A NOTE has been added to this issue. ====================================================================== http://austingroupbugs.net/view.php?id=1161 ====================================================================== Reported By: steffen Assigned To: ====================================================================== Project: 1003.1(2016)/Issue7+TC2 Issue ID: 1161 Category: Shell and Utilities Type: Clarification Requested Severity: Editorial Priority: normal Status: New Name: steffen Organization: User Reference: Section: command Page Number: 2596 Line Number: 84274 ff. Interp Status: --- Final Accepted Text: ====================================================================== Date Submitted: 2017-09-04 13:09 UTC Last Modified: 2018-04-25 17:57 UTC ====================================================================== Summary: command -v must find something executable ======================================================================
---------------------------------------------------------------------- (0003997) McDutchie (reporter) - 2018-04-25 17:57 http://austingroupbugs.net/view.php?id=1161#c3997 ---------------------------------------------------------------------- Of course, kre is right in that 'command -v' and friends ('command -V' and 'type') cannot be made entirely reliable. One thing that has not been mentioned is that, even if the result of 'command -v' can be made 100% accurate, it would still be impossible to eliminate the race condition between that test and the actual attempt to exec the returned file. However, I strongly disagree that this means 'command -v' and friends are not useful at all. One simple use case is a sanity check when initialising a shell script. A script that depends on certain utilities may use 'command -v' to check for their presence and refuse to initialise if they are not found. This would be much preferable to the program continuing and failing halfway (or, much worse, *not* failing properly and continuing to run in an inconsistent state instead, as is all too common for real-world shell scripts). True, such a sanity check cannot be made "reliable". But it will work fine in 99.99...% of cases, which is good enough for it to be quite useful indeed. It should go without saying that this does not remove the need for proper exception handling. So the real purpose of 'command -v' is not "can I definitely execute this utility?" but "can I reasonably expect to succeed at executing this utility, barring exceptions such as race conditions, I/O errors and ACL shenanigans?" This is useful data to decide whether or not to attempt to perform an operation. Checking for the x bit is a minimum requirement for that use case. Being aware of things like 'noexec' mounts and ACLs, if present on the system, would be even better. Unfortunately, ash and derivatives don't even attempt to check the x bit, so their 'command -v' commonly returns files that you cannot reasonably expect to execute. Issue History Date Modified Username Field Change ====================================================================== 2017-09-04 13:09 steffen New Issue 2017-09-04 13:09 steffen Name => steffen 2017-09-04 13:09 steffen Section => command 2017-09-04 13:09 steffen Page Number => 2596 2017-09-04 13:09 steffen Line Number => 84274 ff. 2017-09-04 15:49 kre Note Added: 0003821 2017-09-05 06:58 stephane Note Added: 0003823 2017-09-05 08:30 stephane Note Added: 0003824 2017-09-05 12:34 steffen Note Added: 0003825 2017-09-05 12:39 steffen Note Added: 0003826 2017-09-05 12:47 kre Note Added: 0003827 2017-09-05 13:37 steffen Note Added: 0003828 2017-09-05 15:37 chet_ramey Note Added: 0003829 2017-09-05 15:41 stephane Note Added: 0003830 2017-09-05 16:01 stephane Note Added: 0003831 2017-09-05 16:36 stephane Note Added: 0003832 2017-09-05 17:30 stephane Note Added: 0003833 2017-09-05 20:17 steffen Note Added: 0003834 2017-09-05 20:19 steffen Note Added: 0003835 2017-09-05 20:27 kre Note Added: 0003836 2017-09-05 20:29 chet_ramey Note Added: 0003837 2017-09-05 21:27 eblake Note Added: 0003838 2017-09-05 21:50 kre Note Added: 0003839 2017-09-05 21:52 kre Note Edited: 0003839 2017-09-05 22:21 eblake Note Added: 0003840 2017-09-05 22:23 Don Cragun Note Added: 0003841 2017-09-05 22:24 Don Cragun Note Deleted: 0003841 2017-09-06 00:00 kre Note Added: 0003842 2017-09-06 00:00 kre Note Deleted: 0003839 2017-09-06 08:30 stephane Note Added: 0003843 2017-09-06 08:32 stephane Note Edited: 0003843 2017-09-06 09:30 stephane Note Added: 0003844 2017-09-06 09:37 stephane Note Added: 0003845 2017-09-06 09:50 joerg Note Added: 0003846 2017-09-06 09:50 joerg Note Edited: 0003846 2017-09-06 09:50 joerg Note Edited: 0003846 2017-09-06 09:54 stephane Note Edited: 0003845 2017-09-06 12:55 steffen Note Added: 0003847 2017-09-06 13:41 stephane Note Added: 0003848 2017-09-07 17:31 kre Note Deleted: 0003842 2017-09-07 17:58 kre Note Added: 0003849 2017-09-07 18:29 chet_ramey Note Added: 0003850 2017-09-07 21:17 kre Note Added: 0003851 2017-09-07 21:18 kre Note Edited: 0003851 2017-09-09 19:10 chet_ramey Note Added: 0003852 2018-04-25 17:57 McDutchie Note Added: 0003997 ======================================================================
