0000698: shell builtins should not be required to be exec'able

ID	Project	Category	View Status	Date Submitted	Last Update

0000698	1003.1(2013)/Issue7+TC1	Shell and Utilities	public	2013-05-16 22:22	2013-05-23 16:37

Reporter	eblake	Assigned To
Priority	normal	Severity	Objection	Type	Error
Status	Closed	Resolution	Rejected

Name	Eric Blake
Organization	Red Hat
User Reference	ebb.builtins
Section	XCU 1.6
Page Number	2318
Line Number	73547
Interp Status	---
Final Accepted Text


Summary	0000698: shell builtins should not be required to be exec'able
Description	The standard describes a list of utilities that are not special built-ins, but are typically provided as shell built-ins because they sometimes or always need particular access to the current shell's environment in a manner that an exec'able alternative would fail to have. Because these particular utilities are already provided by the shell, and required to be recognized even without a PATH search, some implementations have proceeded to provide exec'able alternatives by doing: $ printf '#!/bin/sh\ncommand "$(basename -- "$0")" "$@"\n' > /usr/bin/alias $ chmod +x /usr/bin/alias $ for util in bg cd command false fd fg getopts jobs kill newgrp pwd read true umask unalias wait; do ln /usr/bin/alias /usr/bin/$util done However, as discussed in this bug report: https://bugzilla.redhat.com/show_bug.cgi?id=698670 there was a question on whether it even makes sense to mandate some of the shell builtins to be exec-able. Some of these 17 built-ins are often provided as an exec'able variant (true and false have no dependency on the current environment ; at least bash requires newgrp to be exec'able because it does not provide a built-in variant; pwd is often exec'able because a suid program can gather information about otherwise-unsearchable directories; kill is often exec'able with the caveat that the exec'd version cannot honor %1 job specs the way a built-in can). However, others are rather pointless as an exec'able version (about the only use I could see for an exec'able cd is its side-effect of printing the location that a CDPATH lookup would find; an exec'able fg will always fail; and so forth). I propose that implementations still be allowed to provide exec'able alternatives of these 17 built-ins, as a quality of implementation, but that the requirement be relaxed to allow for implementations that do not want to install what would normally be pointless unused shim file around the built-ins.
Desired Action	At page 46 line 1442 [XCU 3.83 Built-In Utility], change Regular built-ins are not required to be actually built into the shell on the implementation, but they do have special command-search qualities. to Regular built-ins are not required to be actually built into the shell on the implementation, but some of them do have special command-search qualities. At line 1445 [XCU 3.83 Built-In Utility], change Regular Built-In Utilities are defined in detail in XCU Section 2.9.1.1 (on page 2339). to Regular Built-In Utilities are defined in detail in XCU Section 1.6 (on page 2318). At page 2318 line 73552 [XBD 1.6 Built-In Utilities], change However, all of the standard utilities, including the regular built-ins in the table, but not the special built-ins described in Section 2.14 (on page 2356), shall be implemented in a manner so that they can be accessed via the exec family of functions as defined in the System Interfaces volume of POSIX.1-2008 and can be invoked directly by those standard utilities that require it (env, find, nice, nohup, time, xargs). to: All built-in standard utilities that are neither special built-ins described in Section 2.14 (on page 2356) nor those with special command search requirements described in section 2.9.1.1 (on page 2339) shall be implemented in a manner so that they can be accessed via the exec family of functions as defined in the System Interfaces volume of POSIX.1-2008 and can be invoked directly by those standard utilities that require it (env, find, nice, nohup, time, xargs). It is unspecified whether the regular built-in utilities with special command search requirements will be accessible via the exec family of functions; for the ones that are accessible, they may have limited behavior when invoked via exec.
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eblake 2013-05-16 22:25 manager bugnote:0001611	I've also noticed some inconsistencies in the various utilities in describing in non-normative text what happens when trying to use these built-ins in subshells or separate execution environments (for example, alias mentions things in RATIONALE, while bg mentions it in APPLICATION USAGE); I am considering auditing them and adding a note that cleans this up as part of this bug.

geoffclare 2013-05-17 09:35 manager bugnote:0001613	See XRAT C.1.7 for the original POSIX.2 developers' views on this issue.

Don Cragun 2013-05-23 16:36 manager bugnote:0001626	This was discussed during the May 23, 2013 conference call. XRAT section C.1.7 clearly states why the authors of this section of the standard decided to require these built-ins to be exec-able. We do not believe that the arguments presented in this change request override the concerns stated by the original authors. Therefore, we reject this proposed change.

Date Modified	Username	Field	Change
2013-05-16 22:22	eblake	New Issue
2013-05-16 22:22	eblake	Name	=> Eric Blake
2013-05-16 22:22	eblake	Organization	=> Red Hat
2013-05-16 22:22	eblake	User Reference	=> ebb.builtins
2013-05-16 22:22	eblake	Section	=> XCU 1.6
2013-05-16 22:22	eblake	Page Number	=> 2318
2013-05-16 22:22	eblake	Line Number	=> 73547
2013-05-16 22:22	eblake	Interp Status	=> ---
2013-05-16 22:25	eblake	Note Added: 0001611
2013-05-17 09:35	geoffclare	Note Added: 0001613
2013-05-23 16:36	Don Cragun	Note Added: 0001626
2013-05-23 16:37	Don Cragun	Status	New => Closed
2013-05-23 16:37	Don Cragun	Resolution	Open => Rejected

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