Linux Agent: Update all instances of '[...]' to '[[...]]'

[rawiriblundell]

Hi,
bash, ksh and zsh all support [[ ]], which is more capable, more robust/safer and it is faster. This is technically an (admittedly miniscule) optimisation as well as a style change.

[svenpanne]

I am very reluctant to make such a change for portability reasons. Currently we have 10 different shell-based agents, and our goal is to reduce these to 2: The OpenVMS agent is totally different, but the 9 other ones can and should be merged at some point. Furthermore, the agents have to run on really old platforms, like ancient Solaris/AIX versions etc. with really ancient and primitive shells. So we simply can't assume even slightly modern features, everything should run on some #!/bin/sh, be it effectively an ash, sh, bash, dash, ... We are not there yet, but we should use less bash-isms, not more, so this pull request goes into the wrong direction.

FYI: https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/Portable-Shell.html has quite some info about things to avoid. As a general rule of thumb: If a shell feature is not used by a standard configure script, avoid it.

[mike1098]

@svenpanne : IN AIX /bin/sh is linked to korn shell:

`sh Command

Purpose

   Invokes the default shell.

Syntax

   Refer to the syntax of the ksh command. The /usr/bin/sh file is linked to the Korn shell.

Description

   The sh command invokes the default shell and uses its syntax and flags. The shell linked to the /usr/bin/sh path is the default shell. The standard configuration of the operating system links the /usr/bin/sh path to the Korn
   shell.

Flags

   Refer to the flags for the Korn shell (ksh command).

Files

   Item
        Description
   /usr/bin/sh
        Contains the sh command.

`