Add check/shellcheck (#1144)

This adds `check/shellcheck` for checking shell scripts. It's adapted
from Cirq's check/shellcheck.

Author: mhucka

check/shellcheck

@@ -0,0 +1,80 @@
+#!/usr/bin/env bash
+
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Summary: execute shellcheck for all shell scripts in this repository
+#
+# Usage:
+#     check/shellcheck [--dry-run] [shellcheck-arguments]
+#
+# Use the '--dry-run' option to print out the shellcheck command line without
+# running it.  This displays all shell script files found in the repository.
+
+set -uo pipefail
+
+# Change working directory to the repository root.
+thisdir=$(dirname "${BASH_SOURCE[0]:?}")
+repo_dir=$(git -C "${thisdir}" rev-parse --show-toplevel)
+cd "${repo_dir}" || exit $?
+
+# Process command line arguments
+opt_dry_run=0
+declare -a shellcheck_options
+declare -a our_shell_scripts
+
+for arg in "$@"; do
+    if [[ "${arg}" == "--dry-run" ]]; then
+        opt_dry_run=1
+    else
+        shellcheck_options+=( "${arg}" )
+    fi
+done
+
+# Find all shell scripts in this repository.
+IFS=$'\n' read -r -d '' -a our_shell_scripts <<< "$(
+    git ls-files -z -- \
+        ':(exclude)*.'{hdf5,ipynb,json,md,py,rst,toml,ts,txt,yaml} | \
+    xargs -0 file | grep -i 'shell script' | cut -d: -f1
+)"
+
+# Verify our_shell_scripts array - require it must contain files below.
+declare -a required_shell_scripts
+required_shell_scripts=(
+    # items below must be sorted
+    check/format-incremental
+    check/mypy
+    check/pylint
+    check/pytest
+)
+
+scripts_not_found=$(comm -13 \
+    <(printf "%s\n" "${our_shell_scripts[@]}") \
+    <(printf "%s\n" "${required_shell_scripts[@]}") )
+
+if [[ -n "${scripts_not_found}" ]]; then
+    echo "Identification of shell scripts failed - files not found:" >&2
+    printf "\n%s\n\n" "${scripts_not_found}" >&2
+    echo "Please fix $0." >&2
+    exit 2
+fi
+
+# Ready to run here.
+if (( opt_dry_run )); then
+    printf '%s ' '>>' 'shellcheck' "${shellcheck_options[@]}"
+    printf '\\\n  %s ' "${our_shell_scripts[@]}"
+    printf '\\\n;\n'
+else
+    shellcheck "${shellcheck_options[@]}" "${our_shell_scripts[@]}"
+fi