Fix issues reported by shellcheck (quantumlib#5910)

- exit on cd failure, SC2164
- needless double negative, SC2236
- fix useless use of cat, SC2002
- replace legacy backticks, SC2006
- clean unused variables, SC2034
- avoid globing/splitting on variable expansion, SC2086
- expand variables at trap execution, SC2064
- remove redundant loop over single value, SC2043
- export variable separately from value assignment, SC2155
- ${name} is not needed in arithmetic expressions, SC2004
- do not assign array to string, SC2124
- disable shellcheck for zsh block
- add shebang to `dev_tools/pypath` to identify it as a bash script

No change in code logic.
The updated code passes the following test

```
$ git ls-files | xargs file | grep -i shell.script | cut -d: -f1 |
      xargs shellcheck
```

check/all

@@ -31,8 +31,9 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 # Parse arguments.
 apply_arg=""
@@ -44,7 +45,7 @@ for arg in "$@"; do
     elif [[ "${arg}" == "--apply-format-changes" ]]; then
         apply_arg="--apply"
     elif [ -z "${rev}" ]; then
-        if [ "$(git cat-file -t ${arg} 2> /dev/null)" != "commit" ]; then
+        if [ "$(git cat-file -t "${arg}" 2> /dev/null)" != "commit" ]; then
             echo -e "\033[31mNo revision '${arg}'.\033[0m" >&2
             exit 1
         fi

check/asv_run

@@ -8,7 +8,8 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 asv run "$@"

check/build-changed-protos

@@ -26,12 +26,13 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 # Figure out which revision to compare against.
-if [ ! -z "$1" ] && [[ $1 != -* ]]; then
-    if [ "$(git cat-file -t $1 2> /dev/null)" != "commit" ]; then
+if [ -n "$1" ] && [[ $1 != -* ]]; then
+    if [ "$(git cat-file -t "$1" 2> /dev/null)" != "commit" ]; then
         echo -e "\033[31mNo revision '$1'.\033[0m" >&2
         exit 1
     fi
@@ -63,7 +64,7 @@ dev_tools/build-protos.sh
 # but the error logic will still work.
 uncommitted=$(git status --porcelain 2>/dev/null | grep -E "^?? cirq-google" | cut -d " " -f 3)
 
-if [[ ! -z "$uncommitted" ]]; then
+if [[ -n "$uncommitted" ]]; then
     echo -e "\033[31mERROR: Uncommitted generated files found! Please generate and commit these files using dev_tools/build-protos.sh:\033[0m"
     for generated in $uncommitted
     do

check/doctest

@@ -14,8 +14,9 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 source dev_tools/pypath
 

check/format-incremental

@@ -32,8 +32,9 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 
 # Parse arguments.
@@ -46,7 +47,7 @@ for arg in "$@"; do
     elif [[ "${arg}" == "--all" ]]; then
         only_changed=0
     elif [ -z "${rev}" ]; then
-        if [ "$(git cat-file -t ${arg} 2> /dev/null)" != "commit" ]; then
+        if [ "$(git cat-file -t "${arg}" 2> /dev/null)" != "commit" ]; then
             echo -e "\033[31mNo revision '${arg}'.\033[0m" >&2
             exit 1
         fi
@@ -82,7 +83,7 @@ if (( only_changed == 1 )); then
 
     # Get the modified, added and moved python files.
     IFS=$'\n' read -r -d '' -a format_files < \
-        <(git diff --name-only --diff-filter=MAR ${rev} -- '*.py' ':(exclude)cirq-google/cirq_google/cloud/*' ':(exclude)*_pb2.py')
+        <(git diff --name-only --diff-filter=MAR "${rev}" -- '*.py' ':(exclude)cirq-google/cirq_google/cloud/*' ':(exclude)*_pb2.py')
 else
     echo -e "Formatting all python files." >&2
     IFS=$'\n' read -r -d '' -a format_files < \

check/misc

@@ -8,8 +8,9 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$( dirname "${BASH_SOURCE[0]}" )"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 # Check for non-contrib references to contrib.
 results=$(grep -Rl "\bcirq\.contrib\b" cirq-core | grep -v "cirq/contrib" | grep -v "__")

check/mypy

@@ -8,15 +8,17 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 CONFIG_FILE='mypy.ini'
 
