head: missing support for implicit -n (e.g. `head -5`)

[AlexandreYang]

Bug

head does not support the shorthand form head -5 (passing a number directly without -n), while tail handles it correctly.

Expected behavior (bash-compatible):

$ echo -e "1\n2\n3\n4\n5\n6" | head -5
1
2
3
4
5

Actual behavior:
head -5 likely returns an "unknown flag" or similar error.

Source

Reported internally via Slack (2026-04-02).

Fix

When parsing flags in the head builtin, handle the case where a bare numeric argument (e.g. -5, -42) is passed — treating it as equivalent to -n 5 / -n 42. This matches POSIX/bash/GNU behaviour.

tail already implements this correctly and can serve as a reference.