On MacOS, the default bash on the OS is 3.2.  If I run help trap it says that if the signal_spec is DEBUG, that the command is run after each command, but the bash manpage says it is run before each command.  The actual behavior of this bash agrees with the manpage (i.e. before the command).  I downloaded the source of 4.4.18(1) and built it.  Both the manpage and help trap say that the trap is executed before each command.  However, even then, the text of each of those pieces of documentation are slightly different.  So my naive assumption that the help builtin reads/parses the manpage is clearly incorrect.  So where does help get its info that it relays to the user?
help  displays information about builtin commands. What is the source of help? Does it maintain a database for builtin commands, or does it read some files of each builtin commands (similar to the manpage of each utilities)?  Sometimes I find its information seems to expand that by --help  $ cd --help bash: cd: --: invalid option cd: usage: cd [-L|[-P [-e]]] [dir]  $ help cd cd: cd [-L|[-P [-e]]] [dir]     Change the shell working directory.      Change the current directory to DIR.  The default DIR is the value of the     HOME shell variable.      The variable CDPATH defines the search path for the directory containing     DIR.  Alternative directory names in CDPATH are separated by a colon (:).     A null directory name is the same as the current directory.  If DIR begins     with a slash (/), then CDPATH is not used.      If the directory is not found, and the shell option `cdable_vars' is set,     the word is assumed to be  a variable name.  If that variable has a value,     its value is used for DIR.      Options:         -L  force symbolic links to be followed         -P  use the physical directory structure without following symbolic         links         -e  if the -P option is supplied, and the current working directory         cannot be determined successfully, exit with a non-zero status      The default is to follow symbolic links, as if `-L' were specified.      Exit Status:     Returns 0 if the directory is changed, and if $PWD is set successfully when     -P is used; non-zero otherwise.   I thought help extract the help part of the executable, but for a  python script , it isn't  $ help ./pdf-merge.py bash: help: no help topics match `./pdf-merge.py'.  Try `help help' or `man -k ./pdf-merge.py' or `info ./pdf-merge.py'.  $ ./pdf-merge.py --help usage: pdf-merge.py [-h] [-v] [--ask] [--output OUTPUT] [--title TITLE]                     [--author AUTHOR] [--keyword KEYWORD] [--pdftk PDFTK]                     [--gs GS] [--pdfmarks PDFMARKS] [--unicode]                     PDF [PDF ...]  Merge PDFs preserving bookmarks. Thanks to Larry Cai for suggesting that Unicode be supported and for discussion about the `--pdfmarks` option.  positional arguments:   PDF                  an input PDF to merge  optional arguments:   -h, --help           show this help message and exit   -v, --version        show program's version number and exit   --ask                pause for manual pdfmark tweaking   --output OUTPUT      name of the output PDF   --title TITLE        title of output PDF   --author AUTHOR      author of output PDF   --keyword KEYWORD    keywords for the output PDF   --pdftk PDFTK        path to the pdftk executable   --gs GS              path to the gs (Ghostscript) executable   --pdfmarks PDFMARKS  path to pdfmarks file. If not given, a temporary file                        is used. If given and the file is missing, execution                        will stop after the file is created (before the                        Ghostscript run). If given and the file exists, no                        attempt will be make to use pdftk to generate the mark                        file (I assume your input file is what you want).   --unicode            instead of merging PDFs, convert PDF-formatted unicode                        strings. For example `--unicode '&lt;FEFF03B103B203B3&gt;'                        \u03b1\u03b2\u03b3`
Why isn't it $E \approx 27.642 \times mc^2$? Repeat copies of array elements: Run-length decoding in MATLAB I have a homework question that asks     Find necessary and sufficient conditions on $1 \leq i &lt; j \leq n$ so that $(i \, j)$ and $(1 \, 2 \, \dotsc \, n)$ generate $S_n$.   Here is what I have done so far.  Call $c = (1 \, 2 \, \dotsc \, n)$.  I made the observation that $$ \overbrace{c c \dotsb c}^{n - j + 1} (i \, j) \overbrace{c^{-1} c^{-1} \dotsb c^{-1}}^{n - j + 1} = (1 + i - j + n \, 1). $$ Thus, $\langle (i \, j), c \rangle = \langle (1 \, i - j + n + 1), c \rangle$, so it suffices to look at $\langle (1 \, k), c \rangle$ for positive integers $2 \leq k \leq n$.  I've checked that, for the value $k=2$, $\langle (1 \, 2), c \rangle = S_n$.  