-CIRQ_PACKAGES=$(env PYTHONPATH=. python dev_tools/modules.py list --mode package-path)
+read -r -a CIRQ_PACKAGES < \
+    <(env PYTHONPATH=. python dev_tools/modules.py list --mode package-path)
 
 echo -e -n "\033[31m"
-mypy --config-file=dev_tools/conf/$CONFIG_FILE "$@" $CIRQ_PACKAGES dev_tools examples
+mypy --config-file=dev_tools/conf/$CONFIG_FILE "$@" "${CIRQ_PACKAGES[@]}" dev_tools examples
 result=$?
 echo -e -n "\033[0m"
 

check/nbformat

@@ -25,8 +25,9 @@ for arg in "$@"; do
 done
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 pip show tensorflow-docs > /dev/null || exit 1
 
@@ -35,7 +36,7 @@ FORMAT_CMD="python3 -m tensorflow_docs.tools.nbfmt --indent=1"
 # Test the notebooks
 unformatted=$($FORMAT_CMD --test docs 2>&1 | grep "\- docs" || true)
 needed_changes=0
-if [ ! -z "${unformatted}" ]; then
+if [ -n "${unformatted}" ]; then
     needed_changes=1
     if (( only_print == 0 )); then
         $FORMAT_CMD docs

check/npm

@@ -23,7 +23,8 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 npm --prefix 'cirq-web/cirq_ts' "$@"

check/npx

@@ -22,8 +22,8 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
 
-cd 'cirq-web/cirq_ts'
+cd "${topdir}/cirq-web/cirq_ts" || exit $?
 npx "$@"

check/pylint

@@ -8,10 +8,12 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
-CIRQ_MODULES=$(env PYTHONPATH=. python dev_tools/modules.py list --mode package-path)
+read -r -a CIRQ_MODULES < \
+    <(env PYTHONPATH=. python dev_tools/modules.py list --mode package-path)
 
 # Add dev_tools to $PYTHONPATH so that pylint can find custom checkers
-env PYTHONPATH=dev_tools pylint --jobs=0 --rcfile=dev_tools/conf/.pylintrc "$@" $CIRQ_MODULES dev_tools examples
+env PYTHONPATH=dev_tools pylint --jobs=0 --rcfile=dev_tools/conf/.pylintrc "$@" "${CIRQ_MODULES[@]}" dev_tools examples

check/pylint-changed-files

@@ -25,12 +25,13 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 # Figure out which revision to compare against.
-if [ ! -z "$1" ] && [[ $1 != -* ]]; then
-    if [ "$(git cat-file -t $1 2> /dev/null)" != "commit" ]; then
+if [ -n "$1" ] && [[ $1 != -* ]]; then
+    if [ "$(git cat-file -t "$1" 2> /dev/null)" != "commit" ]; then
         echo -e "\033[31mNo revision '$1'.\033[0m" >&2
         exit 1
     fi
@@ -55,7 +56,7 @@ fi
 
 typeset -a changed
 IFS=$'\n' read -r -d '' -a changed < \
-    <(git diff --name-only ${rev} -- '*.py' ':(exclude)cirq-google/cirq_google/cloud/*' ':(exclude)*_pb2.py' \
+    <(git diff --name-only "${rev}" -- '*.py' ':(exclude)cirq-google/cirq_google/cloud/*' ':(exclude)*_pb2.py' \
     | grep -E "^(cirq|dev_tools|examples).*.py$"
 )
 

check/pytest

@@ -12,8 +12,9 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 # Run in parallel by default.  Pass the `-n0` option for a single-process run.
 # (the last `-n` option wins)

check/pytest-and-incremental-coverage

@@ -79,7 +79,7 @@ pytest_result=$?
 
