View Issue Details
| ID | Project | Category | View Status | Date Submitted | Last Update |
|---|---|---|---|---|---|
| 0001063 | 1003.1(2013)/Issue7+TC1 | Shell and Utilities | public | 2016-07-23 22:13 | 2024-06-11 08:56 |
| Reporter | izabera | Assigned To | |||
| Priority | normal | Severity | Editorial | Type | Clarification Requested |
| Status | Closed | Resolution | Accepted As Marked | ||
| Name | Isabella | ||||
| Organization | |||||
| User Reference | |||||
| Section | sh | ||||
| Page Number | 3189 | ||||
| Line Number | 106700-106701 | ||||
| Interp Status | --- | ||||
| Final Accepted Text | 0001063:0003881 | ||||
| Summary | 0001063: missing specification for sh -h | ||||
| Description | The synopsis in http://pubs.opengroup.org/onlinepubs/9699919799/utilities/sh.html specifies several options for sh, including -h. Then the options section in the same page says: > "The -a, -b, -C, -e, -f, -m, -n, -o option, -u, -v, and -x options are > described as part of the set utility in Special Built-In Utilities." Someone forgot to add -h in that list. | ||||
| Desired Action | Change this sentence > "The -a, -b, -C, -e, -f, -m, -n, -o option, -u, -v, and -x options are > described as part of the set utility in Special Built-In Utilities." to this > "The -a, -b, -C, -e, -f, -h, -m, -n, -o option, -u, -v, and -x options are > described as part of the set utility in Special Built-In Utilities." | ||||
| Tags | tc3-2008 | ||||
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Why not just mark -h as deprecated, and forget about this change? It is, after all, a more or less useless (and underspecified) option as it is now. Its definition is: Locate and remember utilities invoked by functions as those functions are defined but as best I can tell, as a function is being defined, no utilities are invoked, ever, so there is nothing to remember. Nor is it specified anywhere what is the point (or effect) of this memory if there was something to remember. When if this language were to be corrected to say what it probably means, which is, while parsing, to remember the location of utilities that will be invoked by a function when it is invoked sometime later, how is that supposed to be done in general, consider ... fn() { "$@" } what does -h cause to be remembed in that case? Then, if we can ever work out just what we should remember (if anything) we then need to figure out what to do with that... For normal command invocation, the file system location (that is, from the PATH search) of commands with no /'s in their names may be remembered, until PATH is changed, when they are all forgotten (this is really just an efficiency hack, and aside from perhaps allowing the shell to fail to notice a utility newly installed earlier in PATH than where the old one was previously found, should be invisible to the user.) If this is the intent of -h , then there's really no need, remembering the location of commands actually invoked when the function is invoked, is a better solution. So, that cannot be it, the only possible intent for this would be so that something like PATH=.:other:stuff fn() { cmd } where ./cmd exists, would turn "cmd" into the value of ${PWD}/cmd at the time of definition. I can see some utility in that, but it isn't reliable if any change to PATH would cause the memory to be deleted. Nor is it a good idea (IMO) to create something which is only practiclaly useful when "." is in PATH. If "cmd" was "./cmd" then remembering its location (replacing "." with ${PWD}) would be a most unusual operation. There are all kinds of other issues - such as the relationship between aliases and functions - most shells (I believe) interpret aliases during function definition, so that in... fn() { w true; d sleep 10; dd; } alias w=while alias d=do alias dd=done the aliases do not affect the function (which would run the 'w', 'd', amd 'dd' commands) but if the function is defined after the aliases, then the function instead contains a while loop. But there is nothing in the definition of aliases that requires this that I'm aware of, it would be OK for a shell to take the above and implement a while loop (expanding the aliases as the function is invoked.) In that case, the word which is just "true" here is a utility invoked by the function, so -h (if defined properly) should cause its location to be remembered. But when the function was defined, the shell could not possibly know that. The same for "sleep". To use the same example, if -h were in effect, would that prevent the aliases being expanded, as (on most systems) 'w' and 'dd' are utilities that a PATH search will locate (and we could use something else instead of 'd' and make it 3 out of 3...) If the function has 'dd' replaced by /usr/bin/dd then the 'dd' alias would never be seen. Let's say the function was instead fn() { wh true; d sleep 10; dd; } and the 'w' alias (defined later) was a 'wh" alias instead. Given normal system commands, the lookup for 'wh' and 'd' will fail, so there is nothing to remember in that case, so their aliases could be expanded later, but 'dd' is a utility that should be found, so now we have a function containing a while loop, with no "done" - but which was not detected as a syntax error during parsing? This actually makes another (unspecified) assumption of course, which is that when a function is defined (with -h set) that a utility that will (or might) later be invoked, which cannot be found (at the time of function definition) should simply be left as it is, and searched for later. But nothing anywhere says that is what should happen, perhaps when -h is set, utilities to be invoked by the function which cannot be found are intended to give a "command not found" error at function definition time, and the function definition should exit with status 127 ? All of this is simply a gigantic quagmire, and (apart from that set of shells that treat tne -h definition literally, and so make the option a no-op - which is what the NetBSD shell does, and always has (since it learned about -h at all), and always will, I expect) I really cannot believe that there are any two (unrelated) shells that implement -h the same way in all respects. That is, there is no existing standard -h option that can be defined, just a whole bunch of different ones. That makes it a prime example for doing to it the same as was done recently to "-o nolog", and retain the existence of the option but explicitly state that shells may ignore it, and perhaps even make the operation of the shell unspecified if the option is set. In that case, it would be better to remove -h from the synposis, than to add it to the later list. |