This immediately implies $\langle (1 \, n), c \rangle = S_n$.  I suspect that no other values of $k$ will work, but am not sure how to prove it. Cannot log in with Apple ID after performing password reset My wife forgot the password to her Apple ID account. Her Macbook Air (OS X Yosemite) restarted to install some updates. She has her user account set up through her Apple ID account.  Since she forgot the password she reset it from my laptop.  She can log in to iCloud and Apple ID with the new password, but cannot log in with the new password on the Air.  When she tries to log in she is greeted with this message:     "This account uses an iCloud password to log in. If you need to change   the password, use another device to change it at appleid.apple.com." Move the Add Favorite / Ignored Tags boxes to the top It seems that most people who use the ignored tags feature end up with a fairly long list of those tags.  Although , it is sometimes necessary to add new ignored tags manually (typically when adding some-tag*).  It would be a lot easier if the add box was above the long list of ignored tags rather than below. I suppose the same would apply to the favorites tags for consistency's sake. Prove the equality: $\det\left[\begin{smallmatrix} -2a &a+b &a+c \\ b+a&-2b &b+c \\ c+a&c+b &-2c \end{smallmatrix}\right] = 4(a+b)(b+c)(c+a)$ i have to prove:   $$\begin{vmatrix} -2a &amp;a+b  &amp;a+c \\   b+a&amp;-2b  &amp;b+c \\   c+a&amp;c+b  &amp;-2c  \end{vmatrix} = 4(a+b)(b+c)(c+a)$$   I have tried many calculations between the rows and columns of the determinant to get to the answer i want to solve the exercise however none of them gave me something right. Can someone help? I get this error:  Can't locate Foo.pm in @INC  Is there an easier way to install it than downloading, untarring, making, etc? Zooming / Focusing certain parts of a chart in latex I am using pgfplot to draw graphs in latex. Now this is the problem I am facing:  \begin{tikzpicture}[scale=1] \begin{axis}[ybar,grid=major,bar width=2pt,width=8cm, height=8cm,xmin=1,xmax=540,ymin=1,ymax=240] \addplot [draw=black,fill=green]  table [x=size, y=cluster, col sep=comma] {sim-0.4-0.6.csv}; \end{axis} \end{tikzpicture}   It generates the following graph:    Since, I have values mainly between 0-50 on x-axis, I want to zoom there. After that, values comes around 500. S0, there are no graph points between 50 - 450. How should I do that? How to make UV loft like 3ds Max in Blender using curves? I am try to make a car Body using the technique like UV loft in 3ds Max.  How to apply this technique in  Blender using curves or closer to this technique? I have seen this technique in a Blender tutorial, but I forgot about it. How to mount ufs file system under Debian testing? In order to mount my /dev/sda4  under /mnt/ufs_mount on Debian Testing , i have tried the following commands (it works fine under Debian Jessie):  modprobe ufs mount -r -t ufs -o ufstype=ufs2 /dev/sda4 /mnt/ufs_mount   and:  mount -t ufs -o ufstype=ufs2 /dev/sda4 /mnt/ufs_mount   But i get this error:  mount: /dev/sda4 is write-protected, mounting read-only mount: wrong fs type, bad option, bad superblock on /dev/sda4,    missing codepage or helper program, or other error     In some cases useful info is found in syslog - try    dmesg | tail or so.   The output of: dmesg | tail :  [ 1136.965142] ufs: ufs_fill_super(): bad magic number [ 1255.758946] ufs: ufs_fill_super(): bad magic number [ 2098.945757] ufs: ufs was compiled with read-only support, can't be mounted as read-write [ 2098.946045] ufs: You didn't specify the type of your ufs filesystem        mount -t ufs -o ufstype=sun|sunx86|44bsd|ufs2|5xbsd|old|hp|nextstep|nextstep-cd|openstep ...        &gt;&gt;&gt;WARNING&lt;&lt;&lt; Wrong ufstype may corrupt your filesystem, default is ufstype=old [ 2098.967212] ufs: ufs_fill_super(): bad magic number [ 2927.982112] perf: interrupt took too long (2504 &gt; 2500), lowering kernel.perf_event_max_sample_rate to 79750   Edit My sources.list:  deb http://httpredir.debian.org/debian/ stretch main deb-src http://httpredir.debian.org/debian/ stretch main  deb http://security.debian.org/debian-security stretch/updates main deb-src http://security.debian.org/debian-security stretch/updates main   And uname -a:  Linux debian 4.6.0-1-amd64 #1 SMP Debian 4.6.4-1 (2016-07-18) x86_64 GNU/Linux   How to mount ufs file system under Debian testing? pgfplots axis scaling Is it possible to define in pgfplots that y should correspond to Y cm in my plot?   