[bash/en] Improved bash file

Changes:
* Added conditional execution
* Improved Variable info
    * Added info on builtin variables
    * Added string substitution for string variables
    * Added info about strings in single quotes, which doesn't expand variables
* Added info about input, output redirection to file
* Improved for description
* Improved if example
* Added functions
* Added list of useful commands

NOTE: I didn't add while loops because there was a pull request with that.