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It also seems that different shells implement -h with different meanings: bash: Remember the location of commands as they are looked up for execution. This is enabled by default. (Unclear if this is what POSIX suggests) zsh: Do not enter command lines into the history list if they are duplicates of the previous event. (Not what POSIX requires) ksh: Each command becomes a tracked alias when first encountered. (As POSIX) yash: When a function is defined when this option is enabled, the shell immediately performs command path search for each command that appears in the function and caches the command’s full path. (As POSIX) dash: Does not support -h at all mksh: If the trackall option is set (i.e. set -o trackall or set -h), the shell tracks all commands. This option is set automatically for non-interactive shells. (As POSIX?) |
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For bash and mksh, I'd say no, it's not what POSIX suggests; as the text doesn't mention setting or changing alias definitions that ksh version suspect also. I tend to agree with kre in Note 3306 that it should be unspecified behavior if aliases used in the definition or the PATH variable change before the function is invoked. I feel it should be clarified too that 1) aliases are expanded both at definition and execution time when tokenizing the compund_list elements of a definition's compound_command, in XCU 2.9.5 and 2) executing set +h does not affect definitions created with -h in effect; an unset of fname also required. With those I'd agree the yash behavior most conforming as to what is suggested. As an optimization that can speed up execution of functions called in a loop I'd prefer not to deprecate or remove it, however. |
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Maybe it makes sense to look at the Bourne Shell, where the feature has been taken from. The Bourne Shell tries to enter all command names into the hash table while parsing the command line. |
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Along with desired action, Change in set -h P2410, L77147 of 2016 edition: Locate and remember utilities invoked by functions as those functions are defined (the utilities are normally located when the function is executed). to: [OB] Setting this option may speed up PATH searches (see [xref to XBD Chapter 8]). This option may be enabled by default.[/OB] On page 2413 line 77273 of 2016 edition (set RATIONALE), change: The −h flag is related to command name hashing. See [xref to hash]. to: The −h option is related to command name hashing. See [xref to hash]. The normative description is deliberately vague because the way this option works varies between shell implementations. Earlier versions of this standard specified -h as a way to locate and remember utilities to be invoked by functions as those functions are defined (the utilities are normally located when the function is executed). However, this did not match existing practice in most shells. On page 2847, line 93798 of 2016 edition (last paragraph of hash DESCRIPTION), change: Utilities provided as built-ins to the shell shall not be reported by hash. to: Utilities provided as built-ins to the shell and functions shall not be reported by hash. On page 2848, line 93830-93834 of 2016 edition (hash STDOUT), add to the end of the paragraph: This list shall be cleared when the contents of the PATH environment variable are changed. On page 3226 line 108276-108280 of 2016 edition (sh SYNOPSIS) add OB shading to all six occurrences of the h option letter. |
| Date Modified | Username | Field | Change |
|---|---|---|---|
| 2016-07-23 22:13 | izabera | New Issue | |
| 2016-07-23 22:13 | izabera | Name | => Isabella |
| 2016-07-23 22:13 | izabera | Section | => (section number or name, can be interface name) |
| 2016-07-23 22:13 | izabera | Page Number | => (page or range of pages) |
| 2016-07-23 22:13 | izabera | Line Number | => (Line or range of lines) |
| 2016-07-23 22:53 | Don Cragun | Section | (section number or name, can be interface name) => sh |
| 2016-07-23 22:53 | Don Cragun | Page Number | (page or range of pages) => 3189 |
| 2016-07-23 22:53 | Don Cragun | Line Number | (Line or range of lines) => 106700-106701 |
| 2016-07-23 22:53 | Don Cragun | Interp Status | => --- |
| 2016-07-24 02:23 | kre | Note Added: 0003306 | |
| 2016-07-25 16:57 | nick | Note Added: 0003307 | |
| 2016-07-25 18:58 | shware_systems | Note Added: 0003308 | |
| 2016-07-26 09:25 | joerg | Note Added: 0003309 | |
| 2017-11-16 17:26 | geoffclare | Note Added: 0003881 | |
| 2017-11-16 17:27 | geoffclare | Final Accepted Text | => 0001063:0003881 |
| 2017-11-16 17:27 | geoffclare | Status | New => Resolved |
| 2017-11-16 17:27 | geoffclare | Resolution | Open => Accepted As Marked |
| 2017-11-16 17:28 | geoffclare | Tag Attached: tc3-2008 | |
| 2019-10-28 10:53 | geoffclare | Status | Resolved => Applied |
| 2024-06-11 08:56 | agadmin | Status | Applied => Closed |
