The Backtick Operator
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 "d by spaces.
For safety reasons, the full path to the executed program should be given. In addition, the string is split into arguments prior to dynamic expansion in order to prevent the expanded strings from being erroneously interpreted as more 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 language may be sufficient.
This operator is permitted only in conditional expressions and when assigning values to an attribute. In versions 2.1.11 and later, using it in an invalid context will return a syntax error, and the server will refuse to start. In versions of the server prior to 2.1.11, the data was treated as a single quoted string, and no run-time expansion or program execution is performed.