Singularity: Prefer `${var}` over `$var`

doc.zih.tu-dresden.de/docs/software/singularity_recipe_hints.md

@@ -74,9 +74,9 @@ From: ubuntu:20.04
 
 %environment
     export MPICH_DIR=/opt/mpich
-    export SINGULARITY_MPICH_DIR=$MPICH_DIR
-    export SINGULARITYENV_APPEND_PATH=$MPICH_DIR/bin
-    export SINGULAIRTYENV_APPEND_LD_LIBRARY_PATH=$MPICH_DIR/lib
+    export SINGULARITY_MPICH_DIR=${MPICH_DIR}
+    export SINGULARITYENV_APPEND_PATH=${MPICH_DIR}/bin
+    export SINGULAIRTYENV_APPEND_LD_LIBRARY_PATH=${MPICH_DIR}/lib
 
 %post
     echo "Installing required packages..."
@@ -88,23 +88,23 @@ From: ubuntu:20.04
     echo "Installing MPICH"
     export MPICH_DIR=/opt/mpich
     export MPICH_VERSION=4.1
-    export MPICH_URL="https://www.mpich.org/static/downloads/$MPICH_VERSION/mpich-$MPICH_VERSION.tar.gz"
+    export MPICH_URL="https://www.mpich.org/static/downloads/${MPICH_VERSION}/mpich-${MPICH_VERSION}.tar.gz"
     mkdir -p /tmp/mpich
     mkdir -p /opt
     # Download
-    cd /tmp/mpich && wget -O mpich-$MPICH_VERSION.tar.gz $MPICH_URL && tar -xf mpich-$MPICH_VERSION.tar.gz
-    
+    cd /tmp/mpich && wget -O mpich-${MPICH_VERSION}.tar.gz ${MPICH_URL} && tar -xf mpich-${MPICH_VERSION}.tar.gz
+
     # Configure and compile/install
-    cd /tmp/mpich/mpich-$MPICH_VERSION
-    ./configure --prefix=$MPICH_DIR && make install
-    
-    
+    cd /tmp/mpich/mpich-${MPICH_VERSION}
+    ./configure --prefix=${MPICH_DIR} && make install
+
+
     # Set env variables so we can compile our application
-    export PATH=$MPICH_DIR/bin:$PATH
-    export LD_LIBRARY_PATH=$MPICH_DIR/lib:$LD_LIBRARY_PATH
-    export MANPATH=$MPICH_DIR/share/man:$MANPATH
-    
-    
+    export PATH=${MPICH_DIR}/bin:${PATH}
+    export LD_LIBRARY_PATH=${MPICH_DIR}/lib:${LD_LIBRARY_PATH}
+    export MANPATH=${MPICH_DIR}/share/man:${MANPATH}
+
+
     echo "Compiling the MPI application..."
     cd /opt && mpicc -o mpitest mpitest.c
 ```
@@ -195,13 +195,13 @@ This image may be run with
 singularity exec xeyes.sif xeyes.
 ```
 
-This works because all the magic is done by Singularity already like setting `$DISPLAY` to the outside
-display and mounting `$HOME` so `$HOME/.Xauthority` (X11 authentication cookie) is found. When you are
+This works because all the magic is done by Singularity already like setting `${DISPLAY}` to the outside
+display and mounting `${HOME}` so `${HOME}/.Xauthority` (X11 authentication cookie) is found. When you are
 using `--contain` or `--no-home` you have to set that cookie yourself or mount/copy it inside
-the container. Similar for `--cleanenv` you have to set `$DISPLAY`, e.g., via
+the container. Similar for `--cleanenv` you have to set `${DISPLAY}`, e.g., via
 
 ```console
-export SINGULARITY_DISPLAY=$DISPLAY
+export SINGULARITY_DISPLAY=${DISPLAY}
 ```
 
 When you run a container as root (via `sudo`) you may need to allow root for your local display