Bug #21635: Reading code from STDIN unexpectedly sets $_ in 3.4.6+

• Status: Closed
• Assignee: prism
• Target version: -
• ruby -v: ruby 3.4.6 (2025-09-16 revision dbd83256b1) +PRISM [arm64-darwin23]
• Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[sisyphus (Sisyphus P)]

It seems from Ruby 3.4.6+ if you pass in code that doesn't end with a newline to the ruby interpreter it sets $_:

# ruby -v
ruby 3.4.6 (2025-09-16 revision dbd83256b1) +PRISM [arm64-darwin23]
# printf 'p $_' | ruby
"p $_"

This only happens if the code passed in doesn't end with a newline and doesn't happen in ruby 3.4.5 or earlier:

# ruby -v
ruby 3.4.5 (2025-07-16 revision 20cda200d3) +PRISM [arm64-darwin23]
# printf 'p $_' | ruby
nil

I would expect that $_ is nil at the start of program execution.

NB. this does result in the world's shortest Ruby quine (printf 'print' | ruby)

[kddnewton (Kevin Newton)]

• Assignee set to prism

[kddnewton (Kevin Newton)]

This is happening because of prism's use of IO#gets on STDIN. We will fix this.

[kddnewton (Kevin Newton)]

• Status changed from Open to Closed

Applied in changeset git|d036dc0a794d87309f912e7585749c681c2438f5.

---

For prism parser, do not update $_ from STDIN

Fixes [Bug #21635]