python/private/autodetecting_toolchain_interpreter.sh

#!/bin/sh

# Don't set -e because we don't have robust trapping and printing of errors.
set -u

# We use /bin/sh rather than /bin/bash for portability. See discussion here:
# https://groups.google.com/forum/?nomobile=true#!topic/bazel-dev/4Ql_7eDcLC0
# We do lose the ability to set -o pipefail.

FAILURE_HEADER="\
Error occurred while attempting to use the default Python toolchain \
(@rules_python//python:autodetecting_toolchain)."

die() {
  echo "$FAILURE_HEADER" 1>&2
  echo "$1" 1>&2
  exit 1
}

# We use `which` to locate the Python interpreter command on PATH. `command -v`
# is another option, but it doesn't check whether the file it finds has the
# executable bit.
#
# A tricky situation happens when this wrapper is invoked as part of running a
# tool, e.g. passing a py_binary target to `ctx.actions.run()`. Bazel will unset
# the PATH variable. Then the shell will see there's no PATH and initialize its
# own, sometimes without exporting it. This causes `which` to fail and this
# script to think there's no Python interpreter installed. To avoid this we
# explicitly pass PATH to each `which` invocation. We can't just export PATH
# because that would modify the environment seen by the final user Python
# program.
#
# See also:
#
#     https://github.com/bazelbuild/continuous-integration/issues/578
#     https://github.com/bazelbuild/bazel/issues/8414
#     https://github.com/bazelbuild/bazel/issues/8415

# Try the "python3" command name first, then fall back on "python".
PYTHON_BIN="$(PATH="$PATH" which python3 2> /dev/null)"
if [ -z "${PYTHON_BIN:-}" ]; then
  PYTHON_BIN="$(PATH="$PATH" which python 2>/dev/null)"
fi
if [ -z "${PYTHON_BIN:-}" ]; then
  die "Neither 'python3' nor 'python' were found on the target \
platform's PATH, which is:

$PATH

Please ensure an interpreter is available on this platform (and marked \
executable), or else register an appropriate Python toolchain as per the \
documentation for py_runtime_pair \
(https://github.com/bazelbuild/rules_python/blob/master/docs/python.md#py_runtime_pair)."
fi

exec "$PYTHON_BIN" "$@"