# 9.2. Prerequisites

Successful launch of jobs by PRRTE requires the ability to find the PRRTE daemon executables and shared libraries on all nodes at run time.

In general, if the PRRTE executables and libraries can be found via in system-default search paths (i.e., without the user needing to set or modify `PATH` or `LD_LIBRARY_PATH`), then nothing additional needs to be done. However, if the PRRTE binaries and libraries are not found, the instructions below may be used to locate them.

In general, PRRTE requires that its executables are in your `PATH` on every node on which you will run and if PRRTE was compiled as dynamic libraries (which is the default), the directory where its libraries are located must be in your `LD_LIBRARY_PATH` on every node. For example:

- If PRRTE is installed in `/usr/bin` and `/usr/lib`), that is usually sufficient, and the user does not need to do anything extra.

- If PRRTE is installed in a location that is not searched by default, users may need to add `$prefix/bin` to their `PATH` and `$libdir` (which defaults to `$prefix/lib`) to their `LD_LIBRARY_PATH`.

  Caution

  In scheduled environments, ensuring PRRTE’s executables and libraries can be found on the node that executes `prterun` may be sufficient.

  In non-scheduled environments, users may need to set the `PATH` and `LD_LIBRARY_PATH` environment variables in their shell setup files (e.g., `$HOME/.bashrc`) so that non-interactive `ssh`-based logins will be able to find the PRRTE executables and libraries.

  For example, if PRRTE was installed with a prefix of `/opt/prrte`, then the following should be in your `PATH` and `LD_LIBRARY_PATH`

  Environment variable | Value to add
  --- | ---
  `PATH` | `/opt/prrte/bin`
  `LD_LIBRARY_PATH` | `/opt/prrte/lib`

  Depending on your environment, you may need to set these values in your shell startup files (e.g., `.bashrc`, `.cshrc`, etc.).

Additionally, PRRTE requires that jobs can be started on remote nodes without any input from the keyboard. For example, if using `ssh` as the remote agent, you must have your environment setup to allow execution on remote nodes without entering a password or passphrase.

## 9.2.1. Adding PRRTE to `PATH` and `LD_LIBRARY_PATH`

PRRTE must be able to find its executables in your `PATH` on every node (if PRRTE was compiled as dynamic libraries, then its library path must appear in `LD_LIBRARY_PATH` as well). As such, your configuration/initialization files need to add PRRTE to your `PATH` / `LD_LIBRARY_PATH` properly.

How to do this may be highly dependent upon your local configuration; you may need to consult with your local system administrator. Some system administrators take care of these details for you, some don’t. Some common examples are included below, however.

You must have at least a minimum understanding of how your shell works to get PRRTE in your `PATH` / `LD_LIBRARY_PATH` properly. Note that PRRTE must be added to your `PATH` and `LD_LIBRARY_PATH` in the following situations:

1. When you login to an interactive shell

   If your interactive login environment is not configured properly, executables like `prterun` will not be found, and it is typically obvious what is wrong. The PRRTE executable directory can manually be added to the `PATH`, or the user’s startup files can be modified such that the PRRTE executables are added to the `PATH` every login. This latter approach is preferred.

   All shells have some kind of script file that is executed at login time to set things like `PATH` and `LD_LIBRARY_PATH` and perform other environmental setup tasks. This startup file is the one that needs to be edited to add PRRTE to the `PATH` and `LD_LIBRARY_PATH`. Consult the manual page for your shell for specific details (some shells are picky about the permissions of the startup file, for example). The table below lists some common shells and the startup files that they read/execute upon login:

   Shell | Interactive login startup files
   --- | ---
   `bash` | `.bash_profile` if it exists, or `.bash_login` if it exists, or `.profile` if it exists (in that order). Note that some Linux distributions automatically come with `.bash_profile` scripts for users that automatically execute `.bashrc` as well. Consult the `bash(1)` man page for more information.
   `zsh` | `.zshrc` followed by `.zshenv`
   `sh` (or Bash named `sh`) | `.profile`
   `csh` | `.cshrc` followed by `.login`
   `tcsh` | `.tcshrc` if it exists, `.cshrc` if it does not, followed by `.login`

2. When you login to non-interactive shells on remote nodes

   If your non-interactive remote environment is not configured properly, executables like `prterun` will not function properly, and it can be somewhat confusing to figure out.

   The startup files in question here are the ones that are automatically executed for a non-interactive login on a remote node (e.g., `ssh othernode ps`). Note that not all shells support this, and that some shells use different files for this than listed for interactive logins. Some shells will supersede non-interactive login startup files with files for interactive logins. That is, running non-interactive login startup file may automatically invoke interactive login startup file. The following table lists some common shells and the startup file that is automatically executed, either by PRRTE or by the shell itself:

   Shell | Non-interactive login startup files
   --- | ---
   `bash` | `.bashrc` if it exists
   `zsh` | `.zshrc` followed by `.zshenv`
   `sh` (or Bash named `sh`) | This shell does not execute any file automatically, so PRRTE will execute the `.profile` script before invoking PRRTE executables on remote nodes
   `csh` | `.cshrc`
   `tcsh` | `.tcshrc` if it exists, `.cshrc` if it does not

## 9.2.2. Using the `--prefix` option with prterun

If users are unable to add the relevant directories to `PATH` and `LD_LIBRARY_PATH`, the `prterun` `--prefix` option may be sufficient.

There are some situations where you cannot modify the `PATH` or `LD_LIBRARY_PATH` — e.g., some ISV applications prefer to hide all parallelism from the user, and therefore do not want to make the user modify their shell startup files.

In such cases, you can use the `prterun```–prefix`` command line option, which takes as an argument the top-level directory where PRRTE was installed. While relative directory names are possible, they can become ambiguous depending on the job launcher used; using absolute directory names is strongly recommended.

For example, say that PRRTE was installed into `/opt/prrte-VERSION`. You would use the `--prefix` option thusly:

    shell$ prterun --prefix /opt/prrte-VERSION -n 4 a.out

This will prefix the `PATH` and `LD_LIBRARY_PATH` on both the local and remote hosts with `/opt/prrte-VERSION/bin` and `/opt/prrte-VERSION/lib`, respectively. This is usually unnecessary when using resource managers to launch jobs (e.g., Slurm, Torque, etc.) because they tend to copy the entire local environment — to include the `PATH` and `LD_LIBRARY_PATH` — to remote nodes before execution. As such, if `PATH` and `LD_LIBRARY_PATH` are set properly on the local node, the resource manager will automatically propagate those values out to remote nodes. The `--prefix` option is therefore usually most useful in `ssh`-based environments (or similar), OR when the cluster has been configured with PRRTE located in a different location on the remote nodes.

It is possible to make this the default behavior by passing to `configure` the flag `--enable-prterun-prefix-by-default`. This will make `prterun` behave exactly the same as `prterun --prefix $prefix ...`, where `$prefix` is the value given to `--prefix` in `configure`.

Finally, note that specifying the absolute pathname to `prterun` is equivalent to using the `--prefix` argument. For example, the following is equivalent to the above command line that uses `--prefix`:

    shell$ /opt/prrte-VERSION/bin/prterun -n 4 a.out