 # assume successful cover_result in case coverage is not run
 cover_result=0
-if (( $ANALYZE_COV )); then
+if (( ANALYZE_COV )); then
     # Convert to .py,cover files.
     coverage annotate
 
@@ -92,7 +92,7 @@ if (( $ANALYZE_COV )); then
 fi
 
 # Report result.
-if (( ${pytest_result} || ${cover_result} )); then
+if (( pytest_result || cover_result )); then
     exit 1
 fi
 exit 0

check/pytest-changed-files

@@ -28,18 +28,19 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 # Figure out which branch to compare against.
-rest=$@
-if [ ! -z "$1" ] && [[ $1 != -* ]]; then
-    if [ "$(git cat-file -t $1 2> /dev/null)" != "commit" ]; then
+rest=( "$@" )
+if [ -n "$1" ] && [[ $1 != -* ]]; then
+    if [ "$(git cat-file -t "$1" 2> /dev/null)" != "commit" ]; then
         echo -e "\033[31mNo revision '$1'.\033[0m" >&2
         exit 1
     fi
     rev=$1
-    rest=${@:2}
+    rest=( "${@:2}" )
 elif [ "$(git cat-file -t upstream/master 2> /dev/null)" == "commit" ]; then
     rev=upstream/master
 elif [ "$(git cat-file -t origin/master 2> /dev/null)" == "commit" ]; then
@@ -77,4 +78,4 @@ fi
 
 source dev_tools/pypath
 
-pytest ${rest} "${changed[@]}"
+pytest "${rest[@]}" "${changed[@]}"

check/pytest-changed-files-and-incremental-coverage

@@ -33,8 +33,8 @@ cd "$( dirname "${BASH_SOURCE[0]}" )" || exit 1
 cd "$(git rev-parse --show-toplevel)" || exit 1
 
 # Figure out which revision to compare against.
-if [ ! -z "$1" ] && [[ $1 != -* ]]; then
-    if [ "$(git cat-file -t $1 2> /dev/null)" != "commit" ]; then
+if [ -n "$1" ] && [[ $1 != -* ]]; then
+    if [ "$(git cat-file -t "$1" 2> /dev/null)" != "commit" ]; then
         echo -e "\033[31mNo revision '$1'.\033[0m" >&2
         exit 1
     fi

check/ts-build

@@ -22,7 +22,8 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 check/npx webpack --mode production "$@"

check/ts-build-current

@@ -26,7 +26,7 @@ check/ts-build
 # Find the changed bundle.js files, if any
 untracked=$(git status --porcelain 2>/dev/null | grep "cirq-web/cirq_ts/dist/" | cut -d " " -f 3)
 
-if [[ ! -z "$untracked" ]]; then
+if [[ -n "$untracked" ]]; then
     echo -e "\033[31mERROR: Uncommitted changes to bundle file(s) found! Please commit these files:\033[0m"
     for generated in $untracked
     do
@@ -35,4 +35,4 @@ if [[ ! -z "$untracked" ]]; then
     exit 1
 fi
 
-exit 0
+exit 0

check/ts-coverage

@@ -22,7 +22,8 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 check/npm run coverage "$@"

check/ts-lint

@@ -22,7 +22,8 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 check/npm run lint "$@"

check/ts-lint-and-format

@@ -22,7 +22,8 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 check/npm run fix "$@"

check/ts-test

@@ -24,7 +24,8 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 check/npm run test "$@"

check/ts-test-e2e

@@ -24,7 +24,8 @@
 ################################################################################
 
 # Get the working directory to the repo root.
-cd "$(dirname "${BASH_SOURCE[0]}")"
-cd "$(git rev-parse --show-toplevel)"
+thisdir="$(dirname "${BASH_SOURCE[0]}")" || exit $?
+topdir="$(git -C "${thisdir}" rev-parse --show-toplevel)" || exit $?
+cd "${topdir}" || exit $?
 
 check/npm run test-e2e "$@"

dev_tools/conf/pip-install-minimal-for-pytest-changed-files.sh

@@ -20,10 +20,10 @@ set -e
 cd "$( dirname "${BASH_SOURCE[0]}" )"
 cd "$(git rev-parse --show-toplevel)"
 
-REQS="-r dev_tools/requirements/pytest-minimal.env.txt"
+reqs=( -r dev_tools/requirements/pytest-minimal.env.txt )
 
 # Install contrib requirements only if needed.
 changed=$(git diff --name-only origin/master | grep "cirq/contrib" || true)
-[ "${changed}" = "" ] || REQS="$REQS -r cirq-core/cirq/contrib/requirements.txt"
+[ "${changed}" = "" ] || reqs+=( -r cirq-core/cirq/contrib/requirements.txt )
 
-pip install $REQS
+pip install "${reqs[@]}"