Probe grep capability instead of matching for GNU grep

lib/helpers.sh

@@ -183,23 +183,37 @@ function error_and_proceed() {
 };
 export -f error_and_proceed;
 
-function check_dependencies() {
-  if [[ "$(uname)" == 'Darwin' ]]; then
-    # Prefer ggrep (typically from Homebrew) if available
-    if command -v ggrep >/dev/null 2>&1; then
-      shopt -s expand_aliases;
-      alias grep=ggrep;
-      return;
-    fi;
+# Probes the two grep behaviors tfenv depends on beyond POSIX:
+#   `\+` repetition in a BRE, per the 'latest' regex in lib/tfenv-version-name.sh
+#   `-o` reporting every match on a line, per libexec/tfenv-list-remote
+function grep_is_sufficient() {
+  local grep_bin="${1}";
+  local bre_result;
+  local multi_match_result;
 
-    # Fall back to checking if the system grep is GNU grep
-    # (e.g. installed via nix or manually)
-    if grep --version 2>&1 | grep -q 'GNU grep'; then
-      return;
-    fi;
+  bre_result="$("${grep_bin}" -e '^[0-9]\+$' <<< '42' 2>/dev/null)";
+  [ "${bre_result}" = '42' ] || return 1;
+
+  multi_match_result="$("${grep_bin}" -o -E '[0-9]+' <<< '1 2' 2>/dev/null | tr '\n' ' ')";
+  [ "${multi_match_result}" = '1 2 ' ] || return 1;
+
+  return 0;
+};
+export -f grep_is_sufficient;
 
-    log 'error' 'GNU Grep is a requirement and your Mac does not have it. Consider "brew install grep" or "nix profile install nixpkgs#gnugrep"';
+function check_dependencies() {
+  # ggrep, typically from Homebrew, takes precedence when it is capable
+  if command -v ggrep >/dev/null 2>&1 && grep_is_sufficient ggrep; then
+    shopt -s expand_aliases;
+    alias grep=ggrep;
+    return;
+  fi;
+
+  if grep_is_sufficient grep; then
+    return;
   fi;
+
+  log 'error' 'The grep on PATH does not support `\+` in basic expressions or `-o` multi-match, both of which tfenv requires. Consider "brew install grep" or "nix profile install nixpkgs#gnugrep"';
 };
 export -f check_dependencies;
 

test/test_dependencies.sh

@@ -0,0 +1,57 @@
+#!/usr/bin/env bash
+
+# Source common test setup
+source "$(dirname "${0}")/test_common.sh";
+
+#####################
+# Begin Script Body #
+#####################
+
+declare -a errors=();
+
+log 'info' '### Test Suite: dependencies';
+
+log 'info' '## grep_is_sufficient: accepts the grep already on PATH';
+(
+  grep_is_sufficient grep || exit 1;
+  exit 0;
+) && log 'info' '## grep_is_sufficient: system grep accepted' \
+  || error_and_proceed 'system grep rejected; tfenv needs only `\+` in a BRE and `-o` multi-match';
+
+log 'info' '## grep_is_sufficient: rejects a grep whose -o returns only the first match';
+(
+  # Satisfies the BRE probe but truncates -o output, so only the multi-match check rejects it
+  declare stub_dir;
+  stub_dir="$(mktemp -d)" || exit 1;
+  trap 'rm -rf "${stub_dir}"' EXIT;
+  cat > "${stub_dir}/grep" <<'STUB';
+#!/usr/bin/env bash
+if [ "${1}" = '-o' ]; then
+  /usr/bin/grep "${@}" | head -n 1;
+else
+  /usr/bin/grep "${@}";
+fi;
+STUB
+  chmod +x "${stub_dir}/grep" || exit 1;
+  grep_is_sufficient "${stub_dir}/grep" && exit 1;
+  exit 0;
+) && log 'info' '## grep_is_sufficient: truncating grep rejected' \
+  || error_and_proceed 'grep_is_sufficient accepted a grep whose -o returns only the first match';
+
+log 'info' '## grep_is_sufficient: rejects a grep that cannot be run';
+(
+  grep_is_sufficient /nonexistent/grep && exit 1;
+  exit 0;
+) && log 'info' '## grep_is_sufficient: missing binary rejected' \
+  || error_and_proceed 'grep_is_sufficient accepted a grep binary that does not exist';
+
+log 'info' '## check_dependencies: succeeds with a capable grep on PATH';
+(
+  check_dependencies || exit 1;
+  exit 0;
+) && log 'info' '## check_dependencies: passed' \
+  || error_and_proceed 'check_dependencies failed despite a capable grep being on PATH';
+
+finish_tests 'dependencies';
+
+exit 0;