more updates to apptainer guide

documentation/htc_workloads/using_software/containers-docker.md

@@ -162,7 +162,7 @@ Apptainer to convert it to a SIF image:
 
 After converting the Docker image to a sif format, you can use the 
 image in your job as described in the
-[Apptainer/Singularity Guide](../containers-singularity). 
+[Apptainer/Singularity Guide](../containers-singularity#using-singularity-or-apptainer-images-in-an-htcondor-job). 
 
 ## Special Cases
 

documentation/htc_workloads/using_software/containers-singularity.md

@@ -10,7 +10,30 @@ in the Open Science Pool. You can use your own custom container to run
 jobs in the Open Science Pool. This guide describes how to create your
 own Apptainer/Singularity container "image" (the blueprint for the container).
 
-## Identify Components
+## Do You Need to Build a Container? 
+
+If there is an existing Docker container or Apptainer/Singularity container with 
+the software you need, you can proceed with using these options to submit a job. 
+* [See OSPool-provided containers here](../available-containers-list/)
+* [Using an existing Docker container](../containers-docker/)
+* [Using an existing Apptainer/Singularity container](#using-singularity-or-apptainer-images-in-an-htcondor-job)
+
+If you can't find a good option among existing containers, you may need to 
+build your own. See [this section of the guide](#building-your-own-apptainer-singularity-container) for more information. 
+
+## OSG-Provided Apptainer/Singularity Images
+
+The OSG Team maintains a set of images that are already in the OSG
+Apptainer/Singularity repository. [A list of ready-to-use containers can be found on this page](../available-containers-list/).
+
+If the software you need isn't already supported in a listed container,
+you can create your own container or use any container image in Docker Hub.
+
+How to explore these containers is shown [below](#exploring-apptainer-singularity-images-on-the-access-points). 
+
+## Building Your Own Apptainer/Singularity Container
+
+### Identify Components
 
 What software do you want to install? Make sure that you have either the source
 code or a command that can be used to install it through Linux (like `apt-get` or
@@ -22,15 +45,7 @@ as your starting point. See the available containers on Docker Hub here:
 [OSG Docker Containers](https://hub.docker.com/u/opensciencegrid)
 The best candidates for you will be containers that have "osgvo" in the name.
 
-## OSG-Provided Apptainer/Singularity Images
-
-The OSG Team maintains a set of images that are already in the OSG
-Apptainer/Singularity repository. [A list of ready-to-use containers can be found on this page](../available-containers-list/).
-
-If the software you need isn't already supported in a listed container,
-you can create your own container or any container image in Docker Hub.
-
-## Apptainer/Singularity Build
+### Apptainer/Singularity Build
 
 If you are building an image for the first time, the temporary cache directory of the apptainer image needs to be defined. The following commands define the cache location of the apptainer image to be built. Please run the commands in the terminal of your access point.
 
@@ -128,8 +143,7 @@ Once your build spec is ready, you can "build" the container image by running th
 Once the image is built, test it on an OSG-managed access point,
 and use it in your HTCondor jobs.
 
-
-### Exploring Apptainer/Singularity Images on the Access Points
+## Exploring Apptainer/Singularity Images on the Access Points
 
 Just like it is important to test your codes and jobs at a small scale,
 you should make sure that your Apptainer/Singularity container is working correctly before using it in jobs. One way
@@ -142,31 +156,42 @@ to how containers are started for jobs, is:
 
     apptainer shell my-container.sif
 
+If you want to test an existing container produced by OSG Staff, use the 
+full path provided in [this guide](../available-containers-list/). 
+
 This example will give you an interactive shell. You can explore the
 container and test your code with your own inputs from your `/home`
 directory, which is automatically mounted (but note - $HOME will not be
 available to your jobs later). Once you are down exploring, exit the
-container by running `exit` or with `CTRL+D`
+container by running `exit` or with `CTRL+D`. 
 
+## Using Singularity or Apptainer Images in an HTCondor Job
 
-## Pre-existing .sif Apptainer/Singularity Images
+Once you have a ".sif" container image file with all your needed software, 
+you can use this file as part of an HTCondor job. 
+
+### Upload the Container Image to the OSDF
 
 The image will be resused for
 each job, and thus the preferred transfer method is [OSDF](../../managing_data/osdf/).
-Store the .sif file under the "protected" area on your access point
-(see table [here](../../managing_data/overview/)), and then use the OSDF
+Store the .sif file under your personal data area on your access point
+(see table [here](../../managing_data/osdf/#where-to-put-your-files)). 
+
+### Use the Container in an HTCondor Job
+
+Once the image is placed in your OSDF space, you can use an OSDF 
 url directly in the `+SingularityImage` attribute. Note that you can not
 use shell variable expansion in the submit file - be sure to replace the
 username with your actual OSPool username. Example:
 
-    +SingularityImage = "osdf:///ospool/protected/USERNAME/my-custom-image-v1.sif"
+    +SingularityImage = "osdf:///ospool/apXX/data/USERNAME/my-custom-image-v1.sif"
 
     <other usual submit file lines>
     queue
 
-Be aware that OSDF aggressively caches the image based on file naming.
+**Be aware that OSDF aggressively caches the image based on file naming.
 If you need to do quick changes, please use versioning of the .sif file
-so that the caches see a "new" name. In this example, replacing
+so that the caches see a "new" name.** In this example, replacing
 `my-custom-image-v1.sif` with new content will probably mean that some
 nodes get the old version and some nodes the new version. Prevent this
 by creating a new file named with v2.