Add ksh support for OpenBSD (and mksh/pdksh)

[Copilot]

wp shell fails immediately on OpenBSD because the REPL's prompt command uses bash-specific builtins (read -e, read -p, history -r/-s/-w) that ksh does not support.

Changes

• is_ksh_shell() — detects ksh-compatible shells by binary basename (ksh, ksh93, ksh88, mksh, pdksh)
• is_supported_shell() — helper combining bash + ksh checks; used by the $SHELL env-var fallback so OpenBSD users with SHELL=/bin/ksh no longer need to set WP_CLI_CUSTOM_SHELL
• ksh command path in create_prompt_cmd() — when a ksh shell is detected, replaces bash-specific flags with POSIX equivalents; history is skipped (unavailable in non-interactive ksh mode):

# bash path (unchanged)
read -re -p 'wp> ' LINE; history -s -- "$LINE"; history -w /path/to/history; ...

# new ksh path
printf %s 'wp> ' >&2; IFS= read -r LINE || exit; printf '%s\n' "$LINE";

The prompt is written to stderr so it remains visible to the user without being captured by popen.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Feature request: support ksh under OpenBSD</issue_title>
<issue_description>## Feature Request

Add support for default shell on OpenBSD (ksh).

Describe your use case and the problem you are facing

Trying to use wp shell cmd under OpenBSD results in errors and exits prematurely.

$ wp shell
/bin/ksh: fc: history functions not available
/bin/ksh: read: -e: unknown option
$ echo $?
0

$ wp cli info
OS:     OpenBSD 6.6 GENERIC.MP#584 amd64
Shell:  /bin/ksh
PHP binary:     /usr/local/bin/php-7.3
PHP version:    7.3.13
php.ini used:   /etc/php-7.3.ini
WP-CLI root dir:        phar://wp-cli.phar/vendor/wp-cli/wp-cli
WP-CLI vendor dir:      phar://wp-cli.phar/vendor
WP_CLI phar path:       /var/www/htdocs
WP-CLI packages dir:
WP-CLI global config:
WP-CLI project config:
WP-CLI version: 2.4.0
$ env | grep WP_CLI_CUSTOM_SHELL
WP_CLI_CUSTOM_SHELL=/bin/ksh

Describe the solution you'd like

Quick win:

Add some more detection at shell instantiation to detect shell and platform to enable running in ksh on OpenBSD.

Nice to have:

Test in a few different shell/OS environments and modify to run across more - else, fail gracefully when unsupported.

Happy to work on this</issue_description>

Comments on the Issue (you are @copilot in this section)

@danielbachhuber Thanks for the report, @leonstafford !

It looks like you're actually passing WP_CLI_CUSTOM_SHELL=/bin/ksh correctly. The problem is that this series of commands are failing:

shell-command/src/WP_CLI/Shell/REPL.php

Lines 130 to 137 in 3417708

	$cmd = 'set -f; '
	. "history -r {$history_path}; "
	. 'LINE=""; '
	. "read -re -p {$prompt} LINE; "
	. '[ $? -eq 0 ] || exit; '
	. 'history -s "$LINE"; '
	. "history -w {$history_path}; "
	. 'echo $LINE; ';

Do you know what it would take to rewrite this for compatibility with /bin/ksh?

• Fixes Feature request: support ksh under OpenBSD #43

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[codecov]

Codecov Report

❌ Patch coverage is 64.00000% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/WP_CLI/Shell/REPL.php	64.00%	9 Missing ⚠️

📢 Thoughts on this report? Let us know!

[Copilot]

Pull request overview

Adds ksh compatibility to the built-in wp shell --basic REPL so it can run on OpenBSD (and similar environments) where /bin/ksh is the default and bash-specific read/history builtins are unavailable.

Changes:

• Add ksh-family shell detection (is_ksh_shell()) and a combined is_supported_shell() helper.
• Expand $SHELL fallback to accept supported shells (bash or ksh-family) instead of bash-only.
• Introduce a ksh-specific prompt command path using POSIX read and printing the prompt to stderr (skipping history).

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