenced under CC-by-NC 4.0.
// Development of this documentation was sponsored by Network RADIUS SAS.
