In nim, you can define a symbol via -d:value and test if it was defined with defined(value). It is possible however to define a key and retrieve its value? I'm looking for something in the vein of --colors:on but user defined.
I also looked for this and didn't find anything for my nim-small-coreutils. I ended up using environment variables as a hack and read them in a nonportable manner:
const colors = staticExec "echo \"$colors\""
when colors == "on":
echo "It's on!"
else:
echo "I guess it's off? The value is: ", colors
This takes the environment variable at compile time, so you can use it like this:
colors=on nim c example
or alternatively:
nim c --putEnv:colors=on c example
Related
I've seen that some people when writing bash script they define local variables inside an if else statement like example 1
Example 1:
#!/bin/bash
function ok() {
local animal
if [ ${A} ]; then
animal="zebra"
fi
echo "$animal"
}
A=true
ok
For another example, this is the same:
Example 2:
#!/bin/bash
function ok() {
if [ ${A} ]; then
local animal
animal="zebra"
fi
echo "$animal"
}
A=true
ok
So, the example above printed the same result but which one is the best practice to follow. I prefer the example 2 but I've seen a lot people declaring local variable inside a function like example 1. Would it be better to declare all local variables on top like below:
function ok() {
# all local variable declaration must be here
# Next statement
}
the best practice to follow
Check your scripts with https://shellcheck.net .
Quote variable expansions. Don't $var, do "$var". https://mywiki.wooledge.org/Quotes
For script local variables, prefer to use lowercase variable names. For exported variables, use upper case and unique variable names.
Do not use function name(). Use name(). https://wiki.bash-hackers.org/scripting/obsolete
Document the usage of global variables a=true. Or add local before using variables local a; then a=true. https://google.github.io/styleguide/shellguide.html#s4.2-function-comments
scope best practice
Generally, use the smallest scope possible. Keep stuff close to each other. Put local close to the variable usage. (This is like the rule from C or C++, to define a variable close to its usage, but unlike in C or C++, in shell declaration and assignment should be on separate lines).
Note that your examples are not the same. In the case variable A (or a) is an empty string, the first version will print an empty line (the local animal variable is empty), the second version will print the value of the global variable animal (there was no local). Although the scope should be as smallest, animal is used outside of if - so local should also be outside.
The local command constrains the variables declared to the function scope.
With that said, you can deduce that doing so inside an if block will be the same as if you did outside of it, as long as it's inside of a function.
I have a additionaly script help_script.sh:
....
result="example"
and the
main.bash
var=`sh help_script.sh`
echo $var
Is there any way to assign the result from help_script.sh to the var value in the main.sh? I know solution with "echo", but I'm looking for another solution.
Use the source keyword to inline the help_script.sh
This will run the script in the same context so any variables should remain in scope.
See this Super User question for more details https://superuser.com/q/46139/149483
I'm trying to lazily evaluate configuration option. I want to issue a Make error only if the variable is actually used (substituted).
Consider the following Makefile:
VAR = $(error "E")
NFS_DIR = NFS_DIR is $(VAR)
T = $(NFS_DIR) is 1
all:
echo Test
If I run it with my environment (which has /srv/nfs value), the output is following:
➜ ~ make
echo Test
Makefile:3: *** "E". Stop.
So the recursive definition acts like simple definition.
If I clear the environment, it works as expected:
➜ ~ env -i make
echo Test
Test
I couldn't find any mention that recursively-expanded variable, when defined with same name as environment variable, will act like simply-expanded variable.
So the questions are:
Why the observed behavior?
How to implement the desired behavior?
Update: to clarify — I don't want to use ?= since the options are configuration options I want them to come strictly from Makefile and not from environment.
Any variable which is in the environment when make starts, will be exported to the environment of any command make runs (as part of a recipe, etc.) In order for make to send the value of the variable to the command, make first has to expand it. It's not acting like a simply-expanded variable. It's just that running a command forces the expansion.
This is kind of a nasty side-effect but I'm not sure there's anything that can be done directly: this behavior is traditional and mandated by POSIX and lots of makefiles would break if it were changed.
You have two choices I can think of. The first is to use the unexport make command to tell make to not send that variable in the command's environment.
The second is to change the name of the variable in make to something that is not a valid environment variable: make will only export variables whose names are legal shell variables (contain only alphanumeric plus _). So using a name like VAR-local instead of VAR would do it.
The question appear to be extremely clear in the title but the actual request get lost in details so the only other reply left it mostly unanswered. Directly answering to the question in the title, which is very interesting, to define a variable in a Makefile with same name as environment variable you can get its value with printenv:
PATH=${shell printenv PATH}:/opt/bin
echo:
echo $(PATH)
Other techniques to achieve the same result without relying on evaluation with external commands are welcome.
I have some code that creates a variable of some name automatically and assigns some value to it. The code is something like the following:
myVariableName="zappo"
eval "${myVariableName}=zappo_value"
How would I access the value of this variable using the automatically generated name of the variable? So, I'm looking for some code a bit like the following (but working):
eval "echo ${${myVariableName}}"
(... which may be used in something such as myVariableValue="$(eval "echo ${${myVariableName}}")"...).
Thanks muchly for any assistance
If you think this approach is madness and want offer more general advice, the general idea I'm working on is having variables defined in functions in a library with such names as ${usage} and ${prerequisiteFunctions}. These variables that are defined within functions would be accessed by an interrogation function that can, for instance, ensure that prerequisites etc. are installed. So a loop within this interrogation function is something like this:
for currentFunction in ${functionList}; do
echo "function: ${currentFunction}"
${currentFunction} -interrogate # (This puts the function variables into memory.)
currentInterrogationVariables="${interrogationVariables}" # The variable interrogationVariables contains a list of all function variables available for interrogation.
for currentInterrogationVariable in ${currentInterrogationVariables}; do
echo "content of ${currentInterrogationVariable}:"
eval "echo ${${currentInterrogationVariable}}"
done
done
Thanks again for any ideas!
IIRC, indirection in bash is by !, so try ${!myVariableName}
Try:
echo ${!myVariableName}
It will echo the variable who's name is contained in $myVariableName
For example:
#!/bin/bash
VAR1="ONE"
VAR2="TWO"
VARx="VAR1"
echo ${VARx} # prints "VAR1"
echo ${!VARx} # prints "ONE"
I'm trying to determine the existing HDDs in each system using a for loop as show below, the problem is when I try to set the variable using the below code i get sda=true: command not found. What is the proper way to do this?
#!/bin/bash
for i in a b c d e f
do
grep -q sd$i /proc/partitions
if [ $? == 0 ]
then
sd$i=true
else
sd$i=false
fi
done
You need to use an array or declare:
declare sd$i=true
I would use an array in this case. For example:
$ i=a
$ sd[$i]=true
$ echo ${sd[a]}
true
As another poster stated, if you want to do this without an array, you can instead make a local variable by using syntax like declare sd$i=true. If you want to make a global variable, use export sd$i=true.
BASH FAQ entry #6: "How can I use variable variables (indirect variables, pointers, references) or associative arrays?": "Assigning indirect/reference variables"