I know how to do this in tkz-fct, so let me add an tkz-fct example to make more clear what I want:  \documentclass{article}  \usepackage{pgfplots} \usepackage{tkz-fct} \usetkzobj{all}  \begin{document}  \begin{tikzpicture}[scale=.5] \tkzInit[xmax=10,ystep=3,ymax=10] \tkzAxeXY \tkzGrid \tkzFct[color=red, thick,domain=0:10]{\x} \tkzFct[domain=0:10]{\x**2} \end{tikzpicture} \begin{tikzpicture}[scale=.5] \tkzInit[xmax=10,ystep=1.5,ymax=10] \tkzAxeXY \tkzGrid \tkzFct[color=red, thick,domain=0:10]{\x} \tkzFct[domain=0:10]{\x**2} \end{tikzpicture}   \end{document}     In one case 3 corresponds to 0.5 cm on the y-axis in the picture, in the other case 1.5 corresponds to 0.5 cm on the y axis. This is achieved by using the scale=0.5 option together with the ystep option.  Not that it is important for me to have grids as in the tkz-fct example. Does the point at which salt is added to a dish affect how the flavor is changed? What is the difference between an SLR and a DSLR? What are the differences between SLRs and DSLRs? (I know that DSLRs are Digital SLRs but are there any actual differences?)  Also, what are the advantages/disadvantages of both? Disable commandButton after first click to prevent double submission I have an apex:commandButton on a visualforce page that invokes a method on the controller. There is currently no rerender value set for the button. Instead the controller method returns a PageReference to redirect the user as required.  E.g.  Visualforce:  &lt;apex:commandButton value="Save" action="{!save}"/&gt;   Controller:  public PageReference save() {     // save body ...      // return the user the a parent opportunity     return new PageReference('/' + opp.Id); }   Users are currently able to click on the resulting save button multiple times before the controller completes and returns the PageReference to redirect the browser to an opportunity.  How can I disable the commandButton after the first click?    I tried wrapping the commandButton in an actionStatus and facet, but the need to define the rerender property prevented the resulting PageReference redirect.  E.g.  This will render nicely in the browser and prevent multiple clicks, but doesn't redirect to the Opportunity on completion.  &lt;apex:actionStatus id="saveStatus"&gt;     &lt;apex:facet name="stop"&gt;         &lt;apex:commandButton value="Save" action="{!save}" status="saveStatus" rerender="saveParentBlock" /&gt;     &lt;/apex:facet&gt;     &lt;apex:facet name="start"&gt;         &lt;apex:commandButton value="Saving..." disabled="true" status="saveStatus"/&gt;     &lt;/apex:facet&gt; &lt;/apex:actionStatus&gt;     There is a similar question . I need to support an existing PageReference redirect, which slightly alters the requirements.  Force.com Discussion Boards:   Ideas: I am trying to use Stack Exchange Andoid App to use my accounts in Stack Overflow &amp; Area 51.   For the Area 51, there is only Area 51 Discussion. Even Area 51 Discussion isn't working in the App!   VERSION 1.0.51  When adding this site, it's showing the correct full name as in this screen-shot:     However, when I pin this site to my fav sites in Stack Exchange, it is showing: Unknown Site. When clicking it, it shows black screen then just quit the App!  Here is the screen-shot:       Is the site not yet capable with Android? or is it a bug in the App itself?  PS: this question isn't duplicate:   In the previous versions of the App, the site was opening but with Unknown Site name. However, in this version, it quits the app when pressing the area site! It isn't opening at all! This question is also to track this bug in the App. As it's about the site name issue &amp; the content as well. SD card not showing files when connected to pc When my phone MOTOROLA G3(2015) is updated to Marshmallow, its not showing files when connected to PC. While connecting, it shows options as charing only, PTP, MTP, MIDI. When I select MTP, it shows the external card, but not showing the contents, where as internal memory is detected and all files are shown. Since I'm confused with these two phrases, I did a search in Google Books hoping to find some examples. But then I found :     Two private spaces or replicas cannot contain the same object with two distinct references. For example, during a disconnected phase, the pump can be present in two different private spaces and still be designated with the same name. During a meeting, two replicas use a same name to access the pump object(local or distant).   This made me more confused. I don't really understand why "the same name" was used in one sentence while "a same name" was used in another. Are there any differences between them? Potential energy of an infinitesimal length of elastic rod Why is $\arctan(x)=x-x^3/3+x^5/5-x^7/7+\dots$?  Can someone point me to a proof, or explain if it's a simple answer?  What I'm looking for is the point where it becomes understood that trigonometric functions and pi can be expressed as series.  A lot of the information I find when looking for that seems to point back to arctan. How do I fix my locale issue?