I create a cron by crontab -e and enter: 00 17 * * * mkdir /root/file1  at time 17:00 I don't find a file1 created by cron.... Please tell me what I do....
This is a  about using cron &amp; crontab.   You have been directed here because the community is fairly sure that the answer to your question can be found below. If your question is not answered below then the answers will help you gather information that will help the community help you. This information should be edited into your original question.  The answer for 'Why is my crontab not working, and how can I troubleshoot it?' can be seen below.  This addresses the cron system with the crontab highlighted.
Examples and Counterexamples of Relations which Satisfy Certain Properties Definition: Given a set $X$, a relation $R$ on $X$ is any subset of $X\times X$.  A relation $R$ on $X$ is said to be  reflexive if $(x,x) \in R$ for all $x \in X$, irreflexive if $(x,x) \not\in R$ for all $x \in X$, transitive if $(x,y) \in R$ and $(y,z) \in R$ implies that $(x,z)\in R$, intransitive (or antitransitive) if $(x,y) \in R$ and $(y,z) \in R$ implies that $(x,z)\not\in R$, symmetric if $(x,y) \in R$ implies that $(y,x) \in R$, antisymmetric if $(x,y) \in R$ and $(y,x) \in R$ implies that $x=y$.   Given any combination of the properties listed above, is there a nontrivial (i.e. nonempty) relation which satisfies that combination of properties? This question attempts to collect a community-maintained list of quality books on the  programming language, targeted at various skill levels.  C is a complex programming language that is difficult to pick up on-the-go by reading online tutorials. A comprehensive book is often the best way to learn the language, and finding a good book is the first step. It is important to avoid badly-written books, and even more importantly, books that contain serious technical errors.  Please suggest edits to the accepted answer to add quality books, with an approximate skill level and a short blurb/description about each book. (Note that the question is locked, so no new answers will be accepted. A single answer is being maintained with the list)  Feel free to debate book choices, quality, headings, summaries, skill levels, and anything else you see that is wrong. Books that are deemed satisfactory by the C community here will stick around on the list; the rest will be regularly removed.  For books that have reviews by the Association of C and C++ Users (ACCU), a link to those reviews should be added along with the book.  See also:   Other C-related resources in the   A similar list for :      This question was discussed on  as part of the .The consensus was to keep it undeleted and actively maintained. Can my child attend a private school whilst I'm on USA ESTA? I am British, currently staying in the USA for 6 weeks on an ESTA Visa Waiver. I am living with a local family and whilst here my child (9 years old) has been attending a private (non-state) school with the children of the local family.  Today somebody told me that by having my child attend a private school I may be in violation of the terms of the ESTA Visa Waiver.  I have found nothing about this at   Where can I find official documentation / online advice about this? Not enough space on /tmp I'm not able to run update manager as I get an error saying that there is not enough free space in the /tmp directory. I've practically cleaned out the tmp directory but the error persists.  here's df-h  /dev/loop0       13G   11G  952M  92% / udev            2.0G  4.0K  2.0G   1% /dev tmpfs           785M  920K  784M   1% /run none            5.0M     0  5.0M   0% /run/lock none            2.0G  584K  2.0G   1% /run/shm /dev/sda6        20G   14G  6.4G  68% /host  overflow        1.0M   16K 1008K   2% /tmp Will metering from a DSLR apply to a film SLR? I still like to shoot with film (I use a Nikon F3) but I like the immediate feedback of digital.  I'm thinking of buying a used DSLR to help me get the framing and lighting right when I shoot static subjects.  Then I'd take the final shot with my F3. But would the metering be transferable?  If the shot looks perfect at 1/125 with f/2 on a DSLR, will the same settings work on a film camera? Proof of a formula involving Euler's totient function: $\varphi (mn) = \varphi (m) \varphi (n) \cdot \frac{d}{\varphi (d)}$ Arch on WSL: libQt5Core.so.5 not found, despite being installed I was hit by some hardcore power-tripping forum mods on the Arch Linux forum, so here I am.  I'm trying to run GUI apps on WSL, such as dolphin or kate. However, none of them can find libQt5Core.so.5, despite it being installed in /usr/lib as you'd expect. Here's a terminal transcript:  /u/lib » DISPLAY=:0.0 dolphin dolphin: error while loading shared libraries: libQt5Core.so.5: cannot open shared object file: No such file or directory /u/lib » pwd /usr/lib /u/lib » ls -la | grep Qt5Core -rw-r--r-- 1 root root     1143 Aug  7 06:05 libQt5Core.prl lrwxrwxrwx 1 root root       20 Aug  7 06:05 libQt5Core.so -&gt; libQt5Core.so.5.11.1* lrwxrwxrwx 1 root root       20 Aug  7 06:05 libQt5Core.so.5 -&gt; libQt5Core.so.5.11.1* lrwxrwxrwx 1 root root       20 Aug  7 06:05 libQt5Core.so.5.11 -&gt; libQt5Core.so.5.11.1* -rwxr-xr-x 1 root root  5234312 Aug  7 06:05 libQt5Core.so.5.11.1* /u/lib » ldd `which dolphin`         linux-vdso.so.1 (0x00007fffdb426000) ...         libQt5Core.so.5 =&gt; not found /u/lib » pacman -Qk qt5-base qt5-base: 3546 total files, 0 missing files   Note that other GUI apps work fine - anything compiled against GTK, including Firefox, work perfectly. I just have to set the $DISPLAY variable. Not setting the var doesn't help, as you'd expect.  Any ideas? Thanks! Who did that to the priest's dog? In John Michael McDonagh's wonderful philosophical black comedy , a good priest receives a death threat during a confessional giving him a week to sort out his affairs.   During the course of the week there are a lot of encounters between the priest and his parishioners, many acting as red herrings to distract the audience form the identity of the potential killer. Several threatening acts also take place: the church is burned and the priest's dog is killed.   But in the final encounter between the priest and the mystery antagonist (I'm not saying who to avoid spoilers) the antagonist admits burning the church but claims not to have killed the priest's dog.  