I have an existing script in my Linux host with these statements:  local variable=$1 if [ "z${variable}" != "zfalse" ]; then   local flag="--some_flag" fi   I haven't found an explanation of these "z${variable}" and "zfalse" notation or syntax in my Shell Scripting book. Hope someone can help explain what they mean. Thanks in advance.
I see this often in the build scripts of projects that use autotools (autoconf, automake).  When somebody wants to check the value of a shell variable, they frequently use this idiom:  if test "x$SHELL_VAR" = "xyes"; then ...   What is the advantage to this over simply checking the value like this:  if test $SHELL_VAR = "yes"; then ...   I figure there must be some reason that I see this so often, but I can't figure out what it is.
mysql_fetch_array, mysql_fetch_assoc, mysql_fetch_object Man is sitting in his room and gets contradictory advice from two men from the future Where does "pay through the nose" originate? In Java, are enum types inside a class static? Appropriate data transformation Is the function $f(x)=|x|^{1/2}$ Lipschitz continuous near $0$? If yes, find a constant for some interval containing $0$.   I think the answer is yes since I can find $L=1$ that satisfies Lipschitz continuity criteria in a interval close to $0$, am I right? What is the maximum length of a URL in different browsers? What is the maximum length of a URL in different browsers? Does it differ among browsers? Is a maximum URL length part of the HTTP specification? Does a passive Perception (Wisdom) check add WIS mod + Perception skill? I'm new to D&amp;D. There seems to be a discrepancy between the 5e Starter Set Rulebook and the associated character sheets. The Rulebook says that a passive wisdom (percep) score is "10 + the creature's wisdom modifier, as well as any bonuses."  In the example they have a 1st-level character (with a proficiency bonus of +2) Wisdom of 15 (+2) and a  proficiency in Perception, he or she has a passive Wisdom (Perception) of 14 (10+2+2).  But the Human Fighter Folk hero character has a WIS mod of +1, and a +3 written beside perception, and a passive Wis percep of 13 (not 14?).  The Dwarven Cleric has Wis mod of +3 and a +3 written beside perception, and only a 13 passive wisdom.  I guess this has to do with proficiency in the skill, but I'm confused. Can someone explain these instances and maybe also when proficiency is used, vs when just the modifier or just the skill without proficiency? How do I get a remote host IP address if I don't have ping, and don't have any bind utilities like dig, nslookup, etc?  I need an answer that does not include 'install X' or 'use sidecar container'. I am looking for something that relies on nothing more than bash and the basic shell commands. Travelling with two different passports Let $x,y,z&gt;0$: Prove that:   $\frac{1}{(x+2y)^2}+\frac{1}{(y+2z)^2}+\frac{1}{(z+2x)^2} \geq\frac{1}{xy+yz+zx}$  I tried to apply Cauchy - Schwarz's inequality but I couldn't prove this inequality! I'm reading Nano: The Essentials by T. Pradeep and I came upon this statement in the section explaining the basics of scanning electron microscopy.     However, the equation breaks down when the electron velocity approaches the speed of light as mass increases. At such velocities, one needs to do relativistic correction to the mass so that it becomes[...]   We all know about the famous theory of relativity, but I couldn't quite grasp the "why" of its concepts yet. This might shed new light on what I already know about time slowing down for me if I move faster.  Why does the (relativistic) mass of an object increase when its speed approaches that of light? "[" as first character in table row Whenever I try to compile a table that has a "[" as first character in a row it gives me the error:  Missing number, treated as zero. Illegal unit of measure   Example:  \begin{table}[htb]     \centering     \begin{tabular}{r|r}         [some text ] &amp; ... \\         [some more text ] &amp; ...     \end{tabular}     \caption{...}     \label{tab:...} \end{table}   How can I print "[" as first character in a row? Conditional distributions of the multivariate normal How does the Southwest Airlines boarding process work? I'm flying this week on Southwest Airlines for the first time. I paid for "Early Bird" automatic checkin and the FAQ mentions something about "A" boarding passes. I have no idea what that means, and I know that Southwest doesn't have assigned seats.  How does the boarding process work and is there anything I should be aware of? Inequality $\frac{1}{n+1}+\frac{1}{n+2}+...+\frac{1}{3n+1} \ge 1$ What are review tests (audits) and how do they work? I recently received this rather amusing message while reviewing a user's first post:  Congratulations! This was only a test, designed to make sure you were paying attention. This post has already been removed, but thanks for taking time to leave feedback for the author.   What is the purpose of this test? How does the audit work? What happens if I fail? Why was the system expecting me to negatively review a good post, or positively review a bad post? A review is obviously an audit. Should I report it? Do review audits count toward the Reviewer badge? Do robo-reviewers dream of electric sheep? 2 prong to 3 prong outlet help The following code snippet (correctly) gives a warning in C and an error in C++ (using gcc &amp; g++ respectively, tested with versions 3.4.5 and 4.2.1; MSVC does not seem to care):  char **a; const char** b = a;   I can understand and accept this. The C++ solution to this problem is to change b to be a const char * const *, which disallows reassignment of the pointers and prevents you from circumventing const-correctness ().  char **a; const char* const* b = a;   However, in pure C, the corrected version (using const char * const *) still gives a warning, and I don't understand why. Is there a way to get around this without using a cast?  To clarify: 1) Why does this generate a warning in C?  It should be entirely const-safe, and the C++ compiler seems to recognize it as such. 2) What is the correct way to go about accepting this char** as a parameter while saying (and having the compiler enforce) that I will not be modifying the characters it points to? For example, if I wanted to write a function:  void f(const char* const* in) {   // Only reads the data from in, does not write to it }   And I wanted to invoke it on a char**, what would be the correct type for the parameter? I'm trying to install Linux alongside my 64-bit Windows 8 and went into my partition manager to find this structure.    In order to create partitions for my Linux install I need to free up some space. I see that the last partition is an empty recovery partition, as you can see highlighted, it's 100% free.  I was wondering is it okay to just remove this partition seeing as it doesn't appear I'll be losing anything?  Not sure if this helps but this is on a Fujitsu AH532 running Windows 8 64-bit with UEFI.