parallel_parse.sh: Cosmetic changes

Make it a bit more readable.
Use non-abbreviated arguments when available.
Use lower-case for variable names, which is the de-facto for non-exported
variables.
Shift once instead of after each argument.
Make the script exit if it expands an unset variable or any call fails.

parallel_parse.sh

 #!/bin/bash
+set -eu
 usage() {
   echo "Usage:"
   echo "  find [...] -print0 | $0 \\"
@@ -22,16 +23,26 @@ if [ "$1" = "-h" -o "$1" = "--help" ]; then
   exit 0
 fi
 
-PARALLELISM="$1"
-shift
-STATE_DIR="$1"
-mkdir -p "$STATE_DIR" || exit $?
-shift
-OUT_FILE="$1"
-shift
+parallelism="$1"
+state_dir="$2"
+out_file="$3"
+shift 3
+mkdir -p "$state_dir"
 
 # XXX: any simpler way ?
-xargs -0 -r -n 1 -P "$PARALLELISM" -I "@FILE@" -- "$SHELL" -c 'INFILE="$1";shift;STATE_DIR="$1";shift;echo -n .;exec "$@" -Q --format json --out "$STATE_DIR/$(sed s:/:@:g <<< "$INFILE").json" "$INFILE"' "$0" "@FILE@" "$STATE_DIR" "$@"
+xargs \
+  -0 \
+  --no-run-if-empty \
+  --max-args=1 \
+  --max-procs="$parallelism" \
+  -I "@FILE@" \
+  -- \
+  "$SHELL" \
+    -c 'infile="$1";stte_dir="$2";shift 2;echo -n .;exec "$@" -Q --format json --out "$state_dir/$(sed s:/:@:g <<< "$infile").json" "$infile"' \
+    "$0" \
+    "@FILE@" \
+    "$state_dir" \
+    "$@"
 echo
 # XXX: what if there are too many state files for a single execution ?
-find "$STATE_DIR" -type f -print0 | xargs -0 -r "$@" --out "$OUT_FILE" --state-file
+find "$state_dir" -type f -print0 | xargs -0 --exit --no-run-if-empty "$@" --out "$out_file" --state-file