cipd/python: optimize cat usage

Bash has a builtin for reading files, so use that instead of running
the cat program to save on time & overhead.

Change-Id: Ia90fc22a47f2ead4ef056020a79ebdebd7bd70ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4111082
Reviewed-by: Brian Ryner <bryner@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>

cipd

@@ -90,13 +90,13 @@ CIPD_BACKEND="https://chrome-infra-packages.appspot.com"
 VERSION_FILE="${MYPATH}/cipd_client_version"
 
 CLIENT="${MYPATH}/.cipd_client"
-VERSION=`cat "${VERSION_FILE}"`
+VERSION=$(<"${VERSION_FILE}")
 PLATFORM="${OS}-${ARCH}"
 
 # A value in .cipd_client_platform overrides the "guessed" platform.
 PLATFORM_OVERRIDE_FILE="${MYPATH}/.cipd_client_platform"
 if [ -f "${PLATFORM_OVERRIDE_FILE}" ]; then
-  PLATFORM=`cat ${PLATFORM_OVERRIDE_FILE}`
+  PLATFORM=$(<"${PLATFORM_OVERRIDE_FILE}")
 fi
 
 URL="${CIPD_BACKEND}/client?platform=${PLATFORM}&version=${VERSION}"
@@ -232,7 +232,7 @@ function self_update() {
 # so mismatches are very unlikely.
 INSTALLED_VERSION_FILE="${MYPATH}/.versions/.cipd_client.cipd_version"
 if [ -f "${INSTALLED_VERSION_FILE}" ]; then
-  JSON_BODY=`cat "${INSTALLED_VERSION_FILE}"`
+  JSON_BODY=$(<"${INSTALLED_VERSION_FILE}")
   if [[ "$JSON_BODY" != *"infra/tools/cipd/${PLATFORM}"* ]]; then
     >&2 echo "Detected CIPD client platform change to ${PLATFORM}."
     >&2 echo "Deleting the existing client to trigger the bootstrap..."

python-bin/python3

@@ -6,7 +6,7 @@ if [ "$OSTYPE" = "msys" ]
 then
   PYTHON3_BIN_DIR="$DEPOT_TOOLS/$(sed -e 's-\\-/-g' $DEPOT_TOOLS/python3_bin_reldir.txt)"
 else
-  PYTHON3_BIN_DIR="$DEPOT_TOOLS/$(cat $DEPOT_TOOLS/python3_bin_reldir.txt)"
+  PYTHON3_BIN_DIR="$DEPOT_TOOLS/$(<"$DEPOT_TOOLS/python3_bin_reldir.txt")"
 fi
 PATH="$PYTHON3_BIN_DIR":"$PYTHON3_BIN_DIR/Scripts":"$PATH"
 "$PYTHON3_BIN_DIR/python3" "$@"

python2-bin/python2

@@ -8,7 +8,7 @@ then
   # this converts them to the forward slashes a unix environment expects.
   PYTHON_BIN_DIR="$DEPOT_TOOLS/$(sed -e 's-\\-/-g' $DEPOT_TOOLS/python_bin_reldir.txt)"
 else
-  PYTHON_BIN_DIR="$DEPOT_TOOLS/$(cat $DEPOT_TOOLS/python_bin_reldir.txt)"
+  PYTHON_BIN_DIR="$DEPOT_TOOLS/$(<"$DEPOT_TOOLS/python_bin_reldir.txt")"
 fi
 PATH="$PYTHON_BIN_DIR":"$PATH"
 "$PYTHON_BIN_DIR/python" "$@"