# Environment Customization

## Before Starting

Consider these important facts:

* Environment variables **are all upper case**.
* To use their values, precede the name with a `$`.

## Initializing Your Environment

Linux utilizes Bash as the default shell and when a session started it reads commands from `~/.bash_profile`.

Environment variables are set in the file `~/.bashrc`.

📝 **Note:** The files `~/.bash_profile` and `.bashrc` are hidden. To list hidden files, type `ls -a`.

## Know the Environment Variables

Here is a list of some common environment variables:

* `$HOME` - Path of your home directory
* `$PATH` - List of directories where the system checks for programs to run
* `$LD_LIBRARY_PATH` - List of directories where the system checks for shared libraries to load
* `$HOSTNAME` - The name of the host, e.g. `tigris.doane.edu`.

📝 **Note:** See the values of all your environment variables by typing `env` on your terminal.

## Working with the Environment Variables

* Display the value of an environment variable using `echo`:

  ```bash
  echo $HOME
  /home/UID
  ```
* Modify the value of environment variables with `export`:

  ```bash
  export PATH=$PATH:/home/UID
  export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/home/UID/custom_lib_directory
  ```
* Set a value for environment variables:

  ```bash
  export OMP_NUM_THREADS=12
  ```

  *This command sets the value of the variable called OMP\_NUM\_THREADS (an OpenMP parameter) to 12.*


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