Re: If the variable has its 'integer' attribute set,
On Sun, Nov 21, 2021 at 07:18:09PM +, Darac Marjal wrote: > You probably already know that if you write: > > i = 65 > > then several things happen: Only one thing happens: bash tries to run the command named "i" and you will almost certainly get an error message saying "command not found". If you want a variable assignment, you must not have spaces around the = sign. i=65 This assigns the string "65" to the variable named i. > So, that's a very basic method of storing the variable. At this point, > we know that the variable "i" equals 65, right? Well, how do we know > that the variable doesn't hold the character "A", or that it's not an > array? So, there must be a table somewhere else that tells us "what KIND > of data is stored in 'i'?". Bash does not store integer variables. Ever. Only string variables. Using "declare -i" does not change that. It only makes bash undergo some weird gyrations every time a new string value is assigned to the variable in question. (And overloads the += operator, because life wasn't confusing enough yet.) Many people think "Oh, if I use declare -i, bash will store my values as integers, and arithmetic will become more efficient, because bash won't have to keep doing string-to-integer conversions all the time." This is incorrect. Everything is stored as a string.
Re: If the variable has its 'integer' attribute set,
On 21/11/2021 01:37, sim sim wrote: > Hi, all. > My question may be in the wrong place but I'm already exhausted. > Started reading https://www.gnu.org/software/bash/manual/bash.pdf. > > 3.4 Shell Parameters > . > > A parameter is set if it has been assigned a value. The null string > is a valid value. Once a variable is set, it may be unset only by > using the 'unset' builtin command. > A variable may be assigned to by a statement of the form > NAME=[VALUE] > If VALUE is not given, the variable is assigned the null string. All > VALUEs undergo tilde expansion, parameter and variable expansion, > command substitution, arithmetic expansion, and quote removal > (detailed below). If the variable has its 'integer' attribute set, > then VALUE is evaluated as an arithmetic expression even if the > '$((...))' expansion is not used. > ......... > ...... > I do not understand "If the variable has its 'integer' attribute > set," where the variable has an 'integer' attribute, after all, this > is not a function? > I do not ask to explain in detail, because it is long and I will not > understand because of my poor English, but if you can just write a > simple variable for: > name=[value] > where "If the variable has its 'integer' attribute set", > for example it will help a lot. This manual is difficult for a > beginner to read. The same concept in different places denotes: either > an attribute or a parameter or an option. > And advise a smart forum on this topic, where you can ask questions, > Google found trash. The way I would explain it is to assume that bash stores variables with "data" and "metadata" separately. You probably already know that if you write: i = 65 then several things happen: * a block of memory is allocated * the address of the allocated memory is stored somewhere, and a reference to that address is associated with the variable "i" * The value "65" is stored in that memory location So, that's a very basic method of storing the variable. At this point, we know that the variable "i" equals 65, right? Well, how do we know that the variable doesn't hold the character "A", or that it's not an array? So, there must be a table somewhere else that tells us "what KIND of data is stored in 'i'?". From a naive point of view, we might use an enumeration for the type (if the type is 1 it's an integer, if it's 2 it's a string, if it's 3 it's an array and so on), but the authors of bash, for whatever reason, seem to use type flags. Perhaps a variable can be an integer *and* a string? I'm not sure. In summary, though, I would read "If the variable has its 'integer' attribute set" as synonymous with "If the variable is declared an integer". The bit to be careful about is that the value is NOT evaluated as an arithmetic if the variable merely LOOKS like an integer (e.g. the string "65"). > Thanks, > Sim. OpenPGP_signature Description: OpenPGP digital signature
Re: If the variable has its 'integer' attribute set,
On Sun, Nov 21, 2021 at 01:37:14AM +, sim sim wrote: > I do not understand "If the variable has its 'integer' attribute set," where > the variable has an 'integer' attribute, after all, this is not a function? They're talking about "declare -i". Which is really a stupid thing and I wish it didn't exist, but it does, and people are silly enough to use it, and I cannot stop them. For those who don't understand the context of the question, this is about bash, and variables in bash. All variables in bash hold string values. Even if you use "declare -i", the value that is stored in memory is a string. What "declare -i" does is make it so that when you assign new strings to the variable, dark magic occurs. The value that gets assigned will be interpreted by the shell's math context parser, and the result will be a value that looks like a number, or else an error. unicorn:~$ bash unicorn:~$ declare -i foo unicorn:~$ foo=bar unicorn:~$ declare -p foo declare -i foo="0" So... what happened here? We tried to assign the string "bar" to a variable with the "-i" flag set. Since bar doesn't look like a number, it was sent into the math context parser. The math context parser saw that "bar" looks like a variable name, so it recursively evaluated "bar" in a math context, retrieve the value from the variable "bar". Which did not exist. So that value was assigned as "0". Which is a string that looks like a number. Because, as I said, all bash variables hold strings. unicorn:~$ bar='hello world' unicorn:~$ foo=bar bash: hello world: syntax error in expression (error token is "world") Here, we got an error, because "hello world" could not be evaluated by the shell's math parser. unicorn:~$ bar=7*9 unicorn:~$ foo=bar unicorn:~$ declare -p foo bar declare -i foo="63" declare -- bar="7*9" Here, bar which is an ordinary value holds the string value "7*9", which happens to be a valid expression in the eyes of the shell's math parser. When we assigned "bar" to "foo", which has the "-i" flag set, the string value was fed to the math parser, and "7" and "9" were temporarily turned into integers, and a multiplication was performed, and then the resulting number 63 was turned back into a string, and the string "63" was stored in "foo". Why people think this is a good idea is utterly beyond me. I can only advise you never to use this thing. It's confusing and dumb. If you want to perform arithmetic while assigning a value to a variable, I advise you to use an *explicit* arithmetic expression: foo=$(( math stuff here )) That way you can see what's happening when you read the script, and there won't be as many surprises. Bash is full of enough landmines without introducing more.
If the variable has its 'integer' attribute set,
Hi, all. My question may be in the wrong place but I'm already exhausted. Started reading https://www.gnu.org/software/bash/manual/bash.pdf. 3.4 Shell Parameters . A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be unset only by using the 'unset' builtin command. A variable may be assigned to by a statement of the form NAME=[VALUE] If VALUE is not given, the variable is assigned the null string. All VALUEs undergo tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal (detailed below). If the variable has its 'integer' attribute set, then VALUE is evaluated as an arithmetic expression even if the '$((...))' expansion is not used. . .. I do not understand "If the variable has its 'integer' attribute set," where the variable has an 'integer' attribute, after all, this is not a function? I do not ask to explain in detail, because it is long and I will not understand because of my poor English, but if you can just write a simple variable for: name=[value] where "If the variable has its 'integer' attribute set", for example it will help a lot. This manual is difficult for a beginner to read. The same concept in different places denotes: either an attribute or a parameter or an option. And advise a smart forum on this topic, where you can ask questions, Google found trash. Thanks, Sim.
