Added SHELL known problems

pages/merlin6/99 support/known-problems.md

@@ -12,11 +12,47 @@ permalink: /merlin6/known-problems.html
 
 | Topic                                                                                      |
 |:-----------------------------------------------------------------------------------------  |
+| [Default Shell](/merlin6/known-problems.html#default-shell)                                |
 | [OpenGL vs Mesa](/merlin6/known-problems.html#opengl-vs-mesa)                              |
 | [Paraview](/merlin6/known-problems.html#OpenGL)                                            |
 | [ANSYS](/merlin6/known-problems.html#opengl-support-paraview-ansys-etc)                    |
 | [Illegal instructions error](i/merlin6/known-problems.html#illegal-instructions)           |
 
+## Default SHELL
+
+In general, **`/bin/bash` is the recommended default user's SHELL** when working in Merlin.
+
+Some users might notice that BASH is not the default SHELL when login to Merlin systems, or they might need to run a different SHELL. 
+This is probably because when the PSI account was requested, no SHELL description was specified or a different one was requested explicitly by the requestor.
+Users can check which is the default SHELL specified in the PSI account with the following command:
+
+```bash
+getent passwd $USER | awk -F: '{print $NF}'
+```
+
+If SHELL does not correspond to the one you need to use, you should request a central change for it. 
+This is because Merlin accounts are central PSI accounts. Hence, **change must be requested via [PSI Service Now](/merlin6/contact.html#psi-service-now)**.
+
+Alternatively, if you work on other PSI Linux systems but for Merlin you need a different SHELL type, a temporary change can be performed during login startup. 
+You can update one of the following files:
+* `~/.login`
+* `~/.profile`
+* Any `rc` or `profile` file in your home directory (i.e. `.cshrc`, `.bashrc`, `.bash_profile`, etc.)
+
+with the following lines:
+
+```bash
+# Replace MY_SHELL with the bash type you need
+MY_SHELL=/bin/bash
+exec $MY_SHELL -l
+```
+
+Notice that available *shells* can be found in the following file: 
+
+```bash
+cat /etc/shells
+```
+
 ## OpenGL vs Mesa
 
 Some applications can run with OpenGL support. This is only possible when the node contains a GPU card.