Path: csiph.com!goblin1!goblin.stu.neva.ru!usenet.stanford.edu!not-for-mail From: Martijn Dekker Newsgroups: gnu.bash.bug Subject: Re: How to use PROMPT_COMMAND(S) without breaking other scripts Date: Mon, 24 Aug 2020 17:58:27 +0100 Lines: 54 Approved: bug-bash@gnu.org Message-ID: References: <17a0ba52-32ee-b9bc-72ff-3587b2050fbd@case.edu> <0c10b76f-a7f0-1c52-77b7-4b208035956b@case.edu> NNTP-Posting-Host: lists.gnu.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit X-Trace: usenet.stanford.edu 1598288325 6831 209.51.188.17 (24 Aug 2020 16:58:45 GMT) X-Complaints-To: action@cs.stanford.edu To: bug-bash@gnu.org Envelope-to: bug-bash@gnu.org User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 In-Reply-To: <0c10b76f-a7f0-1c52-77b7-4b208035956b@case.edu> Content-Language: en-GB Received-SPF: none client-ip=2a02:2770::21a:4aff:fec6:e3d8; envelope-from=martijn@inlv.org; helo=freekahlil.inlv.org X-detected-operating-system: by eggs.gnu.org: No matching host in p0f cache. That's all we know. X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, NICE_REPLY_A=-2.25, SPF_HELO_NONE=0.001, SPF_NONE=0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: bug-bash@gnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Bug reports for the GNU Bourne Again SHell List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-Mailman-Original-Message-ID: X-Mailman-Original-References: <17a0ba52-32ee-b9bc-72ff-3587b2050fbd@case.edu> <0c10b76f-a7f0-1c52-77b7-4b208035956b@case.edu> Xref: csiph.com gnu.bash.bug:16802 Op 24-08-20 om 15:57 schreef Chet Ramey: > I sometimes think I should have stuck with converting PROMPT_COMMAND to > an array. Either way, there's going to be a transition, and maybe that > would have been the easiest. Is it too late? I think that would actually be cleaner than adding a separate array, per Koichi's report. One problem is that if a script does the obvious PROMPT_COMMAND+=("some command here") if PROMPT_COMMAND is not yet set, then it starts adding at array index 0, so a subsequent traditional usage from some other script PROMPT_COMMAND="some command here" would overwrite it. So array usage should not use the 0 index. To avoid using the 0 index, one possibility is: PROMPT_COMMAND[$(( ${#PROMPT_COMMAND[@]} + 1 ))]="some command here" which, if PROMPT_COMMAND is unset, starts adding at index 1, not 0, and otherwise acts identically. However, 'set -u'/'set -o nounset' kills that. That option makes the ${#PROMPT_COMMAND[@]} expansion error out if there are no array elements (even though ${#@} works with no positional parameters). It's also an unwieldy command. So maybe that idea is not the best. Another way to avoid using the 0 index, which is 'set -u' compatible, would be PROMPT_COMMAND=${PROMPT_COMMAND-} PROMPT_COMMAND+=("some command here") The first command sets PROMPT_COMMAND[0] to the empty value if it doesn't exist yet, and otherwise leaves it unchanged. It's a bit of an ugly hack though. But then, maybe it's best if bash itself just sets PROMPT_COMMAND[0] to the empty value on initialisation. IMO that would be a reasonably clean and reliable way to ensure a smooth transition. Just my 2ยข, - M. -- || modernish -- harness the shell || https://github.com/modernish/modernish || || KornShell lives! || https://github.com/ksh93/ksh