Probe grep capability instead of matching for GNU grep

[arubdesu]

Description

tfenv install currently cannot run on a macOS machine that has neither Homebrew nor nix/etcetera-installed grep, because check_dependencies in lib/helpers.sh tests for GNU grep by matching grep --version against the literal string GNU grep, so on Darwin the check fails and the install aborts. Stock macOS answer:

$ /usr/bin/grep --version
grep (BSD grep, GNU compatible) 2.6.0-FreeBSD

This blocking/error state is only reachable on a 'manual' or git install - test/install_deps.sh runs brew install grep on Darwin, so CI always has ggrep and would not exercise the failing path. That is also why I hit it and most users won't: I avoid brew and installed tfenv by cloning the repo. (I understand a golang rewrite is underway, which is awesome/exciting!)

Workaround

I wanted to spell out that I may very well be a relative corner case installation-wise, and before submitting this I did just workaround it: symlinking the system grep as ggrep on my shell's PATH satisfies the existing check, because helpers.sh accepts ggrep on presence alone. I don't want to influence the use of the core maintainer teams time, your judgement call on accepting this vs. having me use this trivial stopgap would be totally understood either way.

I opened this anyway for two reasons. The workaround asks every person who hits this to first work out why an install failed, and BSD grep already does what tfenv needs, so the requirement looked removable rather than something to route around, and in general lowers cognitive load for users. Worth noting the symlink keeps working after this change: it now passes on merit via the probe rather than on its name, so if anyone was like me but not mentioning it, nobody's existing workaround breaks.

Also probably a good time to mention I wrote this with the help of a clanker/coding agent, but every line of code of this description and the PR itself was reviewed by human me before submitting. Pardon I submitted as a draft since this is the first time you're seeing code submitted by me, I was a bit timid about targeting master right off the bat 😅

Fix overview

This PR replaces the vendor-string test with a probe of the two grep behaviors tfenv actually relies on beyond POSIX:

• \+ as a repetition operator in a basic expression, used by the latest regex built in lib/tfenv-version-name.sh
• -o reporting every match on a line rather than only the first, used to extract versions in libexec/tfenv-list-remote

All other grep invocations I found in lib/ and libexec/ use POSIX-guaranteed options (-e -v -q -F -E -Hn -h --). BSD grep implements both of the outliers above, which is what its "GNU compatible" claim refers to, so the "GNU grep" check rejects a working version.

I want to be clear this is not proposing a return to installing ggrep automatically. The 3.0.0 note on that ("it's just not good practice") remains right to me. The argument here is narrower: the prerequisite itself appears unnecessary, because the grep already present can do the job.

Consistency with existing behavior

libexec/tfenv-install already handles the same BSD/GNU split for a different tool by probing capability rather than identity in the case of mktemp:

tmpdir_arg="-t";
if mktemp --help 2>&1 | grep -- '--tmpdir' >/dev/null; then
  tmpdir_arg="--tmpdir";
fi;

That difference is adapted to, whereas the grep difference was fatal.

Two secondary fixes in the same function

• The ggrep branch previously returned on command -v ggrep alone. It is now gated by the same probe, so the check is stricter on that path than it was.
• The check was wrapped in if [[ "$(uname)" == 'Darwin' ]]. A minimal POSIX grep on Linux breaks the same \+ regex and was never checked. The probe is cheap, so it now runs everywhere. I am happy to restore the Darwin gate if you would rather keep this PR's scope smaller.

CHANGELOG

The clanker proposed I actually submit an entry for an 'UNRELEASED' version to CHANGELOG.md with the following text, which I'd not really seen before, I'll just include it on the off chance you BOTH accept and like it enough to just paste it in 😅:

* FIX: `check_dependencies` probes grep for the `\+` BRE repetition and `-o` multi-match
  behaviors tfenv uses, instead of matching `grep --version` for "GNU grep" (which stock
  macOS never reports), blocking `tfenv install` on a Mac without Homebrew or nix. The
  probe also gates the `ggrep` branch, and runs on all platforms rather than only Darwin.

Issue Link

When it comes to this issue itself, I didn't find an exact existing match, happy to open one independently if that's preferable or helpful.
For related issues, #442 removed the brew assumption but kept the GNU requirement. #488 cites "macOS needs ggrep" as motivation for the Go rewrite.

Testing

• Ran ./test/run.sh locally and all tests pass
• Tested on at least one platform (note which below)
• New tests added for new functionality (if applicable)

Platform tested: macOS 27.0 beta (build 26A5416b), arm64, stock /usr/bin/grep 2.6.0-FreeBSD

Full suite: 16 suites, 0 failures. I ran it with ggrep removed from PATH, so check_dependencies fell through to the system grep on every invocation, which is the exact condition that aborts on unpatched tfenv. On CI, where install_deps.sh puts ggrep on Darwin runners, the ggrep branch is what gets exercised instead; both paths are now probed identically.

The run referenced above was functional rather than mocked: it downloaded real Terraform releases from releases.hashicorp.com, verified SHA256 hashes, and extracted them, so the install, verification and extraction paths were all exercised end to end. It also incidentally confirms the mktemp --tmpdir probe cited above behaves correctly on this platform.

test/test_dependencies.sh is new and covers four cases:

• the system grep passes the probe
• a stub grep whose -o returns only the first match is rejected, so the probe is shown to have teeth rather than passing everything
• a nonexistent binary is rejected
• check_dependencies succeeds end to end

Behavior before and after on this machine:

/usr/bin/grep → grep (BSD grep, GNU compatible) 2.6.0-FreeBSD
   version-string check: FAIL
   capability probe:     PASS

Nothing here is specific to macOS 27: the grep version string is unchanged from earlier releases, so the bug and the fix are not version-dependent. That is simply where it was verified.

Quality Checklist

• Shell quoting verified — all variables quoted, braces used
• Cross-platform considered — BSD vs GNU tool differences
• No unintended changes to other commands
• README.md updated (if user-facing change)
• CHANGELOG.md updated (if notable change)

The GNU grep requirement was never documented in the README, only in the error message, which this updates. The CHANGELOG is left to you, as per the note above.

---

The claims about which grep options tfenv uses, the BSD grep behavior, and the before/after results above are all things I verified on my own machine rather than taking on faith.

Happy to adjust the target branch, approach/scope/wording, and/or split the Darwin-gate removal out if that makes review easier. Thank you for your consideration!