Editing Bash (Unix shell) (section)

=== Deprecated syntax ===

* Back-tick style command substitutions: <code>`...`</code> is deprecated in favor of <code>$(...)</code>;
* Use of -a or -o in <code>test</code>/<code>[</code>/<code>[[</code> commands,
** for example, <code>[ -r ./file -a ! -l ./file ]</code> is deprecated in favor of <code>[ -r ./file ] && ! [ -l ./file ]</code>;
* Use of the arithmetic syntax <code>$[...]</code> is deprecated in favor of <code>$((...))</code> or <code>((...))</code>, as appropriate;
* Use of <code>^</code> as a pipeline is deprecated in favor of <code>|</code>;
* Any uses of <code>expr</code> or <code>let</code>.