Are they lying? Or is there a good reason that someone else might have killed the dog? # ~/.profile: executed by the command interpreter for login shells. # This file is not read by bash(1), if ~/.bash_profile or ~/.bash_login # exists. # see /usr/share/doc/bash/examples/startup-files for examples. # the files are located in the bash-doc package.  # the default umask is set in /etc/profile; for setting the umask # for ssh logins, install and configure the libpam-umask package. #umask 022  # if running bash if [ -n "$BASH_VERSION" ]; then     # include .bashrc if it exists     if [ -f "$HOME/.bashrc" ]; then     . "$HOME/.bashrc"     fi fi  # set PATH so it includes user's private bin if it exists if [ -d "$HOME/bin" ] ; then     PATH="$HOME/bin:$PATH" fi   Can anyone explain the contents of the ~/.profile file? So when you enter into the ~/.profile file what does all the writing mean? Where should the period be put when an entire sentence is quoted at the end of a sentence? The number of permutations of $\{1,2,\ldots,n\}$ that have exactly one ascent (rise). Sloane's  counts the number of $n$-permutations with exactly one ascent.  For example $a(3)=4$ because we have: $1\wedge32$, $21\wedge3$, $2\wedge31$, $31\wedge2$ where I have marked the unique ascent with $\wedge$.  The same sequence also counts the number of length $n$ binary words that contain at least two $1$'s. For example $a(3)=4$ because we have: $011, 101, 110, 111.$  Is there a simple bijection between these two classes?  The formula for the sequence is $a(n) = 2^n - n - 1$.  I see how this counts the number of such binary words.  Is there a combinatorial derivation of this formula demonstrating that it counts the number of such n-permutations?  Both the exponential and ordinary generating functions of the sequence are given.  Again, I understand their symbolic derivation for enumeration of such binary words but can these functions be derived symbolically by considering the structure of such n-permutations? What prevents photons from getting mass from higher order Feynman diagrams The Higgs boson and gluons have no electric charge and photons couple to charge, so there is no tree level interaction between them and photons.  But what prevents higher order diagrams from contributing a non-zero mass term to the photon, for instance    where a photon couples to some fermion (say an electron, or a top quark) which can interact with the Higgs field.   Or consider that same diagram but with quarks and a gluon interacting between them? Or any higher diagram with even more loops?  I have heard charge conservation depends on gauge invariance, which in turn depends on photons being massless.  So it appears the photon has no mass, and these diagrams must all cancel somehow.  So I'm hoping there is a very nice symmetry explanation for why they all disappear, but if I say "because of gauge invariance" that would be circular logic, so there must be another symmetry at stake here?     What prevents photons from obtaining a mass from high-order self-energy loop diagrams? How to show that $f$ is differentiable at $x=0.$ What's the past participle form of the word exit? Is it exit (irregular, like set)? exited? exitted? On one page I found exited but if that's the case why isn't it exitted (double t) like with the word emit - emitted? Is there a rule when the consonant at the end is doubled and when not? How can I extract a page range / a part of a PDF? Do you have any idea how to extract a part of a PDF document and save it as PDF? On OS X it is absolutely trivial by using Preview. I tried PDF editor and other programs but to no avail. I would like a program where I select the part that I want and then save it as a PDF file with a simple command like CMD+N on OS X. I want the extracted part to be saved as PDF and not as JPEG, etc. What are the fundamental differences between the mainstream *NIX shells? What are the fundamental differences between the mainstream *NIX shells and what scenarios might prompt you to use one over the other?  I understand that some of it probably comes down to user preference but I've only ever used Bash and I'm interested to hear where another shell might be useful. Also, is there an impact on user-written shell scripts when running under one shell or another or is it simply a matter of changing the shell at the top of the file?  My instinct says it's not that easy. Undecidable language problems. In the 3rd and 4th editions of D&amp;D, there were explicit rules for determining if a character knew anything about a monster before them.  In 3rd edition, for example, use of the  skill with a general DC of 10 + the monster's HD allowed for determining one fact, plus one fact per 5 points over the check.  Looking at the Intelligence section of the 5e PHB, I don't see any similar notation.  Does 5e provide any guidance as to when a player could use their out-of-game knowledge about a monster, or when the player might be told things their character would probably know in-game? iPhone asks 'Trust This Computer' again (I am on macOS 10.14.6 and iOS 12.4)   I am certain that I have trusted my Mac before. Without the latter trust, I can also access the phone in iTunes.   I have tried reset "Location and Privacy" on my phone. Then   When connecting the iPhone with iTunes running, it asks for the trust and I trust it.  Reboot my Mac and the phone asks for another trust.   Is there some known malware mimics system behavior and steal data from an iPhone?  (I do have installed some pir*ted software recently. That's my fault.) Using the validation data I'm unclear on the exact process of using the validation data.  Let's say that I fit my neural network model and adjust hyperparameters using the training set and validation set. Do I then evaluate the test set on this model? Or do I recombine the validation and training sets and fit a fresh model with the hyperparameters that I found during the validation phase, and then evaluate on the test data? I have seen a number of different notebooks and examples that do both ways.  Surely, once I've found my hyperparameters, it makes sense to fit a fresh model using the full training set (recombined with validation set), since the validation loss has no effect on the weights.